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The Asteroid Belt

© Copyright 2001, Jim Loy

On the left we see 24 Hubble Space Telescope images of the minor planet Vesta (as it rotated). Click here or on the picture to see a larger copy of the picture.

The Asteroid Belt is between the orbits of Mars and Jupiter. "Asteroid" means "star-like object," and that is what they appear like when viewed through a telescope. But of course, they are not anything like stars. "Minor Planet" is the more appropriate alternate name, as they do orbit the sun, much like the other planets. They vary in size from a few meters to hundreds of kilometers. Ceres, the largest minor planet, and the first to be discovered (by Giuseppe Piazzi, in 1801), is spherical and 940 km in diameter. Smaller minor planets are irregular in shape. Here is a list of the 20 largest minor planets (The first number shows the order in which they were discovered):

Europa is also the name of a moon of Jupiter. There are about 9000 known (numbered) minor planets, and nowadays about 30 are discovered every month. At one time, it was thought likely that spacecraft would have to leave the plane of the planetary orbits (the ecliptic) in order to avoid the asteroid belt. But all space probes which have pass through the belt have done so without any collisions. The asteroid belt is so sparsely populated by large objects that it is likely that a manned spacecraft would pass through without anyone seeing a minor planet. The asteroid belt is where most meteors come from, although many meteors do come from comets.

Minor planets come in families. In other words, there are parts of the asteroid belt which contain no minor planets, and there are parts which contain many minor planets. Any family of asteroids shares the same distance (semimajor axis, actually) from the sun. The main cause of these families is the gravitational pull of Jupiter. For example, a minor planet with a semimajor axis of 2.5 AU (1 AU is the semimajor axis of the Earth) would encounter Jupiter in the same direction, in the same part of its orbit, repeatedly, a kind of resonance between Jupiter and the minor planet. This would push the minor planet toward the sun, or pull it away from the sun, and change its orbit. And so, there are very few minor planets with semimajor axes of 2.5 AU. There are other gaps in the asteroid belt. Two unusual families of minor planets are the Trojan Asteroids. These are the same distance from the sun as Jupiter. These minor planets are near the two Lagrangian points, one of which orbits ahead of Jupiter by 60 degrees (the Sun, Jupiter, and this Lagrangian point form the vertices of an equilateral triangle), and the other follows Jupiter by 60 degrees. There are 416 known Trojan Asteroids.

Minor planets are classified according to what they are made of. They vary from rock to metal to organic compounds, as do meteorites. At least one minor planet (243 Ida) has a small moon (Dactyl) orbiting it (see the Galileo spacecraft photo on the left).

A few minor planets come near the Earth. This is of some concern to us, as a minor nudge to their orbits (by passing near another planet) may cause a collision between Earth and one of these minor planets. 65 million years ago, the dinosaurs were apparently wiped out by such a collision. Such destructive events are rare, and (at the moment) nothing much can be done to prevent a catastrophe.

Planets (including minor planets) formed by accretion of smaller pieces that collided and stuck together. The minor planets never formed into a larger planet, because of the pull of Jupiter's gravity. There were probably many more minor planets early in the history of the Solar System, most of which were flung out of the Solar System by close encounters with major planets.

See The Titius/Bode Law.

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