As of today the count of known asteroids is 444,080, according to tabulations by the International Astronomical Union's clearinghouse for such matters, the Minor Planet Center in Cambridge, Massachusetts. The total number of finds is growing by about 10% per year.
Yet today the realm between Mars and Jupiter is an interplanetary ghost town compared to the close packing it used to have. Recent calculations suggest that the asteroid belt has lost perhaps 99.9% of the mass it had 4½ billion years ago. Almost all of it vacated in the first 100 million years of solar-system history due to early run-ins with the planets and with massive super-asteroids that stirred up among the locals before themselves being ejected.
One ejection mechanism, first recognized by Daniel Kirkwood in 1867, involves orbital resonances. For example, asteroids circling at an average of about 2½ a.u. from the Sun complete three orbits in the time it takes Jupiter to circle once. This 3:1 coupling creates repetitive tugs on the asteroids by Jupiter that perturb them into eccentric and ultimately unstable orbits. What's left behind is an asteroid-free zone at that orbital slot.
This 3:1 location, along with others corresponding to 5:2, 7:3, and 2:1 orbital resonances with Jupiter, are now called Kirkwood gaps.
The primordial demolition derby might still have left us with 10 times more asteroids than we see today, were it not for positional shuffling among the giant planets that took place about a half billion years the solar system formed. This orbital migration, first postulated by theorist Renu Malhotra in 1993, resulted from countless close encounters with small objects.
Here's how it works: Every time Jupiter heaves a small body out to the Kuiper Belt or beyond, the planet itself shifts infinitesimally closer to the Sun. Do this to a trillion times, and the laws of physics say that Jupiter simply must move elsewhere.
"This migration happened, and you ought to see evidence of it in the asteroid belt," notes David Minton, a graduate student at the University of Arizona. He and Malhotra used high-performance computers to model how migrating planets (and their migrating resonances) sculpted the population and distribution of asteroids we see today. They assumed that Jupiter moved inward by 0.2 astronomical unit (about 18 million miles) and Saturn, Uranus, and Neptune outward by an estimated 0.8, 3.0, and 7.0 a.u., respectively.
Their results, published recently in Nature, show that the distribution of the 690 largest asteroids is a good match to that resulting from migrating planets and a poor match had Jupiter and Saturn not moved at all. In particular, they find evidence that the inward-moving resonances greatly thinned the ranks around the Kirkwood gaps.
Moreover, the main belt's inner edge, roughly 2.1 a.u. from the Sun, marks the final location of a powerful resonance called ν6 that's associated with the precession of Saturn's orbit. Highly sensitive to the ringed planet's orbital location, the ν6 resonance swept through the entire belt like a snowplow, scattering untold millions of objects.
As Minton and Malhotra point, this "great escape" coincides with a pulse of intense inner-planet bombardment that left the Moon with the vast majority of the craters seen today.