Small Bodies: Profile

Asteroids

Definition Minor planets, small rocky bodies.
Composition Leftover material from our solar system.
Largest asteroid 1 Ceres
Total discovered to date Over 250,000 designations have been assigned by the Minor Planet Center; over 100,000 have been observed more than once.
Total numbered asteroids to date. Over 30,000.
Near-Earth asteroids Around 1.0 AU from earth, with perihelia less than or equal to 1.3 AU.
  • Aten group
  • Innermost, semi-major axis less than 1.0 AU, perihelion greater than 0.983 AU.
  • Apollo group
  • Semi-major axis greater than 1.0 AU, perihelion less than 1.017 AU.
  • Amor group
  • Semi-major axis greater than 1.0 AU, perihelion between 1.017 and 1.3 AU.
Main Belt Asteroids Located between orbits of Mars and Jupiter (1.8 to 4.0 AU) in the "asteroid belt".
Trojan Asteroids Two groups of asteroids located along the orbital path of Jupiter, 60 degrees ahead and behind the planet.
Centaurs Asteroids in the outer solar system having orbits between Mars and beyond Neptune.
Type Classification Based on albedo (reflectivity), and color or spectra. Most common are C, S, and M.
C-type Very dark (albedo=0.03), flat spectra, grayish in color.
S-type Moderately bright (albedo=0.10-0.22), spectra with moderate to strong absorption bands, greenish to reddish in color.
M-type Moderately bright (albedo=0.10-0.18), spectra with few absorption bands, reddish in color.

Comets

Definition Icy planetesimals formed in the outer solar system.
Composition Mainly ice and dust
Orbits Highly elliptical, taking them very close to the Sun and back out into deep space, often far beyond the orbit of Pluto.
Orbit duration Less than 200 years to more than several millions of years.
  • Short-period
  • Orbits range from a few years (Encke, Chiron) to about 200 years. Referred to as "periodic".
  • Long-period
  • Gravitationally bound, with orbits over 200 years.
  • Single-apparition
  • Parabolic and hyperbolic orbits (not bound).
Periodic comets Those with short orbit durations
Nucleus Main body of the comet, composed of dust particles trapped in a mixture of ices of water, carbon monoxide, carbon dioxide, methane, and ammonia. Typically only a few kilometers in diameter.
Coma A halo of evaporated gas and dust which forms when the comet approaches the inner solar system. The coma grows larger as the comet gets closer to the sun.

Meteorites

Definition Meteorites are pieces of solar system debris that have fallen to Earth from space. Prior to reaching Earth's surface, a meteorite is called a "meteor" or "meteoroid.")
Composition Rock or metal
Size Most meteorites are tiny, smaller than grains of sand, and vaporize as they pass through the earth's atmosphere. Larger meteorites may be as great as a few miles wide, such as one 65 million years ago that may have wiped out the dinosaurs. The largest intact meteorite found in the USA is the Williamette Meteorite, at 15 and a half tons.

Trans-Neptunian Objects

Definition Trans-Neptunian objects (TNOs) are any objects in the solar system that have an orbit beyond Neptune. Pluto is a trans-Neptunian object; another of the named Trans-Neptunian Objects is Varuna.
Total known There are estimated to be perhaps 70,000 TNOs, each at least 100 km across, between 30 and 50 astronomical units from the Sun.