1.
Double-planet system
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A planetary system is a set of gravitationally bound non-stellar objects in orbit around a star or star system. The Sun together with its system, which includes Earth, is known as the Solar System. The term exoplanetary system is used in reference to other planetary systems. There are at least 2,701 known planetary systems, including 610 systems consisting of planets as of 1 April 2017. These systems contain more than 3,607 known exoplanets, debris disks are also known to be common, though other objects are more difficult to observe. Of particular interest to astrobiology is the zone of planetary systems where planets could have surface liquid water. Historically, heliocentrism was opposed to geocentrism, the notion of a heliocentric Solar System, with the Sun at the center, is possibly first suggested in the Vedic literature of ancient India, which often refer to the Sun as the centre of spheres. Some interpret Aryabhattas writings in Āryabhaṭīya as implicitly heliocentric, the idea was first proposed in western philosophy and Greek astronomy as early as the 3rd century BC by Aristarchus of Samos, but had received no support from most other ancient astronomers. De revolutionibus orbium coelestium by Nicolaus Copernicus, published in 1543, was the first mathematically predictive heliocentric model of a planetary system and he was burned at the stake for his ideas by the Roman Inquisition. In the 18th century the same possibility was mentioned by Isaac Newton in the General Scholium that concludes his Principia and his theories gained traction through the 19th and 20th centuries despite a lack of supporting evidence. The first confirmed detection of an exoplanet was in 1992, with the discovery of several planets orbiting the pulsar PSR B1257+12. The first confirmed detection of exoplanets of a star was made in 1995. The frequency of detections has increased since then, particularly through advancements in methods of detecting extrasolar planets, planetary systems come from protoplanetary disks that form around stars as part of the process of star formation. During formation of a system much material is gravitationally scattered into far-flung orbits, Planets orbiting pulsars have been discovered, and pulsars are the remnants of the supernova explosions of high-mass stars. Planets found around pulsars may have formed as a result of pre-existing stellar companions that were almost entirely evaporated by the supernova blast, alternatively, planets may form in an accretion disk of fallback matter surrounding a pulsar. Fallback disks of matter that failed to orbit during a supernova may also form planets around black holes. As the star mass, planets that are not engulfed move further out from the star. Planets in evolved binary systems, Hagai B, perets,13 Jan 2011 Can Planets survive Stellar Evolution
2.
Earth
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Earth, otherwise known as the World, or the Globe, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets, according to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago. Earths gravity interacts with objects in space, especially the Sun. During one orbit around the Sun, Earth rotates about its axis over 365 times, thus, Earths axis of rotation is tilted, producing seasonal variations on the planets surface. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earths orientation on its axis, Earths lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earths surface is covered with water, mostly by its oceans, the remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet, Earths interior remains active with a solid iron inner core, a liquid outer core that generates the Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect the Earths atmosphere and surface, some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties, in the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely, over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival. Humans have developed diverse societies and cultures, politically, the world has about 200 sovereign states, the modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō, originally, earth was written in lowercase, and from early Middle English, its definite sense as the globe was expressed as the earth. By early Modern English, many nouns were capitalized, and the became the Earth. More recently, the name is simply given as Earth. House styles now vary, Oxford spelling recognizes the lowercase form as the most common, another convention capitalizes Earth when appearing as a name but writes it in lowercase when preceded by the. It almost always appears in lowercase in colloquial expressions such as what on earth are you doing, the oldest material found in the Solar System is dated to 4. 5672±0.0006 billion years ago. By 4. 54±0.04 Gya the primordial Earth had formed, the formation and evolution of Solar System bodies occurred along with the Sun
3.
Moon
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The Moon is an astronomical body that orbits planet Earth, being Earths only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, following Jupiters satellite Io, the Moon is second-densest satellite among those whose densities are known. The average distance of the Moon from the Earth is 384,400 km, the Moon is thought to have formed about 4.51 billion years ago, not long after Earth. It is the second-brightest regularly visible celestial object in Earths sky, after the Sun and its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its cycle of phases have made the Moon an important cultural influence since ancient times on language, calendars, art. The Moons gravitational influence produces the ocean tides, body tides, and this matching of apparent visual size will not continue in the far future. The Moons linear distance from Earth is currently increasing at a rate of 3.82 ±0.07 centimetres per year, since the Apollo 17 mission in 1972, the Moon has been visited only by uncrewed spacecraft. The usual English proper name for Earths natural satellite is the Moon, the noun moon is derived from moone, which developed from mone, which is derived from Old English mōna, which ultimately stems from Proto-Germanic *mǣnōn, like all Germanic language cognates. Occasionally, the name Luna is used, in literature, especially science fiction, Luna is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of our moon. The principal modern English adjective pertaining to the Moon is lunar, a less common adjective is selenic, derived from the Ancient Greek Selene, from which is derived the prefix seleno-. Both the Greek Selene and the Roman goddess Diana were alternatively called Cynthia, the names Luna, Cynthia, and Selene are reflected in terminology for lunar orbits in words such as apolune, pericynthion, and selenocentric. The name Diana is connected to dies meaning day, several mechanisms have been proposed for the Moons formation 4.51 billion years ago, and some 60 million years after the origin of the Solar System. These hypotheses also cannot account for the angular momentum of the Earth–Moon system. This hypothesis, although not perfect, perhaps best explains the evidence, eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists, You have eighteen months. Go back to your Apollo data, go back to computer, do whatever you have to. Dont come to our conference unless you have something to say about the Moons birth, at the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most popular. Afterward there were only two groups, the giant impact camp and the agnostics. Giant impacts are thought to have been common in the early Solar System, computer simulations of a giant impact have produced results that are consistent with the mass of the lunar core and the present angular momentum of the Earth–Moon system
4.
Pluto
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Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered, Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. After 1992, its planethood was questioned following the discovery of objects of similar size in the Kuiper belt. In 2005, Eris, which is 27% more massive than Pluto, was discovered and this led the International Astronomical Union to define the term planet formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet, Pluto is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris, like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has an eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, light from the Sun takes about 5.5 hours to reach Pluto at its average distance. Pluto has five moons, Charon, Styx, Nix, Kerberos. Pluto and Charon are sometimes considered a system because the barycenter of their orbits does not lie within either body. The IAU has not formalized a definition for binary dwarf planets, on July 14,2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto. During its brief flyby, New Horizons made detailed measurements and observations of Pluto, on October 25,2016, at 05,48 pm ET, the last bit of data was received from New Horizons from its close encounter with Pluto on July 14,2015. In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranuss orbit was being disturbed by another planet besides Neptune, by 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death in 1916, unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7,1915, but they were not recognized for what they were. There are fourteen other known prediscovery observations, with the oldest made by the Yerkes Observatory on August 20,1909. Percivals widow, Constance Lowell, entered into a legal battle with the Lowell Observatory over her late husbands legacy. Tombaughs task was to image the night sky in pairs of photographs, then examine each pair
5.
Charon (moon)
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Charon, also known as Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory in Washington, with half the diameter and one eighth the mass of Pluto, it is a very large moon in comparison to its parent body. Its gravitational influence is such that the barycenter of the Pluto–Charon system lies outside Pluto, the New Horizons spacecraft is the only probe that has visited the Pluto system. It approached Charon to within 27,000 km, Charon was discovered by United States Naval Observatory astronomer James Christy, using the 1. 55-meter Flagstaff telescope two months prior. Christy noticed that a slight elongation appeared periodically, later, the bulge was confirmed on plates dating back to April 29,1965. Subsequent observations of Pluto determined that the bulge was due to an accompanying body. The periodicity of the bulge corresponded to Plutos rotation period, which was known from Plutos light curve. This indicated a synchronous orbit, which suggested that the bulge effect was real. Doubts about Charons existence were erased when it and Pluto entered a period of mutual eclipses. This occurs when the Pluto–Charon orbital plane is edge-on as seen from Earth and it was fortuitous that one of these intervals happened to occur soon after Charons discovery. Charon was originally known by the temporary designation S/1978 P1, on June 24,1978, Christy first suggested the name Charon as a scientific-sounding version of his wife Charlenes nickname, Char. The IAU officially adopted the name in late 1985 and it was announced on January 3,1986, there is minor debate over the preferred pronunciation of the name. These indicate only one pronunciation of Charon when referring specifically to Plutos moon, speakers of many languages other than English, and many English-speaking astronomers as well, follow this pronunciation. However, Christy himself pronounced the ch as sh, after his wife Charlene, simulation work published in 2005 by Robin Canup suggested that Charon could have been formed by a collision around 4.5 billion years ago, much like Earth and the Moon. In this model, a large Kuiper belt object struck Pluto at high velocity, destroying itself and blasting off much of Plutos outer mantle, however, such an impact should result in an icier Charon and rockier Pluto than scientists have found. It is now thought that Pluto and Charon might have been two bodies that collided before going into orbit about each other, the collision would have been violent enough to boil off volatile ices like methane but not violent enough to have destroyed either body. The very similar density of Pluto and Charon implies that the parent bodies were not fully differentiated when the impact occurred, Charon and Pluto orbit each other every 6.387 days. The two objects are locked to one another, so each keeps the same face towards the other
6.
Astronomical object
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An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that current astronomy has demonstrated to exist in the observable universe. In astronomy, the object and body are often used interchangeably. Examples for astronomical objects include planetary systems, star clusters, nebulae and galaxies, while asteroids, moons, planets, and stars are astronomical bodies. A comet may be identified as both body and object, It is a body when referring to the nucleus of ice and dust. The universe can be viewed as having a hierarchical structure, at the largest scales, the fundamental component of assembly is the galaxy. Galaxies are organized groups and clusters, often within larger superclusters. Disc galaxies encompass lenticular and spiral galaxies with features, such as spiral arms, at the core, most galaxies have a supermassive black hole, which may result in an active galactic nucleus. Galaxies can also have satellites in the form of dwarf galaxies, the constituents of a galaxy are formed out of gaseous matter that assembles through gravitational self-attraction in a hierarchical manner. At this level, the fundamental components are the stars. The great variety of forms are determined almost entirely by the mass, composition. Stars may be found in systems that orbit about each other in a hierarchical organization. A planetary system and various objects such as asteroids, comets and debris. The various distinctive types of stars are shown by the Hertzsprung–Russell diagram —a plot of stellar luminosity versus surface temperature. Each star follows a track across this diagram. If this track takes the star through a region containing a variable type. An example of this is the instability strip, a region of the H-R diagram that includes Delta Scuti, RR Lyrae, the table below lists the general categories of bodies and objects by their location or structure. International Astronomical Naming Commission List of light sources List of Solar System objects Lists of astronomical objects SkyChart, Sky & Telescope Monthly skymaps for every location on Earth
7.
Planet
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The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System can be seen with the naked eye and these were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, in 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of mass based on where or what they orbit. The planets were thought by Ptolemy to orbit Earth in deferent, at about the same time, by careful analysis of pre-telescopic observation data collected by Tycho Brahe, Johannes Kepler found the planets orbits were not circular but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the planets share characteristics such as volcanism, hurricanes, tectonics. Planets are generally divided into two types, large low-density giant planets, and smaller rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System, in order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites, several thousands of planets around other stars have been discovered in the Milky Way. e. in the habitable zone. On December 20,2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way, around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include not only in the Solar System. The ambiguities inherent in defining planets have led to much scientific controversy, the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the fixed stars, ancient Greeks called these lights πλάνητες ἀστέρες or simply πλανῆται, from which todays word planet was derived. In ancient Greece, China, Babylon, and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the planets circled Earth. The first civilization known to have a theory of the planets were the Babylonians
8.
