1.
Reflecting telescope
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A reflecting telescope is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century as an alternative to the telescope which. Although reflecting telescopes produce other types of aberrations, it is a design that allows for very large diameter objectives. Almost all of the telescopes used in astronomy research are reflectors. Reflecting telescopes come in many variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is referred to as a catoptric telescope. The idea that curved mirrors behave like lenses dates back at least to Alhazens 11th century treatise on optics, the potential advantages of using parabolic mirrors, primarily reduction of spherical aberration with no chromatic aberration, led to many proposed designs for reflecting telescopes. The most notable being James Gregory, who published a design for a ‘reflecting’ telescope in 1663. It would be ten years, before the experimental scientist Robert Hooke was able to build this type of telescope, Isaac Newton has been generally credited with building the first reflecting telescope in 1668. It used a spherically ground metal primary mirror and a diagonal mirror in an optical configuration that has come to be known as the Newtonian telescope. A curved primary mirror is the reflector telescopes basic optical element that creates an image at the focal plane, the distance from the mirror to the focal plane is called the focal length. The primary mirror in most modern telescopes is composed of a glass cylinder whose front surface has been ground to a spherical or parabolic shape. A thin layer of aluminum is deposited onto the mirror. Some telescopes use primary mirrors which are made differently, molten glass is rotated to make its surface paraboloidal, and is kept rotating while it cools and solidifies. The resulting mirror shape approximates a desired paraboloid shape that requires grinding and polishing to reach the exact figure needed. Reflecting telescopes, just like any other system, do not produce perfect images. The use of mirrors avoids chromatic aberration but they produce other types of aberrations, to avoid this problem most reflecting telescopes use parabolic shaped mirrors, a shape that can focus all the light to a common focus. Field curvature - The best image plane is in general curved and it is sometimes corrected by a field flattening lens
2.
Apsis
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An apsis is an extreme point in an objects orbit. The word comes via Latin from Greek and is cognate with apse, for elliptic orbits about a larger body, there are two apsides, named with the prefixes peri- and ap-, or apo- added to a reference to the thing being orbited. For a body orbiting the Sun, the point of least distance is the perihelion, the terms become periastron and apastron when discussing orbits around other stars. For any satellite of Earth including the Moon the point of least distance is the perigee, for objects in Lunar orbit, the point of least distance is the pericynthion and the greatest distance the apocynthion. For any orbits around a center of mass, there are the terms pericenter and apocenter, periapsis and apoapsis are equivalent alternatives. A straight line connecting the pericenter and apocenter is the line of apsides and this is the major axis of the ellipse, its greatest diameter. For a two-body system the center of mass of the lies on this line at one of the two foci of the ellipse. When one body is larger than the other it may be taken to be at this focus. Historically, in systems, apsides were measured from the center of the Earth. In orbital mechanics, the apsis technically refers to the distance measured between the centers of mass of the central and orbiting body. However, in the case of spacecraft, the family of terms are used to refer to the orbital altitude of the spacecraft from the surface of the central body. The arithmetic mean of the two limiting distances is the length of the axis a. The geometric mean of the two distances is the length of the semi-minor axis b, the geometric mean of the two limiting speeds is −2 ε = μ a which is the speed of a body in a circular orbit whose radius is a. The words pericenter and apocenter are often seen, although periapsis/apoapsis are preferred in technical usage, various related terms are used for other celestial objects. The -gee, -helion and -astron and -galacticon forms are used in the astronomical literature when referring to the Earth, Sun, stars. The suffix -jove is occasionally used for Jupiter, while -saturnium has very rarely used in the last 50 years for Saturn. The -gee form is used as a generic closest approach to planet term instead of specifically applying to the Earth. During the Apollo program, the terms pericynthion and apocynthion were used when referring to the Moon, regarding black holes, the term peri/apomelasma was used by physicist Geoffrey A. Landis in 1998 before peri/aponigricon appeared in the scientific literature in 2002
3.
Astronomical unit
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The astronomical unit is a unit of length, roughly the distance from Earth to the Sun. However, that varies as Earth orbits the Sun, from a maximum to a minimum. Originally conceived as the average of Earths aphelion and perihelion, it is now defined as exactly 149597870700 metres, the astronomical unit is used primarily as a convenient yardstick for measuring distances within the Solar System or around other stars. However, it is also a component in the definition of another unit of astronomical length. A variety of symbols and abbreviations have been in use for the astronomical unit. In a 1976 resolution, the International Astronomical Union used the symbol A for the astronomical unit, in 2006, the International Bureau of Weights and Measures recommended ua as the symbol for the unit. In 2012, the IAU, noting that various symbols are presently in use for the astronomical unit, in the 2014 revision of the SI Brochure, the BIPM used the unit symbol au. In ISO 80000-3, the symbol of the unit is ua. Earths orbit around the Sun is an ellipse, the semi-major axis of this ellipse is defined to be half of the straight line segment that joins the aphelion and perihelion. The centre of the sun lies on this line segment. In addition, it mapped out exactly the largest straight-line distance that Earth traverses over the course of a year, knowing Earths shift and a stars shift enabled the stars distance to be calculated. But all measurements are subject to some degree of error or uncertainty, improvements in precision have always been a key to improving astronomical understanding. Improving measurements were continually checked and cross-checked by means of our understanding of the laws of celestial mechanics, the expected positions and distances of objects at an established time are calculated from these laws, and assembled into a collection of data called an ephemeris. NASAs Jet Propulsion Laboratory provides one of several ephemeris computation services, in 1976, in order to establish a yet more precise measure for the astronomical unit, the IAU formally adopted a new definition. Equivalently, by definition, one AU is the radius of an unperturbed circular Newtonian orbit about the sun of a particle having infinitesimal mass. As with all measurements, these rely on measuring the time taken for photons to be reflected from an object. However, for precision the calculations require adjustment for such as the motions of the probe. In addition, the measurement of the time itself must be translated to a scale that accounts for relativistic time dilation
4.
Orbital eccentricity
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The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is an orbit, values between 0 and 1 form an elliptical orbit,1 is a parabolic escape orbit. The term derives its name from the parameters of conic sections and it is normally used for the isolated two-body problem, but extensions exist for objects following a rosette orbit through the galaxy. In a two-body problem with inverse-square-law force, every orbit is a Kepler orbit, the eccentricity of this Kepler orbit is a non-negative number that defines its shape. The limit case between an ellipse and a hyperbola, when e equals 1, is parabola, radial trajectories are classified as elliptic, parabolic, or hyperbolic based on the energy of the orbit, not the eccentricity. Radial orbits have zero angular momentum and hence eccentricity equal to one, keeping the energy constant and reducing the angular momentum, elliptic, parabolic, and hyperbolic orbits each tend to the corresponding type of radial trajectory while e tends to 1. For a repulsive force only the trajectory, including the radial version, is applicable. For elliptical orbits, a simple proof shows that arcsin yields the projection angle of a circle to an ellipse of eccentricity e. For example, to view the eccentricity of the planet Mercury, next, tilt any circular object by that angle and the apparent ellipse projected to your eye will be of that same eccentricity. From Medieval Latin eccentricus, derived from Greek ἔκκεντρος ekkentros out of the center, from ἐκ- ek-, eccentric first appeared in English in 1551, with the definition a circle in which the earth, sun. Five years later, in 1556, a form of the word was added. The eccentricity of an orbit can be calculated from the state vectors as the magnitude of the eccentricity vector, e = | e | where. For elliptical orbits it can also be calculated from the periapsis and apoapsis since rp = a and ra = a, where a is the semimajor axis. E = r a − r p r a + r p =1 −2 r a r p +1 where, rp is the radius at periapsis. For Earths annual orbit path, ra/rp ratio = longest_radius / shortest_radius ≈1.034 relative to center point of path, the eccentricity of the Earths orbit is currently about 0.0167, the Earths orbit is nearly circular. Venus and Neptune have even lower eccentricity, over hundreds of thousands of years, the eccentricity of the Earths orbit varies from nearly 0.0034 to almost 0.058 as a result of gravitational attractions among the planets. The table lists the values for all planets and dwarf planets, Mercury has the greatest orbital eccentricity of any planet in the Solar System. Such eccentricity is sufficient for Mercury to receive twice as much solar irradiation at perihelion compared to aphelion, before its demotion from planet status in 2006, Pluto was considered to be the planet with the most eccentric orbit
5.
