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This Mariner 10 image of Mercury shows a gray, rocky and heavily-cratered world. You can see the curvature of the planet.
Mercury's heavily-cratered south pole.
Mercury:

The small and rocky planet Mercury is the closest planet to the Sun; it speeds around the Sun in a wildly elliptical (non-circular) orbit that takes it as close as 47 million km and as far as 70 million km from the Sun. Mercury completes a trip around the Sun every 88 days, speeding through space at nearly 50 km per second, faster than any other planet. Because it is so close to the Sun, temperatures on its surface can reach a scorching 467 degrees Celsius. But because the planet has hardly any atmosphere to keep it warm, nighttime temperatures can drop to a frigid -183 degrees Celsius.

Because Mercury is so close to the Sun, it is hard to see from Earth except during twilight. Until 1965, scientists thought that the same side of Mercury always faced the Sun. Then, astronomers discovered that Mercury completes three rotations for every two orbits around the Sun. If you wanted to stay up for a Mercury day, you'd have to stay up for 176 Earth days.

Read More About Mercury

Just the Facts
Distance from the Sun: 
57,909,175 km
Equatorial Radius: 
2,439.7 km
Volume: 
60,827,200,000 km3
Mass: 
330,220,000,000,000,000,000,000 kg
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