Chasing Comets, Asteroids, and Mysterious Space Objects
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Contents CHAPTER 1 What Are Comets and Asteroids 4 CHAPTER 2 Chasing Comets CHAPTER 3 Visiting Asteroids CHAPTER 4 Avoiding Impacts CHAPTER 5 Weird and Wonderful CHAPTER 6 What’s Next ACTIVITY Your Space Science Mission Glossary Learning More Index and About the Author 10 20 28 34 40 44 46 47 48
CHAPTER 1 What Are Comets and Asteroids Earth is part of a solar system made up of objects that orbit the Sun These objects include eight planets Earth is one of them and millions of smaller objects Some travel close to the Sun while others stay in the outer reaches of our Solar System far from the Sun Many of these small objects are asteroids and comets Asteroids Asteroids are small lumps of rock and metals scarred with craters The largest is 329 miles 530 km across but others are the size of a house or smaller So far about 780,000 asteroids have been identified but there are many more Most asteroids lie in a belt between the orbits of Mars and Jupiter Asteroids dislodged from the asteroid belt travel through our Solar System on paths that sometimes bring them close to planets 4
A comet’s tail is a stream of dust and gas extending away from the comet as it travels around the Sun The asteroid belt contains millions of asteroids Other asteroids called Trojans share an orbit with a planet Many Trojans share Jupiter’s orbit and one shares Earth’s orbit Other asteroids travel close to Earth’s orbit Comets Comets are also small and lumpy but they contain ice and dust as well as rock Most comets come from the outer edges of our Solar System They travel around the Sun in a long looping path As they approach the Sun they warm up and some of their ice turns into a gas The gas forms an atmosphere called a coma which can reach out into a long tail Comets often become visible to the naked eye when they pass close to Earth Some comets appear regularly Halley’s Comet for example is visible about once every 76 years Other comets take millions of years to complete their journey around the Sun New Discoveries Comets and asteroids are small and difficult to reach but that has not kept astronomers from studying them The information astronomers have gathered has helped them understand more about the beginning of our Solar System and the early days of our own planet Earth 5
