NASA's Messenger spacecraft is scheduled to launch Sunday on a 5-billion-mile trip to Mercury -- the first visit in 30 years to the most extreme and least studied of the inner planets.
It will take seven years for the $427-million probe to reach Mercury, the closest planet to the sun. The probe will orbit the planet for one year, taking color pictures of the entire surface, as well as gathering data on the composition and structure of the crust, the shape and strength of the magnetic field, the makeup of the core and the nature of the material in the polar craters.
Mercury is surprisingly different from the other rocky planets -- Venus, Earth and Mars. The planet is made mostly of iron, making it the densest for its size in the solar system.
It has a magnetic field, even though scientists say such a diminutive planet -- only Pluto is smaller -- should not be able to maintain one. And it may have some water ice hiding in the perpetual shadows of the polar craters -- despite temperatures as high as 850 degrees in the sun.
"It's a pretty bizarre place," said Sean Solomon, Messenger's principal investigator from the Carnegie Institution in Washington.
Scientists are hoping that the planet's strangeness will illuminate fundamental questions about the formation of the early solar system.
"What we're really chasing is the processes that led to the formation of Venus and Mars and the Earth, and why they produced such different outcomes," he said.
Mercury was last visited by Mariner 10, which swung by three times in 1974 and 1975, and glimpsed barely half of the planet's wrinkled surface.
Messenger, managed by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., will launch from Cape Canaveral Air Force Station in Florida.
For protection from the sun, which will be shining up to 11 times brighter than on Earth, the 1.2-ton spacecraft is constructed of heat-resistant carbon-fiber material and shielded by a ceramic cloth.
The cloth is such a good insulator that the front can be 700 degrees and the back will be room temperature, said Robert Gold, the mission's payload manager.
With all the solar energy bombarding the craft, one might think that powering it would be simple. But too much power is also a problem, Gold said. The solar panels must tilt so that they are almost edge-on to the sun to avoid frying in the radiation, and even then they are mirrored to help reflect excess energy.
