For many people, asteroids are big rocks that drift menacingly through space and are great places to have a laser cannon dogfight. Conventional scientific wisdom holds that they are the leftover scraps of planet formation. Their full story, though, is rather more complex and still only dimly glimpsed. What planetary scientists lump together as asteroids are far too diverse—from boulders to floating heaps of gravel to mini planets with signs of past volcanic activity and even liquid water—to have a single common origin.
Only the largest, more than about 100 kilometers across, date to the dawn of our solar system 4.6 billion years ago. Back then, the system was basically one big swarm of asteroids or, as researchers call them at this early stage, planetesimals. How it got that way is a puzzle, but the leading idea is that primordial dust swirling around the nascent sun coagulated into progressively larger bodies. Some of those bodies then agglomerated into planets; some, accelerated by the gravity of larger bodies, were flung into deep space; some fell into the sun; and a tiny few did none of the above. Those survivors linger in pockets where the planets have left them alone, notably the gap between the orbits of Mars and Jupiter. Gradually they, too, are being picked off. Fewer than one in 1,000, and perhaps as few as one in a million, of the asteroids originally in the main belt remain.
Smaller asteroids are not relics but debris. They come in an assortment of sizes that indicate they are products of a chain reaction of collisions: asteroids hit and shatter, the fragments hit and shatter, and so on. Some are rocky; some are metal—suggesting they came from different layers within the original bodies. About a third of asteroids belong to families with similar orbits, which can be rewound in time to a single point in space, namely, the location of the collision that birthed them. Because families should disperse after 10 million to 100 million years, asteroid formation by collision must be an ongoing process.
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Indeed, so is planet formation. Whenever an asteroid hits a planet, it helps to bulk it up. Asteroids are not the leftovers of planet formation so much as they are the finishing touches.