The Dawn space probe is seen near the asteroid Vesta left and the dwarf planet Ceres right in this illustration Dawn was launched in 2007 On September 27 2007 the National Aeronautics and Space Administration NASA launched the Dawn spacecraft Its mission is to explore the asteroid belt It is scheduled to visit the asteroid Vesta in 2011 and Ceres in February 2015 As the spacecraft swings close to Ceres it will take photos and scan the surface of the asteroid This will help scientists learn what Ceres is made of and how it was formed Seven Wonders of Asteroids Comets and Meteors THE ASTEROID ZOO Asteroids come in all sizes and shapes They range from rocks only a few hundred feet across to flying mountains 500 miles 800 km wide The very biggest are round But smaller objects have weaker gravity A large body such as Earth tends to be round Every point on the surface of Earth is just about the same distance from the center This is because its gravity is strong enough to flatten out any large lumps But with much less gravity to pull at an asteroid's surface it can be any shape at all The first asteroid to be explored by a spacecraft was 433 Eros The Near Earth Asteroid Rendezvous NEAR Shoemaker spacecraft launched in 1996 10 The asteroid belt is a great grinding mill producing smaller and smaller pieces down to motes of dust Carl Sagan American astronomer Cosmos 1980
Ceres 11 photographed its surface The photos showed that 433 Eros is shaped like a giant cashew nut measuring 22 x 8 x 8 miles 35 x 13 x 13 km A deep saddlelike dent in the middle separates two bulging ends While the rest of Eros is heavily cratered the saddle is remarkably smooth The saddle might be a huge dent caused by a collision with another asteroid Gravity The amount of gravity a body has depends on how much mass it has The bigger and heavier something is the more gravity it produces Earth has about 6 trillion tons 5 trillion metric tons of mass Mass is the amount of stuff that an object contains Earth's gravity makes you weigh what you do and pulls you back down when you jump A small asteroid like 624 Hektor has much less mass than Earth As a result Hektor's gravity is 150 times less than Earth's You would be able to jump from one end of Hektor to the other easily In fact you would have to be very careful not to launch yourself off into space Top Lumpy 433 Eros was the first asteroid to be photographed up close by a spacecraft Its saddle shaped dent can be seen at the lower edge Bottom The NEAR Shoemaker spacecraft took this photo of 433 Eros's horizon
Other asteroids have even more unusual shapes For instance 4179 Toutatis and 1620 Geographos are shaped like rounded off boxcars On the other hand 216 Kleopatra looks like a dog bone and 4769 Castalia resembles a hamburger bun An even more interesting asteroid is 624 Hektor It is shaped like two beach balls just touching each other It is probably the result of two asteroids colliding But the collision was a gentle one Instead of hitting so hard they shattered the two asteroids stuck together Visiting Hektor would be a weird experience If you were to hike to the contact region you would see a mountain of rock 93 miles 150 km high hanging directly over your head Astronomers have discovered asteroids with many unusual shapes The asteroid 216 Kleopatra resembles a dog bone
The asteroid 25143 Itokawa has a surface that seems to be made up of thousands of boulders The small asteroid 243 Ida is fairly typical It is only 20 miles 32 km wide and shaped like a potato It wouldn't be very interesting except for one thing Ida has a moon It was discovered in 1993 by scientist Ann Harch She had been looking closely at some photos of Ida taken by the Galileo spacecraft and there it was a tiny speck of light near the asteroid She realized that this speck was a little moon which she named Dactyl It is only 0 9 miles 1 4 km across Since then other asteroids have been found to have moons Astronomers call them minor moons because they orbit minor planets One of the most unique of all asteroids is 25143 Itokawa When the Japanese Spacecraft Hayabusa visited the asteroid in 2005 it found something very strange Most asteroids resemble relatively smooth boulders pitted with meteoroid craters of all sizes But Itokawa has almost no craters at all Instead it appears to be made of thousands of loosely attached rocks and boulders This small asteroid measuring 1 755 x 964 X 686 feet 535 x 294 x 209 meters may have formed when the gentle pull of its gravity caused gravel and rocks to slowly drift together WHERE DID THEY COME FROM Throughout history scientists have had many theories about the origin of the solar system and Earth Most modern scientists tell us that the Sun and the planets formed about 4 5 billion years ago 13 Ceres The minor planets still revolving today may well be regarded as the survivors of ten thousand times as many which once filled the sky like migrating birds Fritz Kahn German science writer and illustrator Design of the Universe 1954
