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Mercury's surface closely resembles the Moon's. It is covered by impact craters, ancient lava flows, and quake fault lines. Mile-high cliffs stretch for hundreds of miles across the planet's surface. The huge Caloris impact basin, 800 miles (1,300 km) wide, decorates one side of the planet.


For astronomers, though, a comet's beauty is not just in its appearance, but in its content: A comet is an icy time capsule that contains some of the original material from the cloud of gas and dust that gave birth to Earth and the other planets.

Current theory says that the planets formed as small grains of ice and rock stuck together to form larger bodies, called planetesimals, which then merged to form the planets.


But the next year, astronomers found a second object in a similar orbit, then another, and another. Instead of a planet, they had found the first of the asteroids -- large chunks of rock that were left over from the formation of the solar system. Ceres is the largest, with a diameter of almost 600 miles (1,000 km).

Astronomers have discovered hundreds of thousands of these boulders, and with automated searches they are discovering thousands more each year. Most orbit the Sun in a broad region known as the asteroid belt, which is between the orbits of Mars and Jupiter.


Because it is so far from the Sun, astronomers had a hard time measuring Pluto's size. They finally got it right in the 1980s, after James Christy discovered a companion object. By watching Pluto and the companion, named Charon, eclipse each other, they measured Pluto's diameter at about 1,400 miles -- about one-third less than the diameter of Earth's Moon.

Pluto is basically a ball of frozen nitrogen, methane, and carbon dioxide wrapped around a small core of rock. On average, it's farther from the Sun than any other of the major planets, so its surface is bitterly cold.