International Astronomical Union
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The International Astronomical Union is an international association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy. Among other activities, it acts as the recognized authority for assigning designations to celestial bodies. The IAU is a member of the International Council for Science and its main objective is to promote and safeguard the science of astronomy in all its aspects through international cooperation. The IAU maintains friendly relations with organizations that include amateur astronomers in their membership, the IAU has its head office on the second floor of the Institut dAstrophysique de Paris in the 14th arrondissement of Paris. The IAU is also responsible for the system of astronomical telegrams which are produced and distributed on its behalf by the Central Bureau for Astronomical Telegrams, the Minor Planet Center also operates under the IAU, and is a clearinghouse for all non-planetary or non-moon bodies in the Solar System. The Working Group for Meteor Shower Nomenclature and the Meteor Data Center coordinate the nomenclature of meteor showers, the IAU was founded on July 28,1919, at the Constitutive Assembly of the International Research Council held in Brussels, Belgium. The 7 initial member states were Belgium, Canada, France, Great Britain, Greece, Japan, the first executive committee consisted of Benjamin Baillaud, Alfred Fowler, and four vice presidents, William Campbell, Frank Dyson, Georges Lecointe, and Annibale Riccò. Thirty-two Commissions were appointed at the Brussels meeting and focused on topics ranging from relativity to minor planets, the reports of these 32 Commissions formed the main substance of the first General Assembly, which took place in Rome, Italy, May 2–10,1922. By the end of the first General Assembly, ten nations had joined the Union. Although the Union was officially formed eight months after the end of World War I, the first 50 years of the Unions history are well documented. Subsequent history is recorded in the form of reminiscences of past IAU Presidents, twelve of the fourteen past General Secretaries in the period 1964-2006 contributed their recollections of the Unions history in IAU Information Bulletin No.100. Six past IAU Presidents in the period 1976–2003 also contributed their recollections in IAU Information Bulletin No.104, the IAU includes a total of 12,664 individual members who are professional astronomers from 96 countries worldwide. 83% of all members are male, while 17% are female, among them the unions current president. Membership also includes 79 national members, professional astronomical communities representing their countrys affiliation with the IAU, the sovereign body of the IAU is its General Assembly, which comprises all members. The Assembly determines IAU policy, approves the Statutes and By-Laws of the Union, the right to vote on matters brought before the Assembly varies according to the type of business under discussion. On budget matters, votes are weighted according to the subscription levels of the national members. A second category vote requires a turnout of at least two-thirds of national members in order to be valid, an absolute majority is sufficient for approval in any vote, except for Statute revision which requires a two-thirds majority. An equality of votes is resolved by the vote of the President of the Union, since 1922, the IAU General Assembly meets every three years, with the exception of the period between 1938 and 1948, due to World War II
9.
IAU definition of planet
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A non-satellite body fulfilling only the first two of these criteria is classified as a dwarf planet. According to the IAU, planets and dwarf planets are two classes of objects. A non-satellite body fulfilling only the first criterion is termed a small Solar System body, an alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against this proposal. The definition was a one, and has drawn both support and criticism from different astronomers, but has remained in use. According to this definition, there are eight planets in the Solar System. The definition distinguishes planets from smaller bodies and is not useful outside the Solar System, extrasolar planets, or exoplanets, are covered separately under a complementary 2003 draft guideline for the definition of planets, which distinguishes them from dwarf stars, which are larger. Before the discoveries of the early 21st century, astronomers had no real need for a definition for planets. With the discovery of Pluto in 1930, astronomers considered the Solar System to have nine planets, along with thousands of bodies such as asteroids. Pluto was thought to be larger than Mercury, in 1978, the discovery of Plutos moon Charon radically changed this picture. By measuring Charons orbital period, astronomers could accurately calculate Plutos mass for the first time, in the 1990s, astronomers began finding other objects at least as far away as Pluto, now known as Kuiper Belt objects, or KBOs. Many of these some of Plutos key orbital characteristics and are now called plutinos. Pluto came to be seen as the largest member of a new class of objects, Plutos eccentric and inclined orbit, while very unusual for a planet in the Solar System, fits in well with the other KBOs. New York Citys newly renovated Hayden Planetarium did not include Pluto in its exhibit of the planets when it reopened as the Rose Center for Earth, astronomers also knew that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly. They were also concerned about the classification of planets in other planetary systems, in 2006, the matter came to a head with the first measurement of the size of 2003 UB313. That measurement had showed Eris to appear to be larger than Pluto. The process of new discoveries spurring a contentious refinement of Plutos categorization echoed a debate in the 19th century that began with the discovery of Ceres on January 1,1801, astronomers immediately declared the tiny object to be the missing planet between Mars and Jupiter. Within four years, however, the discovery of two objects with comparable sizes and orbits had cast doubt on this new thinking. By 1851, the number of planets had grown to 23, astronomers began cataloguing them separately and began calling them asteroids instead of planets
10.
SMART-1
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SMART-1 was a Swedish-designed European Space Agency satellite that orbited around the Moon. It was launched on September 27,2003 at 23,14 UTC from the Guiana Space Centre in Kourou, SMART-1 stands for Small Missions for Advanced Research in Technology-1. On September 3,2006, SMART-1 was deliberately crashed into the Moons surface, SMART-1 was about one metre across, and lightweight in comparison to other probes. Its launch mass was 367 kg or 809 pounds, of which 287 kg was non-propellant and it was propelled by a solar-powered Hall effect thruster using xenon propellant, of which there was 82 kg at launch. The thrusters used a field to ionize the xenon and accelerate the ions to a high speed. This ion engine setup achieved a specific impulse of 16.1 kN·s/kg, therefore,1 kg of propellant produced a delta-v of about 45 m/s. The electric propulsion subsystem had a weight of 29 kg with a power consumption of 1,200 watts. SMART-1 is the first in the program of ESAs Small Missions for Advanced Research and Technology. The solar arrays made 1,190 W available for powering the thruster, giving a nominal thrust of 68 mN, as with all ion-engine powered craft, orbital maneuvers were not carried out in short bursts but very gradually. The particular trajectory taken by SMART-1 to the Moon required thrusting for about one third to one half of every orbit, when spiralling away from the Earth thrusting was done on the perigee part of the orbit. SMART-1 was designed and developed by the Swedish Space Corporation on behalf of ESA, assembly of the spacecraft was carried out by Saab Space in Linköping. Tests of the spacecraft were directed by Swedish Space Corporation and executed by Saab Space, the project manager at ESA was Giuseppe Racca and the project manager at the Swedish Space Corporation was Peter Rathsman, the Principal Project Scientist was Bernard Foing. The Advanced Moon micro-Imager Experiment was a colour camera for lunar imaging. The CCD camera with three filters of 750,900 and 950 nm was able to take images with a pixel resolution of 80 m. The camera weighed 2.1 kg and had a consumption of 9 watts. The Demonstration of a Compact X-ray Spectrometer was an X-ray telescope for the identification of elements on the lunar surface. The detection of iron, calcium and titanium depended on the solar activity, the detection range for x-rays was 0.5 to 10 keV. The spectrometer and XSM together weighed 5.2 kg and had a consumption of 18 watts
11.
European Space Agency
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The European Space Agency is an intergovernmental organisation of 22 member states, dedicated to the exploration of space. Established in 1975 and headquartered in Paris, France, ESA has a staff of about 2,000. The main European launch vehicle Ariane 5 is operated through Arianespace with ESA sharing in the costs of launching, after World War II, many European scientists left Western Europe in order to work with the United States. The meeting was attended by representatives from eight countries, including Harrie Massey. The Western European nations decided to have two different agencies, one concerned with developing a system, ELDO, and the precursor of the European Space Agency. The latter was established on 20 March 1964 by an agreement signed on 14 June 1962, from 1968 to 1972, ESRO launched seven research satellites. ESA in its current form was founded with the ESA Convention in 1975, ESA has 10 founding member states, Belgium, Denmark, France, Germany, Italy, the Netherlands, Spain, Sweden, Switzerland and the United Kingdom. These signed the ESA Convention in 1975 and deposited the instruments of ratification by 1980, during this interval the agency functioned in a de facto fashion. ESA launched its first major scientific mission in 1975, Cos-B, ESA joined NASA in the IUE, the worlds first high-orbit telescope, which was launched in 1978 and operated very successfully for 18 years. A number of successful Earth-orbit projects followed, and in 1986 ESA began Giotto, its first deep-space mission, to study the comets Halley and Grigg–Skjellerup. Hipparcos, a mission, was launched in 1989 and in the 1990s SOHO, Ulysses. Recent scientific missions in co-operation with NASA include the Cassini–Huygens space probe, as the successor of ELDO, ESA has also constructed rockets for scientific and commercial payloads. Ariane 1, launched in 1979, brought mostly commercial payloads into orbit from 1984 onward, the successor launch vehicle of Ariane 5, the Ariane 6 is already in the definition stage and is envisioned to enter service in the 2020s. The beginning of the new millennium saw ESA become, along with agencies like NASA, JAXA, ISRO, CSA and Roscosmos, one of the major participants in scientific space research. Although ESA had relied on co-operation with NASA in previous decades, especially the 1990s, changed circumstances led to decisions to rely more on itself, a 2011 press issue thus stated, Russia is ESAs first partner in its efforts to ensure long-term access to space. ESA maintains its scientific and research projects mainly for astronomy-space missions such as Corot, launched on 27 December 2006 and this is the reason space exploration is an integral part of overall space activities. It has always been so, and it will be more important in the future. ESA describes its work in two overlapping ways, For the general public the various fields of work are described as Activities, Member states participate to varying degrees in the mandatory and optional space programmes
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Binary asteroid
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A binary asteroid is a system of two asteroids orbiting their common barycenter. The binary nature of 243 Ida was discovered when the Galileo spacecraft flew by the asteroid in 1993, since then numerous binary asteroids and several triple asteroids have been detected. The mass ratio of the two components of a system is an important characteristic. Most binary asteroids have a mass ratio, i. e. a relatively small satellite in orbit around the main component. Systems with a satellite, or moon, include 22 Kalliope,45 Eugenia,87 Sylvia,107 Camilla,121 Hermione,130 Elektra,243 Ida,283 Emma. Some binary systems have a ratio near unity, i. e. two components of similar mass. They include 90 Antiope and Hermes, several theories have been posited to explain the formation of binary-asteroid systems. The satellites orbiting large main-belt asteroids such as 22 Kalliope,45 Eugenia or 87 Sylvia may have formed by disruption of a parent body after impact or fission after an oblique impact. Trans-Neptunian binaries may have formed during the formation of the Solar System by mutual capture or three-body interaction, near-Earth asteroids, which orbit in the inner part of the Solar System, most likely form by spin-up and mass shedding, likely as a result of the YORP effect. Numerical simulations confirm that when solar energy spins a “rubble pile” asteroid to a sufficiently fast rate by the YORP effect and this process also exposes fresh material at the poles of the asteroid. 2001 SN263 and 1994 CC Minor-planet moon Yarkovsky–OKeefe–Radzievskii–Paddack effect
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Minor planet
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A minor planet is an astronomical object in direct orbit around the Sun that is neither a planet nor exclusively classified as a comet. Minor planets can be dwarf planets, asteroids, trojans, centaurs, Kuiper belt objects, as of 2016, the orbits of 709,706 minor planets were archived at the Minor Planet Center,469,275 of which had received permanent numbers. The first minor planet to be discovered was Ceres in 1801, the term minor planet has been used since the 19th century to describe these objects. The term planetoid has also used, especially for larger objects such as those the International Astronomical Union has called dwarf planets since 2006. Historically, the asteroid, minor planet, and planetoid have been more or less synonymous. This terminology has become complicated by the discovery of numerous minor planets beyond the orbit of Jupiter. A Minor planet seen releasing gas may be classified as a comet. Before 2006, the IAU had officially used the term minor planet, during its 2006 meeting, the IAU reclassified minor planets and comets into dwarf planets and small Solar System bodies. Objects are called dwarf planets if their self-gravity is sufficient to achieve hydrostatic equilibrium, all other minor planets and comets are called small Solar System bodies. The IAU stated that the minor planet may still be used. However, for purposes of numbering and naming, the distinction between minor planet and comet is still used. Hundreds of thousands of planets have been discovered within the Solar System. The Minor Planet Center has documented over 167 million observations and 729,626 minor planets, of these,20,570 have official names. As of March 2017, the lowest-numbered unnamed minor planet is 1974 FV1, as of March 2017, the highest-numbered named minor planet is 458063 Gustavomuler. There are various broad minor-planet populations, Asteroids, traditionally, most have been bodies in the inner Solar System. Near-Earth asteroids, those whose orbits take them inside the orbit of Mars. Further subclassification of these, based on distance, is used, Apohele asteroids orbit inside of Earths perihelion distance. Aten asteroids, those that have semi-major axes of less than Earths, Apollo asteroids are those asteroids with a semimajor axis greater than Earths, while having a perihelion distance of 1.017 AU or less. Like Aten asteroids, Apollo asteroids are Earth-crossers, amor asteroids are those near-Earth asteroids that approach the orbit of Earth from beyond, but do not cross it
14.
