More than 7,000 asteroids have been discovered. Several hundred more are discovered each year. There are undoubtedly hundreds of thousands more that are too small to be seen from Earth.
Only 26 known asteroids are larger than 200 km in diameter. Astronomers know a lot about the asteroids that are larger than 100 km in diameter, but they don't know much about smaller ones. There could be as many as a million or more 1 km-sized asteroids. At any rate, the total mass of all the asteroids probably is less than that of the Moon.
Biggest. The largest asteroid by far is 1 Ceres. It is 933 km in diameter. It alone contains about 25 percent of the mass of all the asteroids combined. The next largest asteroids are 2 Pallas, 4 Vesta and 10 Hygiea which are between 400 and 525 km in diameter. All other known asteroids are less than 340 km across.
Close-ups. 243 Ida and 951 Gaspra were photographed by the Galileo spacecraft on its way to Jupiter. The NEAR spacecraft sent back many photos as it flew by 253 Mathilde in 1997. NEAR is orbiting 433 Eros.
Debate. There are questions about the differences among asteroids, comets and moons. For instance, are many moons around planets actually captured asteroids?
Small moons that are more like asteroids than they are like the larger moons of the Solar System include Mars's tiny moons Deimos and Phobos, Jupiter's eight outer moons, Saturn's outermost moon, Phoebe, and some newly discovered moons of Uranus and Neptune.
Solar System: The Sun Inner System: Mercury Venus Earth Mars Outer System: Jupiter Saturn Uranus Neptune Pluto Other Bodies: Moons Rings Asteroids Comets
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