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Even though Mercury is about 75% the diameter of Mars, it has about the same gravity

Mercury ain't for sissies. Say what you will about the difficulty of sending a spacecraft to a distant place like Saturn, it's Mercury, the first rock from the sun — with its crush of solar gravity and its inferno of solar heat — that presents the greatest challenges. NASA has dared go to Mercury only once before, 30 years ago, when Mariner 10 made three flybys. Now NASA is set to try again, with a spacecraft called MESSENGER, which is scheduled to take off as early as this week on a mission to become the first probe to orbit the planet.

Even to astronomers, Mercury is a cosmic oddity. It spins so slowly and scoots around the sun so fast that a Mercury year — just 88 Earth days — is half as long as a Mercury day. On the planet's illuminated side — where the sun looks three times as big as it does from Earth and is 11 times as bright — temperatures climb to 840F. When that side rotates into darkness, the thermometer plunges to --300F. Eons of this rotisserie roll have cooked Mercury down to a nub with a metal core that represents three-quarters of its diameter. Yet there may be water ice in permanently shadowed polar craters.

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MESSENGER will study these and other Mercurian curiosities when it arrives at the planet in 2011. Completing a full orbit in 12 hours and swooping as close as 124 miles to the surface, it will need just six months to photograph the entire globe. After a three-decade wait for a return visit, that ought to seem like no time at all.

A Scorched Surface ...
Mercury is closer to the sun than any other planet, but it is not quite the hottest of the group. That distinction goes to nearby Venus, where the temperature climbs to 900F (482C), owing to the planet's dense, heat-trapping atmosphere

Reasons for Mercury's wild temperature swings are its lack of any appreciable atmosphere and its reflective surface that bounces 90% of the sun's heat back into space

... with Mysteries Below
Mercury has a large core of mostly iron, making the planet extraordinarily dense. This means that although Mercury is about 75% the diameter of Mars, it has about the same gravity

Diameter -- Core as % of diameter

Mercury -- 3,031 miles (4,878 km) -- 75%
Venus -- 7,504 miles (12,104 km) -- 50%
Earth -- 7,909 miles (12,756 km) -- 55%
Mars -- 4,208 miles (6,787 km) -- 45%
Moon -- 2,155 miles (3,476 km) -- 20%

Mercury's crater-pocked surface looks much like that of our moon. Its most prominent feature is the Caloris Basin, an 800-mile (1,300-km)-wide crater probably formed by an asteroid impact when the planet was still young. MESSENGER will provide the first picture of the whole basin, half of which was in darkness when the Mariner 10 probe flew by 30 years ago

Mercury's average distance from the sun is 36 million miles (58 million km), about two-thirds closer than Earth. From Mercury's surface, the sun appears as much as three times as large and 11 times as bright as it does on Earth

Is there ice on the surface? Radar imaging has detected what appears to be ice reflections in craters near the planet's poles. The walls of these craters may be so steep that sunlight never reaches their floors, putting them in permanent deep freeze

Earth's magnetic field is believed to be generated by the movement of molten iron in the core. But Mercury is so much smaller than Earth that its core should have solidified long ago. MESSENGER will try to determine whether the field is just a dying remnant or is still being driven by a molten core

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