Discovering Pluto by Dale P. Cruikshank

Author:Dale P. Cruikshank
Language: eng
Format: epub
ISBN: 9780816538317
Publisher: University of Arizona Press

Asteroids

While the comets consist largely of ice, the asteroids, which have long been regarded as distinct from comets, have a mostly unexplored association with ices of any kind. Why are they thought of as different from comets? The first few thousand asteroids discovered, starting with the discovery of Ceres in 1799, unlike the comets, were all seen to be in roughly circular orbits and largely confined to the zone between Mars and Jupiter—the asteroid belt. They tumbled along their paths around the Sun without showing any signs of comet-like activity. This picture is now obsolete.

The discovery in recent years of wholesale quantities—tens of thousands—of asteroids by patrol telescopes and spacecraft shows that a vast number of the smaller ones (a few hundred meters to a few kilometers in size) travel paths that cross the orbits of Mars, Earth, and Venus, and thereby pose the risk of a planetary collision.26 So comets are not the only hazard in our region of the Solar System. The population of Mars- and Earth-crossing asteroids is large now—more than fifty thousand estimated—but was much larger in the early days of the Solar System. Many of them have already been swept up by the inner planets, as the crater-scarred surface of the Moon faithfully records. Chips and fragments from violent collisions between the asteroids frequently fall to Earth as meteorites, providing free samples from which we can deduce their mineralogical composition. The break-up and fall of a piece of an asteroid the size of a bus in February 2013 near the Siberian city of Chelyabinsk is a recent example of a noteworthy meteorite strike on planet Earth. Window glass shattered from the blast wave as the meteor entered the atmosphere, sending a thousand people to hospitals, and at least one building collapsed.

Some meteorites also contain complex organic matter that has been sequestered in the parent asteroids since their formation during the origin of the Solar System. These organics and some minerals found in meteorites bespeak an ancient environment on the asteroid in which liquid water was present; and some contain small percentages of water trapped in their minerals even today. But if the present-day asteroids are mostly just dry rocks, they do not really bear on the question of ices or the delivery of water to the ancient Earth. Or do they? In 2006, Henry Hsieh and David Jewitt announced their finding of “activity” associated with an object formerly considered an asteroid because of its orbit. Since then, a handful of other objects that would normally be classified as asteroids have shown surrounding hazy dust clouds, and in some cases a tail-like projection. Not all these “active asteroids” are comets in the sense of evaporating ices because other phenomena could cause them to shed a puff of dust, but the presence of ices in at least some asteroids now seems to be generally accepted, although the definitive observations are still lacking. Still, the amount of water present in asteroids’ water-bearing minerals make them a reasonable potential

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