A NASA spacecraft is closing in on Mercury to snap pictures of uncharted territory Tuesday when it whips around the planet for the final time.

The Messenger probe will skim just 142 miles above Mercury at its closest approach during the flyby, the last of three designed to guide the spacecraft into orbit around the planet in 2011. More than 1,500 photographs of Mercury are expected to be taken, some of regions never before observed up close.

"A planetary flyby is really like Christmas morning for scientists," said Messenger principal investigator Sean Solomon of the Carnegie Institution of Washington. "We expect to be surprised and we expect to be delighted."

On Sunday, Messenger snapped a photo of Mercury that revealed the planet as a half-lit, but desolate-looking world seen from a distance of about 418,000 miles away.

Messenger will make its closest approach to Mercury Tuesday at about 6 p.m. EDT when it speeds by at about 12,000 mph. The planet's gravity is expected to slow Messenger by about 6,000 mph during the flyby and place it on track to enter orbit around Mercury in March 2011.

The $446 million spacecraft flew by Mercury twice in 2008 to map the planet in unprecedented detail while using the rocky world's gravitational pull to refine its flight path through space.

In all, Messenger has photographed about 90 percent of Mercury's surface and is expected to cover another 5 percent of unmapped terrain when it zooms by Tuesday. The spacecraft is the first probe to visit Mercury since NASA's Mariner 10 mission in the mid-1970s.

Unlike Messenger's first two flybys — which revealed the first up-close views of Mercury in decades — Tuesday's rendezvous is aimed at observing specific points on the planet's surface. The probe will target its camera eyes at interesting craters and measure Mercury's magnetosphere and tenuous atmosphere, as well as the planet's odd, comet-like tail of trace gases.

"This is the last look at Mercury's equatorial region, and it's the last time we fly through the tail," Solomon said.

When Messenger arrives in its final orbit around Mercury, it will begin a long-awaited observation phase that will complete its new maps of the planet.

"We're all anxiously looking forward to the great science coming out of this final flyby," said Anthony Carro, Messenger's program executive at NASA Headquarters in Washington, D.C.

NASA launched Messenger — short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging — in 2004. The probe swung past Earth once and Venus twice before beginning its three Mercury flybys.