NASA spacecraft zooms above surface of Mercury

WASHINGTON (Reuters) - A car-sized NASA spacecraft zoomed above the surface of Mercury on Monday, viewing rocky terrain never before seen up close on our solar system's sun-baked innermost planet, scientists said.

The MESSENGER probe flew as low as 124 miles near the equator of Mercury as part of its ongoing exploration of the planet nearest the sun, said project scientist Ralph McNutt of Johns Hopkins University Applied Physics Laboratory.

Initial images sent back to Earth showed newly discovered cliffs on Mercury's surface, with the bulk of the data to be transmitted on Tuesday, McNutt said.

"This is all covering about 30 percent of the planet that has never been seen by a spacecraft before," McNutt said in a telephone interview. "As far as we can tell, everything executed just as it was supposed to."

This was the second of three scheduled encounters before MESSENGER enters into orbit around Mercury in 2011. It flew past Mercury on January 14 and will return in September 2009.

McNutt said combining the data collected in January with that expected from Monday's encounter should give scientists a nice idea of the planet's topographical features. The spacecraft snapped about 1,200 images on its latest pass.

Data from the January fly-by showed that volcanic activity played a key role in forging Mercury's surface and that the planet has been shrinking more than expected over time.

The only previous occasions Mercury was visited by a spacecraft were in 1974 and 1975 when NASA's Mariner 10 flew past it three times and mapped about 45 percent of its surface. MESSENGER's January encounter covered another 20 percent.

With the latest fly-by, at a speed of 14,800 mph, studying an area about the same size as South America, only about 5 percent of the surface will remain unseen by a spacecraft, McNutt said.

Mercury's surface is a mixture of plains, craters caused by long-ago impacts with space rocks, and long, winding cliffs like the ones seen on Monday.

MESSENGER, which stands for Mercury Surface, Space Environment, Geochemistry and Ranging, was launched in 2004.

With many scientists now classifying Pluto a dwarf planet, Mercury is considered the solar system's smallest planet, a third the size of Earth and only a bit bigger than the moon.

Separately, NASA said a spacecraft due to study the region where the outer edge of the solar system meets interstellar space launches on October 19 from Kwajalein Atoll in the Pacific.

(Editing by Julie Steenhuysen and Eric Walsh)