Orbital inclination
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Orbital inclination measures the tilt of an objects orbit around a celestial body. It is expressed as the angle between a plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth directly above the equator, the plane of the orbit is the same as the Earths equatorial plane. The general case is that the orbit is tilted, it spends half an orbit over the northern hemisphere. If the orbit swung between 20° north latitude and 20° south latitude, then its orbital inclination would be 20°, the inclination is one of the six orbital elements describing the shape and orientation of a celestial orbit. It is the angle between the plane and the plane of reference, normally stated in degrees. For a satellite orbiting a planet, the plane of reference is usually the plane containing the planets equator, for planets in the Solar System, the plane of reference is usually the ecliptic, the plane in which the Earth orbits the Sun. This reference plane is most practical for Earth-based observers, therefore, Earths inclination is, by definition, zero. Inclination could instead be measured with respect to another plane, such as the Suns equator or the invariable plane, the inclination of orbits of natural or artificial satellites is measured relative to the equatorial plane of the body they orbit, if they orbit sufficiently closely. The equatorial plane is the perpendicular to the axis of rotation of the central body. An inclination of 30° could also be described using an angle of 150°, the convention is that the normal orbit is prograde, an orbit in the same direction as the planet rotates. Inclinations greater than 90° describe retrograde orbits, thus, An inclination of 0° means the orbiting body has a prograde orbit in the planets equatorial plane. An inclination greater than 0° and less than 90° also describe prograde orbits, an inclination of 63. 4° is often called a critical inclination, when describing artificial satellites orbiting the Earth, because they have zero apogee drift. An inclination of exactly 90° is an orbit, in which the spacecraft passes over the north and south poles of the planet. An inclination greater than 90° and less than 180° is a retrograde orbit, an inclination of exactly 180° is a retrograde equatorial orbit. For gas giants, the orbits of moons tend to be aligned with the giant planets equator, the inclination of exoplanets or members of multiple stars is the angle of the plane of the orbit relative to the plane perpendicular to the line-of-sight from Earth to the object. An inclination of 0° is an orbit, meaning the plane of its orbit is parallel to the sky. An inclination of 90° is an orbit, meaning the plane of its orbit is perpendicular to the sky
6.
Oort cloud
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The Oort cloud, sometimes called the Öpik–Oort cloud, is a theoretical cloud of predominantly icy planetesimals believed to surround the Sun to as far as somewhere between 50,000 and 200,000 AU. It is divided into two regions, a disc-shaped inner Oort cloud and a spherical outer Oort cloud, both regions lie beyond the heliosphere and in interstellar space. The Kuiper belt and the disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth as far from the Sun as the Oort cloud. The outer limit of the Oort cloud defines the boundary of the Solar System. The outer Oort cloud is only bound to the Solar System. These forces occasionally dislodge comets from their orbits within the cloud, based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud. In 1932, the Estonian astronomer Ernst Öpik postulated that long-period comets originated in a cloud at the outermost edge of the Solar System. The idea was revived by Dutch astronomer Jan Oort as a means to resolve a paradox. Thus, Oort reasoned, a comet could not have formed while in its current orbit, there are two main classes of comet, short-period comets and long-period comets. Ecliptic comets have relatively small orbits, below 10 AU, and follow the ecliptic plane, all long-period comets have very large orbits, on the order of thousands of AU, and appear from every direction in the sky. Oort noted that there was a peak in numbers of comets with aphelia of roughly 20,000 AU. The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU to as far as 50,000 AU from the Sun, some estimates place the outer edge at between 100,000 and 200,000 AU. The region can be subdivided into a spherical outer Oort cloud of 20, 000–50,000 AU, the outer cloud is only weakly bound to the Sun and supplies the long-period comets to inside the orbit of Neptune. The inner Oort cloud is known as the Hills cloud, named after Jack G. Hills. The Hills cloud explains the existence of the Oort cloud after billions of years. The outer Oort cloud may have trillions of objects larger than 1 km, earlier it was thought to be more massive, but improved knowledge of the size distribution of long-period comets led to lower estimates. The mass of the inner Oort cloud has not been characterized, if analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of ices such as water, methane, ethane, carbon monoxide and hydrogen cyanide. The Oort cloud is thought to be a remnant of the protoplanetary disc that formed around the Sun approximately 4.6 billion years ago
7.
Comet
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A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to evolve gasses, a process called outgassing. This produces an atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of collections of ice, dust. The coma may be up to 15 times the Earths diameter, if sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° across the sky. Comets have been observed and recorded since ancient times by many cultures, Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars, hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition, Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma and the tail, however, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System, the discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. As of November 2014 there are 5,253 known comets, however, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System is estimated to be one trillion. Roughly one comet per year is visible to the eye, though many of those are faint. Particularly bright examples are called Great Comets, the word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης, Κομήτης was derived from κομᾶν, which was itself derived from κόμη and was used to mean the tail of a comet. The astronomical symbol for comets is ☄, consisting of a disc with three hairlike extensions. The solid, core structure of a comet is known as the nucleus, cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen gases such as carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are described as dirty snowballs after Fred Whipples model
8.
Apparent magnitude
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The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value, the Sun, at apparent magnitude of −27, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere, furthermore, the magnitude scale is logarithmic, a difference of one in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry, apparent magnitudes are used to quantify the brightness of sources at ultraviolet, visible, and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or often simply as V, the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes. The brightest stars in the sky were said to be of first magnitude, whereas the faintest were of sixth magnitude. Each grade of magnitude was considered twice the brightness of the following grade and this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest, and is generally believed to have originated with Hipparchus. This implies that a star of magnitude m is 2.512 times as bright as a star of magnitude m +1 and this figure, the fifth root of 100, became known as Pogsons Ratio. The zero point of Pogsons scale was defined by assigning Polaris a magnitude of exactly 2. However, with the advent of infrared astronomy it was revealed that Vegas radiation includes an Infrared excess presumably due to a disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures, however, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the scale was extrapolated to all wavelengths on the basis of the black body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, with the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30, astronomers have developed other photometric zeropoint systems as alternatives to the Vega system. The AB magnitude zeropoint is defined such that an objects AB, the dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of exactly 100. Since an increase of 5 magnitudes corresponds to a decrease in brightness by a factor of exactly 100, each magnitude increase implies a decrease in brightness by the factor 5√100 ≈2.512. Inverting the above formula, a magnitude difference m1 − m2 = Δm implies a brightness factor of F2 F1 =100 Δ m 5 =100.4 Δ m ≈2.512 Δ m
9.
Elongation (astronomy)
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In astronomy, a planets elongation is the angle between the Sun and the planet, with Earth as the reference point. The greatest elongation of a given planet occurs when this inner planet’s position, when a planet is at its greatest elongation, it is farthest from the Sun as viewed from Earth, so its view is also best at that point. When an inferior planet is visible after sunset, it is near its greatest eastern elongation, when an inferior planet is visible before sunrise, it is near its greatest western elongation. The value of the greatest elongation, for Mercury, is between 18° and 28°, and for Venus between 45° and 47° and this value varies because the orbits of the planets are elliptical, rather than perfect circles. Another minor contributor to this inconsistency is orbital inclination, each orbit is in a slightly different plane. Refer to astronomical tables and websites such as heavens-above to see when the planets reach their next maximum elongations, greatest elongations of a planet happen periodically, with a greatest eastern elongation followed by a greatest western elongation, and vice versa. The period depends on the angular velocity of Earth and the planet. The time it takes to complete this period is the period of the planet. Let T be the period, ω be the angular velocity, ωe Earths angular velocity. For example, Venuss year is 225 days, and Earths is 365 days, thus Venuss synodic period, which gives the time between two subsequent eastern greatest elongations, is 584 days. These values are approximate, because the planets do not have circular, coplanar orbits. Superior planets, dwarf planets and asteroids undergo a different cycle, in other words, as seen from an observer on the superior planet at opposition, the Earth appears at inferior conjunction with the Sun. Technically, the moment of opposition is slightly different from the moment of maximum elongation. For example, Pluto, whose orbit is inclined to the Earths orbital plane. All superior planets are most easily visible at their oppositions because they are near their closest approach to Earth and are also above the horizon all night, the variation in magnitude caused by changes in elongation are greater the closer the planets orbit is to the Earths. As one moves out, the difference in magnitude caused by the difference in elongation gradually fall. Since asteroids travel in a not much larger than the Earths. Sometimes elongation may instead refer to the distance of a moon of another planet from its central planet
10.
