Asteroids are rocky and metallic objects that orbit the Sun but aretoo small to be considered planets. They are known as minor planets.Asteroids range in size from Ceres, which has a diameter of about 1000 km,down to the size of pebbles.

Sixteen asteroids have a diameter of 240 kmor greater. They have been found inside Earth’s orbit to beyond Saturn’sorbit. Most, however, are contained within a main belt that exists betweenthe orbits of Mars and Jupiter. Some have orbits that cross Earth’s pathand some have even hit the Earth in times past.

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One of the best-preservedexamples is Barringer Meteor Crater near Winslow, Arizona.Asteroids are material left over from the formation of the solar system.It was once believed that the asteroids were the debris of a large planet,orbiting between Mars and Jupiter, which had suffered a major catastropheand fragmented into many smaller parts. However, it is now thought that noplanet could ever have formed in this zone, because the stronggravitational influence of the newly formed Jupiter would have preventedthe smaller planet from forming. Instead, the “building blocks” of rock,built up through collisions with the smaller particles present at theformation of the solar system, were left, and are what we call theasteroids today. Much of our understanding about asteroids comes fromexamining pieces of space debris that fall to the surface of Earth.Because the asteroids orbit in the gap between Mars and Jupiter, it is notsurprising that the massive planet affects them. Astronomers in the 1800’snoticed that the asteroid belt has gaps in it, particularly at distances of2.

5 and 3.28 astronomical units from the Sun. The astronomer DanielKirkwood explained these gaps by considering the orbit, which a body atthis distance would have. He discovered that any asteroids in these gapswould be lined up with Jupiter very often, and so it would be pulled by thegravitational influence of the planet, out of the gap. For this reason,these are now called the Kirkwood gaps, and now there are several known.This, however, is not the only effect, which the largest planet in oursolar system has on these small objects.Asteroids that are on a collision course with Earth are called meteoroids.

When a meteoroid strikes our atmosphere at high velocity, friction causesthis chunk of space matter to incinerate in a streak of light known as ameteor. If the meteoroid does not burn up completely, what is left strikesEarth’s surface and is called a meteorite. Of all the meteorites examined,92.

8 percent are composed of silicate (stone), and 5.7 percent are composedof iron and nickel; the rest are a mixture of the three materials. Stonymeteorites are the hardest to identify since they look very much liketerrestrial rocks. Because asteroids are material from the very earlysolar system, scientists are interested in their composition. Spacecraftthat have flown through the asteroid belt have found that the belt isreally quite empty and that asteroids are separated by very largedistances.

Before 1991, the only information obtained on asteroids wasthough Earth based observations. Then on October 1991 asteroid 951 Gasprawas visited by the Galileo spacecraft and became the first asteroid to havehi-resolution images taken of it. Again on August 1993 Galileo made aclose encounter with asteroid 243 Ida. This was the second asteroid to bevisited by spacecraft. Both Gaspra and Ida are classified as S-typeasteroids composed of metal-rich silicates.Throughout the history of the solar system, the Earth and otherplanets have been subjected to impacts from smaller bodies such as cometsand asteroids and sometimes with catastrophic consequences.

Learning andtrying to understand these space objects can help us identify thecomposition of the universe and maybe find out how it started.