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1. Apsis – 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. Orbital inclination – 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

4. International Standard Book Number – The International Standard Book Number is a unique numeric commercial book identifier. An ISBN is assigned to each edition and variation of a book, for example, an e-book, a paperback and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, the method of assigning an ISBN is nation-based and varies from country to country, often depending on how large the publishing industry is within a country. The initial ISBN configuration of recognition was generated in 1967 based upon the 9-digit Standard Book Numbering created in 1966, the 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108. Occasionally, a book may appear without a printed ISBN if it is printed privately or the author does not follow the usual ISBN procedure, however, this can be rectified later. Another identifier, the International Standard Serial Number, identifies periodical publications such as magazines, the ISBN configuration of recognition was generated in 1967 in the United Kingdom by David Whitaker and in 1968 in the US by Emery Koltay. The 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108, the United Kingdom continued to use the 9-digit SBN code until 1974. The ISO on-line facility only refers back to 1978, an SBN may be converted to an ISBN by prefixing the digit 0. For example, the edition of Mr. J. G. Reeder Returns, published by Hodder in 1965, has SBN340013818 -340 indicating the publisher,01381 their serial number. This can be converted to ISBN 0-340-01381-8, the check digit does not need to be re-calculated, since 1 January 2007, ISBNs have contained 13 digits, a format that is compatible with Bookland European Article Number EAN-13s. An ISBN is assigned to each edition and variation of a book, for example, an ebook, a paperback, and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, a 13-digit ISBN can be separated into its parts, and when this is done it is customary to separate the parts with hyphens or spaces. Separating the parts of a 10-digit ISBN is also done with either hyphens or spaces, figuring out how to correctly separate a given ISBN number is complicated, because most of the parts do not use a fixed number of digits. ISBN issuance is country-specific, in that ISBNs are issued by the ISBN registration agency that is responsible for country or territory regardless of the publication language. Some ISBN registration agencies are based in national libraries or within ministries of culture, in other cases, the ISBN registration service is provided by organisations such as bibliographic data providers that are not government funded. In Canada, ISBNs are issued at no cost with the purpose of encouraging Canadian culture. In the United Kingdom, United States, and some countries, where the service is provided by non-government-funded organisations. Australia, ISBNs are issued by the library services agency Thorpe-Bowker

5. Comet – 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

6. Comet nucleus – 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

7. Coma (cometary) – 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

8. Comet tail – 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

9. Antitail – 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

10. Comet dust – 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

11. Meteor shower – 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

12. Lost comet – 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

13. Main-belt comet – Main-belt comets are bodies orbiting within the asteroid belt that have shown comet-like activity during part of their orbit. The Jet Propulsion Laboratory defines a main-belt asteroid as an asteroid with an axis of more than 2 AU but less than 3.2 AU. The first main-belt comet discovered is 7968 Elst–Pizarro and it was discovered in 1979 and was found to have a tail by Eric Elst and Guido Pizarro in 1996 and given the cometary designation 133P/Elst-Pizarro. Although quite a few comets have semimajor axes well within Jupiters orbit, main-belt comets differ in having small eccentricities. The first three identified main-belt comets all orbit within the part of the asteroid belt. It is not known how an outer Solar System body like the other comets could have made its way into a low-eccentricity orbit typical of the asteroid belt, some main-belt comets display a cometary dust tail only for a part of their orbit near perihelion. Activity in 133P/Elst–Pizarro is recurrent, having been observed at each of the last three perihelia, the activity persists for a month or several out of each 5-6 year orbit, and is presumably due to ice being uncovered by minor impacts in the last 100 to 1000 years. These impacts are suspected to excavate these subsurface pockets of volatile material helping to expose them to solar radiation, observations of Scheila indicated that large amounts of dust were kicked up by the impact of another asteroid of approximately 35 meters in diameter. In October 2013, observations of P/2013 R3, taken with the 10.4 m Gran Telescopio Canarias on the island of La Palma showed that this comet was breaking apart. The brightest A fragment was also detected at the position in CCD images obtained at the 1.52 m telescope of the Sierra Nevada Observatory in Granada on October 12. NASA reported on a series of images taken by the Hubble Space Telescope between October 29,2013 and January 14,2014 that show the separation of the four main bodies. The Yarkovsky–OKeefe–Radzievskii–Paddack effect, caused by sunlight, increased the rate until the centrifugal force caused the rubble pile to separate. The term main-belt comet is a based on orbit and the presence of an extended morphology. It does not imply that these objects are comets or that the material surrounding their nuclei was ejected by the sublimation of volatiles, identified members of this morphology class include, Centaur Extinct comet Henry Hsiehs Main-Belt Comets page has extensive details on Main-belt comets David Jewitt. J. Licandro New images obtained with the GTC

