Mercury is the planet closest to the Sun.
It is slightly larger than Earth's Moon, and looks very much like the Moon, with ancient impact craters scarring its rocky surface. Mercury flies along in its orbit at an average speed of 29 miles per second[faster than any other planet.]
Because Mercury is so close to the Sun, noontime temperatures soar to 700 degrees Fahrenheit (370 C). But because it has almost no atmosphere to hold in the heat, temperatures plummet to almost 300 degrees below zero Fahrenheit (-185 C) at night.

Mercury is a dense, rocky planet. Its core probably consists of iron and nickel. As Mercury spins on its axis, the iron in its core may generate a weak magnetic field that surrounds the planet.

Because Mercury orbits so close to the Sun, it never strays far from the Sun in Earth's sky. Mercury is visible only for a brief time before sunrise or after sunset. It looks like a bright star shining just above the horizon. It moves quickly from morning sky to evening sky. It is named for the Roman messenger god, who had wings on his heels to carry him swiftly from Earth to the heavens.

In 1991, astronomers found that despite the heat, Mercury may have small ice caps at its north and south poles. The ice exists inside deep craters. The floors of these craters remain in perpetual shadow, so the Sun cannot melt the ice.

For many decades, scientists thought the same side of Mercury always faced the Sun. But in 1965, astronomers discovered that Mercury completes three full turns on its axis for every two orbits around the Sun. Because of this motion, we see the same side of Mercury each time the planet comes closest to Earth.

Only one spacecraft, Mariner 10, has ever visited Mercury. It made three passes near the planet in 1974 and '75. It snapped pictures of half of the planet's surface, measured temperatures, and discovered a weak magnetic field. American and European space agencies are drafting plans for future missions to Mercury, but none will take place before the next century.

If Mercury ever had an atmosphere, it has long since escaped into space because of the heat of the nearby Sun. There is no air, no water, and nothing can grow on the planet's surface. Astronauts will need strong shielding to protect them from the heat and the Sun's harsh radiation. But bulky spacesuits will feel light because Mercury's surface gravity is only about one-third Earth's.

The Sun would look almost three times as large as it does from Earth. A single "day" on Mercury [the interval from sunrise to sunset] lasts 88 Earth days, and the night lasts 88 days more. When Mercury is closest to the Sun, it moves so quickly that for a time the Sun appears to move backward in the sky.

Mercury's surface is hard and rocky, pitted with craters, and probably is covered with a thin layer of fine dust. Huge cliffs tower as much as two miles above the surrounding landscape.