Binoculars
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Binoculars or field glasses are two telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes when viewing distant objects. Most are sized to be held using both hands, although sizes vary widely from opera glasses to large pedestal mounted military models. Almost from the invention of the telescope in the 17th century the advantages of mounting two of them side by side for binocular vision seems to have been explored, most early binoculars used Galilean optics, that is, they used a convex objective and a concave eyepiece lens. The Galilean design has the advantage of presenting an image but has a narrow field of view and is not capable of very high magnification. This type of construction is used in very cheap models. They also have large exit pupils making centering less critical and the field of view works well in those applications. These are typically mounted on a frame or custom-fit onto eyeglasses. An improved image and higher magnification is achieved in binoculars employing Keplerian optics, since the Keplerian configuration produces an inverted image, different methods were used to turn the image right way up. In aprismatic binoculars with Keplerian optics each tube has one or two additional lenses between the objective and the ocular and these lenses are used to erect the image. The binoculars with erecting lenses have a disadvantage, their length is too long. Such binoculars were popular in the 1800s, but became obsolete shortly after the Karl Zeiss company introduced improved prism binoculars in the 1890s, Optical prisms added to the design are another way to turn the image right way up, usually in a Porro prism or roof-prisms design. Porro prism binoculars are named after Italian optician Ignazio Porro who patented this image erecting system in 1854, Binoculars of this type use a Porro prism in a double prism Z-shaped configuration to erect the image. This feature results in binoculars that are wide, with lenses that are well separated. Thus, the size of binoculars is reduced, Binoculars using roof prisms may have appeared as early as the 1870s in a design by Achille Victor Emile Daubresse. Most roof prism binoculars use either the Abbe-Koenig prism or the Schmidt-Pechan prism designs to erect the image and they have objective lenses that are approximately in line with the eyepieces. Roof-prisms designs create an instrument that is narrower and more compact than Porro prisms, there is also a difference in image brightness. Roof-prisms designs also require tighter tolerances for alignment of their optical elements and this adds to their expense since the design requires them to use fixed elements that need to be set at a high degree of collimation at the factory. Porro prisms binoculars occasionally need their prism sets to be re-aligned to bring them into collimation, the fixed alignment in roof-prism designs means the binoculars normally will not need re-collimation
11.
Celestial equator
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The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earths equator. In other words, it is a projection of the terrestrial equator out into space, as a result of the Earths axial tilt, the celestial equator is inclined by 23. 4° with respect to the ecliptic plane. An observer standing on the Earths equator visualizes the celestial equator as a semicircle passing directly overhead through the zenith, as the observer moves north, the celestial equator tilts towards the opposite horizon. Celestial objects near the equator are visible worldwide, but they culminate the highest in the sky in the tropics. The celestial equator currently passes through these constellations, Celestial bodies other than Earth also have similarly defined celestial equators, Celestial pole Celestial sphere Declination Equatorial coordinate system
12.
Southern Hemisphere
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The Southern Hemisphere is the half sphere of Earth which is south of the equator. It contains all or parts of five continents, four oceans and its surface is 80. 9% water, compared with 60. 7% water in the case of the Northern Hemisphere, and it contains 32. 7% of Earths land. Due to the tilt of Earths rotation relative to the Sun, September 22 or 23 is the vernal equinox and March 20 or 21 is the autumnal equinox. The South Pole is in the middle of the southern hemispherical region, Southern Hemisphere climates tend to be slightly milder than those at similar latitudes in the Northern Hemisphere, except in the Antarctic which is colder than the Arctic. This is because the Southern Hemisphere has significantly more ocean and much land, water heats up. In the Southern Hemisphere the sun passes from east to west through the north, sun-cast shadows turn anticlockwise throughout the day and sundials have the hours increasing in the anticlockwise direction. Cyclones and tropical storms spin clockwise in the Southern Hemisphere due to the Coriolis effect, the southern temperate zone, a subsection of the Southern Hemisphere, is nearly all oceanic. Forests in the Southern Hemisphere have special features which set apart from those in the Northern Hemisphere. Both Chile and Australia share, for example, unique species or Nothofagus. The eucalyptus is native to Australia but is now planted in Southern Africa and Latin America for pulp production and, increasingly. Approximately 800,000,000 humans live in the Southern Hemisphere and this is due to the fact that there is significantly less land in the Southern Hemisphere than in the Northern Hemisphere. Africa Antarctica Asia Australia South America Zealandia Media related to Southern Hemisphere at Wikimedia Commons
13.
Kilometre
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The kilometre or kilometer is a unit of length in the metric system, equal to one thousand metres. K is occasionally used in some English-speaking countries as an alternative for the kilometre in colloquial writing. A slang term for the kilometre in the US military is klick, there are two common pronunciations for the word. It is generally preferred by the British Broadcasting Corporation and the Australian Broadcasting Corporation, many scientists and other users, particularly in countries where the metric system is not widely used, use the pronunciation with stress on the second syllable. The latter pronunciation follows the pattern used for the names of measuring instruments. The problem with this reasoning, however, is that the meter in those usages refers to a measuring device. The contrast is more obvious in countries using the British rather than American spelling of the word metre. When Australia introduced the system in 1975, the first pronunciation was declared official by the governments Metric Conversion Board. However, the Australian prime minister at the time, Gough Whitlam, by the 8 May 1790 decree, the Constituent assembly ordered the French Academy of Sciences to develop a new measurement system. In August 1793, the French National Convention decreed the metre as the length measurement system in the French Republic. The first name of the kilometre was Millaire, although the metre was formally defined in 1799, the myriametre was preferred to the kilometre for everyday use. The term myriamètre appeared a number of times in the text of Develeys book Physique dEmile, ou, Principes de la de la nature. French maps published in 1835 had scales showing myriametres and lieues de Poste, the Dutch, on the other hand, adopted the kilometre in 1817 but gave it the local name of the mijl. It was only in 1867 that the term became the only official unit of measure in the Netherlands to represent 1000 metres. In the US, the National Highway System Designation Act of 1995 prohibits the use of highway funds to convert existing signs or purchase new signs with metric units. Although the State DOTs had the option of using metric measurements or dual units, all of them abandoned metric measurements, the Manual on Uniform Traffic Control Devices since 2000 is published in both metric and American Customary Units. Some sporting disciplines feature 1000 m races in major events, but in other disciplines, even though records are catalogued
14.
Mile
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The mile is an English unit of length of linear measure equal to 5,280 feet, or 1,760 yards, and standardised as exactly 1,609.344 metres by international agreement in 1959. The Romans divided their mile into 5,000 feet but the importance of furlongs in pre-modern England meant that the statute mile was made equivalent to 8 furlongs or 5,280 feet in 1593. This form of the mile then spread to the British-colonized nations who continue to employ the mile, the US Geological Survey now employs the metre for official purposes but legacy data from its 1927 geodetic datum has meant that a separate US survey mile continues to see some use. Derived units such as miles per hour and miles per gallon, however, continue to be abbreviated as mph, mpg. The modern English word mile derives from Middle English myl and Old English mīl, the present international mile is usually what is understood by the unqualified term mile. When this distance needs to be distinguished from the nautical mile, in British English, the statute mile may refer to the present international miles or to any other form of English mile since the 1593 Act of Parliament which set it as a distance of 1,760 yards. Under American law, however, the statute mile refers to the US survey mile, the mile has been variously abbreviated—with and without a trailing period—as m, M, ml, and mi. The American National Institute of Standards and Technology now uses and recommends mi in order to avoid confusion with the SI metre and millilitre. Derived units such as miles per hour and miles per gallon, however, continue to be abbreviated in the United States, United Kingdom, the BBC style holds that There is no acceptable abbreviation for ‘miles’ and so it should be spelt out when used in describing areas. The Roman mile consisted of a thousand paces as measured by every other step—as in the distance of the left foot hitting the ground 1,000 times. The ancient Romans, marching their armies through uncharted territory, would push a carved stick in the ground after each 1000 paces. Well-fed and harshly driven Roman legionaries in good weather thus created longer miles, the distance was indirectly standardised by Agrippas establishment of a standard Roman foot in 29 BC, and the definition of a pace as 5 feet. An Imperial Roman mile thus denoted 5,000 Roman feet, surveyors and specialized equipment such as the decempeda and dioptra then spread its use. In modern times, Agrippas Imperial Roman mile was empirically estimated to have been about 1,481 metres in length, in Hellenic areas of the Empire, the Roman mile was used beside the native Greek units as equivalent to 8 stadia of 600 Greek feet. The mílion continued to be used as a Byzantine unit and was used as the name of the zero mile marker for the Byzantine Empire. The Roman mile also spread throughout Europe, with its local variations giving rise to the different units below, also arising from the Roman mile is the milestone. All roads radiated out from the Roman Forum throughout the Empire –50,000 miles of stone-paved roads, at every mile was placed a shaped stone, on which was carved a Roman numeral, indicating the number of miles from the center of Rome – the Forum. Hence, one knew how far one was from Rome
15.