69230 Hermes
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69230 Hermes is an Apollo, Mars- and Venus-crosser binary asteroid that passed Earth at about twice the distance of the Moon on October 30,1937. It is named after the Greek god Hermes, at the time, this was the closest known approach of an asteroid to the Earth. Not until 1989 was a closer approach observed, at closest approach, Hermes was moving 5° per hour across the sky and reached 8th magnitude. It was discovered by Karl Reinmuth in images taken at Heidelberg Observatory on October 28,1937, only four days of observations could be made before Hermes became too faint to be seen in the telescopes of the day. This was not enough to calculate an orbit, and Hermes was lost and it thus did not receive a number, but Reinmuth nevertheless named it after the Greek god Hermes. It was the only unnumbered but named asteroid, having only the provisional designation 1937 UB, on October 15,2003, Brian A. Skiff of the LONEOS project made an observation that, when the orbit was calculated backwards in time. The orbit is now known, and Hermes has been assigned sequential number 69230. In retrospect it turned out that Hermes came even closer to the Earth in 1942 than in 1937, within 1.7 Moon distances, without being observed. On October 30,1937, it passed 0.00494 AU from the Earth and on April 26,1942, Hermes is an S-type asteroid, a classification first reported by Andy Rivkin and Richard Binzel. Radar observations led by Jean-Luc Margot at Arecibo Observatory and Goldstone in October and November 2003 showed Hermes to be a binary asteroid, the primary and secondary components have nearly identical radii of 300–450 m, and their orbital separation is only 1,200 metres. In the 1978 novel The Hermes Fall by John Baxter, the asteroid endangers the Earth in 1980, lightcurve plot of 69230 Hermes, Palmer Divide Observatory, B. D. Warner Arecibo 2003 press release Hermes radar results at UCLA Asteroids with Satellites, Robert Johnston, johnstonsarchive
15.
90 Antiope
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90 Antiope is a double asteroid in the outer asteroid belt. It was discovered on October 1,1866, by Robert Luther, in 2000, it was found to consist of two almost-equally-sized bodies orbiting each other. At average diameters of about 88 km and 84 km, both components are among the 500 largest asteroids, Antiope is a member of the Themis family of asteroids that share similar orbital elements. The number in 90 Antiopes name denotes that it was the 90th asteroid to be discovered and its proper name comes from Greek mythology, but it is disputed whether this is Antiope the Amazon or Antiope the mother of Amphion and Zethus. However, the name Antiope is also used to refer to the system as a whole. The most remarkable feature of Antiope is that it consists of two components of almost equal size, making it a double asteroid. Its binary nature was discovered on 10 August 2000 by a group of astronomers using adaptive optics at the Keck Telescope on Mauna Kea, before this, IRAS observations had suggested that the asteroid was 120 km in diameter. Antiope orbits in the third of the core region of the asteroid belt. Since each component is about 86±1 km across, with their centers separated by only about 171 kilometers, as a result, the two bodies orbit around the common center of mass which lies in the space between them. The orbital period is approximately 16.50 hours, and the eccentricity below 0.006, every several years, a period of mutual occultations occurs when the asteroid is viewed from Earth. Using Keplers third law, the mass and density of the components can be derived from the orbital period, the axis of the mutual orbit of the two components points towards ecliptic coordinates = with 2 degrees uncertainty. This is tilted about 63° to the orbit of the system. Antiope itself has an diameter of about 88 km, while its twin. Like most bodies in this region, the components of the Antiope system are of the dark C spectral type, the shape model is consistent with slightly non-spherical components, having a size ratio of 0.95, and exhibiting equilibrium figures for homogeneous rotating bodies. A comparison with grazing occultation event lightcurves taken in 2003 suggests that the shape of the components do not depart much from Roche equilibrium figures. The impactor is calculated to have more than 17 km in diameter. The crater can not be resolved using the W. M. Keck II telescope, another observed stellar occultation by Antiope was reported on June 11,1980. Discovery of Companions to Asteroids 762 Pulcova and 90 Antiope SWrI Press Release, an Occultation by the double asteroid Antiope seen in California Beatty, Kelly
16.
Kuiper belt
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It is similar to the asteroid belt, but it is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies, although many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles, such as methane, ammonia and water. The Kuiper belt is home to three officially recognized dwarf planets, Pluto, Haumea, and Makemake, some of the Solar Systems moons, such as Neptunes Triton and Saturns Phoebe, are also thought to have originated in the region. The Kuiper belt was named after Dutch-American astronomer Gerard Kuiper, though he did not actually predict its existence, in 1992,1992 QB1 was discovered, the first Kuiper belt object since Pluto. Since its discovery, the number of known KBOs has increased to over a thousand, the Kuiper belt should not be confused with the theorized Oort cloud, which is a thousand times more distant and is mostly spherical. The objects within the Kuiper belt, together with the members of the scattered disc, Pluto is the largest and most-massive member of the Kuiper belt and the largest and the second-most-massive known TNO, surpassed only by Eris in the scattered disc. Originally considered a planet, Plutos status as part of the Kuiper belt caused it to be reclassified as a planet in 2006. It is compositionally similar to other objects of the Kuiper belt, and its orbital period is characteristic of a class of KBOs, known as plutinos. After the discovery of Pluto in 1930, many speculated that it not be alone. The region now called the Kuiper belt was hypothesized in various forms for decades and it was only in 1992 that the first direct evidence for its existence was found. The number and variety of speculations on the nature of the Kuiper belt have led to continued uncertainty as to who deserves credit for first proposing it. The first astronomer to suggest the existence of a population was Frederick C. That same year, astronomer Armin O. Leuschner suggested that Pluto may be one of many long-period planetary objects yet to be discovered. Kuiper was operating on the common in his time that Pluto was the size of Earth and had therefore scattered these bodies out toward the Oort cloud or out of the Solar System. Were Kuipers hypothesis correct, there would not be a Kuiper belt today, the hypothesis took many other forms in the following decades. Cameron postulated the existence of a mass of small material on the outskirts of the solar system. Observation, however, ruled out this hypothesis, in 1977, Charles Kowal discovered 2060 Chiron, an icy planetoid with an orbit between Saturn and Uranus. He used a blink comparator, the device that had allowed Clyde Tombaugh to discover Pluto nearly 50 years before
17.
Dwarf planet
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A dwarf planet is a planetary-mass object that is neither a planet nor a natural satellite. The International Astronomical Union currently recognizes five dwarf planets, Ceres, Pluto, Haumea, Makemake, another hundred or so known objects in the Solar System are suspected to be dwarf planets. Individual astronomers recognize several of these, and in August 2011 Mike Brown published a list of 390 candidate objects, Stern states that there are more than a dozen known dwarf planets. Only two of these bodies, Ceres and Pluto, have observed in enough detail to demonstrate that they actually fit the IAUs definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto and they subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 are to be named under the assumption that they are dwarf planets. The classification of bodies in other systems with the characteristics of dwarf planets has not been addressed. Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter which were for some decades considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the asteroid for the smaller bodies. With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets and it was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was more than ten times as massive as the largest object in the asteroid belt, Ceres. In the 1990s, astronomers began to find objects in the region of space as Pluto. Many of these shared several of Plutos key orbital characteristics, and Pluto started being seen as the largest member of a new class of objects and this led some astronomers to stop referring to Pluto as a planet. Several terms, including subplanet and planetoid, started to be used for the now known as dwarf planets. By 2005, three trans-Neptunian objects comparable in size to Pluto had been reported and it became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified. Astronomers were also confident that more objects as large as Pluto would be discovered, Eris was discovered in January 2005, it was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet. As a consequence, the became a matter of intense debate during the IAU General Assembly in August 2006. The IAUs initial draft proposal included Charon, Eris, and Ceres in the list of planets, dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits. The IAUs final Resolution 5A preserved this three-category system for the bodies orbiting the Sun
18.
Eris (dwarf planet)
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Eris is the most massive and second-largest dwarf planet known in the Solar System. It is also the known body directly orbiting the Sun. It is measured to be 2,326 ±12 kilometers in diameter, Eris is 27% more massive than dwarf planet Pluto, though Pluto is slightly larger by volume. Eris mass is about 0. 27% of the Earths mass, Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity was verified later that year. It is an object and a member of a high-eccentricity population known as the scattered disk. It has one moon, Dysnomia. As of February 2016, its distance from the Sun is 96.3 astronomical units, because Eris appeared to be larger than Pluto, NASA initially described it as the Solar Systems tenth planet. This, along with the prospect of other objects of similar size being discovered in the future, motivated the International Astronomical Union to define the term planet for the first time. Observations of an occultation by Eris in 2010 showed that its diameter was 2,326 ±12 kilometers, very slightly less than Pluto. Eris was discovered by the team of Mike Brown, Chad Trujillo, the discovery was announced on July 29,2005, the same day as Makemake and two days after Haumea, due in part to events that would later lead to controversy about Haumea. Routine observations were taken by the team on October 21,2003, in January 2005, the re-analysis revealed Eriss slow motion against the background stars. Follow-up observations were carried out to make a preliminary determination of Eriss orbit. More observations released in October 2005 revealed that Eris has a moon, observations of Dysnomias orbit permitted scientists to determine the mass of Eris, which in June 2007 they calculated to be ×1022 kg, 27%±2% greater than Plutos. Eris is named after the Greek goddess Eris, a personification of strife, the regular adjectival form of Eris is Eridian. As a result, for a time the object known to the wider public as Xena. Xena was a name used internally by the discovery team. It was inspired by the character of the television series Xena. The discovery team had saved the nickname Xena for the first body they discovered that was larger than Pluto
19.
