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How do asteroids and comets move in space?
Asteroids are essentially gigantic chunks of rock floating in space. They range in size from a few feet to several miles in diameter. Many asteroids orbit the Sun. Most are located in an area between the orbits of Mars and Jupiter known as the asteroid belt. There are at least 30,000 of these giant boulders in this area, each in it own individual orbit around the Sun. The asteroid belt is somewhat of a mystery. It seems there should be a planet here in this large gap instead of asteroids. Many astronomers believe that billions of years ago a large, rocky planet may have existed here. It may have been blasted apart by some type of cataclysmic event, such as a collision with a large planetoid or even another planet. Another theory states that these asteroids may have formed from the primordial material left over from the formation of the Solar System. The intense gravity of Jupiter may have prevented this material from coalescing into a planet. The asteroids in the asteroid belt range in size form just a few hundred feet to several miles. The largest, Ceres, is over 630 miles in diameter. Other asteroids have highly elliptical orbits that take them very close to the Sun. Occasionally, collisions and gravitational close encounters can push asteroids out of the asteroid belt and send them careening inward towards the Sun. Occasionally, some of these particles may come close enough to the Earth to get caught by its gravity. They may enter the atmosphere as meteors. If they survive the heat of entry and strike the ground, the will be referred to as meteorites. Scientists can study meteorites to learn more about the asteroids from which they originated.

Comets are essentially large balls of rock and ice. Many astronomers refer to them as "dirty snowballs" or "icy mudballs" because that is exactly what they look like. The ice that forms a comet can consist of both water ice and frozen gases. As comets make their way along their orbits, they leave trails of material behind. If the Earth happens to pass through one of these trails, the result will be a meteor shower. There are several meteor showers every year that can be attributed to comet debris. As the particles enter the atmosphere, they leave bright streaks of hot, vaporized gas behind. These events can be quite beautiful to behold.

Asteroids and comets pose a unique problem for life here on Earth. Thousands of these objects orbit the Sun in a kind of celestial pinball game. Their paths take them dangerously close to our home planet. They are known as Earth-crossing asteroids and comets. In addition, it is believed that close encounters with stars may send additional objects from the Oort cloud careening inward towards the planets. One only has to look at our closest neighbor, the Moon, to see the result of millions of years of cosmic bombardment. Our thick atmosphere protects us from most of the smaller objects, but the big ones sometimes get through. Barringer Crater in Arizona is the result of one such incident. It is nearly a mile across and was formed by a meteor only 150 feet in diameter. Dozens of other impact craters have also been identified on our planet. They are difficult to find because erosion tends to erase them over time. Many scientists believe that the dinosaurs were killed when a asteroid or comet about 10 miles in diameter plowed into the Earth. This impact left a crater nearly 200 miles in diameter. This impact would have thrown billions of tons of rock and dust into the upper atmosphere, blocking out the sunlight. The result would have been a scenario frequently referred to as nuclear winter. The cold and lack of sunlight would have killed off all plant life. And without the plants, the animals would have died off. But that was millions of years ago. Could it happen again? Well, something exploded in the atmosphere over Tunguska, Siberia in 1908. The explosion knocked down trees like matchsticks in an area covering hundreds of miles. This explosion has been estimated to have been as strong as 1000 Hiroshima bombs. The resulting shock wave traveled around the Earth twice. Evidence that comets and asteroids still hit planets was witnessed by thousands in 1994 when come Shoemaker-Levy 9 smashed into the planet Jupiter with the force of thousands of hydrogen bombs. The impact left scars on Jupiter's surface that were larger than the Earth. It is almost certain that our planet will be struck again by one of these cosmic leftovers. But these events are extremely rare, and chances are it will be a long time in coming.


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