1.
Asteroid
–
Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids and these terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. As minor planets in the outer Solar System were discovered and found to have volatile-based surfaces that resemble those of comets, in this article, the term asteroid refers to the minor planets of the inner Solar System including those co-orbital with Jupiter. There are millions of asteroids, many thought to be the remnants of planetesimals. The large majority of known asteroids orbit in the belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth objects, individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups, C-type, M-type, and S-type. These were named after and are identified with carbon-rich, metallic. The size of asteroids varies greatly, some reaching as much as 1000 km across, asteroids are differentiated from comets and meteoroids. In the case of comets, the difference is one of composition, while asteroids are composed of mineral and rock, comets are composed of dust. In addition, asteroids formed closer to the sun, preventing the development of the aforementioned cometary ice, the difference between asteroids and meteoroids is mainly one of size, meteoroids have a diameter of less than one meter, whereas asteroids have a diameter of greater than one meter. Finally, meteoroids can be composed of either cometary or asteroidal materials, only one asteroid,4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the eye for a short time. As of March 2016, the Minor Planet Center had data on more than 1.3 million objects in the inner and outer Solar System, the United Nations declared June 30 as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, the first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet. In the early half of the nineteenth century, the terms asteroid. Asteroid discovery methods have improved over the past two centuries. This task required that hand-drawn sky charts be prepared for all stars in the band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, the expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers
2.
Albedo
–
Albedo is a measure for reflectance or optical brightness. It is dimensionless and measured on a scale from zero to one, surface albedo is defined as the ratio of radiation reflected to the radiation incident on a surface. The proportion reflected is not only determined by properties of the surface itself and these factors vary with atmospheric composition, geographic location and time. While bi-hemispherical reflectance is calculated for an angle of incidence. The temporal resolution may range from seconds to daily, seasonal or annual averages, unless given for a specific wavelength, albedo refers to the entire spectrum of solar radiation. Due to measurement constraints, it is given for the spectrum in which most solar energy reaches the surface. This spectrum includes visible light, which explains why surfaces with a low albedo appear dark, albedo is an important concept in climatology, astronomy, and environmental management. The term albedo was introduced into optics by Johann Heinrich Lambert in his 1760 work Photometria, any albedo in visible light falls within a range of about 0.9 for fresh snow to about 0.04 for charcoal, one of the darkest substances. Deeply shadowed cavities can achieve an effective albedo approaching the zero of a black body, when seen from a distance, the ocean surface has a low albedo, as do most forests, whereas desert areas have some of the highest albedos among landforms. Most land areas are in a range of 0.1 to 0.4. The average albedo of Earth is about 0.3 and this is far higher than for the ocean primarily because of the contribution of clouds. Earths surface albedo is regularly estimated via Earth observation satellite sensors such as NASAs MODIS instruments on board the Terra, thereby, the BRDF allows to translate observations of reflectance into albedo. Earths average surface temperature due to its albedo and the effect is currently about 15 °C. If Earth were frozen entirely, the temperature of the planet would drop below −40 °C. If only the land masses became covered by glaciers, the mean temperature of the planet would drop to about 0 °C. In contrast, if the entire Earth was covered by water — a so-called aquaplanet — the average temperature on the planet would rise to almost 27 °C, hence, the actual albedo α can then be given as, α = α ¯ + D α ¯ ¯. Directional-hemispherical reflectance is sometimes referred to as black-sky albedo and bi-hemispherical reflectance as white-sky albedo and these terms are important because they allow the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the surface. The albedos of planets, satellites and asteroids can be used to infer much about their properties, the study of albedos, their dependence on wavelength, lighting angle, and variation in time comprises a major part of the astronomical field of photometry
3.
Nickel
–
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a golden tinge. Nickel belongs to the metals and is hard and ductile. Meteoric nickel is found in combination with iron, a reflection of the origin of elements as major end products of supernova nucleosynthesis. An iron–nickel mixture is thought to compose Earths inner core, use of nickel has been traced as far back as 3500 BCE. Nickel was first isolated and classified as an element in 1751 by Axel Fredrik Cronstedt. The elements name comes from a mischievous sprite of German miner mythology, Nickel, an economically important source of nickel is the iron ore limonite, which often contains 1–2% nickel. Nickels other important ore minerals include garnierite, and pentlandite, major production sites include the Sudbury region in Canada, New Caledonia in the Pacific, and Norilsk in Russia. Nickel is slowly oxidized by air at room temperature and is considered corrosion-resistant, historically, it has been used for plating iron and brass, coating chemistry equipment, and manufacturing certain alloys that retain a high silvery polish, such as German silver. About 6% of world production is still used for corrosion-resistant pure-nickel plating. Nickel-plated objects sometimes provoke nickel allergy, Nickel has been widely used in coins, though its rising price has led to some replacement with cheaper metals in recent years. Nickel is one of four elements that are ferromagnetic around room temperature, alnico permanent magnets based partly on nickel are of intermediate strength between iron-based permanent magnets and rare-earth magnets. The metal is valuable in modern times chiefly in alloys, about 60% of world production is used in nickel-steels, other common alloys and some new superalloys comprise most of the remainder of world nickel use, with chemical uses for nickel compounds consuming less than 3% of production. As a compound, nickel has a number of chemical manufacturing uses. Nickel is a nutrient for some microorganisms and plants that have enzymes with nickel as an active site. Nickel is a metal with a slight golden tinge that takes a high polish. It is one of four elements that are magnetic at or near room temperature. Its Curie temperature is 355 °C, meaning that bulk nickel is non-magnetic above this temperature, the unit cell of nickel is a face-centered cube with the lattice parameter of 0.352 nm, giving an atomic radius of 0.124 nm
4.
Iron
–
Iron is a chemical element with symbol Fe and atomic number 26. It is a metal in the first transition series and it is by mass the most common element on Earth, forming much of Earths outer and inner core. It is the fourth most common element in the Earths crust, like the other group 8 elements, ruthenium and osmium, iron exists in a wide range of oxidation states, −2 to +6, although +2 and +3 are the most common. Elemental iron occurs in meteoroids and other low oxygen environments, but is reactive to oxygen, fresh iron surfaces appear lustrous silvery-gray, but oxidize in normal air to give hydrated iron oxides, commonly known as rust. Unlike the metals that form passivating oxide layers, iron oxides occupy more volume than the metal and thus flake off, Iron metal has been used since ancient times, although copper alloys, which have lower melting temperatures, were used even earlier in human history. Pure iron is soft, but is unobtainable by smelting because it is significantly hardened and strengthened by impurities, in particular carbon. A certain proportion of carbon steel, which may be up to 1000 times harder than pure iron. Crude iron metal is produced in blast furnaces, where ore is reduced by coke to pig iron, further refinement with oxygen reduces the carbon content to the correct proportion to make steel. Steels and iron alloys formed with metals are by far the most common industrial metals because they have a great range of desirable properties. Iron chemical compounds have many uses, Iron oxide mixed with aluminium powder can be ignited to create a thermite reaction, used in welding and purifying ores. Iron forms binary compounds with the halogens and the chalcogens, among its organometallic compounds is ferrocene, the first sandwich compound discovered. Iron plays an important role in biology, forming complexes with oxygen in hemoglobin and myoglobin. Iron is also the metal at the site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants. A human male of average height has about 4 grams of iron in his body and this iron is distributed throughout the body in hemoglobin, tissues, muscles, bone marrow, blood proteins, enzymes, ferritin, hemosiderin, and transport in plasma. The mechanical properties of iron and its alloys can be evaluated using a variety of tests, including the Brinell test, Rockwell test, the data on iron is so consistent that it is often used to calibrate measurements or to compare tests. An increase in the content will cause a significant increase in the hardness. Maximum hardness of 65 Rc is achieved with a 0. 6% carbon content, because of the softness of iron, it is much easier to work with than its heavier congeners ruthenium and osmium. Because of its significance for planetary cores, the properties of iron at high pressures and temperatures have also been studied extensively
5.
