Mercury, the closest planet to the sun, can have a surface temperature exceeding 450 degrees Celsius (840 degrees Fahrenheit). The sun is 11 times hotter at Mercury than it is at Earth. To protect the spacecraft instruments from this intense heat, Messenger has a ceramic fabric sunshade.

"The outside of the sunshade will get as hot as a pizza oven, while the rest of the spacecraft will remain at nearly room temperature," says James Leary, mission systems engineer at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland.

Mercury is not uniformly hot, because the thin atmosphere does not transfer heat from the equator to the poles. Temperatures on the dark side of the planet can drop to -185 degrees C (-300 degrees F). The planet Venus is therefore much hotter, with a surface temperature reaching 482 degrees C (900 degrees F) due to the greenhouse effect of its thick clouds.

Principal Investigator Sean Solomon of the Carnegie Institution of Washington says that one of the most bizarre questions scientists hope to answer about Mercury is whether there is ice lurking in shadowed regions at the poles.

"Because the spin axis of Mercury has almost no tilt, a crater near a pole is in permanent shadow," says Solomon. "The floor of a shadowed crater is cold enough not only to freeze out any volatiles like water, but to keep it in a solid state for the lifetime of the planet."

Images of Mercury's polar regions, taken by the Earth-based Arecibo telescope, show that the floors of the craters are highly reflective at radar wavelengths.

Water ice is the most likely culprit, but any volatile with a freezing point above -180 degrees C also could cause this reflectivity. Alternate hypotheses suggest the crater floors could be covered with elemental sulfur, or perhaps frozen silicates.

Mercury is about the size of our moon, and extremely dense. The planet's density is so high that two-thirds of the planet is believed to be iron metal, yet scientist aren't sure why the planet would be so metal-rich.

The inner planets of the solar system formed from the disk of gas and dust that surrounded the sun, and it could be that there was a gradient in chemistry of disk, with more metal closer to the sun.

Or perhaps Mercury's composition was more Earth-like to start, but the heat from the sun caused Mercury to lose its rocky material. Asteroid impacts also may have caused Mercury to lose its lighter rocky material.

Solomon says that, to him, the most fundamental question about Mercury is how the planet formed.

"The inner planets are all litter-mates, if you will, the products of a single early stage in the evolution of a star, and yet the siblings turned out very differently," says Solomon.

"In order to understand what processes most control the differences in outcomes, we really have to study and learn about the most extreme of those outcomes - and that's Mercury."

The Mariner 10 spacecraft is the first and only spacecraft to visit Mercury thus far. Mariner 10 cruised by the planet three times in 1974 and 1975, and gathered data on about 45 percent of the surface. Messenger will be the first spacecraft to actually orbit around Mercury, and the spacecraft will stay in this orbit for at least one Earth year.

Messenger will have a long looping journey through the solar system on its approach to Mercury, using six gravity-assist maneuvers by Earth, Venus, and Mercury before it enters Mercury's orbit. Its first flyby of Mercury will be in 2008, and it will enter Mercury's orbit in March 2011.

Messenger will travel 7.9-billion kilometers (4.9-billion miles), orbiting around the sun 15 times. This long journey will allow the mostly solar-powered Messenger to carry much less fuel than it would need for a more straightforward path.

Because Messenger will flyby Venus twice, the spacecraft will be able to briefly study the Earth's closest neighbor. Solomon says they may look for signs of lightning on the night side of Venus, and they may also use the spacecraft's spectrometer and laser altimeter to gather information about Venus, although the actual details of the Venus flyby are still under discussion.

Mercury has an unusual rotation rate, turning so slowly that it takes two orbits around the sun before a single day passes. So although Messenger will orbit Mercury for one Earth year, only two Mercury solar days will have passed (one Mercury solar day, from sunrise to sunrise, is equal to 176 Earth days).

Messenger will complete an elliptical orbit around Mercury every 12 hours. The orbit will be highly inclined, measuring 80 degrees from the equator, and will most closely approach Mercury's northern hemisphere. The lowest altitude planned is 200 kilometers (124 miles).

Ultimately, the spacecraft will go much lower than that. Robert Farquhar, mission manager at APL, says that when the spacecraft runs out of fuel, it will crash onto Mercury's surface.

"For that purpose we have a US flag on the spacecraft, so we will have a US flag on that body for the first time," says Farquhar.

Messenger is scheduled for launch from Cape Canaveral Air Force Station, Florida, at 2:16 a.m. EDT on Aug. 2. There is a 13-day window for launch, with a final opportunity on August 14.

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