Chasing Comets, Asteroids, and Mysterious Space Objects
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
Why Study Comets and Asteroids No astronauts have ever visited a comet or an asteroid but in recent decades astronomers have sent many spacecraft to study them Astronomers also use powerful telescopes to learn more about comets and asteroids Telescopes on Earth are often built on mountains and in deserts where the air is clear Other telescopes are in orbit above Earth’s atmosphere where the view of space is clearer still Comets streaking across the sky have fascinated people for thousands of years Ancient peoples saw them as omens or signs that warned of disasters to come It was only natural for people to try to learn more about these phenomena Asteroids on the other hand are smaller and only rarely come close enough to be visible from Earth For that reason they were not even discovered until 1801 Scientists use telescopes to monitor asteroids such as this one and their journeys through space 6
In the Beginning About 4 6 billion years ago our Solar System was nothing more than a cloud of dust and gas At some point the cloud collapsed in on itself forming a disk Most of the dust and gas came together and formed a star the Sun The rest remained in orbit around the new star Tiny pieces collided and clumped together forming larger and larger objects Some of these lumps eventually became planets Asteroids and comets are some of the leftovers of this planet-forming process They never became part of a planet By studying them scientists can learn about our early Solar System Asteroids and comets may help us understand why the eight planets of our Solar System are so different from each other They may also explain how life began on Earth and whether it might exist on other planets Types of Asteroids Astronomers divide asteroids into three types based on what they are made of C-type asteroids are the most common and are made of clay and rock S-type asteroids are made of rock and metal and M-type asteroids are mainly metallic Astronomers use different Scientists have built many special space probes to visit and study asteroids and comets tools to study the composition or makeup of asteroids A few asteroids have been studied closely by visiting spacecraft called probes These probes use tools called spectrometers to analyze what asteroids are made of Scientists study other asteroids using spectrometers on ground-based telescopes They also use radar by sending a radio signal to the asteroid and studying what is reflected 7