Iron meteorite
–
Iron meteorites are meteorites that consist overwhelmingly of an iron–nickel alloy known as meteoric iron that usually consists of two mineral phases, kamacite and taenite. Iron meteorites originate from planetary cores of planetesimals, the iron found in iron meteorites was one of the earliest sources of usable iron available to humans, before the development of smelting that signaled the beginning of the iron age. Although they are rare compared to the stony meteorites, comprising only about 5. 7% of witnessed falls. This is due to several factors, They are easily recognized as unusual even by laymen, modern-day searches for meteorites in deserts and Antarctica yield a much more representative sample of meteorites overall. They are much more resistant to weathering and they are much more likely to survive atmospheric entry, and are more resistant to the resulting ablation. Hence, they are likely to be found as large pieces. They are able to be even when buried by use of surface metal detecting equipment. Because they are denser than stony meteorites, iron meteorites also account for almost 90% of the mass of all known meteorites. All the largest known meteorites are of type, including the largest—the Hoba meteorite. Iron meteorites have been linked to M-type asteroids because both have similar characteristics in the visible and near-infrared. Iron meteorites are thought to be the fragments of the cores of larger ancient asteroids that have been shattered by impacts, the IIE iron meteorites may be a notable exception, in that they probably originate from the crust of S-type asteroid 6 Hebe. Chemical and isotope analysis indicates that at least about 50 distinct parent bodies were involved and this implies that there were once at least this many large, differentiated, asteroids in the asteroid belt – many more than today. The overwhelming bulk of these consists of the FeNi-alloys kamacite and taenite. Minor minerals, when occurring, often form rounded nodules of troilite or graphite, schreibersite and troilite also occur as plate shaped inclusions, which show up on cut surfaces as cm-long and mm-thick lamellae. The troilite plates are called Reichenbach lamellae, the chemical composition is dominated by the elements Fe, Ni and Co, which make up more than 95%. Ni is always present, the concentration is always higher than 5%. Iron meteorites were historically used for their meteoric iron, which was forged into cultural objects, with the advent of smelting and the beginning of the iron age the importance of iron meteorites as a resource decreased, at least in those cultures that developed those techniques. The Inuit used the Cape York meteorite for a longer time
6.
22 Kalliope
–
22 Kalliope is a large M-type asteroid from the asteroid belt discovered by J. R. Hind on November 16,1852. It is named after Calliope, the Greek Muse of epic poetry and it is orbited by a small moon named Linus. Kalliope is somewhat elongated, approximately 166 km in diameter, and slightly asymmetric and this new diameter, which was measured by observing mutual eclipses of Kalliope and Linus, is 8% smaller than that calculated from IRAS observations. The spectrum of Kalliope is an M-type, indicating that its surface may be composed of iron–nickel metal. The asteroids density is about 3.4 g/cm3, spectroscopic studies have shown, however, evidence of hydrated minerals and silicates, which indicate rather a stony surface composition. Kalliope also has a low albedo, which is inconsistent with a purely metallic surface. Lightcurve analysis indicates that Kalliopes pole most likely points towards ecliptic coordinates = with a 10° uncertainty, Kalliopes rotation is then slightly retrograde. Kalliope has one natural satellite, Linus, or Kalliope I Linus. It is quite large, being about 28 km in diameter and it orbits about 1100 km from the center of Kalliope, equivalent to about 13.2 Kalliope radii. Linus was discovered on August 29,2001 by Jean-Luc Margot and Michael E. Brown, based on more than 5 years of regular observations of Kalliope binary system using adaptive optics systems on ground-based telescopes. The observed chords of Linus give an opportunity to estimate the size of the moonlet which was estimated to 20–28 km. Lightcurve plot of 22 Kalliope, Palmer Divide Observatory, B. D,22 Kalliope at the JPL Small-Body Database Discovery · Orbit diagram · Orbital elements · Physical parameters
7.
21 Lutetia
–
21 Lutetia is a large asteroid in the asteroid belt of an unusual spectral type. It measures about 100 kilometers in diameter and it was discovered in 1852 by Hermann Goldschmidt, and is named after Lutetia, the Latin name of Paris. Lutetia has a shape and is heavily cratered, with the largest impact crater reaching 45 km in diameter. The surface is heterogeneous and is intersected by a system of grooves and scarps. It has an average density, meaning that it is made of metal-rich rock. The Rosetta probe passed within 3,162 km of Lutetia in July 2010 and it was the largest asteroid visited by a spacecraft until Dawn arrived at Vesta in July 2011. Lutetia was discovered on November 15,1852, by Hermann Goldschmidt from the balcony of his apartment in Paris, a preliminary orbit for the asteroid was computed in November–December 1852 by German astronomer Georg Rümker and others. In 1903, it was photographed at opposition by Edward Pickering at Harvard College Observatory and he computed an opposition magnitude of 10.8. There have been two reported stellar occultations by Lutetia, observed from Malta in 1997 and Australia in 2003, with one chord each. On July 10,2010, the European Rosetta space probe flew by Lutetia at a distance of 3168 ±7.5 km at a velocity of 15 kilometres per second on its way to the comet 67P/Churyumov-Gerasimenko. The flyby provided images of up to 60 meters per pixel resolution and covered about 50% of the surface, the 462 images were obtained in 21 narrow- and broad-band filters extending from 0.24 to 1 μm. Lutetia was also observed by the imaging spectrometer VIRTIS, and measurements of the magnetic field. Lutetia orbits the Sun at the distance of approximately 2.4 AU in the asteroid belt. Its orbit lies almost in the plane of ecliptic and is moderately eccentric, the orbital period of Lutetia is 3.8 years. The Rosetta flyby demonstrated that the mass of Lutetia is ×1018 kg and it has one of the highest densities seen before in asteroids at 3.4 ±0.3 g/cm3. Taking into account possible porosity of 10–15%, the density of Lutetia exceeds that of a typical stony meteorite. The composition of Lutetia has puzzled astronomers for some time, while classified among the M-type asteroids, most of which are metallic, Lutetia is one of the anomalous members that do not display much evidence of metal on their surface. The Rosetta probe actually found that the asteroid has a red spectrum in the visible light
8.
Silicate
–
A silicate is a compound containing an anionic silicon compound. The great majority of the silicates are oxides, but hexafluorosilicate, orthosilicate is the anion SiO4−4 or its compounds. Related to orthosilicate are families of anions with the formula 2n−, important members are the cyclic and single chain silicates n and the sheet-forming silicates n. Silicates constitute the majority of Earths crust, as well as the terrestrial planets, rocky moons. Sand, Portland cement, and thousands of minerals are examples of silicates, silicate compounds, including the minerals, consist of silicate anions whose charge is balanced by various cations. Myriad silicate anions can exist, and each can form compounds with many different cations, hence this class of compounds is very large. Both minerals and synthetic materials fit in this class, in the vast majority of silicates, including silicate minerals, the Si occupies a tetrahedral environment, being surrounded by 4 oxygen centres. In these structures, the bonds to silicon conform to the octet rule. These tetrahedra sometimes occur as isolated SiO4−4 centres, but most commonly, commonly the silicate anions are chains, double chains, sheets, and three-dimensional frameworks. All these such species have negligible solubility in water at normal conditions, silicates are well characterized as solids, but are less commonly observed in solution. The anion SiO4−4 is the base of silicic acid, Si4. Instead, solutions of silicates are usually observed as mixtures of condensed, the nature of soluble silicates is relevant to understanding biomineralization and the synthesis of aluminosilicates, such as the industrially important catalysts called zeolites. Although the tetrahedron is the coordination geometry for silicon compounds. A well-known example of such a high number is hexafluorosilicate. Octahedral coordination by 6 oxygen centres is observed, at very high pressure, even SiO2 adopts this geometry in the mineral stishovite, a dense polymorph of silica found in the lower mantle of the Earth. This structure is formed by shock during meteorite impacts. In geology and astronomy, the silicate is used to denote types of rock that consist predominantly of silicate minerals. On Earth, a variety of silicate minerals occur in an even wider range of combinations as a result of the processes that form
9.