STEREO
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STEREO is a solar observation mission. Two nearly identical spacecraft were launched in 2006 into orbits around the Sun that cause them to pull farther ahead of. This enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections, the apogee reached the Moons orbit. On December 15,2006, on the orbit, the pair swung by the Moon for a gravity assist. The B spacecraft encountered the Moon again on the same orbital revolution on January 21,2007, Spacecraft B entered a heliocentric orbit outside the Earths orbit. Spacecraft A will take 347 days to complete one revolution of the Sun, the A spacecraft/Sun/Earth angle will increase at 21.650 degree/year. The B spacecraft/Sun/Earth angle will change −21.999 degrees per year.8 km/s and their current locations are shown here. Over time, the STEREO spacecraft will continue to separate from each other at a rate of approximately 44 degrees per year. There are no positions for the spacecraft. They achieved 90 degrees separation on January 24,2009, a known as quadrature. This is of interest because the mass ejections seen from the side on the limb by one spacecraft can potentially be observed by the in situ particle experiments of the other spacecraft. As they passed through Earths Lagrangian points L4 and L5, in late 2009, on February 6,2011, the two spacecraft were exactly 180 degrees apart from each other, allowing the entire Sun to be seen at once for the first time. Even as the angle increases, the addition of an Earth-based view, e. g. from the Solar Dynamics Observatory, in 2015, contact was lost for several months when the STEREO spacecraft passed behind the Sun. They will then start to approach Earth again, with closest approach sometime in 2023 and they will not be recaptured into Earth orbit. On October 1,2014, contact was lost with STEREO-B during a reset to test the crafts automation. The team originally thought the spacecraft had begun to spin, decreasing the amount of power that could be generated by the solar panels, NASA used its Deep Space Network, first weekly and later monthly, to try to re-establish communications. After a silence of 22 months, contact was regained at 22,27 UTC on August 21,2016, when the Deep Space Network established a lock on STEREO-B for 2.4 hours. Engineers will work to develop software to fix the spacecraft, further, while the spacecraft was power positive at the time of contact, its orientation will drift and power levels will fall
16.
Galactic tide
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A galactic tide is tidal force experienced by objects subject to the gravitational field of a galaxy such as the Milky Way. Tidal forces are dependent on the gradient of a field, rather than its strength. Two interacting galaxies will not always collide head-on, and the tidal forces will distort each galaxy along an axis pointing roughly towards, such tails are typically strongly curved. If a tail appears to be straight, it is probably being viewed edge-on, the stars and gas that comprise the tails will have been pulled from the easily distorted galactic discs of one or both bodies, rather than the gravitationally bound galactic centres. Two very prominent examples of collisions producing tidal tails are the Mice Galaxies, just as the Moon raises two water tides on opposite sides of the Earth, so a galactic tide produces two arms in its galactic companion. Secondly, if one of the two galaxies is in the foreground, then the second galaxy — and the bridge between them — may be partially obscured, together, these effects can make it hard to see where one galaxy ends and the next begins. Tidal loops, where a tail joins with its parent galaxy at both ends, are rarer still, because tidal effects are strongest in the immediate vicinity of a galaxy, satellite galaxies are particularly likely to be affected. The stripping mechanism is the same as between two galaxies, although its comparatively weak gravitational field ensures that only the satellite, not the host galaxy, is affected. If the satellite is very small compared to the host, the tidal tails produced are likely to be symmetric. It has been suggested that the discs of gas and stars around some galaxies, such as Andromeda. Tidal effects are present within a galaxy, where their gradients are likely to be steepest. This can have consequences for the formation of stars and planetary systems, typically a stars gravity will dominate within its own system, with only the passage of other stars substantially affecting dynamics. However, at the outer reaches of the system, the gravity is weak. In the Solar System, the hypothetical Oort cloud, believed to be the source of long-period comets, the Oort cloud is believed to be a vast shell surrounding the Solar System, possibly over a light-year in radius. Across such a vast distance, the gradient of the Milky Ways gravitational field plays a far more noticeable role, such a body, being composed of a rock and ice mixture, would become a comet when subjected to the increased solar radiation present in the inner Solar System. It has been suggested that the tide may also contribute to the formation of an Oort cloud. This shows that the effects of the tide are quite complex. Cumulatively the effect can be significant, however, up to 90% of all comets originating from an Oort cloud may be the result of the galactic tide
17.
Center of mass
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The distribution of mass is balanced around the center of mass and the average of the weighted position coordinates of the distributed mass defines its coordinates. Calculations in mechanics are simplified when formulated with respect to the center of mass. It is a point where entire mass of an object may be assumed to be concentrated to visualise its motion. In other words, the center of mass is the equivalent of a given object for application of Newtons laws of motion. In the case of a rigid body, the center of mass is fixed in relation to the body. The center of mass may be located outside the body, as is sometimes the case for hollow or open-shaped objects. In the case of a distribution of separate bodies, such as the planets of the Solar System, in orbital mechanics, the equations of motion of planets are formulated as point masses located at the centers of mass. The center of mass frame is a frame in which the center of mass of a system is at rest with respect to the origin of the coordinate system. The concept of center of mass in the form of the center of gravity was first introduced by the ancient Greek physicist, mathematician, and engineer Archimedes of Syracuse. He worked with simplified assumptions about gravity that amount to a uniform field, in work on floating bodies he demonstrated that the orientation of a floating object is the one that makes its center of mass as low as possible. He developed mathematical techniques for finding the centers of mass of objects of uniform density of various well-defined shapes, Newtons second law is reformulated with respect to the center of mass in Eulers first law. The center of mass is the point at the center of a distribution of mass in space that has the property that the weighted position vectors relative to this point sum to zero. In analogy to statistics, the center of mass is the location of a distribution of mass in space. Solving this equation for R yields the formula R =1 M ∑ i =1 n m i r i, solve this equation for the coordinates R to obtain R =1 M ∭ Q ρ r d V, where M is the total mass in the volume. If a continuous mass distribution has density, which means ρ is constant. The center of mass is not generally the point at which a plane separates the distribution of mass into two equal halves, in analogy with statistics, the median is not the same as the mean. The coordinates R of the center of mass of a system, P1 and P2, with masses m1. The percentages of mass at each point can be viewed as projective coordinates of the point R on this line, another way of interpreting the process here is the mechanical balancing of moments about an arbitrary point
18.
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
19.
