Its heavily-cratered globe is only about 3,000 miles in diameter, making Mercury the second smallest planet in the Solar System, after Pluto. Mercury has no natural satellite.

Mercury orbits the Sun every 88 days at an average distance of about 36 million miles. It is the closest planet to the Sun. Earth, the third planet, is three times as far from the Sun as Mercury.

Mariner 10. The U.S. probe Mariner 10 flew by Mercury three times in 1974 and 1975, measuring a daytime temperature on the surface of 374 degrees Fahrenheit. And that might reach as high as 842 degrees when Mercury is closest to the Sun. At night, temperatures drop to minus-292 degrees.

In photos, Mercury resembles Earth's Moon. Its surface is heavily cratered. The planet's mass is only five percent of Earth's and half of the mass of Mars. It probably has a large nickel-iron core.

Thin atmosphere. Mercury has only a trace of atmosphere, mostly composed of sodium, helium and hydrogen. There appears to be more sodium than anything else in the thin atmosphere.

Mercury has an odd swing, showing first one face and then the other during close approaches to the Sun.

Mercury, closest of all to the Sun, has been studied the least. Scientists don't know much about what's deep inside the planet, or even just below the surface. Only 40 percent of the planet's surface has been photographed. The rest is unmapped. Despite being relatively close to Earth, Mercury is among the least understood of the planets.

In 2004, NASA plans to send a spacecraft named MESSENGER to orbit Mercury.

• Mercury is the eighth largest planet with a diameter only about one-third of Earth¹s diameter.
• The planet closest to the Sun, Mercury is only about one-third as far from the Sun as Earth.
• Mercury has almost no atmosphere.
• The surface of Mercury is similar to Earth¹s Moon in that its solid and covered with craters.
• It looks like there may be ice in the shadows of craters near Mercury's north pole.
• Mercury has phases, like Earth's Moon.
• Mercury looks like a crescent moon when it passes between Earth and the Sun.
• Mercury has no moons.
• Mercury is 36,250,000 miles from the Sun.
• The diameter of Mercury is 3,032 miles.

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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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