C-type asteroid
–
They are the most common variety, forming around 75% of known asteroids. They are distinguished by a low albedo because their composition includes a large amount of carbon, in addition to rocks. Asteroids of this class have very similar to those of carbonaceous chondrite meteorites. The latter are very close in composition to the Sun. C-type asteroids are extremely dark, with albedos typically in the 0.03 to 0.10 range, consequently, whereas a number of S-type asteroids can normally be viewed with binoculars at opposition, even the largest C-type asteroids require a small telescope. The potentially brightest C-type asteroid is 324 Bamberga, but that very high eccentricity means it rarely reaches its maximum magnitude. Their spectra contain moderately strong ultraviolet absorption at wavelengths below about 0.4 μm to 0.5 μm, while at longer wavelengths they are largely featureless but slightly reddish. The so-called water absorption feature around 3 μm, which can be an indication of content in minerals is also present
10.
16 Psyche
–
16 Psyche is one of the ten most massive asteroids in the asteroid belt. It is over 200 kilometers in diameter and contains a less than 1% of the mass of the entire asteroid belt. It is thought to be the iron core of a protoplanet. It is the most massive metallic M-type asteroid, Psyche was discovered by the Italian astronomer Annibale de Gasparis on 17 March 1852 from Naples and named after the Greek mythological figure Psyche. The first fifteen asteroids to be discovered were given symbols by astronomers as a type of shorthand notation, Psyche was given an iconic symbol, as were several other asteroids discovered over the next few years. This symbol, a semicircle topped by a star, represents a butterflys wing, symbol of the soul, Psyche seems to have a surface that is 90% metallic, with small amounts of pyroxene. Therefore, Psyche appears to be a metallic core from a larger differentiated parent body some 500 kilometers in diameter. If Psyche is indeed one, there could be other asteroids on similar orbits, however, Psyche is not part of any identified asteroid family. However, this scenario is considered to have a probability of just 1%, an alternative is that Psyche was broken by impacts, but not catastrophically torn apart. In this case, it may be a candidate for the parent body of the mesosiderites, Psyche is massive enough that its gravitational perturbations on other asteroids can be observed, which enables a mass measurement. IRAS data shows it to have a diameter of 253 km, further estimates in 2006 and 2011 that also suggested a smaller size have resulted in an increase in its estimated density to one that is more appropriate for a metallic asteroid. Psyche appears to have a regular surface and is approximately ellipsoidal in shape. Light-curve analysis has indicated that Psyches pole points towards ecliptic coordinates = or = with a 10° uncertainty. This gives an axial tilt of 95° and it is possible that at least some examples of enstatite chondrite meteorites originated from this asteroid, based on similar spectral analysis results. Presence of water The NASA Infrared Telescope Facility at the Mauna Kea Observatories released evidence of the presence of water or hydroxyl ions on the asteroid in October 2016. The water is believed to have reached Psyche via past impacts from smaller asteroids containing volatile substances such as carbon, hydrogen, no spacecraft has visited Psyche, but in 2014 a mission to Psyche was proposed to NASA. A team led by Lindy Elkins-Tanton, the director of the School for Earth and Space Exploration at Arizona State University and this team argued that 16 Psyche would be a valuable object for study because it is the only metallic core-like body discovered so far. On September 30,2015, the Psyche orbiter mission was one of five Discovery Program semifinalist proposals, shape model deduced from lightcurve JPL Ephemeris 16 Psyche at the JPL Small-Body Database Discovery · Orbit diagram · Orbital elements · Physical parameters
11.
Rosetta (spacecraft)
–
Rosetta was a space probe built by the European Space Agency launched on 2 March 2004. Along with Philae, its lander module, Rosetta performed a study of comet 67P/Churyumov–Gerasimenko. During its journey to the comet, the spacecraft flew by Mars and it was launched as the third cornerstone mission of the ESAs Horizon 2000 programme, after SOHO / Cluster and XMM-Newton. On 6 August 2014, the reached the comet and performed a series of manoeuvres to eventually orbit the comet at distances of 30 to 10 kilometres. On 12 November, its lander module Philae performed the first successful landing on a comet, communications with Philae were briefly restored in June and July 2015, but due to diminishing solar power, Rosettas communications module with the lander was turned off on 27 July 2016. On 30 September 2016, the Rosetta spacecraft ended its mission by hard-landing on the comet in its Maat region, the probe is named after the Rosetta Stone, a stele of Egyptian origin featuring a decree in three scripts. The lander is named after the Philae obelisk, which bears a bilingual Greek and it was one of ESAs Horizon 2000 cornerstone missions. The spacecraft consisted of the Rosetta orbiter, which featured 12 instruments, the Rosetta mission orbited Comet Churyumov–Gerasimenko for 17 months and was designed to complete the most detailed study of a comet ever attempted. The spacecraft was controlled from the European Space Operations Centre, in Darmstadt and it has been estimated that in the decade preceding 2014, some 2,000 people assisted in the mission in some capacity. In 2007, Rosetta made a Mars gravity assist on its way to Comet Churyumov–Gerasimenko, the spacecraft also performed two asteroid flybys. The craft completed its flyby of asteroid 2867 Šteins in September 2008, later, on 20 January 2014, Rosetta was taken out of a 31-month hibernation mode as it approached Comet Churyumov–Gerasimenko. Rosettas Philae lander successfully made the first soft landing on a comet nucleus when it touched down on Comet Churyumov–Gerasimenko on 12 November 2014. On 5 September 2016, ESA announced that the lander was discovered by the narrow-angle camera aboard Rosetta as the made a low,2.7 km pass over the comet. The lander sits on its side wedged into a crevice of the comet. During the 1986 approach of Halleys Comet, international space probes were sent to explore the comet, after the probes returned valuable scientific information, it became obvious that follow-ons were needed that would shed more light on cometary composition and answer new questions. Both ESA and NASA started cooperatively developing new probes, the NASA project was the Comet Rendezvous Asteroid Flyby mission. The ESA project was the follow-on Comet Nucleus Sample Return mission, both missions were to share the Mariner Mark II spacecraft design, thus minimising costs. In 1992, after NASA cancelled CRAF due to budgetary limitations, after the spacecraft launch, Gerhard Schwehm was named mission manager, he retired in March 2014
12.
216 Kleopatra
–
216 Kleopatra is an asteroid orbiting in the asteroid belt. It was discovered by Austrian astronomer Johann Palisa on April 10,1880 and it is named after Cleopatra, the famous queen of Ancient Egypt. The asteroid is notable for its shape that resembles that of a ham-bone. In 2008, it was discovered to be a ternary asteroid, Kleopatra is a relatively large asteroid, measuring 217 ×94 ×81 km. Initial observations with the ESO3.6 m Telescope at La Silla and these results were disputed when radar observations at the Arecibo Observatory showed that the two lobes of the asteroid are connected, resembling the shape of a ham-bone. The radar observations provided a detailed shape model that appeared on the cover of Science Magazine, in 1988 a search for satellites or dust orbiting this asteroid was performed using the UH88 telescope at the Mauna Kea Observatories, but the effort came up empty. In September 2008, Franck Marchis and his collaborators announced that by using the Keck Observatorys adaptive optics system, the outer and inner satellites are about 5 km and 3 km in diameter, respectively. In February 2011 the moons were named Alexhelios and Cleoselene, after Cleopatras children Alexander Helios and it is believed that Kleopatras shape, rotation, and moons are due to an oblique impact perhaps 100 million years ago. The increased rotation would have elongated the asteroid and caused Alexhelios to split off, Cleoselene may have split off later, around 10 million years ago. Kleopatra is a contact binary - if it were spinning much faster, lightcurve plot of 216 Kleopatra, Palmer Divide Observatory, B
13.
