The Earth collides with and accretes about 10,000 tons of dust particles per year as it orbits the Sun. This dust ranges in size from 10 microns to 1 millimeter, and it is identified as the fragments of comets and asteroids. Deposition in the oceans of the world has been documented by the Deep Sea Drilling Project. This material is obviously a significant source of material to Earth’s biogeochemical processes.
Each year dozens of asteroids are discovered through improved telescope techniques. Many of these asteroids are determined to have Earth-crossing orbits. Given enough time—thousands or tens of thousands or millions of years—some of these asteroids are predicted to collide with Earth. In March and early April of this year, for example, we experienced the spectacular passage of a comet (Hyakutake) to within 9 million miles of Earth.
Asteroids pose a greater danger to life on Earth than do comets. Collisions in the main belt are common events. Asteroid fragments are thrown to the farthest reaches of the Solar System or toward the inner Solar System—the terrestrial planets and the Sun. Comets have much greater orbital speeds than do asteroids, and they too produce cataclysmic collisions with Earth. Some, such as Halley’s Comet, orbit the Sun in the opposite direction from the Earth, thereby enhancing the magnitude of impact with Earth. The 1908 explosion of a small comet or asteroid over the Tunguska River region of Siberia resulted in the devastation of more than a thousand square kilometers of forest. This was a small event compared with the total range of possibilities for collisions. Large asteroids enter the inner Solar System every 10 million years and persist for as long; those 1 km diameter and larger pose the greatest risk to life (1000-2000 are in near-Earth orbits; these collide with Earth once every 300,000 years). Perhaps 100,000 Near-Earth Objects (NEOs) of 100 meters diameter come inside Mars’ orbit today.
Oort Cloud comets can be dislodged from their orbits by Alpha Centauri and other nearby stars and enter into highly elliptical, planet-crossing orbits, leading to possible collisions. Kuiper belt comets far from the jovian planets can reside there for billions of years, while those closest to the jovian planets will have their orbits disrupted and be ejected from the Solar System sling-shot fashion or drawn to the inner Solar System. Neptune’s gravity slowly erodes the inner edge of the Kuiper belt at 40 AU, pulling comets into the inner Solar System. Comets may persist in near-Sun orbits for 500 orbits before exhausting themselves; when exhausted, they may resemble or become asteroids. Collision of a comet or asteroid with Earth would result in global disruption: nuclear winter, large dust clouds exploding into the atmosphere and obscuring the Sun, prolonged darkness, subzero temperatures, violent windstorms, enormous fireballs, ejection of rocks and steam into the atmosphere, shock of the Earth’s crust, resultant earthquakes; volcanic activity, global tsunamis, total darkness lasting for months, acid rainfall, settling of dust and creation of a several centimeter layer of sediment over the Earth’s surface, and extinction of a very large percentage of species. Several of the bibliographic references give detailed accounts of the potential effects a collision would have on climate and life.