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Asteroids & Comets

Asteroids & Comets. Debris of the Solar System. Asteroids are rocky or metallic objects orbiting the Sun that are smaller than a major planet, but that show no evidence of an atmosphere and contain little volatile (easily evaporated) material

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Asteroids & Comets

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  1. Asteroids & Comets AST 2010: Chapter 12

  2. Debris of the Solar System • Asteroids are rocky or metallic objects orbiting the Sun that are smaller than a major planet, but that show no evidence of an atmosphere and contain little volatile (easily evaporated) material • Comets are icy bodies that revolve around the Sun and are smaller than a major planet, but that contain frozen water and other volatile materials AST 2010: Chapter 12

  3. Discovery of Asteroids • Most asteroid orbits lie in the asteroid belt, between Mars and Jupiter • too small to be visible without a telescope • They were first discovered when astronomers were hunting for a missing planet between Mars and Jupiter • The 1st asteroid, named Ceres and initially thought to be the “missing planet”, was discovered by Giovanni Piazzi in 1801 • It orbits at 2.8 AU from the Sun • The discovery of other “minor planets” in similar orbits followed in subsequent years • Now, more than 20,000 asteroids are known to have well-determined orbits AST 2010: Chapter 12

  4. Asteroid Nomenclature • Asteroids are given both a number and a name • The names were originally chosen from Greek/Roman goddesses, then other female names were used, and finally all names go! • Asteroids 2410 and 4859 are named after Morrison and Fraknoi Gaspra Ida Mathilde AST 2010: Chapter 12

  5. Asteroid Census • The total number of asteroids in the solar system is very large • It must be estimated on the basis of systematic sampling of the sky • Studies indicate that there are 106 asteroids with diameters greater than 1 km! • The largest is Ceres, with a diameter of ~1000 km • Pallas and Vesta have diameters of ~500 km • 15 more are larger than 250 km across • There are 100 times more objects 10-km across than 100-km across • The total mass of asteroids is less than the mass of the Moon AST 2010: Chapter 12

  6. Asteroid Orbits • All asteroids revolve around the Sun in west-to-east direction, like the planets • Most of their orbits lie near the plane in which the Earth and the other planets circle • The asteroid belt is defined as the region that contains all asteroids with semi-major axes in the range from 2.2 to 3.3 AU • Their orbital periods range from 3.3 to 6 years • 75% of known asteroids are in the main belt • But they are not closely spaced AST 2010: Chapter 12

  7. Asteroid Families • Japanese astronomer Kiyotsuga Hirayama found in 1917 that some asteroids fall into families • The families are groups with similar orbital characteristics • Each family may have resulted from a breakup of a larger body, or from the collision of two asteroids • Members of each family have similar speeds • There are physical similarities among the larger members of a given family • Several dozen families are found AST 2010: Chapter 12

  8. Asteroid Physical Appearance • The majority of asteroids are very dark • They do not reflect much light • Their reflectivities are only 3 to 4 percent • There is, however, a sizable group that is not very dark • Its typical reflectivities are 15 to 20 percent (similar that of the Moon) • A few asteroids even have reflectivities as high as 60% • To understand the reasons for these differences, astronomers performed spectral analysis of the light reflected by asteroids AST 2010: Chapter 12

  9. Asteroid Classification (1) • The dark asteroids • are believed to be primitive bodies • chemically unchanged since the beginning of the solar system • are composed of silicates mixed with dark, organic carbon compounds • include Ceres, Pallas, and most objects in the outer third of the asteroid belt • Most of the primitive asteroids are classed as C asteroids • C stands for carbonaceous (carbon rich) AST 2010: Chapter 12

  10. Asteroid Classification (2) • The second most populous group is the S asteroids • S stands for a “stony” or silicate composition • They have no dark carbon compounds and hence higher reflectivities • Most S asteroids seem to be also primitive • The third group is the M asteroids • M stands for “metal” • Their identification is difficult • done by radar for the largest asteroids such as Psyche • They are much less numerous • Each may have come from a parent body that had earlier differentiated and later shattered in a collision • There is enough metal in a 1-km M-type asteroid to supply the world with iron for a long period of time AST 2010: Chapter 12

  11. Asteroid Classification (3) AST 2010: Chapter 12

  12. Trojan Asteroids • The Trojans are located far beyond main belt • at ~5.2 AU from the Sun, nearly the same distance as Jupiter • dark, primitive objects, like some other asteroids • They have stable orbits because of Jupiter • In Jupiter’s orbit, there are two points near which an asteroid can stay almost indefinitely • They make equilateral triangles with Jupiter and the Sun • Since their first discovery in 1906, several hundreds have been found • The larger Trojans can be up to ~200 km across AST 2010: Chapter 12