Arecibo Observatory
–
The Arecibo Observatory is a radio telescope in the municipality of Arecibo, Puerto Rico. This observatory is operated by SRI International, USRA and UMET, the observatory is the sole facility of the National Astronomy and Ionosphere Center, which refers to the observatory, and the staff that operates it. From its construction in the 1960s until 2011, the observatory was managed by Cornell University and it is used in three major areas of research, radio astronomy, atmospheric science, and radar astronomy. Scientists who want to use the observatory submit proposals that are evaluated by an independent scientific board, the observatory has appeared in film and television productions, gaining more recognition in 1999 when it began to collect data for the SETI@home project. It has been listed on the American National Register of Historic Places starting in 2008 and it was the featured listing in the National Park Services weekly list of October 3,2008. The center was named an IEEE Milestone in 2001 and it has a visitor center that is open part-time. The main collecting dish is 305 m in diameter, constructed inside the left by a karst sinkhole. The dish surface is made of 38,778 perforated aluminum panels, each about 3 by 6 feet, the ground beneath is accessible and supports shade-tolerant vegetation. The observatory has four transmitters, with effective isotropic radiated powers of 20 TW at 2380 MHz,2.5 TW at 430 MHz,300 MW at 47 MHz. The reflector is a reflector, not a parabolic reflector. To aim the device, the receiver is moved to intercept signals reflected from different directions by the dish surface of 270 m radius. A parabolic mirror would have varying astigmatism when the receiver is off the focal point, the platform has a rotating, bow-shaped track 93 m long, called the azimuth arm, carrying the receiving antennas and secondary and tertiary reflectors. This allows the telescope to observe any region of the sky in a cone of visibility about the local zenith. Puerto Ricos location near the Northern Tropic allows Arecibo to view the planets in the Solar System over the Northern half of their orbit. The round trip time to objects beyond Saturn is longer than the 2.6 hour time that the telescope can track a celestial position. The origins of the trace to late 1950s efforts to develop anti-ballistic missile defences as part of the newly formed ARPAs ABM umbrella-effort. Even at this stage it was clear that the use of radar decoys would be a serious problem at the long ranges needed to successfully attack a warhead. Among the many Defender projects were several studies based on the concept that a nuclear warhead would cause unique physical signatures while still in the upper atmosphere
14.
Puerto Rico
–
Puerto Rico, officially the Commonwealth of Puerto Rico and briefly called Porto Rico, is an unincorporated territory of the United States located in the northeast Caribbean Sea. It is an archipelago that includes the island of Puerto Rico and a number of smaller ones such as Mona, Culebra. The capital and most populous city is San Juan and its official languages are Spanish and English, though Spanish predominates. The islands population is approximately 3.4 million, Puerto Ricos rich history, tropical climate, diverse natural scenery, renowned traditional cuisine, and attractive tax incentives make it a popular destination for travelers from around the world. Four centuries of Spanish colonial government transformed the ethnic, cultural and physical landscapes primarily with waves of African captives, and Canarian. In the Spanish imperial imagination, Puerto Rico played a secondary, in 1898, following the Spanish–American War, the United States appropriated Puerto Rico together with most former Spanish colonies under the terms of the Treaty of Paris. Puerto Ricans are natural-born citizens of the United States, however, Puerto Rico does not have a vote in the United States Congress, which governs the territory with full jurisdiction under the Puerto Rico Federal Relations Act of 1950. As a U. S. territory, American citizens residing on the island are disenfranchised at the level and may not vote for president. However, Congress approved a constitution, allowing U. S. citizens on the territory to elect a governor. A fifth referendum will be held in June 2017, with only Statehood, in early 2017, the Puerto Rican government-debt crisis posed serious problems for the government. The outstanding bond debt that had climbed to $70 billion or $12,000 per capita at a time with 12. 4% unemployment, the debt had been increasing during a decade long recession. Puerto Ricans often call the island Borinquen – a derivation of Borikén, its indigenous Taíno name, the terms boricua and borincano derive from Borikén and Borinquen respectively, and are commonly used to identify someone of Puerto Rican heritage. The island is also known in Spanish as la isla del encanto. Columbus named the island San Juan Bautista, in honor of Saint John the Baptist, eventually traders and other maritime visitors came to refer to the entire island as Puerto Rico, while San Juan became the name used for the main trading/shipping port and the capital city. The islands name was changed to Porto Rico by the United States after the Treaty of Paris of 1898, the anglicized name was used by the US government and private enterprises. The name was changed back to Puerto Rico by a joint resolution in Congress introduced by Félix Córdova Dávila in 1931, the ancient history of the archipelago known today as Puerto Rico is not well known. The scarce archaeological findings and early Spanish scholarly accounts from the colonial era constitute the basis of knowledge about them. The first comprehensive book on the history of Puerto Rico was written by Fray Íñigo Abbad y Lasierra in 1786, the first settlers were the Ortoiroid people, an Archaic Period culture of Amerindian hunters and fishermen who migrated from the South American mainland
15.
S-type asteroid
–
S-type asteroids, or silicaceous asteroids, are of a stony composition, hence the name. Approximately 17% of asteroids are of type, making it the second most common after the C-type. S-types are moderately bright and consist mainly of iron- and magnesium-silicates and they are dominant in the inner asteroid belt within 2.2 AU, common in the central belt within about 3 AU, but become rare farther out. The largest is 15 Eunomia, with the next largest members by diameter being 3 Juno,29 Amphitrite,532 Herculina and 7 Iris. Their spectrum has a steep slope at wavelengths shorter than 0.7 µm. The 1 µm absorption is indicative of the presence of silicates, often there is also a broad but shallow absorption feature centered near 0.63 µm. The composition of asteroids is similar to a variety of stony meteorites which share similar spectral characteristics. This whole S assemblage of asteroids is spectrally quite distinct from the carbonaceous C-group, Asteroid spectral types L-type asteroid K-type asteroid X-type asteroid Bus, S. J. Binzel, R. P. Phase II of the Small Main-belt Asteroid Spectroscopy Survey, A feature-based taxonomy
16.
Asteroid impact avoidance
–
Asteroid impact avoidance comprises a number of methods by which near-Earth objects could be diverted, preventing destructive impact events. While the chances of a collision are not great in the near term. NASA warns that the earth is unprepared and it has been estimated that a velocity change of just 3. 5/t × 10−2 m·s−1 is needed to successfully deflect a body on a direct collision trajectory. In addition, under circumstances, much smaller velocity changes are needed. It was then determined that a deflection from this potential return trajectory, several years before the swing-by, an impact by a 10 kilometres asteroid on the Earth has historically caused an extinction-level event due to catastrophic damage to the biosphere. There is also the threat from comets coming into the inner Solar System, impacts from objects as small as 50 metres in diameter, which are far more common, are historically extremely destructive regionally. Finding out the composition of the object is also helpful before deciding which strategy is appropriate. Missions like the 2005 Deep Impact probe have provided information on what to expect. The 1992 NASA-sponsored Near-Earth-Object Interception Workshop hosted by Los Alamos National Laboratory evaluated issues involved in intercepting celestial objects that could hit Earth, in a 1992 report to NASA, a coordinated Spaceguard Survey was recommended to discover, verify and provide follow-up observations for Earth-crossing asteroids. This survey was expected to discover 90% of these larger than one kilometer within 25 years. In 1998, NASA formally embraced the goal of finding and cataloging, by 2008, the impact of an object much larger than 1 km diameter could well result in worldwide damage up to, and potentially including, extinction of the human species. The NASA commitment has resulted in the funding of a number of NEO search efforts that are making progress toward the 90% goal by 2008. The 2009 discovery of an NEO approximately 2 to 3 kilometers in diameter demonstrated there were still large objects to be detected, United States Representative George E. Brown, Jr. This bill to provide for a Near-Earth Object Survey program to detect, track, catalogue, and characterize certain near-Earth asteroids and comets was introduced in March 2005 by Rep. Dana Rohrabacher. It was eventually rolled into S.1281, the NASA Authorization Act of 2005, passed by Congress on December 22,2005, subsequently signed by the President, and stating in part, The U. S. It shall be the goal of the Survey program to achieve 90% completion of its near-Earth object catalogue within 15 years after the date of enactment of this Act, a recommended option and proposed budget to carry out the Survey program pursuant to the recommended option. Analysis of possible alternatives that NASA could employ to divert an object on a collision course with Earth. The result of this directive was a report presented to Congress in early March 2007, the Minor Planet Center in Cambridge, Massachusetts has been cataloging the orbits of asteroids and comets since 1947
17.
