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"Asteroid" is from the Greek αστηρ for "star".

An asteroid is a carbonaceous, rocky, or metallic body orbitting the Sun, but smaller than a planet. An asteroid differs from a comet in that it is inert, its regolith (loose surface material) does not boil with volatile gases when passing near the sun, and in having a nearly circular orbit. There are millions of asteroids and most of them are found in the toroidal (doughnut shaped) asteroid belt between the orbits of Mars and Jupiter. The largest asteroid is Ceres,[1] which is roughly one-fourth the diameter of the Earth's moon, although many astrophysicists now classify the object as a dwarf planet.[2]

Asteroid families

Many of the bodies of the asteroid belt have orbits with no obvious relationship to the orbits of other asteroids, but about a third of them can be grouped into one of several tens of "asteroid families" that share similar orbital characteristics and spectral properties. The orbit of any asteroid will change slowly due to a combination of gravitational and radiation effects in a statistically predictable way. In addition, a collision with another asteroid can destroy both of them. Calculations show that the members of family will remain distinguishable from the background for a period on the order of a billion years. The same calculations that determine the future behavior of an asteroid family can be used to approximate its past behavior. Such calculations applied to observed asteroid families tend to converge to a cluster at some time in the past. The result varies from family to family but usually is at least several million years. Recently a group of four asteroids was discovered whose orbits, when extrapolated into the past, converged at 450±50 thousand years.[3] Previously, the shortest divergence time calculated for an asteroid family was 5.80±0.02 million years for 13 asteroids belonging to the Karin family.[4] Secular astronomers interpret such dates as being when the individual objects of a family originated from a collision.


  1. Asteroids, Nine Planets web-site.
  2. Dwarf Planet Ceres Views of the Solar System web-site.
  3. David Nesvorny, David Vokrouhlicky, and William F. Bottke. The Breakup of a Main-Belt Asteroid 450 Thousand Years Ago. Fri. 9th June, 2006Fri. June 9th, 2006, Vol 312, Science.
  4. The History of a Asteroid Family. September 21, 2011 By Natural Historian.
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