A NASA probe made its second Mercury flyby early Monday as closes in on the closest planet to the sun.

The MESSENGER spacecraft was due to pick up a gravitational boost during the rendezvous today at 4:45 a.m. EDT (0845 GMT) that will help it settle into orbit around Mercury in 2011.

But scientists also directed MESSENGER's cameras and sensors to capture new images and data from areas of the planet that remained uncharted after its first flyby on Jan. 14.

"Mercury has been a real anomaly in that, up until now, we have not seen the entire surface on one of the bodies closest to the Earth," said Sean Solomon, MESSENGER's principal investigator at the Carnegie Institution of Washington, during a teleconference last week.

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At its closest approach, MESSENGER, short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, was expected to swing within 125 miles (200 km) of the planet's surface.

Mission controllers expected to lose contact with the spacecraft at certain times while it twisted and turned for better views of Mercury. MESSENGER also switched from solar to battery power for 17 minutes while flying through the planetary shadow.

Scientists eagerly await the new images from never-before-seen regions of Mercury, which add up to about 30 percent of the planet's surface.

The spacecraft was slated to snap 629 images specifically for nine large image mosaics that will help scientists begin mapping Mercury.

Researchers hope to begin receiving new data from MESSENGER about 21 hours after it leaves Mercury's shadow, in the very early morning hours of U.S. EDT on Tuesday.

MESSENGER is the first spacecraft in 33 years to encounter Mercury up close since NASA's Mariner probe buzzed the planet three times in 1974 and 1975.

The new spacecraft swung by Earth once and Venus twice since launching in August 2004, and has now completed two of three Mercury flybys before going into orbit around the planet.

Mission controllers saved on precious propellant by taking advantage of the solar wind's force to alter the probe's trajectory. The flyby solar sailing maneuvers marked the successful execution of a "3-D complex exercise in threading the needle," Solomon said.

MESSENGER's first flyby on settled an old scientific debate by showing how volcanoes have shaped the planet's flat, smooth plains. It also detected Mercury's magnetic field, which is shaped by the solar wind's bombardment into a tear drop with the flat face toward the sun.

The second flyby today allowed the spacecraft to peer at the opposite side of the planet.

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