Near-Earth Asteroid Tracking
–
Near-Earth Asteroid Tracking was a program run by NASA and the Jet Propulsion Laboratory, surveying the sky for near-Earth objects. NEAT was conducted from December 1995 until April 2007, at GEODSS on Hawaii, with the discovery of more than 40 thousand minor planets, NEAT has been one of the most successful programs in this field, comparable to the Catalina Sky Survey, LONEOS and Mount Lemmon Survey. NEAT was the successor of the Palomar Planet-Crossing Asteroid Survey, the original principal investigator was Eleanor F. Helin, with co-investigators Steven H. Pravdo and David L. Rabinowitz. NEAT has an agreement with the U. S. Air Force to use a GEODSS telescope located on Haleakala, Maui. GEODSS stands for Ground-based Electro-Optical Deep Space Surveillance and these wide field Air Force telescopes were designed to optically observe Earth orbital spacecraft, the NEAT team designed a CCD camera and computer system for the GEODSS telescope. The CCD camera format is 4096 ×4096 pixels and the field of view is 1. 2° ×1. 6°, beginning in April 2001, the Samuel Oschin telescope was also put into service to discover and track near-Earth objects. This telescope is equipped with a camera containing 112 CCDs each 2400 ×600 and this is the telescope that produced the images leading to the discovery of 50000 Quaoar in 2002, and 90377 Sedna in 2003 and the dwarf planet Eris. In addition to discovering thousands of asteroids, NEAT is also credited with the co-discovery of periodic comet 54P/de Vico-Swift-NEAT, the C/2001 Q4 comet was discovered on August 24,2001 by NEAT. An asteroid was named in its honour,64070 NEAT, in early 2005,1996 PW was discovered on 1996 August 9 by a NEAT automated search camera on Haleakalā, Hawaii. It was the first object that was not a comet discovered on an orbit typical of a long-period comets. This is raised the possibility it was a comet or a usual asteroid. List of Near-Earth asteroids by distance from Sun Minor Planet Center Planetary Data System Spaceguard Near Earth Asteroid Tracking
18.
Binary asteroid
–
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
19.
Minor-planet moon
–
A minor-planet moon is an astronomical object that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, the first modern era mention of the possibility of an asteroid satellite was in connection with an occultation of the bright star Gamma Ceti by the minor planet Hebe in 1977. The observer, amateur astronomer Paul D. Maley, detected an unmistakable 0.5 second disappearance of this naked eye star from a site near Victoria, many hours later, several observations were reported in Mexico attributed to the occultation by Hebe itself. Although not confirmed this documents the first formally documented case of a companion of an asteroid. As of October 2016, there are over 300 minor planets known to have moons, in addition to the terms satellite and moon, the term binary is sometimes used for minor planets with moons, and triple for minor planets with two moons. If one object is much bigger it can be referred to as the primary, when binary minor planets are similar in size, the Minor Planet Center refers to them as binary companions instead of referring to the smaller body as a satellite. A good example of a true binary is the 90 Antiope system, small satellites are often referred to as moonlets. As of February 2017, over 330 moons of planets have been discovered. For example, in 1978, stellar occultation observations were claimed as evidence of a satellite for the asteroid 532 Herculina, however, later more-detailed imaging by the Hubble Telescope did not reveal a satellite, and the current consensus is that Herculina does not have a significant satellite. There were other reports of asteroids having companions in the following years. In 1993, the first asteroid moon was confirmed when the Galileo probe discovered the small Dactyl orbiting 243 Ida in the asteroid belt, the second was discovered around 45 Eugenia in 1998. In 2001,617 Patroclus and its same-sized companion Menoetius became the first known asteroids in the Jupiter trojans. The first trans-Neptunian binary after Pluto–Charon,1998 WW31, was resolved in 2002. Triple asteroids, or trinary asteroids, are known since 2005 and this was followed by the discovery of a second moon orbiting 45 Eugenia. Also in 2005, the Kuiper belt object Haumea was discovered to have two moons, making it the second KBO after Pluto known to have more than one moon, additionally,216 Kleopatra and 93 Minerva were discovered to be trinary asteroids in 2008 and 2009 respectively. Since the first few trinary asteroids were discovered, more continue to be discovered at a rate of one a year. Most recently discovered was a moon orbiting the belt asteroid 130 Elektra. List of multiple planets, The data about the populations of binary objects are still patchy
20.
Contact binary (small Solar System body)
–
A contact binary is a small Solar System body that is composed of two bodies that have gravitated toward each other until they touch. This means contact binaries have odd shapes, Comet Churyumov–Gerasimenko and Comet Tuttle are most likely contact binaries. Asteroids suspected of being contact binaries include the unusually elongated 624 Hektor,25143 Itokawa, which was photographed by the Hayabusa probe, also appears to be a contact binary which has resulted in an elongated, bent body. About 10–15% of near-Earth asteroids larger than 200 meters are expected to be binaries with two lobes in mutual contact. Rubble pile Binary asteroid Binary system
21.
Vulcanoid
–
The vulcanoids are a hypothetical population of asteroids that orbit the Sun in a dynamically stable zone inside the orbit of the planet Mercury. They are named after the hypothetical planet Vulcan, whose existence was disproven in 1915, so far, no vulcanoids have been discovered, and it is not yet clear if any exist. If they do exist, the vulcanoids could easily evade detection because they would be very small, due to their proximity to the Sun, searches from the ground can only be carried out during twilight or solar eclipses. Any vulcanoids must be between about 100 metres and 6 kilometres in diameter and are located in nearly circular orbits near the outer edge of the gravitationally stable zone. The vulcanoids, should they be found, may provide scientists with material from the first period of planet formation, celestial bodies interior to the orbit of Mercury have been hypothesized, and searched for, for centuries. The German astronomer Christoph Scheiner thought he had seen small bodies passing in front of the Sun in 1611, in the 1850s, Urbain Le Verrier made detailed calculations of Mercurys orbit and found a small discrepancy in the planets perihelion precession from predicted values. He postulated that the influence of a small planet or ring of asteroids within the orbit of Mercury would explain the deviation. Shortly afterward, an astronomer named Edmond Lescarbault claimed to have seen Le Verriers proposed planet transit the Sun. The new planet was quickly named Vulcan but was never again. The vulcanoids take their name from this hypothetical planet, what Lescarbault saw was probably another sunspot. Vulcanoids, should they exist, would be difficult to due to the strong glare of the nearby Sun. Several searches during eclipses were conducted in the early 1900s, which did not reveal any vulcanoids, conventional telescopes cannot be used to search for them because the nearby Sun could damage their optics. In 1998, astronomers analysed data from the SOHO spacecrafts LASCO instrument, the data taken between January and May of that year did not show any vulcanoids brighter than magnitude 7. This corresponds to a diameter of about 60 kilometres, assuming the asteroids have a similar to that of Mercury. In particular, a large planetoid at a distance of 0.18 AU, later attempts to detect the vulcanoids involved taking astronomical equipment above the interference of Earths atmosphere, to heights where the twilight sky is darker and clearer than on the ground. In 2000, planetary scientist Alan Stern performed surveys of the vulcanoid zone using a Lockheed U-2 spy plane, the flights were conducted at a height of 21,300 metres during twilight. In 2002, he and Dan Durda performed similar observations on an F-18 fighter jet and they made three flights over the Mojave desert at an altitude of 15,000 metres and made observations with the Southwest Universal Imaging System—Airborne. Even at these heights the atmosphere is present and can interfere with searches for vulcanoids
22.