14. Great comet – 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

15. Sungrazing comet – 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

16. Kreutz sungrazer – 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

17. Extinct comet – 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

18. Rock comet – 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

19. Exocomet – 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

20. Interstellar object – 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

21. Naming of comets – 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

22. Centaur (minor planet) – 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

23. Extraterrestrial atmosphere – 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

24. Oort cloud – 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

25. Small Solar System body – 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

26. Asteroid – Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called planetoids and these terms have historically been applied to any astronomical object orbiting the Sun that did not show the disc of a planet and was not observed to have the characteristics of an active comet. As minor planets in the outer Solar System were discovered and found to have volatile-based surfaces that resemble those of comets, in this article, the term asteroid refers to the minor planets of the inner Solar System including those co-orbital with Jupiter. There are millions of asteroids, many thought to be the remnants of planetesimals. The large majority of known asteroids orbit in the belt between the orbits of Mars and Jupiter, or are co-orbital with Jupiter. However, other orbital families exist with significant populations, including the near-Earth objects, individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups, C-type, M-type, and S-type. These were named after and are identified with carbon-rich, metallic. The size of asteroids varies greatly, some reaching as much as 1000 km across, asteroids are differentiated from comets and meteoroids. In the case of comets, the difference is one of composition, while asteroids are composed of mineral and rock, comets are composed of dust. In addition, asteroids formed closer to the sun, preventing the development of the aforementioned cometary ice, the difference between asteroids and meteoroids is mainly one of size, meteoroids have a diameter of less than one meter, whereas asteroids have a diameter of greater than one meter. Finally, meteoroids can be composed of either cometary or asteroidal materials, only one asteroid,4 Vesta, which has a relatively reflective surface, is normally visible to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the eye for a short time. As of March 2016, the Minor Planet Center had data on more than 1.3 million objects in the inner and outer Solar System, the United Nations declared June 30 as International Asteroid Day to educate the public about asteroids. The date of International Asteroid Day commemorates the anniversary of the Tunguska asteroid impact over Siberia, the first asteroid to be discovered, Ceres, was found in 1801 by Giuseppe Piazzi, and was originally considered to be a new planet. In the early half of the nineteenth century, the terms asteroid. Asteroid discovery methods have improved over the past two centuries. This task required that hand-drawn sky charts be prepared for all stars in the band down to an agreed-upon limit of faintness. On subsequent nights, the sky would be charted again and any moving object would, hopefully, the expected motion of the missing planet was about 30 seconds of arc per hour, readily discernible by observers

27. Comet Hopper – 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

28. Comet Rendezvous Asteroid Flyby – Most of CRAFs scientific objectives were later accomplished by the smaller NASA spacecraft Stardust and Deep Impact, and the rest will be accomplished by ESAs flagship Rosetta mission. Designed to be the first of the planned Mariner Mark II series of spacecraft and it was to launch a heavily instrumented penetrator/lander into the comets nucleus to measure temperatures and chemical composition. CRAFs other instruments would collect data on the nucleus, its coma. CRAF was also to provide the first close-up look at how a comets coma and its tail of dust and ions form. CRAF was to be launched aboard a Titan IV-Centaur rocket in August 1995, the trajectory would carry CRAF out to the asteroid belt, where a propulsion maneuver would send the spacecraft back toward Earth for a gravity assist boost. CRAF would fly past Earth in July 1997 to take up its flight path. By using gravity assist, NASA can launch spacecraft aboard rockets that have less thrust than would be needed for a direct flight, the spacecraft was to encounter an asteroid named 449 Hamburga in January 1998 en route to the comet. CRAF would take photographs and deploy other scientific measurements during the encounter period, Asteroid 449 Hamburga is about 88 kilometers in diameter and is a carbonaceous type asteroid. CRAFs planned cometary target was Comet Kopff and it is named for August Kopff, who discovered it on August 22,1906, during an observing session at Koenigstuhl Observatory near Heidelberg, Germany. Ninety-four years after Kopffs discovery, CRAF would arrive at the point with Comet Kopff — in August 2000. Comet and spacecraft would be at the distance of Jupiters orbit, CRAF would fire its penetrator/lander at the comet in August 2001, then would continue to fly beside Kopff. The spacecraft would take data for a total of two and two-thirds years, until about 109 days after they pass closest to the Sun and are outward bound again. It was to be the first time a spacecraft would have flown in formation with a comet, scientists believe comets now reside in a distant region of the solar system called the Oort cloud. Current theory holds that gravitational nudges from stars in the Suns neighborhood send some comets from the Oort cloud falling toward the Sun, the CRAF spacecraft would fly extremely close to the comets nucleus, within 10 kilometers. In December 2002 the spacecraft and the comet would make their closest approach to the Sun and they would then head outward again toward aphelion near Jupiters orbit. On March 31,2003, the Comet Rendezvous Asteroid Flyby, first primary mission of Mariner Mark II, at that time the Cassini mission, with the second Mariner Mark II spacecraft, would be in its Saturnian-tour phase. CRAF was to be the first Mariner Mark II mission, the second mission, called Saturn Orbiter Titan Probe or SOTP, was a Saturn orbiter with a probe designed to plunge into the atmosphere of the ringed planets largest satellite, Titan. CRAF and SOTP were to use identical Mariner Mark II spacecraft, all of the phenomena associated with comets were to be examined by CRAFs instruments

29. Hayabusa Mk2 – Hayabusa Mk2 was a proposed Japan Aerospace Exploration Agency space mission aimed at visiting a small primitive asteroid and returning a sample to Earth for laboratory analysis. It was intended to be the mission to JAXAs Hayabusa mission. The latest proposal for Hayabusa Mk2 stated its target to be the dormant comet 4015 Wilson–Harrington, from 2007 to 2010, it was also considered as a joint JAXA-ESA mission under the name Marco Polo. Thus, it would provide some constraints to the models of planet formation, information on the physical structure will help defining efficient mitigation strategies against a potential threatening object. Small bodies, as primitive building blocks of the Solar System formation process. Moreover, collisions of NEOs with Earth pose a hazard to life. For all these reasons, the exploration of objects is particularly interesting. The principal scientific objective of the Hayabusa Mk2 mission is to return unaltered materials from a NEO, Hayabusa Mk2 will allow us to analyze the samples in terrestrial laboratory, and to obtain measurements that cannot yet be performed from a robotic spacecraft. Moreover, the mission will allow scientists to, Determine the physical and chemical properties of the target body, identify the major events which influenced the history of the target. Determine the elemental and mineralogical properties of the body and their variations with geological context on the surface. Search for pre-solar material yet unknown in meteoritic samples, investigate the nature and origin of organic compounds on the target body. Search for organic compounds which may shed light on the origin of pre-biotic molecules, understand the role of minor body impacts in the origin and evolution of life on Earth. NEOs are among the most accessible bodies of the Solar System, the Hayabusa Mk2 was actually proposed before Hayabusa 2. Unlike Hayabusa 2, which reused most of Hayabusas design, for Hayabusa Mk2 JAXA intended to revise the designs. These are the reasons why this project is called Hayabusa Mk2, Marco Polo was a proposed Japanese-European space mission aimed at visiting a small primitive asteroid and returning a sample to Earth for analysis in laboratories. It was proposed to the program Cosmic Vision 2015-2025 of ESA in June 2007 and this mission was proposed as a joint ESA-JAXA mission. The Lander would perform a landing, anchor to the asteroid surface. Samples will be collected with one or complementary techniques