Dysnomia (moon)
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Dysnomia —officially Eris I Dysnomia—is the only known moon of the dwarf planet Eris. Observations taken on 10 September revealed a moon in orbit around Eris, in keeping with the Xena nickname that was already in use for Eris, the moon was nicknamed Gabrielle by its discoverers, after Xenas sidekick. It is 60 times fainter than Eris in the K band and 480 times fainter in the V band, which means a very different, and quite redder, spectrum, indicating a significantly darker surface. Assuming its albedo is five times lower than Eriss, its diameter would be 685±50 km, of the Solar Systems dwarf planet and minor planet moons, only Charon is definitely larger than Dysnomia. Combining Keck and Hubble observations, the satellite was used to determine the mass of Eris and its orbital period is calculated to be 15. 774±0.002 d. These observations indicate that Dysnomia has an orbit around Eris. This shows that the mass of Eris is 1.27 times that of Pluto, astronomers now know that the four brightest Kuiper belt objects have satellites. Among the fainter members of the only about 10% are known to have satellites. This is thought to imply that collisions between large KBOs have been frequent in the past, impacts between bodies of the order of 1000 km across would throw off large amounts of material that would coalesce into a moon. A similar mechanism is thought to have led to the formation of the Moon when Earth was struck by a giant impactor early in the history of the Solar System. Mike Brown, the discoverer, chose the name Dysnomia due to a number of associations it had for him. Dysnomia, the daughter of Eris, fits the general historically established pattern of naming moons after lesser gods associated with the primary god, also, the English translation of Dysnomia, lawlessness, echoes Lucy Lawless, the actress famous for starring in Xena, Warrior Princess on television. Before receiving their names, Eris and Dysnomia were nicknamed Xena and Gabrielle. James Christy, who discovered Charon, followed the principle established with Pluto by choosing a name which shared its first four letters with his wifes name, Charlene. Dysnomia, similarly, has the same first letter as Browns wife, Diane, and Brown uses the nickname Dy /ˈdaɪ/ for the moon, because of this, Brown pronounces the full name /daɪsˈnoʊmiə/, with a long y. Media related to Dysnomia at Wikimedia Commons
20.
Barycenter
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The barycenter is the center of mass of two or more bodies that are orbiting each other, or the point around which they both orbit. It is an important concept in such as astronomy and astrophysics. The distance from a center of mass to the barycenter can be calculated as a simple two-body problem. In cases where one of the two objects is more massive than the other, the barycenter will typically be located within the more massive object. Rather than appearing to orbit a center of mass with the smaller body. This is the case for the Earth–Moon system, where the barycenter is located on average 4,671 km from the Earths center, when the two bodies are of similar masses, the barycenter will generally be located between them and both bodies will follow an orbit around it. This is the case for Pluto and Charon, as well as for many binary asteroids and it is also the case for Jupiter and the Sun, despite the thousandfold difference in mass, due to the relatively large distance between them. In astronomy, barycentric coordinates are non-rotating coordinates with the origin at the center of mass of two or more bodies, the International Celestial Reference System is a barycentric one, based on the barycenter of the Solar System. In geometry, the barycenter is synonymous with centroid, the geometric center of a two-dimensional shape. The barycenter is one of the foci of the orbit of each body. This is an important concept in the fields of astronomy and astrophysics. If a is the distance between the centers of the two bodies, r1 is the axis of the primarys orbit around the barycenter. When the barycenter is located within the massive body, that body will appear to wobble rather than to follow a discernible orbit. The following table sets out some examples from the Solar System, figures are given rounded to three significant figures. If Jupiter had Mercurys orbit, the Sun–Jupiter barycenter would be approximately 55,000 km from the center of the Sun, but even if the Earth had Eris orbit, the Sun–Earth barycenter would still be within the Sun. To calculate the motion of the Sun, you would need to sum all the influences from all the planets, comets, asteroids. If all the planets were aligned on the side of the Sun. The calculations above are based on the distance between the bodies and yield the mean value r1
21.
New Horizons
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New Horizons is an interplanetary space probe that was launched as a part of NASAs New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory and the Southwest Research Institute, on January 19,2006, New Horizons was launched from Cape Canaveral Air Force Station directly into an Earth-and-solar escape trajectory with a speed of about 16.26 kilometers per second. After a brief encounter with asteroid 132524 APL, New Horizons proceeded to Jupiter, making its closest approach on February 28,2007, most of the post-Jupiter voyage was spent in hibernation mode to preserve on-board systems, except for brief annual checkouts. On December 6,2014, New Horizons was brought online for the Pluto encounter. On January 15,2015, the New Horizons spacecraft began its approach phase to Pluto, on July 14,2015, at 11,49 UTC, it flew 12,500 km above the surface of Pluto, making it the first spacecraft to explore the dwarf planet. On October 25,2016, at 21,48 UTC, the last of the recorded data from the Pluto flyby was received from New Horizons. Having completed its flyby of Pluto, New Horizons has maneuvered for a flyby of Kuiper belt object 2014 MU69, expected to place on January 1,2019. Appointed as the principal investigator, Stern was described by Krimigis as the personification of the Pluto mission. New Horizons was based largely on Sterns work since Pluto 350, the New Horizons proposal was one of five that were officially submitted to NASA. It was later selected as one of two finalists to be subject to a concept study, in June 2001. In November 2001, New Horizons was officially selected for funding as part of the New Frontiers program. However, the new NASA Administrator appointed by the Bush Administration, Sean OKeefe, was not supportive of New Horizons, after an intense campaign to gain support for New Horizons, the Planetary Science Decadal Survey of 2003-2013 was published in the summer of 2002. New Horizons topped the list of projects considered the highest priority among the community in the medium-size category, ahead of missions to the Moon. Weiler stated that it was a result that administration was not going to fight, Alice Bowman became Mission Operations Manager. New Horizons is the first mission in NASAs New Frontiers mission category, larger and more expensive than the Discovery missions, the cost of the mission is approximately $700 million over 15 years. The spacecraft was built primarily by Southwest Research Institute and the Johns Hopkins Applied Physics Laboratory, the missions principal investigator is Alan Stern of the Southwest Research Institute. After separation from the vehicle, overall control was taken by Mission Operations Center at the Applied Physics Laboratory in Howard County. The science instruments are operated at Clyde Tombaugh Science Operations Center in Boulder, New Horizons was originally planned as a voyage to the only unexplored planet in the Solar System
22.
Star
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A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun, many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, indeed, most are invisible from Earth even through the most powerful telescopes. Almost all naturally occurring elements heavier than helium are created by stellar nucleosynthesis during the stars lifetime, near the end of its life, a star can also contain degenerate matter. Astronomers can determine the mass, age, metallicity, and many properties of a star by observing its motion through space, its luminosity. The total mass of a star is the factor that determines its evolution. Other characteristics of a star, including diameter and temperature, change over its life, while the environment affects its rotation. A plot of the temperature of stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that allows the age and evolutionary state of that star to be determined. A stars life begins with the collapse of a gaseous nebula of material composed primarily of hydrogen, along with helium. When the stellar core is sufficiently dense, hydrogen becomes steadily converted into helium through nuclear fusion, the remainder of the stars interior carries energy away from the core through a combination of radiative and convective heat transfer processes. The stars internal pressure prevents it from collapsing further under its own gravity, a star with mass greater than 0.4 times the Suns will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements at the core or in shells around the core, as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled later as new stars. Meanwhile, the core becomes a remnant, a white dwarf. Binary and multi-star systems consist of two or more stars that are bound and generally move around each other in stable orbits. When two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, historically, stars have been important to civilizations throughout the world
23.
Brown dwarf
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Below this range are the sub-brown dwarfs, and above it are the lightest red dwarfs. Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth, unlike the stars in the main-sequence, brown dwarfs are not massive enough to sustain nuclear fusion of ordinary hydrogen to helium in their cores. They are, however, thought to fuse deuterium and to fuse lithium if their mass is above a threshold of 13 MJ and 65 MJ. It is also debated whether brown dwarfs would be defined by their formation processes rather than by their supposed nuclear fusion reactions. Stars are categorized by spectral class, with brown dwarfs designated as types M, L, T, despite their name, brown dwarfs are of different colors. Many brown dwarfs would likely appear magenta to the human eye, Brown dwarfs are not very luminous at visible wavelengths. Planets are known to orbit some brown dwarfs, 2M1207b, MOA-2007-BLG-192Lb, at a distance of about 6.5 light years, the nearest known brown dwarf is Luhman 16, a binary system of brown dwarfs discovered in 2013. DENIS-P J082303. 1-491201 b is listed as the known exoplanet in NASAs exoplanet archive. However, a) the term black dwarf was already in use to refer to a white dwarf, b) red dwarfs fuse hydrogen. Because of this, alternate names for these objects were proposed, in 1975, Jill Tarter suggested the term brown dwarf, using brown as an approximate color. The term black dwarf still refers to a dwarf that has cooled to the point that it no longer emits significant light. The first self-consistent calculation of the minimum mass confirmed a value between 0.08 and 0.07 solar masses for population I objects. The discovery of deuterium burning down to 0.012 solar masses, however, such objects were hard to find as they emit almost no visible light. Their strongest emissions are in the spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs. Since then, numerous searches by various methods have sought these objects, for many years, efforts to discover brown dwarfs were fruitless. In 1988, however, a faint companion to a known as GD165 was found in an infrared search of white dwarfs. The spectrum of the companion GD 165B was very red and enigmatic and it became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until the advent of the Two Micron All Sky Survey which discovered many objects with similar colors, today, GD 165B is recognized as the prototype of a class of objects now called L dwarfs
24.
Isaac Asimov
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Isaac Asimov was an American writer and professor of biochemistry at Boston University. He was known for his works of fiction and popular science. Asimov was a writer, and wrote or edited more than 500 books. His books have published in 9 of the 10 major categories of the Dewey Decimal Classification. Asimov wrote hard science fiction and, along with Robert A. Heinlein, Clarke, he was considered one of the Big Three science fiction writers during his lifetime. Asimovs most famous work is the Foundation Series, his major series are the Galactic Empire series. The Galactic Empire novels are set in earlier history of the same fictional universe as the Foundation series. He wrote hundreds of stories, including the social science fiction Nightfall. Asimov wrote the Lucky Starr series of juvenile science-fiction novels using the pen name Paul French, Asimov also wrote mysteries and fantasy, as well as much nonfiction. Most of his science books explain scientific concepts in a historical way. He often provides nationalities, birth dates, and death dates for the scientists he mentions, as well as etymologies, Asimov was a long-time member and vice president of Mensa International, albeit reluctantly, he described some members of that organization as brain-proud and aggressive about their IQs. He took more joy in being president of the American Humanist Association, the asteroid 5020 Asimov, a crater on the planet Mars, a Brooklyn elementary school, and a literary award are named in his honor. His exact date of birth within that range is unknown, the family name derives from a word for winter crops, in which his great-grandfather dealt. This word is spelled озимые in Russian, and азімыя in Belarusian, phonetically, both words are almost identical because in Russian О in the first unstressed syllable is always pronounced as А. Accordingly, his name originally was Исаак Озимов in Russian, however, he was known in Russia as Ayzek Azimov. Asimov had two siblings, a sister, Marcia, and a brother, Stanley, who was vice-president of New York Newsday. His family emigrated to the United States when he was three years old, since his parents always spoke Yiddish and English with him, he never learned Russian, but he remained fluent in Yiddish as well as English. Growing up in Brooklyn, New York, Asimov taught himself to read at the age of five, after becoming established in the U. S. his parents owned a succession of candy stores, in which everyone in the family was expected to work
25.
