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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.

Mars' Moons

Deimos is farther away and moves slowly from east to west. Deimos would look like a small dot of light in the sky. Phobos is slowly moving closer to Mars. In another 50 to 100 million years, it will crash into Mars.

Phobos is small, dark, and airless. And it's one of the driest bodies in the solar system.

Jupiter's Moons

Io: Fire World

Robotic probes may someday provide close-up views of some of the most remarkable vistas in the solar system, from the canyons of Mars to the ice-geysers of Triton. For a true hot-spot, they might show us the surface of Io, one of the moons of Jupiter. It is an eerie landscape of active volcanoes, tall mountains, and plains covered with frozen sulfur.

Saturn's Moons

Titan: Smog World

Surrounded by a thick atmosphere that's topped by orange smog, and with a landscape that in some ways resembles Earth, Titan is one of the solar system's most intriguing moons.

Titan's frigid atmosphere is about 60 percent denser than Earth's at the surface, and clouds float across the sky. A global haze of organic molecules tops the atmosphere, so it was almost impossible to study the surface until the Cassini spacecraft, which carries instruments that can peer through the haze, arrived at Saturn in 2004.