30. Marco Polo (spacecraft) – It was first proposed to the European Space Agency in collaboration with the Japan aerospace exploration agency JAXA. The concept was rejected fourth times between 2007 and 2015 for the Cosmic Vision programme M medium-class missions, Marco Polo was a mission concept aimed at visiting a small asteroid and returning a sample to Earth for analysis in laboratory. The concept was studied by the European Space Agency in collaboration with the Japan Aerospace eXploration Agency JAXA. Marco Polo was first rejected in June 2007 for the Cosmic Vision program, thus, it would provide some constraints to the models of planet formation and some information on how life ingredients may have been brought to Earth. Information on the structure would help defining efficient mitigation strategies against a potential threatening object. Small bodies, as primitive building blocks of the Solar System formation process. Current exobiological scenarios for the origin of life invoke an exogenous delivery of organic compounds to the early Earth and it has been proposed that carbonaceous chondrite matter could have brought these complex organic molecules capable of triggering the pre-biotic synthesis of biochemical compounds on the early Earth. Moreover, collisions of NEOs with Earth pose a hazard to life. For all these reasons, the exploration of objects is particularly interesting. The Marco Polo proposals were supported by more than 400 scientists worldwide and this concept was in competition for the M1, M2, M3 and M4 missions. It was rejected all four times, identify the major events which influenced the history of the target. Determine the elemental and mineralogical properties of the body and their variations with geological context on the surface. Search for pre-solar material yet unknown in meteoritic samples, investigate the nature and origin of organic compounds on the target body. Understand the role of minor impacts in the origin and evolution of life on Earth. Marco Polos first two came in the competitions for the European Space Agencys Cosmic Vision program M1 and M2 missions. MarcoPolo-R, as it was renamed and re-submitted, then lost out in the M3 competition in 2014. The mission was renamed and re-submitted as MarcoPolo-2D to compete for the M4 opportunity. The lander would perform a landing, anchor to the asteroid surface

31. Vesta (spacecraft) – Vesta was a multiple-asteroid-flyby mission that the Soviet Union was planning in the 1980s. The Vesta mission would have consisted of two probes, to be launched in 1991. Similar to the Vega program, each spacecraft would deploy one or more landers or balloons into the Venusian atmosphere, at Venus, a French satellite dedicated to asteroid flybys would be released. It would return to us for an Earth swing-by, and then reach about 3-3.3 AUs from the Sun, there they would fly by some smaller asteroids, and Vesta, if possible, with a small probe landing there. The exact targets would depend on the launch date, in the initial 1985 study,2700 possible trajectories were analyzed for a launch date in 1991/1992. Considering all constraints, about 12 candidate trajectories were selected, of course, the two identical spacecraft could choose different trajectories and targets. These included 5 Astraea,53 Kalypso,187 Lamberta,453 Tea,1335 Demoulina and 1858 Lobachevskij, around 1985 Vesta was changed to be a Mars mission, with the asteroid-part unchanged. Visiting at least one Apollo-Amor asteroid was also given a preference, images at closest approach could have a resolution of 10 m/pixel. Worst case downlink rate is 600 bit/second, the scientific payload is about 100 kg. The spacecraft has 750 kg dry mass, and carries 750 kg propellants,20 square meters of solar panels provide 350 Watts of power. If DSN support could be obtained, Doppler tracking of the Vesta spacecrafts movement can be used to determine the mass of the encountered bodies. In the other case, another possibility was considered, releasing a test mass, the spacecrafts structure is derived from telecommunication satellites, having the required mass, volume, and delta-v capabilities. The Mars gravity assist constrain the possible trajectories, the asteroid penetrator also imposes limits on the speed of the approach of the target asteroid. Nevertheless,3 possible trajectories were designed, with two Mars gravity assists. A single Mars swing-by is also possible, but the gravity assist increases the mass budget of the spacecraft by 30%. The following trajectories are for the 1994 launch window

32. CONTOUR – The COmet Nucleus TOUR was a NASA Discovery-class space probe that failed shortly after its July 2002 launch. It had as its primary objective close flybys of two nuclei with the possibility of a flyby of a third known comet or an as-yet-undiscovered comet. The two comets scheduled to be visited were Encke and Schwassmann-Wachmann-3, and the target was dArrest. After the solid rocket motor intended to inject the spacecraft into orbit was ignited on August 15,2002. Ground-based telescopes later found three objects along the course of the satellite, leading to the speculation that it had disintegrated, attempts to contact the probe were ended on December 20,2002. The CONTOUR spacecraft was constructed in-house at the Johns Hopkins University Applied Physics Laboratory, the spacecraft was fitted with a 25 cm whipple shield, similar to the one used on Stardust, on its leading face, designed with four layers of nextel fabric and seven layers of kevlar. The results of the earlier tests allowed mission planners to determine a distance from which the CONTOUR would pass by comets targeted on the mission. Three of the four scientific instruments aboard the spacecraft are embedded within this heat shield, power for CONTOUR derives from solar cells, which are mounted onto the spacecraft, decorating the sides and rear and generating up to 670 watts of power. It was launched into a high-apogee Earth orbit with a period of 5.5 days.2 km/s,1.07 AU from the Sun and 0.27 AU from Earth, during the August 2002 injection maneuver, the probe was lost. Three more Earth flybys would have followed, in August 2004, February 2005, on June 18,2006, CONTOUR would have encountered comet Schwassmann-Wachmann-3 at 14 km/s,0.95 AU from the Sun and 0.33 AU from Earth. All flybys would have had a closest encounter distance of about 100 km, after the comet Encke encounter, CONTOUR might have been retargeted towards a new comet if one was discovered with the desired characteristics. According to NASA, An investigation board concluded that the most likely cause of the mishap was failure of the spacecraft due to plume heating during the solid-rocket motor burn. Alternate possible but less likely causes determined were catastrophic failure of the rocket motor, collision with space debris. After the loss of CONTOUR, a replacement spacecraft – CONTOUR2 – was proposed, scheduled for launch in 2006, however, the replacement did not ultimately materialize. CONTOUR Mission Profile by NASAs Solar System Exploration The CONTOUR spacecraft, bradley, Jr. Theron, Gay, Charles, Martin, Patrick, Stepheson, David, Tooley, Craig. Contour Comet Nucleus Tour Mishap Investigation Board Report

