WASHINGTON – It has been almost 30 years since scientists got a chance to see Mercury up close. But early Tuesday, NASA hopes to launch a tiny spacecraft on a seven-year trip toward the sun’s closest planet.

Excited scientists have been waiting for new data since 1975 when Mariner 10 made its last flyby of the planet. Then Mariner mapped only 45 percent of the planet’s surface, leaving the rest a mystery.

“There’s a certain science-fiction aspect to it,” said Timothy Dowling, director of the Comparative Planetology Laboratory at the University of Louisville.

“It’s just a place that’s got lots of extremes,” he said. “They’re actually going to answer some questions that have been open for decades.”

The spacecraft will fly by Earth next summer, then around Venus twice, in October 2006 and June 2007, using each planet’s gravity to propel it towards Mercury. It will pass by Mercury three times – in January and October 2008 and September 2009 – before entering the planet’s orbit in March 2011.

Then, the probe will spend a year examining Mercury, mapping its surface, snapping pictures, and gathering data about the planet’s magnetic field and its mysterious core.

The launch window for Messenger – which stands for Mercury Surface, Space Environment, Geochemistry and Ranging mission – opens at 2:16 a.m. Monday, with additional chances through Aug. 14. The probe’s long journey to Mercury will begin at Cape Canaveral Air Force Station atop a Boeing Delta 2 rocket.

Mercury has the thinnest atmosphere of any of the terrestrial planets and is small but very dense – nearly as dense as Earth, yet only slightly larger than the moon. If Earth were a baseball, Mercury would be roughly the size of a golf ball.

Because of its density, Mercury is thought to be as much as two-thirds metal, mostly iron.

Parts of Mercury are extremely hot – as high as 482 degrees Celsius – because of its proximity to the sun. But because its rotation has no tilt, Mercury’s poles are constantly in shadow.

Based on radar photographs taken from the ground, scientists think the polar craters might be harboring ice, and with it dust or other material trapped there long ago, Dowling said.

“That’s fascinating, because it would be a little ice collector collecting water molecules over the eons, and just keeping them sort of in a pristine refrigerator,” he said.

But what most interests scientists is Mercury’s core, and this will be one of Messenger’s primary areas of study. The planet’s density suggests that it might be almost all core, and scientists theorize that its magnetic field may be driven by a liquid, molten layer outside a solid core, like Earth’s. The spacecraft’s array of instruments will help them figure it out.

Sean Solomon of the Carnegie Institute of Washington is the principal investigator for Messenger, which is being managed for the National Aeronautics and Space Administration by Johns Hopkins University’s Applied Physics Laboratory and the Jet Propulsion Laboratory.

“It’s an unusual planet, even by the standards of the inner solar system,” he said.

As always, mission managers were careful to note that anything could go wrong at any time. But they hope Messenger will become the latest in what has been a string of successes for NASA’s robotic program this year, which has included two rovers on Mars and the arrival of the Cassini probe at Saturn in late June.

Messenger is stocked with seven scientific instruments, including two cameras to help create a map of the planet’s surface and a laser altimeter for topographic maps. Several spectrometers will look for elements in the planet’s crust to determine the composition of Mercury’s rocks, and whether the poles really do have caps of ice.

The Energetic Particle and Plasma Spectrometer, which has a magnetometer on a 12-foot boom, will map Mercury’s magnetic field.

“It’s a wonderful mission, one that will address a range of scientific questions at the scale of an entire planet,” Solomon said.

To protect the instruments – and the spacecraft itself – from the extreme heat of the neighboring sun, engineers devised a lightweight sunshade, based partially on the heat-deflecting technology used to shield the space shuttle during its fiery re-entry into the Earth’s atmosphere.

James Leary, the mission systems engineer for Messenger, said the need to develop thermal protection for the spacecraft made the mission both challenging and exciting.

“The front will get about as hot as a pizza oven, while the rest of the spacecraft will remain at room temperature,” Leary said.

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