NASA probe to fly past little, sun-baked Mercury

By Will Dunham

WASHINGTON (Reuters) - A NASA probe next week will become the first spacecraft in 33 years to fly by Mercury, a sojourn scientists hope will unlock the secrets of the small sun-baked planet.

NASA's car-sized MESSENGER spacecraft is scheduled to zip about 124 miles above the cratered, rocky surface of the closest planet to the sun on Monday, part of a mission designed to place it into orbit around Mercury in 2011.

"I think we're in for some big surprises," Faith Vilas, one of the scientists involved in the mission, told reporters during a conference call on Thursday.

"You know that we can't get cocky about this and say, 'Oh, it's going to look like this.' Every solar system body looks very different from every other solar system body," she added.

MESSENGER is expected to get the first spacecraft measurements of the mineral and chemical makeup of Mercury's surface, and to also retrieve key data about the planet's internal structure including its core.

The probe will also study Mercury's global magnetic field, and scientists also hope the mission will add to the understanding of the planet's gravity field.

MESSENGER's instruments will collect more than 1,200 images and make other observations during this initial fly-by. It will be the first up-close measurements since the Mariner 10 spacecraft's third and final fly-by in 1975.

MESSENGER, which stands for Mercury Surface, Space Environment, Geochemistry and Ranging, was launched in 2004. The probe has meandered through the inner solar system on the way to Mercury, already having flown past Earth once and Venus twice.

THE OTHER SIDE

Scientists said MESSENGER is expected to retrieve far more data about Mercury than did Mariner 10, which surveyed only one hemisphere of the planet. During next week's fly-by, scientists will begin to collect images of the other hemisphere.

"Mercury really has been long ignored -- too long," NASA's planetary science chief James Green said. "With MESSENGER, we believe that many of Mercury's secrets will now be revealed."

With Pluto demoted from planetary status, Mercury is now recognized as the smallest planet in the solar system. Its diameter is about 3,000 miles, about a third the size of Earth and only a bit larger than our moon. Mercury's surface is a mix of craters, plains and long, winding cliffs.

The planet's average distance from the sun is 36 million miles, about two-thirds closer than the Earth. It orbits the sun once every 88 Earth days, or about a quarter of an Earth year.

The spacecraft is due to make its closest approach to Mercury at 2:04 p.m. EST on Monday (1904 GMT).

Mercury, along with Venus, Earth and Mars, are the solar system's family of four rocky inner planets.