Exploring the Solar System

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.

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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