Gravity
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Gravity, or gravitation, is a natural phenomenon by which all things with mass are brought toward one another, including planets, stars and galaxies. Since energy and mass are equivalent, all forms of energy, including light, on Earth, gravity gives weight to physical objects and causes the ocean tides. Gravity has a range, although its effects become increasingly weaker on farther objects. The most extreme example of this curvature of spacetime is a hole, from which nothing can escape once past its event horizon. More gravity results in time dilation, where time lapses more slowly at a lower gravitational potential. Gravity is the weakest of the four fundamental interactions of nature, the gravitational attraction is approximately 1038 times weaker than the strong force,1036 times weaker than the electromagnetic force and 1029 times weaker than the weak force. As a consequence, gravity has an influence on the behavior of subatomic particles. On the other hand, gravity is the dominant interaction at the macroscopic scale, for this reason, in part, pursuit of a theory of everything, the merging of the general theory of relativity and quantum mechanics into quantum gravity, has become an area of research. While the modern European thinkers are credited with development of gravitational theory, some of the earliest descriptions came from early mathematician-astronomers, such as Aryabhata, who had identified the force of gravity to explain why objects do not fall out when the Earth rotates. Later, the works of Brahmagupta referred to the presence of force, described it as an attractive force. Modern work on gravitational theory began with the work of Galileo Galilei in the late 16th and this was a major departure from Aristotles belief that heavier objects have a higher gravitational acceleration. Galileo postulated air resistance as the reason that objects with less mass may fall slower in an atmosphere, galileos work set the stage for the formulation of Newtons theory of gravity. In 1687, English mathematician Sir Isaac Newton published Principia, which hypothesizes the inverse-square law of universal gravitation. Newtons theory enjoyed its greatest success when it was used to predict the existence of Neptune based on motions of Uranus that could not be accounted for by the actions of the other planets. Calculations by both John Couch Adams and Urbain Le Verrier predicted the position of the planet. A discrepancy in Mercurys orbit pointed out flaws in Newtons theory, the issue was resolved in 1915 by Albert Einsteins new theory of general relativity, which accounted for the small discrepancy in Mercurys orbit. The simplest way to test the equivalence principle is to drop two objects of different masses or compositions in a vacuum and see whether they hit the ground at the same time. Such experiments demonstrate that all objects fall at the rate when other forces are negligible
26.
Titan (moon)
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Titan is the largest moon of Saturn. It is the only known to have a dense atmosphere. Titan is the sixth ellipsoidal moon from Saturn, frequently described as a planet-like moon, Titan is 50% larger than Earths Moon, and it is 80% more massive. It is the second-largest moon in the Solar System, after Jupiters moon Ganymede, and is larger than the smallest planet, Mercury, discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, and the sixth known planetary satellite. Titan orbits Saturn at 20 Saturn radii, from Titans surface, Saturn subtends an arc of 5.09 degrees and would appear 11.4 times larger in the sky than the Moon from Earth. Titan is primarily composed of ice and rocky material. The geologically young surface is smooth, with few impact craters, although mountains. The atmosphere of Titan is largely nitrogen, minor components lead to the formation of methane and ethane clouds and nitrogen-rich organic smog. The climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, rivers, lakes, seas, and deltas, Huygens was inspired by Galileos discovery of Jupiters four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his brother Constantijn Huygens, Jr. began building telescopes around 1650 and it was the sixth moon to be discovered. He named it Saturni Luna, publishing in the 1655 tract De Saturni Luna Observatio Nova, after Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers fell into the habit of referring to these and Titan as Saturn I through V. Other early epithets for Titan include Saturns ordinary satellite, Titan is officially numbered Saturn VI because after the 1789 discoveries the numbering scheme was frozen to avoid causing any more confusion. Numerous small moons have been discovered closer to Saturn since then and he suggested the names of the mythological Titans, brothers and sisters of Cronus, the Greek Saturn. In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, Titan orbits Saturn once every 15 days and 22 hours. Because of this, there is a point on its surface. Longitudes on Titan are measured westward, starting from the passing through this point. Its orbital eccentricity is 0.0288, and the plane is inclined 0.348 degrees relative to the Saturnian equator. Viewed from Earth, Titan reaches a distance of about 20 Saturn radii from Saturn
27.
Phoebe (moon)
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Phoebe is an irregular satellite of Saturn with a mean diameter of 213 km. It was the first satellite to be discovered photographically, Phoebe was the first target encountered upon the arrival of the Cassini spacecraft in the Saturn system in 2004, and is thus unusually well-studied for an irregular satellite of its size. Cassinis trajectory to Saturn and time of arrival were specifically chosen to permit this flyby, after the encounter and its insertion into orbit, Cassini did not go much beyond the orbit of Iapetus. Phoebe is roughly spherical and has a differentiated interior and it was spherical and hot early in its history and was battered out of roundness by repeated impacts. It is thought to be a captured planetesimal from the Kuiper belt and it was the first satellite to be discovered photographically. Phoebe was named after Phoebe, a Titan in Greek mythology that was associated with the Moon and it is also designated Saturn IX in some scientific literature. The IAU nomenclature standards have stated that features on Phoebe are to be named characters in the Greek myth of Jason. In 2005, the IAU officially named 24 craters, Phoebes orbit is retrograde, i. e. it orbits Saturn opposite to Saturns rotation. For more than 100 years, Phoebe was Saturns outermost known moon, Phoebe is almost 4 times more distant from Saturn than its nearest major neighbor, and is substantially larger than any of the other moons orbiting planets at comparable distances. All of Saturns regular moons except Iapetus orbit very nearly in the plane of Saturns equator, the outer irregular satellites follow moderately to highly eccentric orbits, and none are expected to rotate synchronously as all the inner moons of Saturn do. The Phoebe ring is one of the rings of Saturn and this ring is tilted 27 degrees from Saturns equatorial plane. It extends from at least 128 to 207 times the radius of Saturn, the ring is about 20 times as thick as the diameter of the planet. Inwardly migrating ring material would thus strike Iapetuss leading hemisphere, created its two-tone coloration, although very large, the ring is virtually invisible—it was discovered using NASAs infrared Spitzer Space Telescope. Phoebe is roughly spherical and has a diameter of 213±1.4 km, Phoebe rotates every nine hours 16 minutes and it completes a full orbit around Saturn in about 18 months. Its surface temperature is 75 K, most of Saturns inner moons have very bright surfaces, but Phoebes albedo is very low, as dark as lampblack. The Phoebean surface is heavily scarred, with craters up to 80 kilometres across. Phoebes dark coloring initially led to scientists surmising that it was a captured asteroid and these are chemically very primitive and are thought to be composed of original solids that condensed out of the solar nebula with little modification since then. In addition, quantities of carbon dioxide have been detected on the surface and it is estimated that Phoebe is about 50% rock, as opposed to the 35% or so that typifies Saturns inner moons
28.
Orbit of the Moon
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Not to be confused with Lunar orbit. The Moon orbits Earth in the direction and completes one revolution relative to the stars in approximately 27.323 days. Earth and the Moon orbit about their barycentre, which lies about 4,600 km from Earths center, on average, the Moon is at a distance of about 385,000 km from Earths centre, which corresponds to about 60 Earth radii. With a mean velocity of 1.022 km/s, the Moon appears to move relative to the stars each hour by an amount roughly equal to its angular diameter. The Moon differs from most satellites of planets in that its orbit is close to the plane of the ecliptic. The plane of the orbit is inclined to the ecliptic by about 5°. The properties of the orbit described in this section are approximations, the Moons orbit around Earth has many irregularities, whose study has a long history. The orbit of the Moon is distinctly elliptical, with an eccentricity of 0.0549. The non-circular form of the lunar orbit causes variations in the Moons angular speed and apparent size as it moves towards, the mean angular movement relative to an imaginary observer at the barycentre is 13. 176° per day to the east. The Moons elongation is its angular distance east of the Sun at any time, at new moon, it is zero and the Moon is said to be in conjunction. At full moon, the elongation is 180° and it is said to be in opposition, in both cases, the Moon is in syzygy, that is, the Sun, Moon and Earth are nearly aligned. When elongation is either 90° or 270°, the Moon is said to be in quadrature, the orientation of the orbit is not fixed in space, but rotates over time. This orbital precession is also called apsidal precession and is the rotation of the Moons orbit within the orbital plane, the Moons apsidal precession is distinct from, and should not be confused with its axial precession. The mean inclination of the orbit to the ecliptic plane is 5. 145°. The rotational axis of the Moon is also not perpendicular to its plane, so the lunar equator is not in the plane of its orbit. Therefore, the angle between the ecliptic and the equator is always 1. 543°, even though the rotational axis of the Moon is not fixed with respect to the stars. The period from moonrise to moonrise at the poles is quite close to the sidereal period, when the sun is the furthest below the horizon, the moon will be full when it is at its highest point. The nodes are points at which the Moons orbit crosses the ecliptic, the Moon crosses the same node every 27.2122 days, an interval called the draconic or draconitic month
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Neso (moon)
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Neso, also known as Neptune XIII, is the outermost natural satellite of Neptune. It is an Irregular moon discovered by Matthew J. Holman, Brett J. Gladman, et al. on August 14,2002, Neso orbits Neptune at a distance of more than 48 Gm, making it the most distant known moon of any planet. At apocenter, the satellite is more than 72 Gm from Neptune and this distance is of such an order that it exceeds Mercurys aphelion, which is approximately 70 Gm from the Sun. Neso is also the moon with the longest orbital period,26.67 years and it follows a highly inclined and highly eccentric orbit illustrated on the diagram in relation to other irregular satellites of Neptune. The satellites above the axis are prograde, the satellites beneath it are retrograde. The yellow segments extend from the pericentre to the apocentre, showing the eccentricity, Neso is about 60 km in diameter based on an assumed albedo, and assuming a mean density of 1.5 g/cm3, its mass is estimated at 2×1017 kg. Given the similarity of the parameters with Psamathe, it was suggested that both irregular satellites could have a common origin in the break-up of a larger moon. Neso is named one of the Nereids. Before the announcement of its name on February 3,2007, Neso was known by its provisional designation, notes Citations MPC, Natural Satellites Ephemeris Service Mean orbital parameters from JPL Matthew Holmans Neptunes page David Jewitts pages Scott Sheppards pages Neso in Fiction
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Psamathe (moon)
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Psamathe, also known as Neptune X, is a retrograde irregular satellite of Neptune. It is named after Psamathe, one of the Nereids, Psamathe was discovered by Scott S. Sheppard and David C. Jewitt in 2003 using the 8.2 meter Subaru telescope, before the announcement of its name on February 3,2007, it was known by the provisional designation S/2003 N1. Psamathe is about 38 kilometers in diameter and it orbits Neptune at a distance of between 25.7 and 67.7 million km and requires almost 25 Earth years to make one orbit. The orbit of this satellite is close to the theoretical stable separation from Neptune for a body in a retrograde orbit, given the similarity of Psamathes orbital parameters with Neso, it was suggested that both irregular satellites could have a common origin in the break-up of a larger moon. Both are further from their primary than any other known moon in the Solar System, irregular satellites Neptunes Known Satellites S/2003 N1 Neptune Satellite Movie Images MPC, Natural Satellites Ephemeris Service Mean orbital parameters IAUC8193