  13. Animation: the Trojans Green circles indicate main-belt asteroids and blue dots on the outermost circle are the Trojans AST 2010: Chapter 12

  14. Asteroids in Outer Solar System • There are asteroids with orbits that carry them far beyond Jupiter • They are hard to detect and only a few have been found • Examples: • Chiron is 200 km across and the largest of them, with a path carrying it from just inside the orbit of Saturn to almost the distance of Uranus • Pholus, with an orbit that takes it 33 AU from the Sun, beyond Neptune, has the reddest surface of any object in the solar system, with unknown composition • They are named after centaurs (mythological half horse, half human) because these objects have some of the properties of both comets and asteroids • In 1988, on its closest approach to the Sun, Chiron’s brightness doubled, much like the comets • Chiron, however, is much bigger than comets AST 2010: Chapter 12

  15. Earth-Approaching Asteroids • Some asteroids may stray far outside the main belt and travel inward along paths that come close to or cross Earth’s orbit • Such asteroids, and other objects that come close to the Earth, are collectively known as Near-Earth Objects (NEOs) • Needless to say, they are of great interest to us • Some of these NEOs have collided with the Earth in the past, and some others are likely to do so in the future • In 1994, a 1-km object passed closer than the Moon • By the end of 2002, more than 640 NEOs larger than 1 km in diameter had been discovered • Astronomers have estimated that there are probably 500 or so NEOs larger than 1 km in diameter that have not yet been found AST 2010: Chapter 12

  16. NEOs • NEOs generally have unstable orbits • Each NEO will meet one of 2 fates: • collide with one of the terrestrial planets — and be destroyed • be ejected gravitationally from the inner solar system after a near-encounter with a planet • The probability for impact is once every 100 million years • Hence the likelihood is very remote than any one of the known NEOs will end up crashing into the Earth in the foreseeable future … • The larger of these impacts will likely generate environmental catastrophes for our planet • This is a good reason for further investigation of NEOs AST 2010: Chapter 12

  17. An NEO Observation • A 5-km-long NEO called Toutatis • approached to within 3 million km of the Earth in 1992 • which is less than 3 times the distance to the Moon • Radar images indicate that it is a double object • consisting of 2 irregular lumps, with diameters of 3 km and 2 km, squashed together • Animation Radar images AST 2010: Chapter 12

  18. Rendezvous with Eros • Eros is an Earth-approaching S-type asteroid • potato-shaped, 34 km long, and 11 km wide • heavily cratered, suggesting that the surface is old • Movies of Eros captured by the NEAR-Shoemaker spacecraft, which orbited and then landed on it in 2000 AST 2010: Chapter 12

  19. Comets • They have been observed since antiquity • Typical comets appear as rather faint, diffuse spots of light • smaller than the Moon and many times less brilliant • They are small chunks of icy material that develop atmospheres as they get closer to the Sun • As a comet gets “very close” to the Sun, the comet may develop a faint, nebulous tail extending far from the main body of the comet • Their appearance is seemingly unpredictable • Comets typically remain visible for periods from a few days to a few months AST 2010: Chapter 12

  20. Comet Orbits • The scientific study of comets dates back to Newton who first recognized that their orbits were very elongated ellipses • Edmund Halley (a contemporary of Newton) in 1705 calculated/published 24 cometary orbits • He noted that the orbits of bright comets seen in 1531, 1607, and 1682 were quite similar — and could belong to the same comet — returning to the perihelion every 76 years • He predicted a return in 1758 • When the comet did appear in 1758, it was given the name Comet Halley AST 2010: Chapter 12

  21. Comet Halley • It has been observed and recorded on every passage near the Sun at intervals from 74 to 79 years since 239 B.C. • The period variations are caused by the jovian planets • In 1910 the Earth was brushed by the comet’s tail, causing much needless public concern • Its last appearance in our skies was in 1986 • met by several spacecraft • It is predicted to return in 2061 • Its nucleus is approximately 16 x 8 x 8 km3 AST 2010: Chapter 12

  22. Comet Census • Records exist for ~1000 comets • Comets are discovered at an average rate of 5 to 10 per year • Most of them are visible only on photographs made with large telescopes • Every few years, a comet may appear that is bright enough to be seen with the naked eye • Recent flybys: • Comet Hyakutake, with a very long tail, was visible for about a month in March 1996 • Comet Hale-Bopp appeared in 1997 AST 2010: Chapter 12

