Impact? The Earth-Crossing Asteroids

Most of the asteroids in the asteroid belt remain there, but some have highly eccentric orbits that take them out of the asteroid belt and across the orbital path of the earth (as well as the paths of other terrestrial planets). Nearly 100 of these so-called Apollo asteroids have been identified so far, and a number of astronomers advise funding efforts to identify and track even more, because the potential for a collision with Earth is real. With advance warning, scientists believe, missiles with thermonuclear warheads could be exploded near an incoming asteroid, sufficiently altering its course to make it avoid the earth, or shattering it into a large number of smaller asteroids. You’re local movie theater or video store is a good source to study Hollywood’s take on these nightmare scenarios, but they are a very real threat. Project NEAT (Near Earth Asteroid Tracking) is funded by NASA. For more information see neat.jpl.nasa.gov.
It is believed that a few asteroids of more than a halfmile diameter might collide with the earth in the course of a million years. Such impacts would be disastrous, each the equivalent of the detonation of several hydrogen bombs. Not only would a great crater, some eight miles across, be formed, but an Earth-enveloping dust cloud would darken the skies. It is thought that the great extinction of dinosaurs 65 million years ago was due to such an impact. Were the impact to occur in the ocean, tidal waves and massive flooding would result.
Earth impacts of smaller objects are not uncommon, but on June 30, 1908, a larger object—apparently the icy nucleus of a very small comet—fell in the sparsely inhabited Tunguska region of Siberia. The falling object outshone the sun, and its explosive impact was felt at a distance of more than six hundred miles. A very wide area of forest was obliterated—quite literally flattened. Pictures from the time show miles of forest with trees stripped and lying on their sides, pointing away from the impact site.

Rocks and Hard Places

Asteroids are composed of stony as well as metallic materials—mostly iron—and are basically tiny planets without atmospheres. Some asteroids have a good deal of carbon in their composition as well. These, called carbonaceous chondrites, are thought to represent the very first materials that came together to form the objects of the solar system. Carbonaceous chondrites are truly the solar system’s fossils, having avoided change for billions upon billions of years.
Earlier astronomers surmised that asteroids were fragments resulting from various meteoric collisions. While some of the smaller meteoroids were likely produced this way, the major asteroids probably came into being at the time of the formation of the solar system as a whole. Theoretical studies show that no planet could have formed at the radius of the asteroid belt (at about 3 A.U. from the sun). The region between Mars and Jupiter is dominated by the gravitational influence of the giant planet Jupiter. These forces stirred up the potential planet-forming material, causing it to collide and break up instead of coming together to create a planet-sized object. The smaller asteroids come in a wide variety of shapes, ranging from nearly spherical, to slab-like, to highly irregular.
During 1993–1994, the Galileo probe passed through the asteroid belt on its way to Jupiter and took pictures of an asteroid orbited by its own miniature moon. Potato-shaped, the asteroid was named Ida, and is about 35 miles (56 km) long, orbited at a distance of roughly 60 miles (97 km) by a rock less than 1 mile in diameter. This little moon is the smallest known natural satellite in the solar system.

The Asteroid Belt

Astronomers have noted and cataloged more than 6,000 asteroids with regular orbits, most of them concentrated in the asteroid belt, between the orbits of Mars and Jupiter. So far, every asteroid that has been noted orbits in the same direction as the earth and other planets—except one, whose orbit is retrograde (backward, or contrary to the direction of the planets). Although the asteroids move in the same direction—and pretty much on the same plane—as the planets, the shape of their orbits is different. Many asteroid orbits are more eccentric (the ellipse is more exaggerated and oblong) than those of the planets.
In early 2001, an asteroid-exploring probe orbited and finally landed on the surface of Eros. As it (slowly) crashed to the asteroid’s surface, it sent back tantalizing close-up images of the surface.