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Giant-impact hypothesis
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The colliding body is sometimes called Theia, from the name of the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. Analysis of lunar rocks, published in 2016, suggests that the impact may have been a direct hit, the giant-impact hypothesis is currently the favoured scientific hypothesis for the formation of the Moon. Supporting evidence includes, Earths spin and the Moons orbit have similar orientations, Moon samples indicate that the Moons surface was once molten. The Moon has a small iron core. The Moon has a lower density than Earth, evidence exists of similar collisions in other star systems. Giant collisions are consistent with the theories of the formation of the solar system. The stable-isotope ratios of lunar and terrestrial rock are identical, implying a common origin, there remain several questions concerning the best current models of the giant-impact hypothesis, however. However, as of 2015 there is no self-consistent model that starts with the giant-impact event, other remaining questions include when the Moon lost its share of volatile elements and why Venus—which experienced giant impacts during its formation—does not host a similar moon. In 1898, George Darwin made the suggestion that the Earth, Darwins hypothesis was that a molten Moon had been spun from the Earth because of centrifugal forces, and this became the dominant academic explanation. Using Newtonian mechanics, he calculated that the Moon had orbited much more closely in the past and was drifting away from the Earth and this drifting was later confirmed by American and Soviet experiments, using laser ranging targets placed on the Moon. Nonetheless, Darwins calculations could not resolve the mechanics required to trace the Moon backward to the surface of the Earth. In 1946, Reginald Aldworth Daly of Harvard University challenged Darwins explanation and their models suggested that, at the end of the planet formation period, several satellite-sized bodies had formed that could collide with the planets or be captured. They proposed that one of objects may have collided with the Earth, ejecting refractory. This collision could potentially explain the unique geological and geochemical properties of the Moon and it is hypothesized that most of the outer silicates of the colliding body would be vaporized, whereas a metallic core would not. Hence, most of the material sent into orbit would consist of silicates. The more volatile materials that were emitted during the collision probably would escape the Solar System, the name of the hypothesized protoplanet is derived from the mythical Greek titan Theia /ˈθiːə/, who gave birth to the Moon goddess Selene. This designation was proposed initially by the English geochemist Alex N. Halliday in 2000 and has accepted in the scientific community. According to modern theories of formation, Theia was part of a population of Mars-sized bodies that existed in the Solar System 4.5 billion years ago
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Origin of the Moon
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Origin of the Moon refers to any of the various explanations for the formation of the Moon, Earths natural satellite. The leading theory has been the giant-impact hypothesis, however, research continues on this matter, and there are a number of variations and alternatives. Other proposed scenarios include captured body, fission, formed together, planetesimal collisions, the standard giant-impact hypothesis suggests a Mars-sized body called Theia impacted Earth, creating a large debris ring around Earth, which then accreted to form the Moon. This collision also resulted in the 23. 5° tilted axis of the earth, however, the Moons oxygen isotopic ratios seem to be essentially identical to Earths. Oxygen isotopic ratios, which may be measured precisely, yield a unique. If Theia had been a separate protoplanet, it probably would have had a different oxygen isotopic signature from Earth, also, the Moons titanium isotope ratio appears so close to the Earths that little if any of the colliding bodys mass could likely have been part of the Moon. Some theories have been stated that presume the Earth had no large moons early in the formation of the Solar System,4.6 billion years ago, Earth being basically rock and lava. Theia, an early protoplanet the size of Mars, hit Earth in such a way that it ejected a considerable amount of material away from Earth. Some proportion of this ejecta escaped into space, but the rest consolidated into a body in orbit about Earth. The hypothesis requires a collision between a body about 90% of the present size of Earth, and another the diameter of Mars, the colliding body has sometimes been referred to as Theia, the mother of Selene, the Moon goddess in Greek mythology. This size ratio is needed in order for the system to have sufficient angular momentum to match the current orbital configuration. Such an impact would have put enough material into orbit around Earth to have accumulated to form the Moon. Computer simulations show a need for a blow, which causes a portion of the collider to form a long arm of material that then shears off. The asymmetrical shape of the Earth following the collision then causes this material to settle into an orbit around the main mass, the energy involved in this collision is impressive, possibly trillions of tons of material would have been vaporized and melted. In parts of the Earth, the temperature would have risen to 10,000 °C, the Moons relatively small iron core is explained by Theias core accreting into that of Earth. The lack of volatiles in the samples is also explained in part by the energy of the collision. The energy liberated during the reaccreation of material in orbit around Earth would have been sufficient to melt a large portion of the Moon, along the way, the Moons rotation became tidally locked to Earth, so that one side of the Moon continually faces toward Earth. Also, the Moon would have collided with and incorporated any small pre-existing satellites of Earth, the geology of the Moon has since been more independent of the Earth
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Solar System
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The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets, of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury. The Solar System formed 4.6 billion years ago from the collapse of a giant interstellar molecular cloud. The vast majority of the mass is in the Sun. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being composed of rock. The four outer planets are giant planets, being more massive than the terrestrials. All planets have almost circular orbits that lie within a flat disc called the ecliptic. The Solar System also contains smaller objects, the asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptunes orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets, identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds. Six of the planets, at least four of the dwarf planets, each of the outer planets is encircled by planetary rings of dust and other small objects. The solar wind, a stream of charged particles flowing outwards from the Sun, the heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium, it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, the Solar System is located in the Orion Arm,26,000 light-years from the center of the Milky Way. For most of history, humanity did not recognize or understand the concept of the Solar System, the invention of the telescope led to the discovery of further planets and moons. The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99. 86% of the known mass. The Suns four largest orbiting bodies, the giant planets, account for 99% of the mass, with Jupiter. The remaining objects of the Solar System together comprise less than 0. 002% of the Solar Systems total mass, most large objects in orbit around the Sun lie near the plane of Earths orbit, known as the ecliptic
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Moons of Mars
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The two moons of Mars are Phobos and Deimos. Both moons were discovered in 1877 by Asaph Hall and are named after the characters Phobos and Deimos who, in Greek mythology, accompanied their father Ares, god of war, Ares was known as Mars to the Romans. It is possible that Mars may have moons smaller than 50–100 meters, of the two moons, Phobos orbits closer to Mars with a semi-major axis of 9,377 km. Deimos orbits Mars with an axis of 23,460 km.5 hours. The actual orbital distances of Phobos and Deimos are 1.4 and 3.5 Martian diameters, and their respective orbital periods are 7.6 and 30.3 hours. In the 20th century, V. G. Perminov, a designer of early Soviet Mars and Venus spacecraft, speculated Swift found. Furthermore, as they had not yet discovered, it was reasoned that they must be small. This would lead Swift to making an accurate estimate of their orbital distances. In addition Swift could have helped in his calculations by his friend. Voltaires 1752 short story Micromégas, about an alien visitor to Earth, Voltaire was presumably influenced by Swift. In recognition of these predictions, two craters on Deimos are named Swift and Voltaire. Asaph Hall discovered Deimos on 12 August 1877 at about 07,48 UTC and Phobos on 18 August 1877, at the US Naval Observatory in Washington, at the time, he was deliberately searching for Martian moons. Hall had previously seen what appeared to be a Martian moon on 10 August, I had barely time to secure an observation of its position when fog from the River stopped the work. This was at half past two oclock on the night of the 11th, cloudy weather intervened for several days. On 15 August the weather looking more promising, I slept at the Observatory, the sky cleared off with a thunderstorm at 11 oclock and the search was resumed. I told him also to keep quiet as I did not wish anything said until the matter was beyond doubt and he said nothing, but the thing was too good to keep and I let it out myself. On 17 August while waiting and watching for the outer moon, the observations of the 17th and 18th put beyond doubt the character of these objects and the discovery was publicly announced by Admiral Rodgers. The telescope used for the discovery was the 26-inch refractor then located at Foggy Bottom, in 1893 the lens was remounted and put in a new dome, where it remains into the 21st century
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Phobos (moon)
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Phobos is the innermost and larger of the two natural satellites of Mars, the other being Deimos. Both moons were discovered in 1877 by American astronomer Asaph Hall, Phobos is a small, irregularly shaped object with a mean radius of 11 km, and is seven times larger than the outer moon, Deimos. Phobos is named after the Greek god Phobos, a son of Ares and Aphrodite, the name Phobos is pronounced /ˈfoʊbəs/ FOH-bəs or /ˈfoʊbɒs/ FOH-bos, or like the Greek Φόβος. Phobos orbits 6,000 km from the Martian surface, closer to its body than any other known planetary moon. It is indeed so close that it orbits Mars much faster than Mars rotates, and completes an orbit in just 7 hours and 39 minutes. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 hours and 15 minutes or less, Phobos is one of the least reflective bodies in the Solar System, with an albedo of just 0.071. Surface temperatures range from about −4 °C on the side to −112 °C on the shadowed side. The defining surface feature is the impact crater, Stickney. Images and models indicate that Phobos may be a rubble pile held together by a thin crust, and that it is being torn apart by tidal interactions. Phobos gets closer to Mars by about 2 meters every one hundred years, Phobos was discovered by astronomer Asaph Hall on 18 August 1877, at the United States Naval Observatory in Washington, D. C. at about 09,14 Greenwich Mean Time. Hall had discovered Deimos, Mars other moon, a few days earlier on 12 August 1877 at about 07,48 UTC. The names, originally spelled Phobus and Deimus respectively, were suggested by Henry Madan, Science Master of Eton, based on Greek mythology, Phobos has dimensions of 27 ×22 ×18 km, and retains too little mass to be rounded under its own gravity. Phobos does not have a due to its low mass. It is one of the least reflective bodies in the Solar System, spectroscopically it appears to be similar to the D-type asteroids, and is apparently of composition similar to carbonaceous chondrite material. Phoboss density is too low to be rock, and it is known to have significant porosity. These results led to the suggestion that Phobos might contain a substantial reservoir of ice, spectral observations indicate that the surface regolith layer lacks hydration, but ice below the regolith is not ruled out. The most prominent of these is the crater, Stickney, an impact crater some 9 km in diameter. As with Mimass crater Herschel, the impact that created Stickney must have nearly shattered Phobos, many grooves and streaks also cover the oddly shaped surface
36.
