A NASA spacecraft will swing by the planet Mercury today to observe the small world up close for the first time in more than three decades.

At precisely 2:04:39 p.m. EST (1904:39 GMT), the MESSENGER space probe is due to pass just 124 miles (200 km) above the surface of Mercury in what will be the spacecraft?s first encounter with its target planet.

?Mercury is in our sights,? said MESSENGER principal investigator Sean Solomon of the Carnegie Institution of Washington. ?It?s been a really long road and we?re about to see a payoff.?

Launched in August 2004, MESSENGER — short for the bulky moniker MErcury Surface, Space ENvironment, GEochemistry, and Ranging — is the first spacecraft to examine Mercury since 1975, when NASA?s Mariner 10 made its third and final swing past the planet. Astronomers hope the probe?s cameras and other onboard instruments will help answer the longstanding mysteries of Mercury?s unusually high density, tenuous atmosphere and uncharted terrain, as well as lead to a better understanding of planetary formation.

?Mercury has been the Cinderella child of the solar system for a long time, caught in the shadow of the stepsisters Venus and Mars,? said Solomon. ?But that?s about to change.?

With a diameter of 3,032 miles (4,880 km), Mercury is about one-third the size of Earth and only slightly bigger than the moon. Like Earth, the planet has its own magnetic field, but only a thin hint of atmosphere and an iron-rich core that apparently makes up 60 percent of its interior.

?With MESSENGER, many of Mercury?s secrets will now be revealed,? said James Green, director of NASA?s planetary science division. ?So this is a tremendously exciting time.?

If all goes well during today?s flyby, MESSENGER will skim over the surface of Mercury at about 16,000 mph (25,749 kph) and take more than 1,200 photographs of the half of the planet that went unseen by Mariner 10. The dip through Mercury?s gravitational field will also slow MESSENGER?s speed by 5,000 mph (8,046 kph) and prepare it for another flyby later this year.

The spacecraft was due to turn away from Earth and gaze toward Mercury on Sunday, and is not expected to begin beaming images and data back to scientists until midday Tuesday, 22 hours after its closest approach, mission managers have said. For 14 minutes, the probe will fly in the shadow of Mercury and draw on its batteries until its solar arrays can once more generate power from sunlight.

?It?s always a little source of anxiety, anytime you stop hearing from your spacecraft,? said Solomon.

But MESSENGER's mission control team has three successful planetary flybys already under its belt, giving Solomon confidence for today's flyby.

?So my fingernails will be in pretty good shape,? Solomon said.

Previous flybys — one by Earth in 2005 and two others past Venus — have prepared the spacecraft for today?s Mercury rendezvous. MESSENGER is also expected to return for two more Mercury flybys, in October of this year and September 2009, respectively, before entering orbit around the planet on March 18, 2011 for a one-year science campaign.

The $446 million mission is a collaborative effort between NASA, the Carnegie Institution of Washington and Laurel, Md.-based Johns Hopkins University Applied Physics Laboratory, which designed and built the MESSENGER spacecraft.