12. Asteroids and meteorites
Origin of the asteroids
In the past, there have been two extreme theories for the origin of asteroids. According to the first, the asteroids represent the fragments of a former planet that has been torn apart. The second theory proposes that the asteroids are the pieces of a planet that never formed. Today, astronomers favor a theory that lies between the extremes. It is now known that the combined mass of all asteroids is far too small to make up a major planet. If all the known asteroids were brought together, they would create a body less than five percent the mass of the Moon. So the first extreme must be discarded. On the other hand, the second extreme can also be excluded because there is strong evidence that many of the asteroids were once collected into a relatively small number of slightly larger parent bodies.
Remnants of a planet that never formed
Why did the asteroids fail to coalesce into a single planet? It is likely that gravitational forces from the rapidly forming and massive Jupiter took charge of its neighborhood, stirring it up and keeping the original asteroids from growing too large. Numerous asteroids in resonant orbits with youthful Jupiter probably permeated the region of the asteroid belt, between 2 and 4 AU. Chaotic zones in the vicinity of these resonances would have pumped up the eccentricities of initially circular orbits, flinging the resident bodies into elongated and inclined orbits, accelerating them to high velocities, and causing them to crash into each other. The colliding objects would be moving too fast to stick to each other. Instead, they would break apart into fragments.
A lifetime of catastrophic collisions
Encounters among the earliest asteroids became increasingly violent as Jupiter stretched and twisted their orbits into eccentric and inclined orientations. These orbits criss-crossed each other, resulting in violent collisions at the place where they meet. Instead of continuing to grow, the largest asteroids were chiseled and blasted apart by mutual collisions. So the original asteroids never grew larger than about 1 million meters across, and they never accumulated into a major planet. The pulverized remnants of these former worlds became the present asteroids, often orbiting the Sun in families with common orbital characteristics and spectral properties.
The Japanese astronomer Kiyotsugu Hirayama (1874-1943) discovered asteroid families. He noticed that groups of main-belt asteroids share very similar orbits, suggesting that they are the broken fragments of larger objects. Hirayama called each group a family because he believed the members shared a common origin as the children of a bigger parent body. He also named a number of families after their largest member asteroids, such as the Eos, Koronis and Themis families.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University