Weblecture
And Easter was on the fourteenth day before the Kalends of May. Then it happened that all through England such a sight in the heavens was seen as no man had seen before. Some men said that it was the star Comet, that some men call the long-haired star; it appeared on the even of Letania Maior, that is the eighth day before the Kalend of May, and so shone for all seven nights.
— Unknown, Anglo Saxon Chronicle, 1066
This is where we get to discuss the objects that don't fit into the groups we've already discussed: planets, their moons, and trans-Neptune dwarf planets. The asteroids and comets are solar system bodies have not cleared their areas of planetesimals, and they are not large enough to collapse into a sphere under their own gravity (with the exception of Ceres).
Take the time to look through the following table, which collects information from a number of sources and allows you to compare the discovery and composition of three classes of asteroids and two types of comets.
Main Belt Body Ceres Dwarf Planet |
Main Belt Bodies (Vesta, Pallas, Hygiea) |
Apollo asteroids ~ 240 Near-Earth Objects crossing E and and Earth's orbit Amor asteroids: ~2000 Near-Earth Objects not crossing Earth's orbit |
Short-period Comet Tempel 1 |
Intermediate period Halley's Comet |
Naked Eye Observation | ||||
Discovered in 1801 by Giuseppe Piazzi. Size: 975*909km |
Discovered by ground-based telescopes (Pallas - 1802, Juno - 1804, Vesta - 1807) | Apollo discovered 1832; not seen again until 1973. Largest: 1866 Sisyphus, 10km in diameter. | Discovered by William Tempel, 1867. Lost due to orbital perturbations by Jupiter which changed periodicity; rediscovered/identified in 1972. | Period determined by Edmund Halley in 1705; corresponds to recorded comet observations as early as 240BC. |
Cuneiform tablets recording the appearance of Halley's Comet around 166 A.D. | ||||
Exploration | ||||
NASA - Dawn launched 9/27/2007; will reached Ceres in February 2015 | Dawn will reach Vesta July 2011 | Hayabusa (Japanese) mission to Apollo asteroid Itokawa, 2005, landed and was directed to collect samples, which were returned to Earth in 2010. | Deep Impact NASA mission 2005, photographed comet and sent impactor to surface | Multiple probes called the Halley armada during 1986 closest pass to Earth (Vega 1 & 2, Giotto, Suisei, Sakigake). US studies delayed by the Space Shuttle Challenger disaster. |
Orbital and Rotational Characteristics | ||||
Orbital period: 4.60 years Small axial tilt(< 4°) Eccentricity: 0.08 Inclination: 10.5° |
Vesta-orbital period: ~ 3.36 years 5.34hr rotation Eccentricity: 0.09 Inclination: 7.135° |
Orbital period: ~ 1 year | Orbital period: 5.5 years Eccentricity: 0.517 Inclination: 10.5° |
Orbital period: 76 years Rotational period varies depending on outgassing Eccentricity: 0.967 Inclination: 162.3° |
Use NASA's NEO Program to plot the orbit of any asteroid or comet | ||||
Magnetosphere | ||||
No asteroids have geothermal activity necessary to create a dynamo that can sustain a magnetic field. | Comets appear to have induced magnetospheres from the interaction of the ionic tail with the solar wind. | |||
Atmosphere | ||||
Possible oxygen/water vapor atmosphere from ice evaporation and outgassing. | No other asteroids have sufficient mass to retain atmospheres. | Dust and ionic tail | Coma primarily vaporized water, carbon monoxide, and carbon dioxide. | |
SurfaceClick on the picture to learn more about the object! | ||||
Possible cratering; apparent water-bearing minerals. Bright spot of unknown cause. | Dry; cratered with distinctive light/dar areas, lava basins and large impact crater (cuts through crust to mantle) | Tend to be cratered and fractured, with no ice due to proximity to sun. | Icy, covered with dust debris: silicates, carbonates,smectite, carbon and hydrocarbons. | Dusty, non-volatile materials |
Core | ||||
Based on rotation data, assumed terrestrial structure (rock core, ice mantle, dust crust). | Highly similar to earth; rocky crust, olivine mantle | Various types occur, depending on iron, carbonaceous, icy, or olivine deposits: Itokawa is considered an S-group asteroid, with a primarily siliceous or stony composition | Primarily ice, consistency of a snow bank. | Ices, including water, carbon dioxide, and ammonia, and rocky bits. |
Formation | ||||
Gravitational collapse of local materials | Possible fragmentation of existing planetesimal | Possible fragmentation of existing planetesimal; composition depends on origin of fragment in original planetesimal (crust = rocky/icy, mantle = olivine, core = iron) | Neptune Oort Cloud | Neptune Oort Cloud, perturbed by Jupiter |
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