Deimos (moon)
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Deimos is the smaller and outer of the two natural satellites of the planet Mars, the other being Phobos. Deimos has a radius of 6.2 km and takes 30.3 hours to orbit Mars. The name Deimos is pronounced /ˈdaɪmɒs/ DY-mos, /deɪmɒs/, or sometimes /ˈdiːməs/ DEE-məs or like the Greek Δεῖμος, in Greek mythology, Deimos was the twin brother of Phobos and personified terror. Deimos is 23,460 km from Mars, much further than Marss other moon, Deimos was discovered by Asaph Hall, Sr. at the United States Naval Observatory in Washington, D. C on 12 August 1877, at about 07,48 UTC. Hall also discovered Phobos on 18 August 1877, at about 09,14 GMT and it is named after Deimos, a figure representing dread in Greek Mythology. The names, at first spelled Phobus and Deimus, were suggested by Henry Madan, Science Master of Eton, from Book XV of the Iliad, where Ares summons Dread and Fear. Deimos, like Marss other moon, Phobos, has spectra, albedos, like most bodies of its size, Deimos is highly non-spherical with triaxial dimensions of 15 ×12.2 ×11 km, making it 0.56 times the size of Phobos. Deimos is composed of rich in carbonaceous material, much like C-type asteroids. It is cratered, but the surface is smoother than that of Phobos. The regolith is highly porous and has a density of only 1.471 g/cm3. It has a velocity of 5.6 m/s and an apparent magnitude of 12.45. Only two geological features on Deimos have been given names, the craters Swift and Voltaire are named after writers who speculated on the existence of two Martian moons before Phobos and Deimos were discovered. DEIMOS Deimoss orbit is circular and is close to Marss equatorial plane. Deimos is possibly an asteroid that was perturbed by Jupiter into an orbit that allowed it to be captured by Mars, though this hypothesis is still controversial and disputed. At its brightest it would be about as bright as Venus is from Earth, with a small telescope, a Martian observer could see Deimoss phases, which take 1.2648 days to run their course. Unlike Phobos, which orbits so fast that it rises in the west and sets in the east, Deimos rises in the east. Because Deimoss orbit is close to Mars and has only a very small inclination to Marss equator. Deimos regularly passes in front of the Sun as seen from Mars and it is too small to cause a total eclipse, appearing only as a small black dot moving across the Sun
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Mars
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Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is referred to as the Red Planet because the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the northern hemisphere covers 40% of the planet, Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Mars trojan, there are ongoing investigations assessing the past habitability potential of Mars, as well as the possibility of extant life. Future astrobiology missions are planned, including the Mars 2020 and ExoMars rovers, liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is about 6⁄1000 that of the Earths, except at the lowest elevations for short periods. The two polar ice caps appear to be largely of water. The volume of ice in the south polar ice cap, if melted. On November 22,2016, NASA reported finding a large amount of ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior, Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches −2.91, which is surpassed only by Jupiter, Venus, the Moon, optical ground-based telescopes are typically limited to resolving features about 300 kilometers across when Earth and Mars are closest because of Earths atmosphere. Mars is approximately half the diameter of Earth with an area only slightly less than the total area of Earths dry land. Mars is less dense than Earth, having about 15% of Earths volume and 11% of Earths mass, the red-orange appearance of the Martian surface is caused by iron oxide, or rust. It can look like butterscotch, other common colors include golden, brown, tan. Like Earth, Mars has differentiated into a metallic core overlaid by less dense materials. Current models of its interior imply a core with a radius of about 1,794 ±65 kilometers, consisting primarily of iron and this iron sulfide core is thought to be twice as rich in lighter elements than Earths. The core is surrounded by a mantle that formed many of the tectonic and volcanic features on the planet
38.
Triton (moon)
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Triton is the largest natural satellite of the planet Neptune. It was discovered on October 10,1846, by English astronomer William Lassell and it is the only large moon in the Solar System with a retrograde orbit, an orbit in the opposite direction to its planets rotation. At 2,700 kilometres in diameter, it is the seventh-largest moon in the Solar System, because of its retrograde orbit and composition similar to Plutos, Triton is thought to have been a dwarf planet captured from the Kuiper belt. Triton has a surface of frozen nitrogen, a mostly water-ice crust, an icy mantle. The core makes up two-thirds of its total mass, Triton has a mean density of 2.061 g/cm3 and is composed of approximately 15–35% water ice. Triton is one of the few moons in the Solar System known to be geologically active, as a consequence, its surface is relatively young with sparse impact craters, and a complex geological history revealed in intricate cryovolcanic and tectonic terrains. Part of its surface has geysers erupting sublimated nitrogen gas, contributing to a nitrogen atmosphere less than 1/70,000 the pressure of Earths atmosphere at sea level. Triton was discovered by British astronomer William Lassell on October 10,1846, a brewer by trade, Lassell began making mirrors for his amateur telescope in 1820. When John Herschel received news of Neptunes discovery, he wrote to Lassell suggesting he search for possible moons, Lassell did so and discovered Triton eight days later. Lassell also claimed to have discovered rings, although Neptune was later confirmed to have rings, they are so faint and dark that it is doubtful that he actually saw them. Triton is named after the Greek sea god Triton, the son of Poseidon, the name was first proposed by Camille Flammarion in his 1880 book Astronomie Populaire, and was officially adopted many decades later. Until the discovery of the second moon Nereid in 1949, Triton was commonly referred to as the satellite of Neptune. Lassell did not name his own discovery, he successfully suggested the name Hyperion, previously chosen by John Herschel. Triton is unique among all large moons in the Solar System for its orbit around its planet. Most of the irregular moons of Jupiter and Saturn also have retrograde orbits. However, these moons are all much more distant from their primaries, and are small in comparison, Tritons orbit is associated with two tilts, the inclination of Neptunes spin to Neptunes orbit, 30°, and the inclination of Tritons orbit to Neptunes spin, 157°. Tritons orbit precesses forward relative to Neptunes spin with a period of about 678 Earth years and that inclination is currently 130°, Tritons orbit is now near its maximum departure from coplanarity with Neptunes. Tritons rotation is locked to be synchronous with its orbit around Neptune
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Neptune
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Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the planet. Neptune is 17 times the mass of Earth and is more massive than its near-twin Uranus. Neptune orbits the Sun once every 164.8 years at a distance of 30.1 astronomical units. It is named after the Roman god of the sea and has the astronomical symbol ♆, Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the remaining known 14 moons were located telescopically until the 20th century. The planets distance from Earth gives it a small apparent size. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989, the advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar. Neptunes composition can be compared and contrasted with the Solar Systems other giant planets, however, its interior, like that of Uranus, is primarily composed of ices and rock, which is why Uranus and Neptune are normally considered ice giants to emphasise this distinction. Traces of methane in the outermost regions in part account for the blue appearance. In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptunes atmosphere has active, for example, at the time of the Voyager 2 flyby in 1989, the planets southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, because of its great distance from the Sun, Neptunes outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 55 K. Temperatures at the centre are approximately 5,400 K. Neptune has a faint and fragmented ring system. On both occasions, Galileo seems to have mistaken Neptune for a star when it appeared close—in conjunction—to Jupiter in the night sky, hence. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned retrograde that day and this apparent backward motion is created when Earths orbit takes it past an outer planet. Because Neptune was only beginning its yearly cycle, the motion of the planet was far too slight to be detected with Galileos small telescope
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2006 definition of planet
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A non-satellite body fulfilling only the first two of these criteria is classified as a dwarf planet. According to the IAU, planets and dwarf planets are two classes of objects. A non-satellite body fulfilling only the first criterion is termed a small Solar System body, an alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against this proposal. The definition was a one, and has drawn both support and criticism from different astronomers, but has remained in use. According to this definition, there are eight planets in the Solar System. The definition distinguishes planets from smaller bodies and is not useful outside the Solar System, extrasolar planets, or exoplanets, are covered separately under a complementary 2003 draft guideline for the definition of planets, which distinguishes them from dwarf stars, which are larger. Before the discoveries of the early 21st century, astronomers had no real need for a definition for planets. With the discovery of Pluto in 1930, astronomers considered the Solar System to have nine planets, along with thousands of bodies such as asteroids. Pluto was thought to be larger than Mercury, in 1978, the discovery of Plutos moon Charon radically changed this picture. By measuring Charons orbital period, astronomers could accurately calculate Plutos mass for the first time, in the 1990s, astronomers began finding other objects at least as far away as Pluto, now known as Kuiper Belt objects, or KBOs. Many of these some of Plutos key orbital characteristics and are now called plutinos. Pluto came to be seen as the largest member of a new class of objects, Plutos eccentric and inclined orbit, while very unusual for a planet in the Solar System, fits in well with the other KBOs. New York Citys newly renovated Hayden Planetarium did not include Pluto in its exhibit of the planets when it reopened as the Rose Center for Earth, astronomers also knew that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly. They were also concerned about the classification of planets in other planetary systems, in 2006, the matter came to a head with the first measurement of the size of 2003 UB313. That measurement had showed Eris to appear to be larger than Pluto. The process of new discoveries spurring a contentious refinement of Plutos categorization echoed a debate in the 19th century that began with the discovery of Ceres on January 1,1801, astronomers immediately declared the tiny object to be the missing planet between Mars and Jupiter. Within four years, however, the discovery of two objects with comparable sizes and orbits had cast doubt on this new thinking. By 1851, the number of planets had grown to 23, astronomers began cataloguing them separately and began calling them asteroids instead of planets
41.
3753 Cruithne
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3753 Cruithne is a Q-type, Aten asteroid in orbit around the Sun in 1,1 orbital resonance with Earth, making it a co-orbital object. It is an asteroid that, relative to Earth, orbits the Sun in an orbit that effectively describes a horseshoe. It has been incorrectly called Earths second moon, Cruithne does not orbit Earth and at times it is on the other side of the Sun, placing Cruithne well outside of Earths Hill sphere. Its orbit takes it inside the orbit of Mercury and outside the orbit of Mars, Cruithne orbits the Sun in about 1 year but it takes 770 years for the series to complete a horseshoe-shaped movement around the Earth. The name Cruithne is from Old Irish and refers to the early Picts in the Annals of Ulster, Cruithne was discovered on October 10,1986, by Duncan Waldron on a photographic plate taken with the UK Schmidt Telescope at Siding Spring Observatory, Coonabarabran, Australia. The 1983 apparition is credited to Giovanni de Sanctis and Richard M. West of the European Southern Observatory in Chile. It was not until 1997 that its orbit was determined by Paul Wiegert and Kimmo Innanen, working at York University in Toronto. Cruithne is approximately 5 kilometres in diameter, and its closest approach to Earth is 12 million kilometres, from 1994 through 2015, Cruithne made its annual closest approach to Earth every November. Although Cruithnes orbit is not thought to be stable over the term, calculations by Wiegert. There is no danger of a collision with Earth for millions of years and its orbital path and Earths do not cross, and its orbital plane is currently tilted to that of the Earth by 19. 8°. Cruithne, having a maximum magnitude of +15.8, is fainter than Pluto. Cruithne is in an elliptic orbit around the Sun. Its period of revolution around the Sun, approximately 364 days at present, is almost equal to that of the Earth, because of this, Cruithne and Earth appear to follow each other in their paths around the Sun. This is why Cruithne is sometimes called Earths second moon, however, it does not orbit the Earth and is not a moon. In 2058, Cruithne will come within 0.09 AU of Mars, the kidney bean will then start to migrate away from the Earth again in the opposite direction – instead of the Earth falling behind the bean, the Earth is pulling away from the bean. The next such series of close approaches will be centred on the year 2292 – in July of that year, more near-resonant near-Earth objects have since been discovered. These include 54509 YORP,1998 UP1,2002 AA29, and 2009 BD which exist in resonant orbits similar to Cruithnes,2010 TK7 is the first and so far only identified Earth trojan. Other examples of natural bodies known to be in horseshoe orbits include Janus and Epimetheus, the orbits these two moons follow around Saturn are much simpler than the one Cruithne follows, but operate along the same general principles
42.