Comet nucleus
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The nucleus is the solid, central part of a comet, popularly termed a dirty snowball or an icy dirtball. A cometary nucleus is composed of rock, dust, and frozen gases, when heated by the Sun, the gases sublimate and produce an atmosphere surrounding the nucleus known as the coma. The force exerted on the coma by the Suns radiation pressure and solar wind cause an enormous tail to form, a typical comet nucleus has an albedo of 0.04. This is blacker than coal, and may be caused by a covering of dust, comets, or their precursors, formed in the outer Solar System, possibly millions of years before planet formation. How and when formed is debated, with distinct implications for Solar System formation, dynamics. Three-dimensional computer simulations indicate the structural features observed on cometary nuclei can be explained by pairwise low velocity accretion of weak cometesimals. The currently favored creation mechanism is that of the nebular hypothesis, astronomers think that comets originate in both the Oort cloud and the scattered disk. Most cometary nuclei are thought to be no more than about 10 miles across, the largest comets that have come inside the orbit of Saturn are C/2002 VQ94, Hale–Bopp, 29P, 109P/Swift–Tuttle, and 28P/Neujmin. The potato-shaped nucleus of Halleys comet contains equal amounts of ice, during a flyby in September 2001, the Deep Space 1 spacecraft observed the nucleus of Comet Borrelly and found it to be about half the size of the nucleus of Halleys Comet. Borrellys nucleus was also potato-shaped and had a black surface. Like Halleys Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight, the nucleus of comet Hale–Bopp was estimated to be 60 ±20 km in diameter. Hale-Bopp appeared bright to the eye because its unusually large nucleus gave off a great deal of dust. The nucleus of P/2007 R5 is probably only 100–200 meters in diameter, the largest centaurs are estimated to be 250 km to 300 km in diameter. Three of the largest would include 10199 Chariklo,2060 Chiron, known comets have been estimated to have an average density of 0.6 g/cm3. Below is a list of comets that have had estimated sizes, densities, about 80% of the Halleys Comet nucleus is water ice, and frozen carbon monoxide makes up another 15%. Much of the remainder is carbon dioxide, methane. Scientists think that comets are chemically similar to Halleys Comet. The nucleus of Halleys Comet is also a dark black
20.
Coma (cometary)
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The coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes close to the Sun on its highly elliptical orbit, as the comet warms, parts of it sublimes. This gives a comet a fuzzy appearance when viewed in telescopes and distinguishes it from stars, the word coma comes from the Greek kome, which means hair and is the origin of the word comet itself. The coma is generally made of ice and comet dust, water dominates up to 90% of the volatiles that outflow from the nucleus when the comet is within 3-4 AU of the Sun. The parent molecule is destroyed primarily through photodissociation and to a smaller extent photoionization. The solar wind plays a role in the destruction of water compared to photochemistry. Larger dust particles are left along the orbital path while smaller particles are pushed away from the Sun into the comets tail by light pressure. Comas typically grow in size as comets approach the Sun, and they can be as large as the diameter of Jupiter, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 also had a coma roughly the diameter of the Sun, even though the coma can become quite large, its size can actually decrease about the time it crosses the orbit of Mars around 1.5 AU from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, Comets were found to emit X-rays in late-March 1996. This surprised researchers, because X-ray emission is associated with very high-temperature bodies. This ripping off leads to the emission of X-rays and far ultraviolet photons, with basic Earth-surface based telescope and some technique, the size of the Coma can be calculated. Called the drift method, one locks the telescope in position and that time multiplied by the cosine of comets declination, times.25 should equal the comas diameter in arcminutes. If the distance to the comet is known, then the apparent size of the coma can be determined. In 2015, it was noted that the ALICE instrument on the ESA Rosetta spacecraft to comet 67/P, detected hydrogen, oxygen, carbon and nitrogen in the Coma, which they also called the Comets atmosphere. Alice is a spectrograph, and it found that electrons created by UV light were colliding and breaking up molecules of water. OAO-2 discovered large halos of hydrogen gas around comets, space probe Giotto detected hydrogen ions at distance of 7.8 million km away from Halley when it did close flyby of the comet in 1986. A hydrogen gas halo was detected to be 15 times the diameter of Sun and this triggered NASA to point the Pioneer Venus mission at the Comet, and it was determined that the Comet emitting 12 tons of water per second. The hydrogen gas emission has not been detected from Earths surface because those wavelengths are blocked by the atmosphere, the process by which water is broken down into hydrogen and oxygen was studied by the ALICE instrument aboard the Rosetta spacecraft
21.
Comet tail
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A comet tail—and coma—are features visible in comets when they are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System. As a comet approaches the inner Solar System, solar radiation causes the materials within the comet to vaporize and stream out of the nucleus. Separate tails are formed of dust and gases, becoming visible through different phenomena, the dust reflects sunlight directly, most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye. In the outer Solar System, comets remain frozen and are difficult or impossible to detect from Earth due to their small size. As a comet approaches the inner Solar System, solar radiation causes the materials within the comet to vaporize and stream out of the nucleus. The streams of dust and gas each form their own distinct tail, the tail of dust is left behind in the comets orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, the ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions. While the solid nucleus of comets is generally less than 50 km across, the coma may be larger than the Sun, the Ulysses spacecraft made an unexpected pass through the tail of the comet C/2006 P1, on February 3,2007. Evidence of the encounter was published in the October 1,2007 issue of the Astrophysical Journal, the observation of antitails contributed significantly to the discovery of solar wind. The ion tail is the result of ultraviolet radiation ejecting electrons off particles in the coma, once the particles have been ionised, they form a plasma which in turn induces a magnetosphere around the comet. The comet and its magnetic field form an obstacle to outward flowing solar wind particles. The comet is supersonic relative to the wind, so a bow shock is formed upstream of the comet. In this bow shock, large concentrations of cometary ions congregate, the field lines drape around the comet forming the ion tail. If the ion tail loading is sufficient, then the field lines are squeezed together to the point where, at some distance along the ion tail. This leads to a disconnection event. This has been observed on a number of occasions, notable among which was on the 20th, april 2007 when the ion tail of comet Encke was completely severed as the comet passed through a coronal mass ejection. This event was observed by the STEREO spacecraft, a disconnection event was also seen with C/2009 R1 on May 26,2010
22.
Antitail
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An antitail is a spike projecting from a comets coma which seems to go towards the Sun, and thus geometrically opposite to the other tails, the ion tail and the dust tail. However, this phenomenon is an illusion that is seen from the Earth. As Earth passes through the orbital plane, this disc is seen side on. The other side of the disc can sometimes be seen, though it tends to be lost in the dust tail, the antitail is therefore normally visible for a brief interval only when Earth passes through the comets orbital plane. Comet tail The coma and tail at the main Comet article, image of Comet Arend-Roland with prominent antitail Emily Lakdawalla. Spot a comet near Saturn tonight, online Encyclopedia of Science - Antitail
23.
Comet dust
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Comet dust refers to cosmic dust that originates from a comet. Comet dust can provide clues to comets origin, when the Earth passes through a comet dust trail, it can produce a meteor shower. Bulk properties of the comet dust such as density as well as the composition can distinguish between the models. For example, the ratios of comet and of interstellar dust are very similar. The 1) interstellar model says that ices formed on dust grains in the cloud that preceded the Sun. The mix of ice and dust then aggregated into a comet without appreciable chemical modification, J. Mayo Greenberg first proposed this idea in 1986. In the 2) Solar System model, the ices that formed in the interstellar cloud first vaporized as part of the disk of gas. The vaporized ices later resolidified and assembled into comets, so the comets in this model would have a different composition than those comets that were made directly from interstellar ice. The 3) primordial rubble pile model for comet formation says that comets agglomerate in the region where Jupiter was forming, the composition of the dust of comet Wild 2 is similar to the composition of dust found in the outer regions of the accretion disks around newly-forming stars. A comet and its dust allow investigation of the Solar System beyond the main planetary orbits, comets are distinguished by their orbits, long period comets have long elliptical orbits, randomly inclined to the plane of the Solar System, and with periods greater than 200 years. A comet will experience a range of conditions as it traverses its orbit. For long period comets, most of the time it will be so far from the Sun that it will be too cold for evaporation of ices to occur, near the Sun, the heating and evaporation rate will be so great, that no dust can be retained. Therefore, the thickness of dust layers covering the nuclei of a comet can indicate how closely, if a comet has an accumulation of thick dust layers, it may have frequent perihelion passages that dont approach the Sun too closely. The accumulation of dust layers over time would change the character of the short-period comet. A dust layer both inhibits the heating of the cometary ices by the Sun, and slows the loss of gases from the nucleus below
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Meteor shower
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A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of debris called meteoroids entering Earths atmosphere at extremely high speeds on parallel trajectories. Most meteors are smaller than a grain of sand, so almost all of them disintegrate, intense or unusual meteor showers are known as meteor outbursts and meteor storms, which may produce greater than 1000 meteors an hour. The Meteor Data Centre lists about 600 suspected meteor showers of which about 100 are well established, the first great storm in modern times was the Leonids of November 1833. American Denison Olmsted explained the event most accurately, after spending the last weeks of 1833 collecting information he presented his findings in January 1834 to the American Journal of Science and Arts, published in January–April 1834, and January 1836. Work continued, however, coming to understand the nature of showers though the occurrences of storms perplexed researchers. In the 1890s, Irish astronomer George Johnstone Stoney and British astronomer Arthur Matthew Weld Downing, were the first to attempt to calculate the position of the dust at Earths orbit. They studied the dust ejected in 1866 by comet 55P/Tempel-Tuttle in advance of the anticipated Leonid shower return of 1898 and 1899, Meteor storms were anticipated, but the final calculations showed that most of the dust would be far inside of Earths orbit. The same results were independently arrived at by Adolf Berberich of the Königliches Astronomisches Rechen Institut in Berlin, although the absence of meteor storms that season confirmed the calculations, the advance of much better computing tools was needed to arrive at reliable predictions. In 1981 Donald K. Yeomans of the Jet Propulsion Laboratory reviewed the history of showers for the Leonids. A graph from it was adapted and re-published in Sky and Telescope and it showed relative positions of the Earth and Tempel-Tuttle and marks where Earth encountered dense dust. In 1985, E. D. Kondrateva and E. A. Reznikov of Kazan State University first correctly identified the years when dust was released which was responsible for several past Leonid meteor storms, in 1995, Peter Jenniskens predicted the 1995 Alpha Monocerotids outburst from dust trails. In anticipation of the 1999 Leonid storm, Robert H. McNaught, David Asher, in 2006 Jenniskens has published predictions for future dust trail encounters covering the next 50 years. Jérémie Vaubaillon continues to update predictions based on each year for the Institut de Mécanique Céleste et de Calcul des Éphémérides. Because meteor shower particles are all traveling in parallel paths, and at the same velocity and this radiant point is caused by the effect of perspective, similar to parallel railroad tracks converging at a single vanishing point on the horizon when viewed from the middle of the tracks. Meteor showers are almost always named after the constellation from which the appear to originate. This fixed point slowly moves across the sky during the due to the Earth turning on its axis. The radiant also moves slightly from night to night against the stars due to the Earth moving in its orbit around the sun
25.