  23. Comet Components (1) • Nucleus: relatively solid and stable, composed mostly of ice and gas, with a small amount of dust and other solids • Coma: a dense cloud of water, carbon dioxide, and other neutral gases sublimed off of the nucleus • Hydrogen cloud: a huge (millions of km in diameter), but very sparse, envelope of neutral H gas • Dust tail: up to 10-million km long, composed of smoke-sized dust particles driven off the nucleus by escaping gases • This is the most prominent part of a comet to the unaided eye • Ion tail: as much as several-hundred-million km long, composed of plasma and laced with rays and streamers caused by interactions with the solar wind AST 2010: Chapter 12

  24. Comet Components (2) ion tail dust tail AST 2010: Chapter 12

  25. Nucleus and Coma of Comet • The nucleus is composed of ancient ice, dust, and gaseous core material • The nucleus has low gravity • It cannot keep dust and gas from escaping • The coma is the bright head of the comet, as seen from the Earth • The coma is a temporary atmosphere of gas and dust around the nucleus • The coma is 100,000's of kilometers across Halley's nucleus Halley's coma AST 2010: Chapter 12

  26. Nucleus of Comet Wild 2 The images were captured by NASA's Stardust spacecraft AST 2010: Chapter 12

  27. Comet’s Ion Tail • The Sun spews out charged particles • This solar wind occurs along the solar magnetic-field lines, extending radially outward from the Sun • Ultraviolet (UV) sunlight ionizes gases in the coma • These ions (charged particles) are pushed by solar-wind particles along field lines to form a tail millions of km long • The blue ion tail acts like a "solar" wind-sock • The tail always points directly away from the Sun because the ions move at very high speed • When the comet is moving away from the Sun, the comet’s ion tail will be almost in front of it! • The blue color is mostly from the light emitted by carbon-monoxide ions, but other types of ions also contribute to the light AST 2010: Chapter 12

  28. Origin and Evolution of Comets • Comets originate from very great distances • The aphelia of new comets are typically ~50,000 AU • This clustering of aphelia was first noted by Dutch astronomer Jan Oort in 1950 • He then proposed an idea for the origin of those comets, which is still accepted by most astronomers today • Oort’s model of comet origin: • The Sun’s sphere of influence extends only a little beyond 50,000 AU, or about 1 LY • Objects in orbit about the Sun at this distance can be easily perturbed by the gravity of passing stars • The comets are some of the perturbed objects, which take on orbits that bring them much closer to the Sun • The reservoir of ancient icy objects from which such comets are presumably derived is called the Oort comet cloud AST 2010: Chapter 12

  29. AST 2010: Chapter 12

  30. Oort Comet Cloud • Astronomers estimate that there may be about a trillion (1012) comets in the Oort cloud • In addition, 10 times this number of comets could be orbiting the Sun between the planets and the Oort cloud • Such cometary objects remain undiscovered probably because they are too faint to be seen directly and because their stable orbits do not bring them closer to the Sun • The total number of comets within the sphere of influence of our Sun could therefore be on the order of ten trillion (1013)! • Their total mass would be similar to that of 1000 Earths • Cometary material could thus be the most important constituent of the solar system after the Sun itself AST 2010: Chapter 12

  31. Kuiper Belt • Another possible source of comets lies just beyond the orbit of Neptune • The existence of this region was first suggested by Gerard Kuiper in 1951 • The first object from this region, now called the Kuiper belt, was discovered in 1992 • The object is ~200 km across • Since then, several hundred more Kuiper-belt objects (KBOs) have been found • It appears that these KBOs are heavily influenced by the gravity of Neptune • Many of the known KBOs have orbits like that of Pluto • Some astronomers have, therefore, suggested that Pluto can be considered the largest member of the Kuiper belt • For this reason, KBOs are sometimes called plutinos AST 2010: Chapter 12

  32. Fate of Comets • Most comets probably spend nearly all their existence in the Oort cloud or Kuiper belt • at a temperature near absolute zero • But once a comet enters the inner solar system, its life likely changes dramatically! • If it survives the initial passage near the Sun, it will return towards the cold aphelion • and may follow a fairly stable orbit for a “while” • It may impact the Sun, or be completely vaporized as it flies by the Sun • It may interact with one or more planets with 3 possible fates: • destroyed after impacting a planet • speeded up and ejected, leaving the solar system forever • perturbed into an orbit of shorter period • Each time it approaches the Sun, a comet loses part of its material • It may end its life catastrophically by breaking apart AST 2010: Chapter 12

  33. Comet Shoemaker-Levy 9 • Some comets die very spectacularly • When comet Shoemaker-Levy 9 passed close to Jupiter in July 1992, the comet broke into about 20 pieces • perhaps due Jupiter’s tidal forces • Fragments of the comets then orbited Jupiter until July 1994 when they crashed into Jupiter, experiencing violent destruction • Animation AST 2010: Chapter 12

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