33. Deep Impact (spacecraft) – Deep Impact was a NASA space probe launched from Cape Canaveral Air Force Station at 18,47 UTC on January 12,2005. It was designed to study the composition of the comet Tempel 1. At 05,52 UTC on July 4,2005, the impactor successfully collided with the comets nucleus, the impact excavated debris from the interior of the nucleus, forming an impact crater. Photographs taken by the spacecraft showed the comet to be more dusty, the impact generated an unexpectedly large and bright dust cloud, obscuring the view of the impact crater. Previous space missions to comets, such as Giotto and Stardust, were fly-by missions and these missions were able to photograph and examine only the surfaces of cometary nuclei, and even then from considerable distances. The Deep Impact mission was the first to eject material from a surface, and the mission garnered large publicity from the media, international scientists. Upon the completion of its mission, proposals were made to further utilize the spacecraft. Consequently, Deep Impact flew by Earth on December 31,2007 on its way to a mission, designated EPOXI, with a dual purpose to study extrasolar planets. By observing the composition of the comet, astronomers hoped to determine how comets form based on the differences between the interior and exterior makeup of the comet, observations of the impact and its aftermath would allow astronomers to attempt to determine the answers to these questions. The missions Principal Investigator was Michael AHearn, an astronomer at the University of Maryland, the Flyby spacecraft is about 3.2 meters long,1.7 meters wide and 2.3 meters high. It includes two panels, a debris shield, and several science instruments for imaging, infrared spectroscopy. The spacecraft also carried two cameras, the High Resolution Imager, and the Medium Resolution Imager.05 to 4.8 micrometres and it has been optimized for observing the comets nucleus. The MRI is the device, and was used primarily for navigation during the final 10-day approach. It also has a wheel, with a slightly different set of filters. The Impactor section of the spacecraft contains an instrument that is identical to the MRI, called the Impactor Targeting Sensor. Its dual purpose was to sense the Impactors trajectory, which could then be adjusted up to four times between release and impact, and to image the comet from close range, the final image taken by the impactor was snapped only 3.7 seconds before impact. The impactors payload, dubbed the Cratering Mass, was 100% copper, including this cratering mass, copper formed 49% of total mass of the impactor, this was to minimize interference with scientific measurements. Since copper was not expected to be found on a comet, instead of using explosives, it was also cheaper to use copper as the payload

34. EPOXI – EPOXI uses the Deep Impact spacecraft in a campaign consisting two missions, the Deep Impact Extended Investigation and Extrasolar Planet Observation and Characterization. After its flyby of Hartley 2, the spacecraft was also set to make a flyby of the apollo asteroid 2002 GT in 2020. The mission was suspended altogether, however, after contact with the spacecraft was suddenly lost in August 2013, mission scientists theorized that a Y2K-like problem had plagued the spacecrafts software. The Deep Impact mission was finished with the visit to comet Tempel 1, but the spacecraft still had plenty of maneuvering fuel left, so NASA approved a second mission, called EPOXI, which included a visit to a second comet as well as observations of extrasolar planets. On July 21,2005, Deep Impact executed a trajectory correction maneuver that placed the spacecraft on course to fly past Earth on December 31,2007, the maneuver allowed the spacecraft to use Earths gravity to begin a new mission in a path towards another comet. In January 2008 Deep Impact began studying the stars with several known extrasolar planets in an attempt to find other such stars nearby, the larger of the spacecrafts two telescopes attempts to find the planets using the transit method. The initial plan was for a December 5,2008 flyby of Comet Boethin, the spacecraft did not carry a second impactor to collide with the comet and would observe the comet to compare it to various characteristics found on 9P/Tempel. He explained that the mission would provide only about half of the information collected during the collision with Tempel 1, Deep Impact would use its spectrometer to study the comets surface composition and its telescopes for viewing the surface features. However, as the Earth gravity assist approached, astronomers were unable to locate Comet Boethin, consequently, its orbit could not be calculated with sufficient precision to permit a flyby. Instead, the decided to send Deep Impact to comet 103P/Hartley requiring an extra two years. NASA approved the funding required and retargeted the spacecraft. Mission controllers at the Jet Propulsion Laboratory began redirecting EPOXI on November 1,2007 and they commanded the spacecraft to perform a three-minute rocket burn that changed the spacecrafts velocity. EPOXI’s new trajectory set the stage for three Earth flybys, the first on December 31,2007 and this placed the spacecraft into an orbital holding pattern so that it could encounter comet 103P/Hartley in 2010. The goal of photometric observations is to measure the quantity of light, an aberration in the primary mirror of the HRI allowed the HRI to spread the light from observations over more pixels without saturating the CCD, effectively obtaining better data. A total of 198,434 images were exposed, ePOChs goals were to study the physical properties of giant planets and search for rings, moons and planets as small as three Earth masses. The spacecraft used Earths gravity for the gravity assist in December 2008. Observations of 103P/Hartley began on September 5 and ended November 25,2010, for a diagram of the EPOXI solar orbits see here. The missions closest approach to 103P/Hartley occurred at 10 am EDT on 4 November 2010, the flyby speed was 12.3 km/s

