The United States will soon embark on a mission to the small planet Mercury, its first since 1973. A spacecraft named Messenger will lift off from Florida to begin a six-and-a-half year journey to a planet slightly larger than our moon and the one closest to the Sun. This proximity requires special protections for the spacecraft so it will not burn up in Mercury's extreme heat. But the planet can become extremely frigid, too.

On Launch Pad 17-B at Cape Canaveral Air Force Station, Boeing workers complete the installation of the fairing around the MESSENGER (Mercury Surface, Space Environment, Geochemistry and Ranging) spacecraft. The fairing is a molded structure that fits flush with the outside surface of the upper stage booster and forms an aerodynamically smooth joint, protecting the spacecraft during launch. MESSENGER is scheduled to launch Aug. 2 aboard a Boeing Delta II rocket and is expected to enter Mercury orbit in March 2011. MESSENGER was built for NASA by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. Image credit:  KSC, NASA.

The only spacecraft to make the trip was the U.S. Mariner 10, which flew by three times in the mid-1970s. But it mapped only half of the surface, so Mercury still remains the least explored of the rocky, inner worlds that include Earth, Venus, and Mars.

Scientists have many questions

"How did Mercury end up mostly metal?" asks Sean Solomon of the Carnegie Institution of Washington, the mission scientists.

He notes that Mercury is mostly iron, making it the densest planet. He wonders why it has less rocky crust than Earth, Venus, and Mars, even though scientists believe they formed the same way from a giant cloud of gas and dust swirling around the Sun.

"We do not know whether that is because closer to the Sun there was more metal than other materials, or it is possible Mercury started out more Earthlike in composition and lost its rocky fraction because of extreme heat or because of mechanical disruption by a giant impact," adds Mr. Solomon.

This question is fundamental, says Mr. Solomon, because Mercury is very different from Earth, Venus, and Mars despite their common origin. "In order to understand what processes most control the differences in outcomes, we really have to study the most extreme of those outcomes, and that is Mercury," he says. "So we think that Mercury has a lot to tell us about how Earth was formed, about how all the inner planets were formed, and how they turned out so differently."

Besides Mercury's extreme density, scientists also wonder why it is the only inner planet besides Earth with a magnetic field. And why do bright radar reflections show what appears to be ice at the bottom of polar craters when daytime temperatures soar to 400 degrees Celsius?

The Messenger spacecraft will try to solve these puzzles by studying the magnetic field, looking to see if the crater substance really is ice or bright silicas as some think, and measuring the planet's extremely thin atmosphere.

Mercury is eccentric in other ways. First, its orbit is eccentric, or highly elongated. This causes the extreme daytime temperatures when it is closest to the sun and night time ones of -200 degrees Celsius when it is furthest away, an immense 600 degree variation. By the way, a day on Mercury lasts two Earth years.

Mission engineer James Leary of Johns Hopkins University says the extreme heat requires Messenger to have special ceramic-fiber protection. "Luckily, most of the spacecraft doesn't have to see those extremes because of an innovative design where we use a sunshade to protect the body of the spacecraft," he says. "The front of this sunshade will get up to about as hot as a pizza oven while the rest of the spacecraft remains at nearly room temperature."

The U.S. mission around Mercury is to last one year. Whether it is extended depends on fuel and instrument health.

Mission manager Robert Farquhar of Johns Hopkins University says Messenger is eventually expected to crash into the surface and leave a souvenir from the country that sent it.

"For that purpose, we have a U.S. flag on the spacecraft and will have a U.S. flag on that body for the first time," says Mr. Farquhar.

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