Asteroid belt
–
The asteroid belt is the circumstellar disc in the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets, the asteroid belt is also termed the main asteroid belt or main belt to distinguish it from other asteroid populations in the Solar System such as near-Earth asteroids and trojan asteroids. About half the mass of the belt is contained in the four largest asteroids, Ceres, Vesta, Pallas, the total mass of the asteroid belt is approximately 4% that of the Moon, or 22% that of Pluto, and roughly twice that of Plutos moon Charon. Ceres, the belts only dwarf planet, is about 950 km in diameter, whereas Vesta, Pallas. The remaining bodies range down to the size of a dust particle, the asteroid material is so thinly distributed that numerous unmanned spacecraft have traversed it without incident. Nonetheless, collisions between large asteroids do occur, and these can form a family whose members have similar orbital characteristics. Individual asteroids within the belt are categorized by their spectra. The asteroid belt formed from the solar nebula as a group of planetesimals. Planetesimals are the precursors of the protoplanets. Between Mars and Jupiter, however, gravitational perturbations from Jupiter imbued the protoplanets with too much energy for them to accrete into a planet. Collisions became too violent, and instead of fusing together, the planetesimals, as a result,99. 9% of the asteroid belts original mass was lost in the first 100 million years of the Solar Systems history. Some fragments eventually found their way into the inner Solar System, Asteroid orbits continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, a Kirkwood gap occurs as they are swept into other orbits. Classes of small Solar System bodies in other regions are the objects, the centaurs, the Kuiper belt objects, the scattered disc objects, the sednoids. On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on Ceres, the detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding was unexpected because comets, not asteroids, are considered to sprout jets. According to one of the scientists, The lines are becoming more and more blurred between comets and asteroids. This pattern, now known as the Titius–Bode law, predicted the semi-major axes of the six planets of the provided one allowed for a gap between the orbits of Mars and Jupiter
23.
Kirkwood gap
–
A Kirkwood gap is a gap or dip in the distribution of the semi-major axes of the orbits of main-belt asteroids. They correspond to the locations of orbital resonances with Jupiter, for example, there are very few asteroids with semimajor axis near 2.50 AU, period 3.95 years, which would make three orbits for each orbit of Jupiter. Other orbital resonances correspond to orbital periods whose lengths are simple fractions of Jupiters, the weaker resonances lead only to a depletion of asteroids, while spikes in the histogram are often due to the presence of a prominent asteroid family. The orbital elements of the asteroids vary chaotically as a result, the 2,1 MMR has a few relatively stable islands within the resonance, however. These islands are depleted due to slow diffusion onto less stable orbits and this process, which has been linked to Jupiter and Saturn being near a 5,2 resonance, may have been more rapid when Jupiters and Saturns orbits were closer together. More recently, a small number of asteroids have been found to possess high eccentricity orbits which do lie within the Kirkwood gaps. Examples include the Alinda family and the Griqua family and these orbits slowly increase their eccentricity on a timescale of tens of millions of years, and will eventually break out of the resonance due to close encounters with a major planet. The most prominent Kirkwood gaps are located at mean orbital radii of,2.06 AU2.5 AU, home to the Alinda family of asteroids 2.82 AU2.95 AU3.27 AU, home to the Griqua family of asteroids. Weaker and/or narrower gaps are found at,1.9 AU2.25 AU2.33 AU2.71 AU3.03 AU3.075 AU3.47 AU3.7 AU. Orbital resonance Alinda family Griqua family Article on Kirkwood gaps at Wolframs scienceworld
24.
Main-belt comet
–
Main-belt comets are bodies orbiting within the asteroid belt that have shown comet-like activity during part of their orbit. The Jet Propulsion Laboratory defines a main-belt asteroid as an asteroid with an axis of more than 2 AU but less than 3.2 AU. The first main-belt comet discovered is 7968 Elst–Pizarro and it was discovered in 1979 and was found to have a tail by Eric Elst and Guido Pizarro in 1996 and given the cometary designation 133P/Elst-Pizarro. Although quite a few comets have semimajor axes well within Jupiters orbit, main-belt comets differ in having small eccentricities. The first three identified main-belt comets all orbit within the part of the asteroid belt. It is not known how an outer Solar System body like the other comets could have made its way into a low-eccentricity orbit typical of the asteroid belt, some main-belt comets display a cometary dust tail only for a part of their orbit near perihelion. Activity in 133P/Elst–Pizarro is recurrent, having been observed at each of the last three perihelia, the activity persists for a month or several out of each 5-6 year orbit, and is presumably due to ice being uncovered by minor impacts in the last 100 to 1000 years. These impacts are suspected to excavate these subsurface pockets of volatile material helping to expose them to solar radiation, observations of Scheila indicated that large amounts of dust were kicked up by the impact of another asteroid of approximately 35 meters in diameter. In October 2013, observations of P/2013 R3, taken with the 10.4 m Gran Telescopio Canarias on the island of La Palma showed that this comet was breaking apart. The brightest A fragment was also detected at the position in CCD images obtained at the 1.52 m telescope of the Sierra Nevada Observatory in Granada on October 12. NASA reported on a series of images taken by the Hubble Space Telescope between October 29,2013 and January 14,2014 that show the separation of the four main bodies. The Yarkovsky–OKeefe–Radzievskii–Paddack effect, caused by sunlight, increased the rate until the centrifugal force caused the rubble pile to separate. The term main-belt comet is a based on orbit and the presence of an extended morphology. It does not imply that these objects are comets or that the material surrounding their nuclei was ejected by the sublimation of volatiles, identified members of this morphology class include, Centaur Extinct comet Henry Hsiehs Main-Belt Comets page has extensive details on Main-belt comets David Jewitt. J. Licandro New images obtained with the GTC
25.
Earth trojan
–
An Earth trojan is an asteroid that orbits the Sun in the vicinity of the Earth–Sun Lagrangian points L4 and L5, thus having an orbit similar to Earths. Only one Earth trojan has so far been discovered, the name trojan was first used in 1906 for the Jupiter trojans, the asteroids that were observed near the Lagrangian points of Jupiters orbit. Current L42010 TK7, A 300-meter-diameter asteroid, discovered using the Wide-field Infrared Survey Explorer satellite and it is the only confirmed Earth trojan as of 2011–2012. L5 No known objects are thought to be L5 trojans of Earth. 2010 TK7 was discovered using the Wide-field Infrared Survey Explorer satellite, in February 2017, the OSIRIS-REx spacecraft performed a search from within the L4 region on its way to asteroid Bennu. The orbits of any Earth trojans could make them less costly to reach than the Moon. Such asteroids could one day be useful as sources of elements that are rare near Earths surface, on Earth, siderophiles such as iridium are difficult to find, having largely sunk to the core of the planet shortly after its formation. A hypothetical planet-sized Earth trojan the size of Mars, given the name Theia, is thought by proponents of the hypothesis to be the origin of the Moon. The hypothesis states that the Moon formed after Earth and Theia collided head-on and this material eventually accreted around Earth and into a single orbiting body, the Moon. At the same time, material from Theia mixed and combined with Earths mantle, supporters of the giant-impact hypothesis theorise that Earths large core in relation to its overall volume is as a result of this combination. Several other small objects have found on an orbital path associated with Earth. Although these objects are in 1,1 orbital resonance, they are not Earth trojans because they do not librate around a definite Sun–Earth Lagrangian point, Earth has another noted companion, asteroid 3753 Cruithne. About 5 km across, it has a type of orbital resonance called an overlapping horseshoe. 2016 HO3, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth
26.