35. Deep Space 1 – Deep Space 1 was a NASA technology demonstration spacecraft which flew by an asteroid and a comet. It was part of the New Millennium Program, dedicated to testing advanced technologies, launched on 24 October 1998, the Deep Space 1 spacecraft carried out a flyby of asteroid 9969 Braille, which was its primary science target. The mission was extended twice to include an encounter with comet 19P/Borrelly, problems during its initial stages and with its star tracker led to repeated changes in mission configuration. While the flyby of the asteroid was a success, the encounter with the comet retrieved valuable information. Three of twelve technologies on board had to work within a few minutes of separation from the rocket for the mission to continue. The Deep Space series was continued by the Deep Space 2 probes, Deep Space 1 was the first NASA spacecraft to use ion propulsion rather than the traditional chemical-powered rockets. The asteroids in the inner Solar System move in relation to other bodies at a noticeable, predictable speed, thus a spacecraft can determine its relative position by tracking such asteroids across the star background, which appears fixed over such timescales. Two or more asteroids let the spacecraft triangulate its position, two or more positions in time let the spacecraft determine its trajectory, existing spacecraft are tracked by their interactions with the transmitters of the NASA Deep Space Network, in effect an inverse GPS. However, DSN tracking requires many skilled operators, and the DSN is overburdened by its use as a communications network, the use of Autonav reduces mission cost and DSN demands. The Autonav system can also be used in reverse, tracking the position of relative to the spacecraft. This is used to acquire targets for the scientific instruments, the spacecraft is programmed with the targets coarse location. After initial acquisition, Autonav keeps the subject in frame, even commandeering the spacecrafts attitude control, the next spacecraft to use Autonav was Deep Impact. ABLE Engineering developed the technology and built the solar array for DS1, with Entech Inc, who supplied the Fresnel optics. The activity was sponsored by the Ballistic Missile Defense Organization, the SCARLET arrays generated 2.5 kilowatts at 1 AU, with less size and weight than conventional arrays. This lack of a history in space meant that despite the potential savings in propellant mass. Furthermore, unforeseen side effects of ion propulsion might in some way interfere with scientific experiments, such as fields. Therefore, it was a mission of the Deep Space 1 demonstration to show long-duration use of an ion thruster on a scientific mission. The NASA Solar Technology Application Readiness electrostatic ion thruster, developed at NASA Glenn and this is an order of magnitude higher than traditional space propulsion methods, resulting in a mass savings of approximately half

36. Giotto (spacecraft) – Giotto was a European robotic spacecraft mission from the European Space Agency. The spacecraft flew by and studied Halleys Comet and in doing so became the first spacecraft to make close up observations of a comet, on 13 March 1986, the mission succeeded in approaching Halleys nucleus at a distance of 596 kilometers. The spacecraft was named after the Early Italian Renaissance painter Giotto di Bondone and he had observed Halleys Comet in 1301 and was inspired to depict it as the star of Bethlehem in his painting Adoration of the Magi. Originally a United States partner probe was planned that would accompany Giotto, there were plans to have observation equipment on board a Space Shuttle in low-Earth orbit around the time of Giottos fly-by, but they in turn fell through with the Challenger disaster. The plan then became a cooperative armada of five spaceprobes including Giotto, because Giotto would pass so very close to the nucleus ESA was mostly convinced it would not survive the encounter due to bombardment from the many high speed cometary particles. The coordinated group of probes became known as the Halley Armada, the later Stardust spacecraft would use a similar Whipple shield. A mock up of the spacecraft resides at the Bristol Aero Collection hangar, at Filton, the mission was given the go-ahead by ESA in 1980, and launched on an Ariane 1 rocket on 2 July 1985 from Kourou, French Guiana. Attitude determination and control used sun pulse and IR Earth sensor data in the telemetry to determine the spacecraft orientation. The Soviet Vega 1 started returning images of Halley on 4 March 1986, and the first ever of its nucleus, Vega 1 closest approach to Halley was 8 889 km. Giotto passed Halley successfully on 14 March 1986 at 596 km distance, one impact sent it spinning off its stabilized spin axis so that its antenna no longer always pointed at the Earth, and importantly, its dust shield no longer protected its instruments. After 32 minutes Giotto re-stabilized itself and continued gathering science data, another impact destroyed the Halley Multicolor Camera, but not before it took photographs of the nucleus at closest approach. Giottos trajectory was adjusted for an Earth flyby and its instruments were turned off on 15 March 1986 at 02,00 UTC. Giotto was commanded to wake up on 2 July 1990 when it flew by Earth in order to sling shot to its next cometary encounter, the probe then flew by the Comet Grigg-Skjellerup on 10 July 1992 which it approached to a distance of about 200 km. Afterwards, Giotto was again switched off on 23 July 1992, in 1999 Giotto made another Earth flyby but was not reactivated. Images showed Halleys nucleus to be a dark peanut-shaped body,15 km long,7 km to 10 km wide, only 10% of the surface was active, with at least three outgassing jets seen on the sunlit side. Analysis showed the comet formed 4.5 billion years ago from volatiles that had condensed onto interstellar dust particles and it had remained practically unaltered since its formation. Measured volume of material ejected by Halley, 80% water, 10% carbon monoxide 2. 5% a mix of methane, other hydrocarbons, iron, and sodium were detected in trace amounts. Giotto found Halleys nucleus was dark, which suggested a thick covering of dust, the nucleuss surface was rough and of a porous quality, with the density of the whole nucleus as low as 0.3 g/cm³