Lost comet
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The D/ designation is for a periodic comet that no longer exists or is deemed to have disappeared. Some astronomers have specialized in this area, such as Brian G. Marsden, there are a number of reasons why a comet might be missed by astronomers during subsequent apparitions. Firstly, cometary orbits may be perturbed by interaction with the giant planets and this, along with nongravitational forces, can result in changes to the date of perihelion. As some comets periodically undergo outbursts or flares in brightness, it may be possible for a faint comet to be discovered during an outburst. Comets can also run out of volatiles and this may have occurred in the case of 5D/Brorsen, which was considered by Marsden to have probably faded out of existence in the late 19th century. Comets are in some known to have disintegrated during their perihelion passage. The best-known example is Bielas Comet, which was observed to split into two components before disappearing after its 1852 apparition, in modern times 73P/Schwassmann–Wachmann has been observed in the process of breaking up. In the case of lost comets this is especially tricky, for example, the comet 177P/Barnard, discovered by Edward Emerson Barnard on June 24,1889, was rediscovered after 116 years in 2006. On July 19,2006, 177P came within 0.36 AU of the Earth, comets can be gone but not considered lost, even though they may not be expected back for hundreds or even thousands of years. With more powerful telescopes it has become possible to observe comets for longer periods of time after perihelion, for example, Comet Hale–Bopp was observable with the naked eye about 18 months after its approach in 1997. It is expected to remain observable with large telescopes until perhaps 2020, comets that have been lost or which have disappeared have names beginning with a D according to current IAU conventions. Comets are typically observed on a periodic return, when they do not they are sometimes found again, while other times they may break up into fragments. These fragments can sometimes be observed, but the comet is no longer expected to return. Other times a comet will not be considered lost until it does not appear at a predicted time, comets may also collide with another object, such as Comet Shoemaker–Levy 9, which collided with Jupiter in 1994
26.
Main-belt comet
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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
27.
Great comet
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A great comet is a comet that becomes exceptionally bright. Great comets are rare, on average, only one will appear in a decade, although comets are officially named after their discoverers, great comets are sometimes also referred to by the year in which they appeared great, using the formulation The Great Comet of. The vast majority of comets are never enough to be seen by the naked eye. However, occasionally a comet may brighten to naked eye visibility, the requirements for this to occur are, a large and active nucleus, a close approach to the Sun, and a close approach to the Earth. A comet fulfilling all three of these criteria will certainly be spectacular, sometimes, a comet failing on one criterion will still be extremely impressive. For example, Comet Hale–Bopp had a large and active nucleus. Equally, Comet Hyakutake was a small comet, but appeared bright because it passed extremely close to the Earth. Cometary nuclei vary in size from a few hundreds of metres across or less to many kilometres across, when they approach the Sun, large amounts of gas and dust are ejected by cometary nuclei, due to solar heating. A crucial factor in how bright a comet becomes is how large, the sudden brightening of comet 17P/Holmes in 2007 showed the importance of the activity of the nucleus in the comets brightness. On October 23–24,2007, the comet suffered a sudden outburst which caused it to brighten by factor of about half a million. It unexpectedly brightened from an apparent magnitude of about 17 to about 2.8 in a period of only 42 hours, all these temporarily made comet 17P the largest object in the Solar System although its nucleus is estimated to be only about 3.4 km in diameter. The brightness of a simple reflective body varies with the square of its distance from the Sun. That is, if a distance from the Sun is halved. However, comets behave differently, due to their ejection of large amounts of gas which then also reflect sunlight. Their brightness varies roughly as the cube of their distance from the Sun, meaning that if a comets distance from the Sun is halved. This means that the brightness of a comet depends significantly on its distance from the Sun. For most comets, the perihelion of their orbit lies outside the Earths orbit, any comet approaching the Sun to within 0.5 AU or less may have a chance of becoming a great comet. For a comet to become spectacular, it needs to pass close to the Earth
28.
Sungrazing comet
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A sungrazing comet is a comet that passes extremely close to the Sun at perihelion – sometimes within a few thousand kilometres of the Suns surface. Although small sungrazers can completely evaporate during such an approach to the Sun. However, the evaporation and tidal forces they experience often lead to their fragmentation. Up until the 1880s, it was thought that all bright comets near the sun were the return of a single sungrazing comet. Very little was known about the population of sungrazing comets until 1979 when coronagraphic observations allowed the detection of sungrazers, as of December 12,2013, there are 1488 known comets that come within ~12 solar radii. This accounts for one third of all comets. Most of these objects vaporize during their approach, but a comet with a nucleus radius larger than 2–3 km is likely to survive the perihelion passage with a final radius of ~1 km. Sungrazer comets were some of the earliest observed comets because they can appear very bright, some are even considered Great Comets. This extreme brightening will allow for naked eye observations from Earth depending on how volatile the gases are. One of the first comets to have its orbit computed was the comet of 1680. It was observed by Isaac Newton and he published the results in 1687. However, this marked the first time that it was hypothesized that Great Comets were related or perhaps the same comet, later Johann Franz Encke computed the orbit of C/1680 V1 and found a period near 9000 years and concluded that Cassinis theory of short period sungrazers was flawed. C/1680 V1 had the smallest measured perihelion distance until 1826 with comet C/1826 U1, advances were made in understanding sungrazing comets in the 19th century with the Great Comets of 1843, C/1880 C1, and 1882. He also hypothesized that the parent body was a comet seen by Aristotle, Comet C/1882 R1 appeared only two years after the previously observed sungrazer so this convinced astronomers that these bright comets were not all the same object. Some astronomers theorized that the comet might pass through a resisting medium near the sun, when astronomers observed C/1882 R1, they measured the period before and after perihelion and saw no shortening in the period which disproved the theory. After perihelion this object was seen to split into several fragments. In an attempt to link the 1843 and 1880 comets to the comet in 1106 and 371 BC, Kreutz measured the fragments of the 1882 comet and he then designated that all sungrazing comets with similar orbital characteristics as these few comets would be part of the Kreutz Group. The 19th century also provided the first spectrum taken of a comet near the sun which was taken by Finlay & Elkin in 1882, later the spectrum was analyzed and Fe and Ni spectral lines were confirmed
29.
