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
CHAPTER 1 AmAzing Asteroids Asteroids are small rocky bodies that orbit or travel around the Sun In general they are materials left over from the formation of planets or are created when other bodies in space collide and break apart Most of the rocky asteroids move around the Sun in elliptical orbits in the same direction of the Sun’s rotation rotation Such motion is termed prograde Looking down on the solar system from a vantage point above Earth’s North Pole an observer would find that prograde orbits are counterclockwise Orbits in a clockwise direction are called retrograde Between the orbits of Mars and Jupiter are a host of asteroids about 1,000 km 600 miles or less in diameter that orbit in the nearby ring called the asteroid belt Many astronomers consider the asteroid belt a demarcation point between the inner solar system consisting mainly of the terrestrial planets and the outer solar system It is because of their small size and large numbers relative 10
AMAzing Asteroids to the major planets that asteroids are also called minor planets The two designations have been used inter changeably though the term asteroid is more widely recognized by the general public Among scientists those who study individual objects with dynamically interesting orbits or groups of objects with similar orbital characteristics generally use the term minor planet whereas those who study the physical properties of such objects usually refer to them as asteroids EARLY OBSERVATIONS The first asteroid was discovered on January 1 1801 by the astronomer Giuseppe Piazzi at Palermo Italy At first Piazzi thought that he had discovered a comet However after the orbital elements of the object had been computed it became clear that the object moved in a planetlike orbit between the orbits of Mars and Jupiter Owing to illness Piazzi was able to observe the object only until February 11 Although the discovery was reported in the press Piazzi only shared details of his observations with a few astronomers at first and did not publish a complete set of his observations until months later The short arc of observations observations did not allow computation of an orbit of sufficient accuracy to predict where the object would reappear when it moved back into the night sky 11
