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Mercury


MESSENGER has safely passed Mercury for a second time, on October 6, 2008. Stay tuned to The Planetary Society Weblog for updates.

Mercury, the closest planet from the Sun, looks much like Earth's Moon -- at first glance. Unlike the Moon, it is crisscrossed by enormous thrust faults that formed as the planet contracted while it cooled. Mercury is among the densest of planets, with its interior dominated by an enormous iron core. Its weak magnetic field suggests that part of Mercury's core may be molten. Scientists were surprised to find that Mercury has a tenuous atmosphere of sodium and potassium that may be related to solar wind reaching the planet's surface.

People once believed that Mercury was locked into spin-orbit resonance with the Sun such that it always kept the same face to the Sun. But the actual situation is stranger than that. Mercury rotates exactly 3 times for every 2 times it goes around the Sun. That makes its days seem 2 Mercury years long, as seen from the surface. The slow rotation gives Mercury the largest diurnal (daily) temperature variation of any known planet -- from nighttime lows of minus 180 degrees Celsius to daytime highs of plus 430 degrees (that's minus 300 to plus 860 Fahrenheit). More about Mercury...

NASA's MESSENGER has begun exploring Mercury, having successfully completed two flybys, the first on January 14, 2008, and the second on October 6, 2008. It will fly by once more, on September 29, 2009, before entering orbit on March 8, 2011.
Only one other mission -- Mariner 10 -- has ever visited Mercury, flying by three times in 1974 and 1975.
Mercury Numbers
Size: 8th largest planet - 4,879 kilometers - 0.3825 Earths across
Calendar: 1 Mercury year = 0.241 Earth years; 1 Mercury day = 175.9 Earth days
Orbit: 57,910,000 kilometers - 0.387 Earth orbits
Axial tilt: 0.01 degrees
Number of moons: none