Asteroids

Beyond Mars is a region of the solar system where small, rocky bodies called asteroids can be found orbiting the sun. This region is referred to as the asteroid belt. Astronomers have discovered more than 10,000 asteroids, and many more are thought to exist. Most asteroids are less than 1 kilometer in diameter; only three are larger than 500 kilometers across. Although most asteroids are found in the asteroid belt, others orbit near Jupiter or range through the inner solar system.

The largest asteroids have enough gravity to have pulled themselves into nearly spherical or oval shapes while forming. Smaller asteroids, however, can have very irregular forms, as seen in Figure 18. These odd shapes are a result of impacts with other asteroids and meteoroids. If these small asteroids were solid, they would be likely to shatter during such collisions. Because they haven't shattered, scientists think that some asteroids are like floating rubble heaps, held together by weak gravity.

In the past, scientists thought that asteroids were the remnants of a shattered planet. Scientists now hypothesize that asteroids are remnants of the early solar system that never came together to form a planet. This hypothesis is supported by the lack of mass in the asteroid belt. If all of the asteroids were put together, they would form a planetary body only about 1500 kilometers in diameter. This would hardly qualify as a planet.

Figure 18 Asteroids come in a variety of sizes and shapes. A The asteroid Ida was photographed by the Galileo probe on its way to Jupiter. B In 2001, the NEAR Shoemaker space probe landed on the asteroid Eros.
Comparing and Contrasting How do the shapes of the small asteroids shown compare to the shapes of the planets?

Critical Thinking

8. Calculating What is the range of temperatures that occur on Mercury's equator during one full revolution around the sun?

9. Comparing and Contrasting How is the composition of Venus's atmosphere different from that of Earth's atmosphere?

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