37. International Cometary Explorer – The International Cometary Explorer spacecraft, was launched August 12,1978, into a heliocentric orbit. ISEE-3 was the first spacecraft to be placed in an orbit at the L1 Earth-Sun Lagrangian point. Renamed ICE, it became the first spacecraft to visit a comet, NASA suspended routine contact with ISEE-3 in 1997, and made brief status checks in 1999 and 2008. On May 29,2014, two-way communication with the spacecraft was reestablished by the ISEE-3 Reboot Project, on July 2,2014, they fired the thrusters for the first time since 1987. However, later firings of the failed, apparently due to a lack of nitrogen pressurant in the fuel tanks. The project team initiated a plan to use the spacecraft to collect scientific data and send it back to Earth. ISEE-3 carries no cameras, instead, its instruments measure energetic particles, waves, plasmas, ISEE-3 originally operated in a halo orbit about the L1 Sun-Earth Lagrangian point,235 Earth radii above the surface. It was the first artificial object placed at a so-called libration point, entering orbit there on November 20,1978, proving that such a suspension between gravitational fields was possible. It rotates at 19.76 rpm about a perpendicular to the ecliptic, to keep it oriented for its experiments, to generate solar power. After completing its mission, ISEE-3 was re-tasked to study the interaction between the solar wind and a cometary atmosphere. On June 10,1982, the spacecraft performed a maneuver which removed it from its orbit around the L1 point. This involved a series of passages between Earth and the Sun-Earth L2 Lagrangian point, through the Earths magnetotail, fifteen propulsive maneuvers and five lunar gravity assists resulted in the spacecraft being ejected from the Earth-Moon system and into a heliocentric orbit. Its last and closest pass over the Moon, on December 22,1983, was only 119.4 km above the surface, following this pass. Its new orbit put it ahead of the Earth on a trajectory to intercept comet Giacobini-Zinner, on September 11,1985, the craft passed through the comets plasma tail. ICE transited between the Sun and Comet Halley in late March 1986, when other spacecraft were near the comet on their early-March comet rendezvous missions, ICE flew through the tail, its minimum distance to the comet nucleus was 28 million kilometres. For comparison, Earths minimum distance to Comet Halley in 1910 was 20.8 million kilometres, an update to the ICE mission was approved by NASA in 1991. It defines a heliospheric mission for ICE consisting of investigations of coronal mass ejections in coordination with ground-based observations, continued cosmic ray studies, by May 1995, ICE was being operated under a low duty cycle, with some data-analysis support from the Ulysses project. On May 5,1997, NASA ended the ICE mission, the ISEE-3/ICE downlink bit rate was nominally 2048 bits per second during the early part of the mission, and 1024 bit/s during the Giacobini-Zinner comet encounter

