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	Mercury At a Glance
	Discovery Known since antiquity Name Roman messenger god Average Distance from Sun 35,983,095 miles 57,909,175 km 0.387 Astronomical Unit Mass 0.055 times Earth's mass Equatorial Diameter 3,032 miles 4,879 km Length of Day 58.65 Earth days Length of Year 0.24 Earth years 88 Earth days Surface Gravity 0.38 that of Earth (If you weigh 100 pounds, you would weigh about 38 pounds on Mercury.) Known Moons None Compare Planets
	Exploration
	Only one spacecraft has visited Mercury. Mariner 10 flew by the planet three times in 1974 and 1975, and mapped about half of its battered, Moon-like surface, which is dominated by impact craters. Mariner found that Mercury's interior consists of a large iron-rich core, which generates a weak magnetic field around the planet. More»

Tiny, cratered Mercury is the closest planet to the Sun. It is larger than Pluto, yet smaller than the largest moons of both Jupiter and Saturn. If Earth were the size of a baseball, Mercury would be the size of a golf ball.

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Big impact craters scar the surface of Mercury. Above: In this radar image, the red dot at Mercury’s north pole points the way to possible water ice. Top: A Mariner 10 mosaic of Mercury reveals craters, basins, and cracks in its surface.

Mercury is only about six percent as massive as Earth, but it is about the same density. It is dominated by its iron core, which makes up 70 percent of its mass. The core may be partly molten, which would explain the presence of Mercury's weak magnetic field, which is just one percent as strong as Earth's. Above the core, Mercury has a thin mantle and crust.

A thin but strong wind from the Sun blasts particles off of Mercury's hard surface, creating a thin atmosphere around the planet. The particles rapidly escape into space, even as the never-ending solar wind blasts more material from the surface.

Mercury speeds around the Sun faster than any other planet. Its 88-day, highly elliptical orbit brings it within 29 million miles (47 million km) of the Sun, and then swings it out to 44 million miles (70 million km).

In the 1800s, astronomers realized that the physics of the day could not correctly predict Mercury's orbital path. The reason remained a mystery until 1915, when Albert Einstein used his new theory of gravity, General Relativity, to correctly predict Mercury's orbit -- and explained the reason for previous errors: Mercury is so close to the Sun that its orbit is affected by the "warp" in space created by the Sun's powerful gravitational field.

From Earth, we view the same side of Mercury each time it passes closest to our planet. For a long time, this caused astronomers to think that the planet was tidally locked with the Sun -- that is, its day and year were of equal length. Not so; in 1965, radar observations showed that Mercury completes three spins on its axis for every two orbits around the Sun.

The planet's surface swings through extremes of temperature. In the daytime, it can reach 700 degrees Fahrenheit (315 C). At night, the temperature drops to �0 F (�3 C). Radar observations suggest that water ice might inhabit shadowed craters at Mercury's poles. These regions never see the Sun, so the ice would not vaporize into space.

Mercury's distance and proximity to the Sun make it difficult to study with ground-based telescopes. It's only visible just after sunset or just before sunrise, forcing astronomers to view it through a thick, turbulent layer of Earth's atmosphere.

Only one spacecraft has visited Mercury. Mariner 10 flew by the planet three times in 1974 and 1975, and mapped about half of Mercury's surface. NASA's MESSENGER spacecraft is now on a long, circuitous trek to Mercury, and should enter orbit in 2011 for a one-year mission to this least-explored of the solar system's inner planets.

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The solar system's smallest planet flits back and forth from morning sky to evening sky several times a year. It never strays far from the Sun in our sky, so it's tough to find in the glare. From the northern hemisphere, it is visible in the morning sky this year in February and early March, June, and October. The late-year appearance is the best, because the planet will stand highest above the horizon. In the evening, Mercury is best seen in April and early May, August and early September, and December. The spring appearance is best.

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This document was last modified: May 04, 2009