Sometimes one can see a flash of light streak across the night sky and disappear This is commonly called a shooting star though they are not stars They are small chunks of stone metal or other materials that orbit the Sun Sometimes they enter Earth’s atmosphere and the friction generated generated by their great speed causes them to burn up The fragments may either vaporize before traveling far or actually hit the ground These objects have different names depending depending on their location One that is beyond Earth’s atmosphere is called a meteoroid A meteoroid that enters Earth’s atmosphere is called a meteor A meteor that actually lands on Earth’s surface is called a meteorite Meteorites which are sturdy enough to reach the ground are mostly pieces of asteroids or huge rocks that orbit the Sun Comets on the other hand are flimsy objects made mostly of frozen water frozen gases and some gritty material They also orbit the Sun Meteoroids vary from small rocks to boulders weighing a ton or more A relative few have orbits that bring them into Earth’s atmosphere and down to the surface as meteorites Most meteorites that have been collected on Earth are probably from 6 INTRODUCTION
Comet Siding Spring appears to streak across the sky like a superhero superhero in this infrared image from NASA’s Wide-field Infrared Survey Explorer or WISE asteroids A few have been identified as being from the Moon Mars or the asteroid Vesta Most meteoroids that enter the atmosphere heat up sufficiently to glow and appear as meteors and the great 7
Asteroids Meteors Meteorites And CoMets majority of these vaporize completely or break up before they reach the surface Some meteorites contain microscopic microscopic crystals whose isotopic proportions are unique and appear to be dust grains that formed in the atmospheres of different stars Most meteors that burn up in Earth’s atmosphere are tiny dustlike particles the remains of disintegrated comets Sometimes a swarm of meteoroids enters Earth’s atmosphere causing a meteor shower with tens or hundreds hundreds of shooting stars flashing across the sky in less than an hour Virtually all these meteors burn up in the upper atmosphere More than 500,000 asteroids with well-established orbits are known and thousands of additional objects are discovered each year Hundreds of thousands more have been seen but their orbits have not been as well determined determined It is estimated that several million asteroids exist but most are small and their combined mass is estimated to be less than a thousandth that of Earth Many comets have been observed over the centuries Comets are considered to come from a vast reservoir the Oort cloud orbiting the Sun and containing trillions of icy objects with the potential to become active comets Most make only a single pass through the inner solar system but some are deflected by Jupiter or Saturn into orbits that allow them to return at predictable times Halley’s Comet 8
introduCtion is the best known of these periodic comets whose next return into the inner solar system is predicted for 2061 Many short-period comets are thought to come from the Kuiper belt a region lying beyond Neptune’s orbit but including part of Pluto’s and housing perhaps hundreds of millions of comet nuclei Very few comet masses have been well determined but most are probably one-billionth the mass of Earth As scientists develop new technologies they can more closely study these celestial bodies and gain insight into the formation of our solar system By understanding what these bodies are composed of researchers can better understand how the solar system originated and has evolved 9