Co-orbital configuration
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There are several classes of co-orbital objects, depending on their point of libration. The most common and best-known class is the trojan, which librates around one of the two stable Lagrangian points, L4 and L5, 60° ahead of and behind the larger body respectively. Another class is the orbit, in which objects librate around 180° from the larger body. Objects librating around 0° are called quasi-satellites, an exchange orbit occurs when two co-orbital objects are of similar masses and thus exert a non-negligible influence on each other. The objects can exchange semi-major axes or eccentricities when they approach each other, orbital parameters that are used to describe the relation of co-orbital objects are the longitude of the periapsis difference and the mean longitude difference. The longitude of the periapsis is the sum of the longitude and the mean anomaly and the mean longitude is the sum of the longitude of the ascending node. Trojan objects orbit 60° ahead of or behind a massive object. The best known example are the asteroids that orbit ahead of or behind Jupiter around the Sun, trojan objects do not orbit exactly at one of either Lagrangian points, but do remain relatively close to it, appearing to slowly orbit it. In technical terms, they librate around =, the point around which they librate is the same, irrespective of their mass or orbital eccentricity. There are several thousand known trojan minor planets orbiting the Sun, most of these orbit near Jupiters Lagrangian points, the traditional Jupiter trojans. As of 2015 there are also 13 Neptune trojans,7 Mars trojans,1 Uranus trojan and 1 Earth trojan known to exist, the Saturnian system contains two sets of trojan moons. Both Tethys and Dione have two moons, Telesto and Calypso in Tethyss L4 and L5 respectively, and Helene and Polydeuces in Diones L4. Polydeuces is noticeable for its wide libration, it wanders as far as ±30° from its Lagrangian point and ±2% from its orbital radius. A pair of co-orbital exoplanets was proposed to be orbiting the star Kepler-223, the possibility of a trojan planet to Kepler-91b was studied but the conclusion was that the transit-signal was a false-positive. One possibility for the zone is a trojan planet of a giant planet close to its star. Objects in a horseshoe orbit librate around 180° from the primary and their orbits encompass both equilateral Lagrangian points, i. e. L4 and L5. The Saturnian moons Janus and Epimetheus share their orbits, the difference in semi-major axes being less than eithers mean diameter and this means the moon with the smaller semi-major axis will slowly catch up with the other. As it does this, the moons gravitationally tug at each other, increasing the semi-major axis of the moon that has caught up and this reverses their relative positions proportionally to their masses and causes this process to begin anew with the moons roles reversed
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Definition of planet
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The definition of planet, since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term asteres planetai, wandering stars, for objects which apparently moved over the sky. Over the millennia, the term has included a variety of different objects, from the Sun and the Moon to satellites and asteroids. By the end of the 19th century the word planet, though it had yet to be defined, had become a term applied only to a small set of objects in the Solar System. After 1992, however, astronomers began to discover many additional objects beyond the orbit of Neptune and these discoveries not only increased the number of potential planets, but also expanded their variety and peculiarity. Some were nearly enough to be stars, while others were smaller than Earths moon. These discoveries challenged long-perceived notions of what a planet could be, the issue of a clear definition for planet came to a head in January 2005 with the discovery of the trans-Neptunian object Eris, a body more massive than the smallest then-accepted planet, Pluto. Under this new definition, Pluto and the other trans-Neptunian objects do not qualify as planets, the IAUs decision has not resolved all controversies, and while many scientists have accepted the definition, some in the astronomical community have rejected it outright. While knowledge of the planets history and is common to most civilizations. The five bodies currently called planets that were known to the Greeks were those visible to the eye, Mercury, Venus, Mars, Jupiter. In his Timaeus, written in roughly 360 BC, Plato mentions, the Sun and Moon and five other stars and his student Aristotle makes a similar distinction in his On the Heavens, The movements of the sun and moon are fewer than those of some of the planets. In his Almagest written in the 2nd century, Ptolemy refers to the Sun, Moon, hyginus explicitly mentions the five stars which many have called wandering, and which the Greeks call Planeta. In his Natural History, written in 77 AD, Pliny the Elder refers to the seven stars, nonnus, the 5th century Greek poet, says in his Dionysiaca, I have oracles of history on seven tablets, and the tablets bear the names of the seven planets. Medieval and Renaissance writers generally accepted the idea of seven planets, indicating that the Sun and the Moon were planets. Even Nicolaus Copernicus, who rejected the model, was ambivalent concerning whether the Sun. And of the five other planets, as the historian of science Thomas Kuhn noted in his book, The Structure of Scientific Revolutions, The Copernicans who denied its traditional title planet to the sun. Were changing the meaning of planet so that it would continue to make distinctions in a world where all celestial bodies. Were seen differently from the way they had seen before
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Ecliptic
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The ecliptic is the apparent path of the Sun on the celestial sphere, and is the basis for the ecliptic coordinate system. It also refers to the plane of this path, which is coplanar with the orbit of Earth around the Sun, the motions as described above are simplifications. Due to the movement of Earth around the Earth–Moon center of mass, due to further perturbations by the other planets of the Solar System, the Earth–Moon barycenter wobbles slightly around a mean position in a complex fashion. The ecliptic is actually the apparent path of the Sun throughout the course of a year, because Earth takes one year to orbit the Sun, the apparent position of the Sun also takes the same length of time to make a complete circuit of the ecliptic. With slightly more than 365 days in one year, the Sun moves a little less than 1° eastward every day, again, this is a simplification, based on a hypothetical Earth that orbits at uniform speed around the Sun. The actual speed with which Earth orbits the Sun varies slightly during the year, for example, the Sun is north of the celestial equator for about 185 days of each year, and south of it for about 180 days. The variation of orbital speed accounts for part of the equation of time, if the equator is projected outward to the celestial sphere, forming the celestial equator, it crosses the ecliptic at two points known as the equinoxes. The Sun, in its apparent motion along the ecliptic, crosses the equator at these points, one from south to north. The crossing from south to north is known as the equinox, also known as the first point of Aries. The crossing from north to south is the equinox or descending node. Likewise, the ecliptic itself is not fixed, the gravitational perturbations of the other bodies of the Solar System cause a much smaller motion of the plane of Earths orbit, and hence of the ecliptic, known as planetary precession. The combined action of two motions is called general precession, and changes the position of the equinoxes by about 50 arc seconds per year. Once again, this is a simplification, periodic motions of the Moon and apparent periodic motions of the Sun cause short-term small-amplitude periodic oscillations of Earths axis, and hence the celestial equator, known as nutation. Obliquity of the ecliptic is the used by astronomers for the inclination of Earths equator with respect to the ecliptic. It is about 23. 4° and is currently decreasing 0.013 degrees per hundred years due to planetary perturbations, the angular value of the obliquity is found by observation of the motions of Earth and other planets over many years. From 1984, the Jet Propulsion Laboratorys DE series of computer-generated ephemerides took over as the ephemeris of the Astronomical Almanac. Obliquity based on DE200, which analyzed observations from 1911 to 1979, was calculated, jPLs fundamental ephemerides have been continually updated. J. Laskar computed an expression to order T10 good to 0″. 04/1000 years over 10,000 years, all of these expressions are for the mean obliquity, that is, without the nutation of the equator included
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Hill sphere
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An astronomical bodys Hill sphere is the region in which it dominates the attraction of satellites. The outer shell of that constitutes a zero-velocity surface. To be retained by a planet, a moon must have an orbit that lies within the planets Hill sphere and that moon would, in turn, have a Hill sphere of its own. Any object within that distance would tend to become a satellite of the moon, one simple view of the extent of the Solar System is the Hill sphere of the Sun with respect to local stars and the galactic nucleus. In more precise terms, the Hill sphere approximates the gravitational sphere of influence of a body in the face of perturbations from a more massive body. It was defined by the American astronomer George William Hill, based on the work of the French astronomer Édouard Roche, for this reason, it is also known as the Roche sphere. In the example to the right, the Hill sphere extends between the Lagrangian points L1 and L2, which lie along the line of centers of the two bodies. The region of influence of the body is shortest in that direction. When eccentricity is negligible, this becomes r ≈ a m 3 M3, in the Earth example, the Earth orbits the Sun at a distance of 149.6 million km. The Hill sphere for Earth thus extends out to about 1.5 million km, all stable satellites of the Earth must have an orbital period shorter than seven months. The previous formula can be re-stated as follows,3 r 3 a 3 ≈ m M and this is also convenient, because many planetary astronomers work in and remember distances in units of planetary radii. The Hill sphere is only an approximation, and other forces can eventually perturb an object out of the sphere and this third object should also be of small enough mass that it introduces no additional complications through its own gravity. The region of stability for retrograde orbits at a distance from the primary, is larger than the region for prograde orbits at a large distance from the primary. This was thought to explain the preponderance of retrograde moons around Jupiter, however, a sphere of this size and mass would be denser than lead. In fact, in any low Earth orbit, a body must be more dense than lead in order to fit inside its own Hill sphere. A spherical geostationary satellite, however, would only need to be more than 6% of the density of water to support satellites of its own. Within the Solar System, the planet with the largest Hill radius is Neptune, with 116 million km, or 0.775 au, an asteroid from the asteroid belt will have a Hill sphere that can reach 220000 km, diminishing rapidly with decreasing mass. The Hill sphere of 1999 KW4, a Mercury-crosser asteroid that has a moon, a typical extrasolar hot Jupiter, HD209458 b, has a Hill sphere radius of 593,000 km, about 8 times its physical radius of approx 71,000 km
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Natural satellite
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A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet. In the Solar System there are six planetary satellite systems containing 178 known natural satellites, four IAU-listed dwarf planets are also known to have natural satellites, Pluto, Haumea, Makemake, and Eris. As of January 2012, over 200 minor-planet moons have been discovered, the Earth–Moon system is unique in that the ratio of the mass of the Moon to the mass of Earth is much greater than that of any other natural-satellite–planet ratio in the Solar System. At 3,474 km across, Earths Moon is 0.27 times the diameter of Earth, the first known natural satellite was the Moon, but it was considered a planet until Copernicus introduction of heliocentrism in 1543. Until the discovery of the Galilean satellites in 1610, however, galileo chose to refer to his discoveries as Planetæ, but later discoverers chose other terms to distinguish them from the objects they orbited. The first to use of the satellite to describe orbiting bodies was the German astronomer Johannes Kepler in his pamphlet Narratio de Observatis a se quatuor Iouis satellitibus erronibus in 1610. He derived the term from the Latin word satelles, meaning guard, attendant, or companion, the term satellite thus became the normal one for referring to an object orbiting a planet, as it avoided the ambiguity of moon. In 1957, however, the launching of the artificial object Sputnik created a need for new terminology, to further avoid ambiguity, the convention is to capitalize the word Moon when referring to Earths natural satellite, but not when referring to other natural satellites. A few recent authors define moon as a satellite of a planet or minor planet, there is no established lower limit on what is considered a moon. Small asteroid moons, such as Dactyl, have also been called moonlets, the upper limit is also vague. Two orbiting bodies are described as a double body rather than primary. Asteroids such as 90 Antiope are considered double asteroids, but they have not forced a clear definition of what constitutes a moon, some authors consider the Pluto–Charon system to be a double planet. In contrast, irregular satellites are thought to be captured asteroids possibly further fragmented by collisions, most of the major natural satellites of the Solar System have regular orbits, while most of the small natural satellites have irregular orbits. The Moon and possibly Charon are exceptions among large bodies in that they are thought to have originated by the collision of two large proto-planetary objects. The material that would have placed in orbit around the central body is predicted to have reaccreted to form one or more orbiting natural satellites. As opposed to planetary-sized bodies, asteroid moons are thought to form by this process. Triton is another exception, although large and in a close, circular orbit, its motion is retrograde, most regular moons in the Solar System are tidally locked to their respective primaries, meaning that the same side of the natural satellite always faces its planet. The only known exception is Saturns natural satellite Hyperion, which rotates chaotically because of the influence of Titan
. Bibcode:2015AJ....150..185M. doi:10.1088/0004-6256/150/6/185.
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