Name Date Asteroids and Comets By Patti Hutchison Caption: drawing of a comet Rocks fly through space all the time. Some of these rocks are asteroids; some are comets. What's the difference? Asteroids are made up of rock and iron. They are similar to the rocky planets closest to the sun. Some asteroids are so large that scientists call them minor planets or planetoids. Like planets, some asteroids have moons. There is an area in space called the asteroid belt. This belt exists between Mars and Jupiter. Here is where millions of asteroids are found. The asteroid belt has an elliptical path, moving in the same direction as the planets. The first asteroid was discovered in 1801; it is called Ceres. It is also the largest. Its circumference is about 960 kilometers. Like a planet, Ceres orbits around the sun. It takes 4.6 Earth years to make one orbit. Not all asteroids are this large, however. Some are only a few hundred meters wide. Scientists think they are "junk" left over from the formation of the solar system. Sometimes an asteroid is pulled out of its orbit. The gravitational pull of a planet or other larger object can attract it. Sometimes an asteroid crashes into a planet. This causes a crater. We can thank Jupiter for protecting us against constant bombardment by asteroids. The Sun's gravity pulls the asteroids toward the inner planets. But the asteroid belt is closer to Jupiter. Jupiter's gravitational pull attracts the asteroids away from Earth. It also protects Mercury, Venus, and Mars. Impacts are not totally impossible, however. Scientists believe that the asteroid 1950 DA will pass close to Earth. This is the greatest chance of asteroid collision for our planet. But you don't need to worry; scientists predict it will not even come close until the year 2880! Like asteroids, comets are more "junk" left over from the formation of the solar system. They are like giant snowballs. They have a small, solid nucleus. It is made of frozen gases and chunks of ice with rocks and dirt in them. The most fascinating part of a comet is its tail. The tail is formed when the comet comes close to the sun. It begins to warm up. The ice turns to gases, which begin to glow. The solar wind blows these glowing gases away from the sun. We see them as a comet's tail. They can flow for thousands of kilometers behind the comet's core. Most comets come from a distant ring of space garbage called the Oort Cloud. Like asteroids, comets also orbit the sun. Most of their orbits are farther away than Pluto. It can take some comets millions of years to make one orbit. Some, however, are close enough to see from Earth. When Earth passes through the tail of a comet, we see a meteor shower. There have been space missions to both asteroids and comets. Scientists study them to get clues about how Earth evolved. For example, some believe that the impact of an asteroid caused a chain of events that led to the extinction of the dinosaurs here on Earth. Impacts from comets may have brought water to our planet. Scientists believe that these space rocks can tell us a lot about the formation of our solar system. Name Date Asteroids and Comets Questions 1. Asteroids are made of: A. ice and dust B. gases C. rock and iron 2. Where is the asteroid belt found? 3. Some asteroids have moons. A. false B. true 4. How does Jupiter protect the inner planets from being hit by asteroids? 5. What is a comet? 6. Where do most comets come from? A. Ceres B. the Sun C. the Oort Cloud.