Comet [videos]
A comet is an icy small Solar System body that, when passing close to the Sun, warms and begins to release gases, a
Comet Tempel collides with Deep Impact's impactor
Comet 67P/Churyumov–Gerasimenko orbited by Rosetta
Comet 17P/Holmes and its blue ionized tail
Comet Wild 2 visited by Stardust probe
Comet nucleus [videos]
The nucleus is the solid, central part of a comet, popularly termed a dirty snowball or an icy dirtball. A cometary
The nucleus of Comet Tempel 1.
The Helix Nebula has a cometary Oort cloud
Tempel 1 and Hartley 2 compared
C/2006 W3 (Chistensen) - emitting carbon gas
Meteor shower [videos]
A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point
Four-hour time lapse exposure of the sky
Leonids from space
Diagram from 1872
Meteor shower on chart
Extraterrestrial atmosphere [videos]
The study of extraterrestrial atmospheres is an active field of research, both as an aspect of astronomy and to gain
Major features of the Solar System (not to scale)
Atmosphere of Venus in UV, by Pioneer Venus Orbiter in 1979
The tenuous atmosphere of Mars visible on the horizon.
Oval BA on the left and the Great Red Spot on the right
Asteroid [videos]
Asteroids are minor planets, especially those of the inner Solar System. The larger ones have also been called
Image: (253) mathilde
243 Ida and its moon Dactyl. Dactyl is the first satellite of an asteroid to be discovered.
First asteroid image (Ceres and Vesta) from Mars – viewed by ''Curiosity'' (20 April 2014).
A composite image, to scale, of the asteroids that have been imaged at high resolution except Ceres. As of 2011 they are, from largest to smallest: 4 Vesta, 21 Lutetia, 253 Mathilde, 243 Ida and its moon Dactyl, 433 Eros, 951 Gaspra, 2867 Šteins, 25143 Itokawa.
Deep Impact (spacecraft) [videos]
Deep Impact was a NASA space probe launched from Cape Canaveral Air Force Station at 18:47 UTC on January 12, 2005. It
Artist's impression of the Deep Impact space probe after deployment of the impactor.
Cameras of the flyby spacecraft, HRI at right, MRI at left
Deep Impact about to be launched with a Delta II rocket
Simulation: The collision of comet Tempel 1 and the Deep Impact impactor, simulated by Celestia software using pre-impact information. The Sun and the Earth are on the right side. Note: The Deep Impact flyby spacecraft faces the wrong direction. The solar array should face the Sun and the high-gain antenna should point to the Earth.
EPOXI [videos]
EPOXI is a compilation of NASA Discovery program missions led by the University of Maryland and principal investigator
The Deep Impact spacecraft at the JPL in July 2004.
EPOXI mission observation of Hartley 2's jets
The nucleus of comet 103P/Hartley measuring approximately 2 kilometers in length and .4 kilometers at its most narrow portion or neck. Jets can be seen streaming out of the nucleus.
Another view of the comet, taken near closest approach.
Stardust (spacecraft) [videos]
Stardust was a 300 kilogram robotic space probe launched by NASA on 7 February 1999. Its primary mission was to collect
Artist's impression of Stardust at comet Wild 2.
Artist's impression of the Stardust spacecraft performing a burn-to-depletion at the end of the Stardust NExT mission.
Landing capsule as seen by the recovery team.
Visible dust grains in the aerogel collector.
Ulysses (spacecraft) [videos]
Ulysses is a decommissioned robotic space probe whose primary mission was to orbit the Sun and study it at all
Image: Ulysses artist rendering b 02
Ulysses spacecraft
Ulysses sits atop the PAM-S and IUS combination
Vega program [videos]
The Vega program (Cyrillic: ВеГа) was a series of Venus missions that also took advantage of the appearance of comet
Vega mission description
Vega solar system probe bus and landing apparatus (model)
Position of Vega landing sites. Red points denote sites returning images from the surface, black central dots sites of surface sample analysis. Map based on mapping from Pioneer Venus Orbiter, Magellan and Venera 15/16.
Vega balloon probe on display at the Udvar-Hazy Center of the Smithsonian Institution. Photo by Geoffrey A. Landis.
C/2014 Q2 (Lovejoy) [videos]
C/2014 Q2 (Lovejoy) is a long-period comet discovered on 17 August 2014 by Terry Lovejoy using a 0.2-meter (8 in)
Image: C2014 Q2
Image: Celestial Nomad Takes Centre Stage
Image: Comet 2014 Q2 Lovejoy
Image: 2015 01 17 Comet C2014 Q2 Lovejoy 4x 60sec f 10 2000mm ISO2500
C/2013 US10 [videos]
C/2013 US10 (Catalina) is an Oort cloud comet discovered on 31 October 2013 by the Catalina Sky Survey at an apparent
C/2013 US10 as seen on 9 Dec 2015. To the upper left is the ion gas tail and to the lower right is the dust tail.
Image: Comet C 2013 US10 Warsaw
Image: C2013US10 CATALINA 06DEC2015 J87
Image: C2013 US10 06DEC2015 J87 BW
C/2013 A1 [videos]
C/2013 A1 (Siding Spring) is an Oort cloud comet discovered on 3 January 2013 by Robert H. McNaught at Siding Spring
Image: NASA 14090 Comet C2013A1 Siding Spring Hubble 20140311
C/2013 A1 – four images
Comet Siding Spring as seen by NEOWISE on 16 January 2014
Image: PIA18611 Mars Comet Siding Spring Flyby 20141009
Deep Space 1 [videos]
Deep Space 1 (DS1) was a NASA technology demonstration spacecraft which flew by an asteroid and a comet. It was part of
Artist concept of Deep Space 1
A Small Deep Space Transponder
Comet 19P/Borrelly imaged just 160 seconds before DS1's closest approach
Image: Ion Engine Being Installed in High Vacuum Tank GPN 2000 000597
Philae (spacecraft) [videos]
Philae (or ) is a robotic European Space Agency lander that accompanied the Rosetta spacecraft until it separated to
Illustration of Philae
Depiction of Philae on Churyumov-Gerasimenko
Rosetta and Philae
Oort cloud [videos]
The Oort cloud , named after the Dutch astronomer Jan Oort, sometimes called the Öpik–Oort cloud, is a theoretical
Image: PIA17046 Voyager 1 Goes Interstellar
Comet Hale–Bopp, an archetypical Oort-cloud comet
This graphic shows the distance from the Oort cloud to the rest of the Solar System and two of the nearest stars measured in astronomical units. The scale is logarithmic, with each specified distance ten times further out than the previous one. Red arrow indicates location of Voyager 1, a space probe that will reach the Oort cloud in 300 years.
Coma (cometary) [videos]
The coma is the nebulous envelope around the nucleus of a comet, formed when the comet passes close to the Sun on its
Structure of Comet Holmes in infrared, as seen by an infrared space telescope
Tempel 1 in X-ray light by Chandra
Artificially colored far-ultraviolet image (with film) of Comet Kohoutek (Skylab, 1973)
Naming of comets [videos]
Comets have been observed for the last 2,000 years. During that time, several different systems have been used to
Comet McNaught, named after its discoverer Robert H. McNaught. It is also known as the Great Comet of 2007 and has the numerical designation C/2006 P1.
The Great January Comet of 1910, named after the date it appeared
Halley's Comet, named after Edmond Halley who first calculated its orbit. It now has the numerical designations 1P/Halley and 1P/1682 Q1.
Main-belt comet [videos]
Main-belt comets (MBCs) are bodies orbiting within the asteroid belt that have shown comet-like activity during part of
Asteroid (596) Scheila displaying a comet-like appearance on December 12, 2010.
Image: 14060 Asteroid P2013R3 Disintegration 20140306
Image: Asteroid P2013 R3 breaks apart
CONTOUR [videos]
