Steve Wood Jr., age 14, of Denton, Texas, for his question:
JUST WHAT IS AN ASTEROID?
Asteroids are small or minor planets that are members of the solar system and which move in elliptical orbits, primarily between the orbits of Mars and Jupiter.
There are about 200 asteroids that have diameters of more than 60 miles, and there are thousands of smaller ones. The total mass of all the asteroids in the solar system combined is much less than the mass of the moon.
The larger asteroids are roughly spherical, but elongated and irregular shapes are common for those with diameters of less than 100 miles. Most asteroids, regardless of size, rotate on their axes every five to 20 hours. Certain asteroids may be binary, or have satellites of their own.
At one time, scientists thought asteroids were the remnants of a former planet, but they no longer believe this. Now they say it is more likely asteroids occupy a place in the solar system where a sizable planet could have formed, but was prevented from doing so by the disruptive gravitational influences of the'nearby giant planet, Jupiter.
Originally, perhaps only a few dozen asteroids existed. They were subsequently fragmented by collisions to produce the population now present.
The so called Trojan asteroids lie in two clouds, one moving 60 degrees ahead of Jupiter in its orbit and the other 60 degrees behind.
The largest asteroids are Ceres, with a diameter of about 640 miles, and Pallas and Vesta, with diameters of about 340 miles. In 1977, an asteriod named Chiron was discovered in orbit between Jupiter and Saturn. Chiron is comparable in size to Pallas and Vesta.
An asteroid named Eros, which measures about 9 by 23 miles in its elongated shape, comes into the inner solar system near Earth, Venus and Mars. It would be a relatively easy target for space missions. Someday it may be mined to aid in constructing space colonies.
Most knowledge about them has been obtained from telescopic and spectroscopic observations. Even the largest objects are too small and distant to reveal discernible surface features through the telescope.
About 90 percent of the asteroids appear to consist of stony materials of a primitive, sedimentary type that accreted from solid grains in the early solar nebula. Thus, unlike Earth and the moon, they have never melted or appreciably reheated since their formation.
A small percentage of the asteroids appear to be entirely metallic in composition and may represent the metallic cores of melted, differentiated bodies from which the outer layers have been eroded away.
A few asteroids show an igneous surface composition like that of many lunar and terrestrial lava flows.