Kreutz sungrazer
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The Kreutz sungrazers are a family of sungrazing comets, characterized by orbits taking them extremely close to the Sun at perihelion. They are believed to be fragments of one large comet that broke up several centuries ago and are named for German astronomer Heinrich Kreutz, several members of the Kreutz family have become great comets, occasionally visible near the Sun in the daytime sky. The most recent of these was Comet Ikeya–Seki in 1965, which may have one of the brightest comets in the last millennium. It has been suggested that another cluster of bright Kreutz system comets may begin to arrive in the inner Solar System in the few years to decades. Many hundreds of members of the family, some only a few meters across, have been discovered since the launch of the SOHO satellite in 1995. None of these smaller comets have survived its perihelion passage, larger sungrazers such as the Great Comet of 1843 and C/2011 W3 have survived their perihelion passage. Amateur astronomers have been successful at discovering Kreutz comets in the data available in time via the Internet. The first comet whose orbit had been found to take it close to the Sun was the Great Comet of 1680. This comet was found to have passed just 200,000 km above the Suns surface and it thus became the first known sungrazing comet. Its perihelion distance was just 1.3 solar radii, astronomers at the time, including Edmond Halley, speculated that this comet was a return of a bright comet seen close to the Sun in the sky in 1106. 163 years later, the Great Comet of 1843 appeared and also passed close to the Sun. Despite orbital calculations showing that it had a period of several centuries, a bright comet seen in 1880 was found to be travelling on an almost identical orbit to that of 1843, as was the subsequent Great Comet of 1882. An alternative suggestion was that the comets were all fragments of an earlier Sun-grazing comet and this idea was first proposed in 1880, and its plausibility was amply demonstrated when the Great Comet of 1882 broke up into several fragments after its perihelion passage. In 1888, Heinrich Kreutz published a paper showing that the comets of 1843,1880, the comet of 1680 proved to be unrelated to this family of comets. After another Kreutz sungrazer was seen in 1887, the one did not appear until 1945. Two further sungrazers appeared in the 1960s, Comet Pereyra in 1963 and Comet Ikeya–Seki, which became bright in 1965. The appearance of two Kreutz Sungrazers in quick succession inspired further study of the dynamics of the group, the group generally has an Inclination of roughly 140 degrees, a perihelion distance of around 0.01 AU, and a Longitude of ascending node of 340–10°. The brightest members of the Kreutz sungrazers have been spectacular, easily visible in the daytime sky, the three most impressive have been the Great Comet of 1843, the Great Comet of 1882 and Comet Ikeya–Seki
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Extinct comet
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Extinct comets are comets that have expelled most of their volatile ice and have little left to form a tail or coma. The volatile material contained in the comet nucleus evaporates away, comets may go through a transition phase as they come close to extinction. A comet may be dormant rather than extinct, if its volatile component is sealed beneath a surface layer. Extinct comets are those that have expelled most of their volatile ice and have left to form a tail or coma. Over time, most of the material contained in a comet nucleus evaporates away. Other related types of comet include transition comets, that are close to becoming extinct, comets such as C/2001 OG108 may represent the transition between extinct comets and typical Halley-type comets or long period comets. Minor planets of the group of damocloids have been studied as possible extinct cometary candidates due to the similarity of their orbital parameters with those of Halley-type comets, dormant comets are those within which volatiles may be sealed, but which have inactive surfaces. For example,14827 Hypnos may be the nucleus of a comet that is covered by a crust several centimeters thick that prevents any remaining volatiles from outgassing. The term dormant comet is also used to describe comets that may become active but are not actively outgassing, for example,60558 Echeclus has displayed a cometary coma and now also has the cometary designation 174P/Echeclus. After passing perihelion in early 2008, centaur 52872 Okyrhoe significantly brightened, when discovered, asteroids were seen as a class of objects distinct from comets, and there was no unified term for the two until small Solar System body was coined by the IAU in 2006. The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near-surface ices by solar radiation, a few objects have ended up being dual-listed because they were first classified as minor planets but later showed evidence of cometary activity. Conversely, some comets are eventually depleted of their volatile ices. A further distinction is that typically have more eccentric orbits than most asteroids. Also, they are theorized to be common objects amongst the bodies orbiting close to the Sun. Roughly six percent of the asteroids are thought to be extinct nuclei of comets which no longer experience outgassing
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Rock comet
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A rock comet is a rare type of small Solar System body that exhibits features of both a comet and an asteroid, mainly in that it outgasses material primarily made up of grains of rock. Rock comets, unlike comets, which outgas primarily ice, have a nucleus made of rock. As a result, they can outgas fairly unpredictably, not just when within the solar frost line, however, if the asteroid is close enough to the Sun for the former, the latter will also happen. Until 2016, the known example of a rock comet iwas 3200 Phaethon. It has been known to brighten on occasions, implying outgassing, and has been observed ejecting dust by the STEREO spacecraft
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Exocomet
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An exocomet, or extrasolar comet, is a comet outside the Solar System, which includes interstellar comets and those that orbit stars other than the Sun. The first exocomets were detected in 1987 around Beta Pictoris, a very young A-type main-sequence star, there are now a total of 11 stars around which exocomets have been observed or suspected. All discovered exocometary systems are very young A-type stars. The exocomets can be detected by spectroscopy as they transit their host stars, the transits of exocomets, like the transits of exoplanets, produce variations in the light received from the star. As the comet comes close enough to the star, cometary gas is evolved from the evaporation of volatile ices, observations of comets, and especially exocomets, improve our understanding of planet formation. Thus, comets are the residuals of the volatile-rich planetesimals that remained in the system without having been incorporated into the planets. They are considered fossil bodies that have seen the physical and chemical conditions prevailing at the time of planet formation, a gaseous cloud around 49 Ceti has been attributed to the collisions of comets in that planetary system
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Interstellar object
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An interstellar comet is a comet located in interstellar space, and not gravitationally bound to a star. Besides known comets within the Solar System, or known extrasolar comets, no interstellar comet, at present, an interstellar comet can only be detected if it passes through the Solar System, and could be distinguished from an Oort cloud comet by its strongly hyperbolic trajectory. The most eccentric known comet, C/1980 E1, only has an eccentricity of 1.057, current models of Oort cloud formation indicate that more comets are ejected into interstellar space than are retained in the Oort cloud, by a factor of 3–100. Other simulations suggest 90–99% of comets are ejected, there is no reason to believe comets formed in other star systems would not be similarly scattered. If interstellar comets exist, they must occasionally pass through the inner Solar System and they would approach the Solar System with random velocities, mostly from the region of the constellation Hercules because the Solar System is moving in that direction. The fact that no comet with a greater than the Suns escape velocity has yet been seen places upper limits to their density in interstellar space. A paper by Torbett indicates that the density is no more than 1013 comets per cubic parsec, other analyses, of data from LINEAR, set the upper limit at 4. 5×10−4/AU3, or 1012 comets per cubic parsec. An interstellar comet could, on occasions, be captured into a heliocentric orbit while passing through the Solar System. Computer simulations show that Jupiter is the only planet massive enough to one. Comets Machholz 1 and Hyakutake C/1996 B2 are possible examples of such comets and they have atypical chemical makeups for comets in the Solar System. List of Solar System objects by greatest aphelion Exocomet Rogue planet An Observational Upper Limit on the Interstellar Number Density of Asteroids and Comets
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Naming of comets
