An asteroid is also called MINOR PLANET or PLANETOID,any of a host of small rocky astronomical objects found primarily between the orbits of Mars and Jupiter. By the 1990's more than 7000 had been observed at two or more oppositions, and 5000 had been assigned numbers, which is done as soon as accurate orbital elements have been determined.
Asteroids are smaller than any of the nine major planets of the solar system. There are very few large asteroids; about 30 have a diameter of more than 124 miles. Aproximately 250 asteroids have a diameter of at least 62 miles. It is estimated that millions of asteroids of boulder size exist in the solar system. These smaller objects are probably formed when larger asteroids collide. A few of them strike the earth's surface in the form of meteorites.
The largest asteroidsare massive enough for their gravity to have molded them into spheres during formation. This hypothesis is supported by the lack of regular variation in their brightnessthat would result if these objects were asymmetric in shape. Smaller asteroids may have a wide range of shapes. Icarus, for example, has a radius of only 1.2 miles. Eros, however, is more of a slab, having dimensions of approximately 6X9X18.6 miles.
Asteroids such as Icarus and Eros, whose orbits cross that of the earth on a nearly continuous basis, are called Apollo asteroids. About 91 of these have definitely been identified. Some astronomers would like to mount a full-scale search for such asteroids, partly out of a fear that they might collide with earth. Knowing of their existence and calculating their orbits would afford a possibility of altering their motion away from the earth. Collisions with larger asteroids are rare, but smaller ones are more numerous. It is estimated that a few asteroids with a diameter of .6 miles may collide with the earth. Knowing of their existence and calculating their orbits would afford a possibility of altering their motion away from the earth. Collisions with larger asteroids are rare, but smaller ones are more numerous. It is estimated that a few asteroids with a diameter of .6 miles may collide with the earth within a period of 1,000,000 years. If an asteroid of this size were to collide with the earth, it would cause an explosion with as much force as several hydrogen bombs. The crater caused by it would be about 8 miles across. A short-term disturbance in the world's climate could result, and a collision in the ocean would be disastrous.Some investagators believe that the extinction of the dinosaurs and many other land and marine animals about 65 million years was triggered by the impact somewhere on the earth's surface of an asteroid or meteorite measuring some 6 miles in diameter.
Asteroids, as well as meteorites appear to be composed of varying proportions of stony and metallic materials. Many objects of both types contain a large amount of carbon, which make them dark with low reflective powers. Such objects are carbonaceous chondrites and are considered to have been the first materials to combine from the primordial nebula which the solar system originated from. They have avoided any melting induced by heat from radioactivity within young asteroids or structural metamorphism induced by meteoric impact. Laboratory studies of carbonaceas chondritic material conducted during the early 1980's suggested the intriguing possibility that one or two nearby supernova explosions may have triggered the compression of the primordial nebula, which then went on to collapse by its own gravity to form the sun and the planets. No planet could have formed in the region between Mars and Jupiter because of the gravitational influence of the latter, which is the largest planet in the solar system and which resembles a star more than a planet. Its influence would have stirred up the pre-planetary material in the asteroid belt during the formation of the solar system, causing the material to crash and break up rather than coalesce to build up to a planet-sized object. Calculations show that if all the asteroids were to fuse into one object, they would form an object comparable in size to the moon or a Galilean satellite of Jupiter.
There are gaps in the orbital distances of the
asteroids from the Sun because of Jupiter's gravitational influence. These
interruptions known as Kirkwood gaps, are a simple fraction of
the orbital period of Jupiter. This resonance phenomanon has the planet
passing by any asteroid in the Kirkwood gaps every two or three asteroid
years depending on which gap.