ESA Science & Technology05-Jul-2005 16:49:45

Asteroid Encounters

Families and Gaps

Asteroids in the main belt are not evenly distributed. The gravitational influence of the planets, particularly giant Jupiter, causes them to collect in groups or 'families'. One of the largest of these is the Flora family, which has about 400 known members.

Other parts of the main belt, known as Kirkwood gaps after their discoverer, are almost empty, due to so-called gravitational resonances. This means that regular gravitational interactions with Jupiter enable the planet to sweep the region clear of asteroids. One very apparent gap occurs at 2.5 AU where an asteroid completes three orbits for every one orbit by Jupiter. This is known as a 3:1 resonance.

Substantial numbers of asteroids also exist outside the main belt. Some of them are thought to be burnt-out comets that have lost much of their water ice after numerous close flybys of the Sun.

Others have been trapped by the gravity of the planets so that they follow unusual orbits. For example, over 200 asteroids share the orbit of Jupiter. These Trojan asteroids are trapped in two groups, one which follows 60 degrees behind the planet and another which is 60 degrees in front.

At least one small object, 3753 Cruithne, is known to travel in a strange horseshoe-shaped orbit around the Earth.

Many thousands of asteroids also inhabit the inner Solar System. The main groups are the Amors, which travel between the Earth and Mars; the Apollos, which cross Earth's orbit; and the Atens, which stay mainly inside Earth's orbit.

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