The COmet Nucleus TOUR (CONTOUR) was a NASA Discovery-class space probe that failed shortly after its July 2002 launch.
Artists' impression of CONTOUR approaching a comet.
The CONTOUR spacecraft at the Kennedy Space Center in May 2002, being prepared for launch.
Long-exposure photograph of the launch of CONTOUR from Cape Canaveral on July 3, 2002.
Apsis [videos]
An apsis (Greek: ἁψίς; plural apsides , Greek: ἁψῖδες) is an extreme point in an object's orbit. The word comes via
Image: Inner Planet Orbits
Image: Outer Planet Orbits
Example of periapsis and apoapsis, with two large bodies in elliptical orbits around their center of mass
Keplerian orbital elements: point F is at the pericenter, point H is at the apocenter, and the red line between them is the line of apsides
Giotto (spacecraft) [videos]
Giotto was a European robotic spacecraft mission from the European Space Agency. The spacecraft flew by and studied
Image: Giotto spacecraft
Image: Giotto Scrovegni 18 Adoration of the Magi
Image: Giotto insignia
Giotto trajectory
Rosetta (spacecraft) [videos]
Rosetta was a space probe built by the European Space Agency launched on 2 March 2004. Along with Philae, its lander
Artist's illustration of Rosetta
Illustration of Rosetta and Philae at the comet
Trajectory of the Rosetta space probe
Interstellar object [videos]
An interstellar object is a body other than a star or substar located in interstellar space, and not gravitationally
Path of the hyperbolic, extrasolar asteroid ʻOumuamua
Image: Eso 1737a shorter
Comet Machholz 1 (96P/Machholz) as viewed by STEREO-A (April 2007)
Kreutz sungrazer [videos]
The Kreutz sungrazers ((listen), pronounced kroits) are a family of sungrazing comets, characterized by orbits taking
An illustration of the sungrazing Great Comet of 1843, as seen from Tasmania
Photograph of the Great Comet of 1882, as seen from South Africa
Approximate relationship of the largest members of the Kreutz sungrazers. Note that the perihelion passage at which fragmentations occurred may not be well established
Vega 1 [videos]
Vega 1 (along with its twin Vega 2) is a Soviet space probe part of the Vega program. The spacecraft was a development
Spacecraft Vega 1
Image: 1986 venera galley nh
Antitail [videos]
An antitail is a spike projecting from a comet's coma which seems to go towards the Sun, and thus geometrically
Comet Lulin antitail to the left, ion tail to right
Showing how a comet may appear to exhibit a short tail pointing in the opposite direction to its type II or dust tail as viewed from Earth i.e. an antitail
Lost comet [videos]
A lost comet is a previously discovered comet that has been missed at its most recent perihelion passage, generally
Biela's Comet was seen in two pieces in 1846, and has not been observed since 1852
5D/Brorsen, which was lost after its 1879 apparition.
Great comet [videos]
A great comet is a comet that becomes exceptionally bright. There is no official definition; often the term is attached
The Great Comet of 1680 over Rotterdam as painted by Lieve Verschuier
Halley's Comet's 1986 apparition was unusually modest in brightness.
Exocomet [videos]
An exocomet, or extrasolar comet, is a comet outside the Solar System, which includes interstellar comets and those
Exocomets and various planet-formation processes around Beta Pictoris, a very young A-type main-sequence star (NASA; artist's conception).
Image: Exocomets plunging into a young star (artist’s impression)
Comet tail [videos]
A comet tail—and coma—are features visible in comets when they are illuminated by the Sun and may become visible from
Comet Lovejoy from orbit
Diagram of a comet showing the dust tail, the dust trail (or antitail) and the ion gas tail, which is formed by the solar wind flow. NASA
A comet's orbit showing the different directions of the gas and dust tails as the comet passes the Sun
Orbital eccentricity [videos]
The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around
Image: Eccentricity rocky planets
Image: Kepler orbits
Extinct comet [videos]
Extinct comets are comets that have expelled most of their volatile ice and have little left to form a tail or coma.
Don Quixote (apmag 15) near perihelion in 2009.
Timeline of Rosetta spacecraft [videos]
Rosetta is a space probe designed to rendezvous with the comet 67P/Churyumov–Gerasimenko, perform flybys of two
67/P on 7 July
Palermo Astronomical Observatory [videos]
The Giuseppe S. Vaiana Astronomical Observatory is an astronomical observatory located in Palermo, Sicily, Italy,
The part of the Palace of the Normans hosting the observatory
Comet dust [videos]
Comet dust refers to cosmic dust that originates from a comet. Comet dust can provide clues to comets' origin. When the
Microscopic view of comet dust particle
Sungrazing comet [videos]
A sungrazing comet is a comet that passes extremely close to the Sun at perihelion – sometimes within a few thousand
Image: A Unique Hubble View of Comet ISON
Rock comet [videos]
A rock comet is a rare type of small Solar System body that exhibits features of both a comet and an asteroid, mainly
The cause of rock comets' outgassing is similar to how mud in a dry lake bottom cracks.
Comet Rendezvous Asteroid Flyby [videos]
The Comet Rendezvous Asteroid Flyby (CRAF) was a cancelled plan for a NASA led exploratory mission designed by the Jet
Conceptual artwork of the CRAF spacecraft
International Cometary Explorer [videos]
The International Cometary Explorer (ICE) spacecraft (designed and launched as the International Sun-Earth Explorer-3
The ISEE-3 (later ICE), undergoing testing and evaluation.
C/2014 E2 (Jacques) [videos]
C/2014 E2 (Jacques), provisionally designated as S002692, is a long-period comet discovered by the Brazilian
Image of 2014 E2 (Jacques) in the constellation Antlia at magnitude 10.5
Centaur (minor planet) [videos]
Centaurs are small solar system bodies with a semi-major axis between those of the outer planets. They have unstable
Image: Kuiper belt plot objects of outer solar system
Image: Comet 38P2067
Colour distribution of centaurs
Image: The Kuiper Belt 42AU Centaurs
C/2016 U1 (NEOWISE) [videos]
C/2016 U1 (NEOWISE) is a hyperbolic comet discovered 21 October 2016 by NEOWISE, the asteroid-and-comet-hunting portion
Image: Comet C2016U1Neowise Orbit 20170114
International Standard Book Number [videos]
The International Standard Book Number (ISBN) is a unique numeric commercial book identifier. — An ISBN is assigned to
A 13-digit ISBN, 978-3-16-148410-0, as represented by an EAN-13 bar code
The parts of a 10-digit ISBN and the corresponding EAN‑13 and barcode. Note the different check digits in each. The part of the EAN‑13 labeled "EAN" is the Bookland country code.
Small Solar System body [videos]
A Small Solar System Body (SSSB) is an object in the Solar System that is neither a planet, nor a dwarf planet, nor a
Euler diagram showing the types of bodies in the Solar System.
Distribution of centaurs and trans-Neptunian objects.
Provisional designation in astronomy [videos]
Provisional designation in astronomy is the naming convention applied to astronomical objects immediately following
Image: Simbolo di Vesta
Orbital period [videos]
The orbital period is the time a given astronomical object takes to complete one orbit around another object, and
The semi-major axis (a) and semi-minor axis (b) of an ellipse
Orbital inclination [videos]
Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle
Fig. 1: One view of inclination i (green) and other orbital parameters