Jupiter trojan
–
The Jupiter trojans, commonly called Trojan asteroids or just Trojans, are a large group of asteroids that share the orbit of the planet Jupiter around the Sun. Relative to Jupiter, each Trojan librates around one of Jupiters two stable Lagrangian points, L4, lying 60° ahead of the planet in its orbit, and L5, 60° behind. Jupiter trojans are distributed in two elongated, curved regions around these Lagrangian points with an average axis of about 5.2 AU. The first Jupiter trojan discovered,588 Achilles, was spotted in 1906 by German astronomer Max Wolf, a total of 6,178 Jupiter trojans have been found as of January 2015. By convention they are named after a mythological figure from the Trojan War. The total number of Jupiter trojans larger than 1 km in diameter is believed to be about 1 million, like main-belt asteroids, Jupiter trojans form families. Jupiter trojans are bodies with reddish, featureless spectra. The Jupiter trojans densities vary from 0.8 to 2.5 g·cm−3, Jupiter trojans are thought to have been captured into their orbits during the early stages of the Solar Systems formation or slightly later, during the migration of giant planets. NASA has announced the discovery of an Earth trojan, the trapped body will librate slowly around the point of equilibrium in a tadpole or horseshoe orbit. These leading and trailing points are called the L4 and L5 Lagrange points, however, no asteroids trapped in Lagrange points were observed until more than a century after Lagranges hypothesis. Those associated with Jupiter were the first to be discovered, E. E. Barnard made the first recorded observation of a Trojan,1999 RM11, in 1904, but neither he nor others appreciated its significance at the time. Barnard believed he saw the recently discovered Saturnian satellite Phoebe, which was only two away in the sky at the time, or possibly an asteroid. The objects identity was not realized until its orbit was calculated in 1999, in 1906–1907 two more Jupiter trojans were found by fellow German astronomer August Kopff. Hektor, like Achilles, belonged to the L4 swarm, whereas Patroclus was the first asteroid known to reside at the L5 Lagrangian point, by 1938,11 Jupiter trojans had been detected. This number increased to 14 only in 1961, as instruments improved, the rate of discovery grew rapidly, by January 2000, a total of 257 had been discovered, by May 2003, the number had grown to 1,600. Asteroids in the L4 group are named after Greek heroes, confusingly,617 Patroclus was named before the Greece/Troy rule was devised, and a Greek name thus appears in the Trojan node. The Greek node also has one misplaced asteroid,624 Hektor, estimates of the total number of Jupiter trojans are based on deep surveys of limited areas of the sky. The L4 swarm is believed to hold between 160–240,000 asteroids with diameters larger than 2 km and about 600,000 with diameters larger than 1 km
27.
Neptune trojan
–
Neptune trojans are bodies in orbit around the Sun that orbit near one of the stable Lagrangian points of Neptune. They therefore have approximately the same period as Neptune and follow roughly the same orbital path. Seventeen Neptune trojans are known, of which thirteen orbit near the Sun–Neptune L4 Lagrangian point 60° ahead of Neptune. The Neptune trojans are termed trojans by analogy with the Jupiter trojans and it is suspected that large Neptune trojans could outnumber Jupiter trojans by an order of magnitude. In 2010, the discovery of the first known L5 Neptune trojan,2008 LC18, was announced, Neptunes trailing L5 region is currently very difficult to observe because it is along the line-of-sight to the center of the Milky Way, an area of the sky crowded with stars. However, New Horizons may not have had sufficient downlink bandwidth, in 2001, the first Neptune trojan was discovered,2001 QR322, near Neptunes L4 region, and with it the fifth known populated stable reservoir of small bodies in the Solar System. In 2005, the discovery of the high-inclination trojan 2005 TN53 has indicated that the Neptune trojans populate thick clouds, on August 12,2010, the first L5 trojan,2008 LC18, was announced. It was discovered by a survey that scanned regions where the light from the stars near the Galactic Center is obscured by dust clouds. This suggests that large L5 trojans are as common as large L4 trojans, to within uncertainty and it would have been possible for the New Horizons spacecraft to investigate L5 Neptune trojans discovered by 2014, when it passed through this region of space en route to Pluto. Some of the patches where the light from the Galactic Center is obscured by dust clouds are along New Horizonss flight path, allowing detection of objects that the spacecraft could image. 2011 HM102, the highest-inclination Neptune trojan known, was just bright enough for New Horizons to observe it in end-2013 at a distance of 1.2 AU. However, New Horizons may not have had sufficient downlink bandwidth, the orbits of Neptune trojans are highly stable, Neptune may have retained up to 50% of the original post-migration trojan population over the age of the Solar System. Neptunes L5 can host stable trojans equally well as its L4, Neptune trojans can librate up to 30° from their associated Lagrangian points with a 10, 000-year period. Neptune trojans that escape enter orbits similar to centaurs, although Neptune cannot currently capture stable trojans, roughly 2. 8% of the centaurs within 34 AU are predicted to be Neptune co-orbitals. Of these, 54% would be in horseshoe orbits, 10% would be quasi-satellites, the unexpected high-inclination trojans are the key to understanding the origin and evolution of the population as a whole. The existence of high-inclination Neptune trojans points to a capture during planetary migration instead of in situ or collisional formation. The estimated equal number of large L5 and L4 trojans indicates that there was no gas drag during capture, the capture of Neptune trojans during a migration of the planets occurs via process similar to the chaotic capture of Jupiter trojans in the Nice model. When Uranus and Neptune are near but not in a mean-motion resonance the period at which the locations where Uranus passes Neptune circulate can resonate with the periods of Neptune trojans
28.
Near-Earth object
–
A near-Earth object is any small Solar System body whose orbit brings it into proximity with Earth. By definition, a solar system body is a NEO if its closest approach to the Sun is less than 1.3 astronomical unit and it is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of the Earth. NEOs have become of increased interest since the 1980s because of increased awareness of the potential danger some of the asteroids or comets pose, and mitigations are being researched. In January 2016, NASA announced the Planetary Defense Coordination Office to track NEOs larger than 30 to 50 meters in diameter and coordinate an effective threat response, NEAs have orbits that lie partly between 0.983 and 1.3 AU away from the Sun. When a NEA is detected it is submitted to the IAUs Minor Planet Center for cataloging, some NEAs orbits intersect that of Earths so they pose a collision danger. The United States, European Union, and other nations are currently scanning for NEOs in an effort called Spaceguard. In the United States and since 1998, NASA has a mandate to catalogue all NEOs that are at least 1 kilometer wide. In 2006, it was estimated that 20% of the objects had not yet been found. In 2011, largely as a result of NEOWISE, it was estimated that 93% of the NEAs larger than 1 km had been found, as of 5 February 2017, there have been 875 NEAs larger than 1 km discovered, of which 157 are potentially hazardous. The inventory is much less complete for smaller objects, which still have potential for scale, though not global. Potentially hazardous objects are defined based on parameters that measure the objects potential to make threatening close approaches to the Earth. Mostly objects with an Earth minimum orbit intersection distance of 0.05 AU or less, objects that cannot approach closer to the Earth than 0.05 AU, or are smaller than about 150 m in diameter, are not considered PHOs. This makes them a target for exploration. As of 2016, three near-Earth objects have been visited by spacecraft, more recently, a typical frame of reference for looking at NEOs has been through the scientific concept of risk. In this frame, the risk that any near-Earth object poses is typically seen through a lens that is a function of both the culture and the technology of human society, NEOs have been understood differently throughout history. Each time an NEO is observed, a different risk was posed and it is not just a matter of scientific knowledge. Such perception of risk is thus a product of religious belief, philosophic principles, scientific understanding, technological capabilities, and even economical resourcefulness.03 E −0.4 megatonnes. For instance, it gives the rate for bolides of 10 megatonnes or more as 1 per thousand years, however, the authors give a rather large uncertainty, due in part to uncertainties in determining the energies of the atmospheric impacts that they used in their determination
29.
