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Asteroids
Irregular (sometimes spherical) lumps of rock and metal that had
never formed into planets during the formation of the solar system
Several hundred thousand asteroids have been observed in the
10km to 100km size range, there maybe millions more of about
1km in size
26 are larger than 200km, the biggest is Ceres (933 km) and
accounts for 25% of the mass of all the asteroids, in fact the total
mass of all the asteroids is less than the mass of the Moon
75% are categorised as C-type (dark, carbon-rich material)
17% are S-type (bright, nickel-iron / magnesium silicate)
Rest are M-type (very bright, pure nickel-iron)
Asteroid Orbits
Main Belt – covering 2-4 AU between orbits of Mars and Jupiter
Near Earth Asteroids (NEA) – asteroids that have orbits that pass
close to the Earth (including Amors, Atens, Apollos)
Trojans – asteroids that orbit with Jupiter at the L4, and L5
Lagrangian points
A crater on asteroid Eros
– observed by the
NEAR asteroid lander
Asteroid Ida has its own
moon, Dactyl. Observed by
the Galileo space probe.
3D construction of asteroid
Eros from NEAR observations
Observing Asteroids
Long exposure photographs reveal asteroid trails from which
asteroid orbits can be calculated. Since asteroids are irregularly
shaped their brightness will vary as they rotate.