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Comets have been observed for the last 2,000 years. During that time, several different systems have used to assign names to each comet. The simplest system names comets after the year in which they were observed, later a convention arose of using the names of people associated with the discovery or the first detailed study of each comet. The original scheme assigned codes in the order that comets passed perihelion and this scheme operated until 1994, when continued increases in the numbers of comets found each year resulted in the creation of a new scheme. This system, which is still in operation, assigns a code based on the type of orbit, before any systematic naming convention was adopted, comets were named in a variety of ways. Prior to the early 20th century, most comets were simply referred to by the year when they appeared e. g. the Comet of 1702. Particularly bright comets which came to public attention would be described as the comet of that year, such as the Great Comet of 1680. If more than one great comet appeared in a single year, occasionally other additional adjectives might be used. Later eponymous comets were named after the astronomer who conducted detailed investigations on them, after Edmond Halley demonstrated that the comets of 1531,1607, and 1682 were the same body and successfully predicted its return in 1759, that comet became known as Halleys Comet. Similarly, the second and third known periodic comets, Enckes Comet, later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their apparition. The first comet to be named after the person who discovered it, however, this convention did not become widespread until the early 20th century. A comet can be named after up to three discoverers, either working together as a team or making independent discoveries. For example, Comet Swift–Tuttle was first found by Lewis Swift and then by Horace Parnell Tuttle a few days later, in recent years many comets have been discovered by large teams of astronomers, in this case comets may be named for the collaboration or instrument they used. For example, 160P/LINEAR was discovered by the Lincoln Near-Earth Asteroid Research team, Comet IRAS–Araki–Alcock was discovered independently by a team using the Infrared Astronomy Satellite and the amateur astronomers Genichi Araki and George Alcock. Today, the numbers of comets discovered by some instruments makes this system impractical. Instead, the comets systematic designations are used to avoid confusion, until 1994, comets were first given a provisional designation consisting of the year of their discovery followed by a lowercase letter indicating its order of discovery in that year. As a result, in 1994 the International Astronomical Union approved a new naming system, prefixes are also added to indicate the nature of the comet, P/ indicates a periodic comet. X/ indicates a comet for which no reliable orbit could be calculated, d/ indicates a periodic comet that has disappeared, broken up, or been lost
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Centaur (minor planet)
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Centaurs are minor planets with a semi-major axis between those of the outer planets. They have unstable orbits that cross or have crossed the orbits of one or more of the giant planets, Centaurs typically behave with characteristics of both asteroids and comets. They are named after the centaurs that were a mixture of horse. It has been estimated there are around 44,000 centaurs in the Solar System with diameters larger than 1 km. The first centaur to be discovered, under the definition of the Jet Propulsion Laboratory, however, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest confirmed centaur is 10199 Chariklo, which at 260 km in diameter is as big as a mid-sized main-belt asteroid, however, the lost centaur 1995 SN55 may be somewhat larger. No centaur has been photographed up close, although there is evidence that Saturns moon Phoebe, imaged by the Cassini probe in 2004, in addition, the Hubble Space Telescope has gleaned some information about the surface features of 8405 Asbolus. As of 2008, three centaurs have been found to display comet-like comas, Chiron,60558 Echeclus, and 166P/NEAT, Chiron and Echeclus are therefore classified as both asteroids and comets. Other centaurs, such as 52872 Okyrhoe and 2012 CG, are suspected of having shown comas, any centaur that is perturbed close enough to the Sun is expected to become a comet. The generic definition of a centaur is a body that orbits the Sun between Jupiter and Neptune and crosses the orbits of one or more of the giant planets. Though nowadays the MPC often lists centaurs and scattered disc objects together as a single group, the Jet Propulsion Laboratory similarly defines centaurs as having a semi-major axis, a, between those of Jupiter and Neptune. In contrast, the Deep Ecliptic Survey defines centaurs using a classification scheme. These classifications are based on the change in behavior of the present orbit when extended over 10 million years. The DES defines centaurs as non-resonant objects whose instantaneous perihelia are less than the osculating semi-major axis of Neptune at any time during the simulation and this definition is intended to be synonymous with planet-crossing orbits and to suggest comparatively short lifetimes in the current orbit. The collection The Solar System Beyond Neptune defines objects with an axis between those of Jupiter and Neptune and a Jupiter – Tisserands parameter above 3. The JPL Small-Body Database lists 324 centaurs, there are an additional 65 trans-Neptunian objects with a perihelion closer than the orbit of Uranus. The Committee on Small Body Nomenclature of the International Astronomical Union has not formally weighed in on either side of the debate, thus far, only the binary objects Ceto and Phorcys and Typhon and Echidna have been named according to the new policy. Other objects caught between these differences in classification methods include 944 Hidalgo which was discovered in 1920 and is listed as a centaur in the JPL Small-Body Database
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Extraterrestrial atmosphere
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The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain insight into Earths atmosphere. In addition to Earth, many of the other objects in the Solar System have atmospheres. These include all the gas giants, as well as Mars, Venus, several moons and other bodies also have atmospheres, as do comets and the Sun. There is evidence that extrasolar planets can have an atmosphere, due to its small size, Mercury has no substantial atmosphere. Its extremely thin atmosphere mostly consists of an amount of helium and traces of sodium, potassium. These gases derive from the wind, radioactive decay, meteor impacts. Mercurys atmosphere is not stable and is constantly being refreshed because of its atoms escaping into space as a result of the planets heat, Venus atmosphere is mostly composed of carbon dioxide. It contains minor amounts of nitrogen and other elements, including compounds based on hydrogen, nitrogen, sulfur, carbon. The atmosphere of Venus is much hotter and denser than that of Earth, as greenhouse gases warm a lower atmosphere, they cool the upper atmosphere, leading to compact thermospheres. By some definitions, Venus has no stratosphere, the troposphere begins at the surface and extends up to an altitude of 65 kilometres. At the top of the troposphere, temperature and pressure reach Earth-like levels, winds at the surface are a few metres per second, reaching 70 m/s or more in the upper troposphere. The stratosphere and mesosphere extend from 65 km to 95 km in height, the thermosphere and exosphere begin at around 95 kilometres, eventually reaching the limit of the atmosphere at about 220 to 250 km. The air pressure at Venus surface is about 92 times that of the Earth, the enormous amount of CO2 in the atmosphere creates a strong greenhouse effect, raising the surface temperature to around 470 °C, hotter than that of any other planet in the Solar System. The Martian atmosphere is thin and composed mainly of carbon dioxide, with some nitrogen. The average surface pressure on Mars is 0. 6-0.9 kPa and this results in a much lower atmospheric thermal inertia, and as a consequence Mars is subject to strong thermal tides that can change total atmospheric pressure by up to 10%. The thin atmosphere also increases the variability of the planets temperature, Martian surface temperatures vary from lows of approximately −140 °C during the polar winters to highs of up to 20 °C in summers. The Mars Reconnaissance Orbiter, though spanning a much shorter dataset, shows no warming of planetary average temperature, MCS MY28 temperatures are an average of 0.9 and 1.7 K cooler than TES MY24 measurements. Locally and regionally, however, changes in pits in the layer of carbon dioxide at the Martian south pole observed between 1999 and 2001 suggest the south polar ice cap is shrinking
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Small Solar System body
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A Small Solar System Body is an object in the Solar System that is neither a planet, nor a dwarf planet, nor a natural satellite. The term was first defined in 2006 by the International Astronomical Union, all other objects, except satellites, orbiting the Sun shall be referred to collectively as Small Solar System Bodies. These currently include most of the Solar System asteroids, most Trans-Neptunian Objects, comets and this encompasses all comets and all minor planets other than those that are dwarf planets. Except for the largest, which are in equilibrium, natural satellites differ from small Solar System bodies not in size. The orbits of satellites are not centered on the Sun, but around other Solar System objects such as planets, dwarf planets. Some of the larger small Solar System bodies may be reclassified in future as dwarf planets, the orbits of the vast majority of small Solar System bodies are located in two distinct areas, namely the asteroid belt and the Kuiper belt. These two belts possess some internal structure related to perturbations by the planets, and have fairly loosely defined boundaries. Other areas of the Solar System also encompass small bodies in smaller concentrations and these include the near-Earth asteroids, centaurs, comets, and scattered disc objects
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Asteroid
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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
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Comet Hopper
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Comet Hopper was a proposed lander to NASAs Discovery Program that, had it been selected, would have orbited and landed multiple times on Comet Wirtanen as it approaches the Sun. The proposed mission is led by Jessica Sunshine of the UMD, working with Lockheed Martin to build the spacecraft, the Comet Hopper mission was one of three Discovery Program finalists that received USD$3 million in May 2011 to develop a detailed concept study. The other two missions were InSight and Titan Mare Explorer, after a review in August 2012, NASA selected the InSight mission. The CHopper mission has three primary science goals over the 7.3 years of its nominal lifetime, the remote mapping will also allow for any nucleus structure, geologic processes, and coma mechanisms to be determined. After arriving at Comet Wirtanen, the spacecraft will approach and land, as the comet approaches the sun, the spacecraft will land and hop multiple times. The final landing will occur at 1.5 AU, as the comet approaches the sun and becomes more active, the spacecraft will be able to record surface changes