Aten asteroid
–
The Aten asteroids are a group of asteroids, whose orbit brings them into proximity with Earth. The group is named after 2062 Aten, the first of its kind, since then, more than 1,000 Atens have been discovered, of which many are classified as potentially hazardous asteroids. For a list of existing articles, see Aten asteroids and List of Aten asteroids, Aten asteroids are defined by having a semi-major axis of less than one astronomical unit, the average distance from the Earth to the Sun. They also have a greater than 0.983 AU. Asteroids orbits can be highly eccentric, an Aten orbit need not be entirely contained within Earths orbit, as nearly all known Aten asteroids have their aphelion greater than 1 AU although their semi-major axis is less than 1 AU. Observation of objects inferior to the Earths orbit is difficult and this difficulty may be the cause of some sampling bias in the apparent preponderance of eccentric Atens, Aten asteroids account for only about 6% of the known near-Earth asteroid population. Many more Apollo-class asteroids are known than Aten-class asteroids, possibly because of the sampling bias, the shortest semi-major axis for any known Aten asteroid is 2008 EY5 at 0.626 AU. A very small possibility of impact remained for 2036, but this was also eliminated, there are also sixteen known Apohele asteroids, traditionally listed as a subclass of Atens, but generally regarded a separate class of their own. Unlike Atens, Apoheles permanently stay within Earths orbit and do not cross it
30.
Amor asteroid
–
The Amor asteroids are a group of near-Earth asteroids named after the asteroid 1221 Amor. They approach the orbit of Earth from beyond, but do not cross it, most Amors cross the orbit of Mars. The two moons of Mars, Deimos and Phobos, may be Amor asteroids that were captured by Marss gravity, the most famous member of this group is 433 Eros, which was the first asteroid to be orbited and then landed upon by a human probe. There are three general criteria which an asteroid must meet to be considered a member of the Amor asteroid class, to be considered near, the asteroid must come closer to Earth than to any other major planet. The closest planet to Earth is Venus, which can come as close as 0.27 AU, therefore, an Amor asteroid must come within 0.30 AU of Earths orbit. The asteroids orbit must be outside the orbit of Earth, asteroids that come close to Earth whose orbits are inside Earths orbit are considered Apohele asteroids. The asteroids orbit must not cross Earths orbit, the most commonly used definition of this is that it never orbits closer to the Sun than Earths average distance from the Sun. A more strict definition is that at any point along the asteroids orbit and this takes into consideration the fact that Earths orbit ranges between 0.983 and 1.016 AU from the Sun. It is more difficult to sort out the Amor asteroids from the non-Amor asteroids using this definition, however. These three criteria boil down to a single test for membership, If an asteroid has a perihelion between 1.000 AU and 1.300 AU, it is an Amor asteroid. Any asteroid with this trait is considered an Amor-class asteroid, regardless of its axis, eccentricity, aphelion, inclination, physical properties, orbital stability. An asteroid belongs to the Amor group if, Its orbital period is greater than one year and this is equivalent to saying that its semi-major axis is greater than 1.0 AU. Its orbit does not cross Earths orbit and that is, its lowest point is higher than Earths highest point. It is an object, that is, its perihelion distance q <1.3 AU. In summary, a >1.0 AU and 1.017 AU < q <1.3 AU, there are 6051 Amor asteroids currently known. 960 of them are numbered, and 73 of them are named, Amor asteroids can be partitioned into four subgroups, depending on their average distance from the Sun. The Amor I subgroup consists of Amor asteroids whose semi-major axes are in between Earth and Mars and that is, they have a semi-major axis between 1.000 and 1.523 AU. Less than one fifth of Amor asteroids belong to this subgroup, Amor I asteroids have lower eccentricities than the other subgroups of Amors
31.
Mars trojan
–
The Mars trojans are a group of objects that share the orbit of the planet Mars around the Sun. They can be found around the two Lagrangian points 60° ahead of and behind Mars, the origin of the Mars trojans is not well understood. One theory suggests that they were captured in its Lagrangian points as the Solar System was forming, however, spectral studies of the Mars trojans indicate this may not be the case. One explanation for this involves asteroids wandering into the Mars Lagrangian points later in the Solar Systems formation and this is also questionable considering the very low mass of Mars
32.
Asteroid group
–
A minor-planet group is a population of minor planets that share broadly similar orbits. Members are generally unrelated to other, unlike in an asteroid family. It is customary to name a group of asteroids after the first member of group to be discovered. There are relatively few asteroids that orbit close to the Sun, several of these groups are hypothetical at this point in time, with no members having yet been discovered, as such, the names they have been given are provisional. Vulcanoid asteroids are asteroids that orbit entirely within the orbit of Mercury. A few searches for vulcanoids have been conducted but none have been discovered so far, Apoheles are asteroids whose aphelion is less than 0.983 AU, meaning they orbit entirely within Earths orbit. Other proposed names for this group are inner-Earth objects or interior Earth objects and Atira asteroids, Mercury-crosser asteroids having a perihelion smaller than Mercurys 0.3075 AU. Venus-crosser asteroids having a smaller than Venuss 0.7184 AU. This group includes the above Mercury-crossers, earth-crosser asteroids having a perihelion smaller than Earths 0.9833 AU. This group includes the above Mercury- and Venus-crossers, apart from the Apoheles and they are also divided into the Aten asteroids having a semi-major axis less than 1 AU, named after 2062 Aten. Apollo asteroids having a semi-major axis greater than 1 AU, named after 1862 Apollo, arjuna asteroids are somewhat vaguely defined as having orbits similar to Earths, i. e. with an average orbital radius of around 1 AU and with low eccentricity and inclination. Due to the vagueness of this definition some asteroids belonging to the Apohele, Amor, the term was introduced by Spacewatch and does not refer to an existing asteroid, examples of Arjunas include 1991 VG. Earth trojans are located in the Earth–Sun Lagrangian points L4. The only known Earth trojan is 2010 TK7, near-Earth asteroids is a catch-all term for asteroids whose orbit closely approaches that of Earth. It includes almost all of the groups, as well as the Amor asteroids. The Amor asteroids, named after 1221 Amor, are asteroids that are not Earth-crossers. Mars-crosser asteroids have orbits that cross that of Mars, but do not necessarily closely approach the Earths, Mars trojans follow or lead Mars on its orbit, at either of the two Lagrangian points 60° ahead or behind. As of March 2008, four are known, the largest appears to be 5261 Eureka
33.
Asteroid family
–
An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions, an asteroid family is a more specific term than asteroid group whose members, while sharing some broad orbital characteristics, may be otherwise unrelated to each other. Large prominent families contain several hundred recognized asteroids, small, compact families can have only about ten identified members. About 33% to 35% of asteroids in the belt are family members. There are about 20 to 30 reliably recognized families, with tens of less certain groupings. One family has been identified associated with the dwarf planet Haumea, some studies have tried to find evidence of collisional families among the trojan asteroids, but at present the evidence is inconclusive. The families are thought to form as a result of collisions between asteroids, in many or most cases the parent body was shattered, but there are also several families which resulted from a large cratering event which did not disrupt the parent body. Such cratering families typically consist of a large body and a swarm of asteroids that are much smaller. Some families have complex structures which are not satisfactorily explained at the moment. Due to the method of origin, all the members have closely matching compositions for most families, notable exceptions are those families which formed from a large differentiated parent body. Asteroid families are thought to have lifetimes of the order of a billion years and this is significantly shorter than the Solar Systems age, so few if any are relics of the early Solar System. Such small asteroids then become subject to such as the Yarkovsky effect that can push them towards orbital resonances with Jupiter over time. Once there, they are relatively rapidly ejected from the asteroid belt, tentative age estimates have been obtained for some families, ranging from hundreds of millions of years to less than several million years as for the compact Karin family. Old families are thought to contain few small members, and this is the basis of the age determinations and it is supposed that many very old families have lost all the smaller and medium-sized members, leaving only a few of the largest intact. A suggested example of old family remains are the 9 Metis and 113 Amalthea pair. Further evidence for a number of past families comes from analysis of chemical ratios in iron meteorites. These show that there must have once been at least 50 to 100 parent bodies large enough to be differentiated, when the orbital elements of main belt asteroids are plotted, a number of distinct concentrations are seen against the rather uniform background distribution of generic asteroids. These concentrations are the asteroid families, the proper elements are related constants of motion that remain almost constant for times of at least tens of millions of years, and perhaps longer
