Artist's impression of MESSENGER. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Earth's first extended visit to the planet Mercury is now one step closer to success after the MESSENGER probe celebrated its birthday one day early on Tuesday with a speedy flyby of its home planet to tweak its course for arrival in orbit in 2011.

MESSENGER made its closest approach to terra firma at 1913 GMT (3:13 p.m. EDT) as it flew 1,458 miles over central Mongolia near the capital of Ulaanbaatar.

In an effort to reduce the amount of propellant MESSENGER had to carry during its launch, engineers designed the mission to include a series of six gravity assist maneuvers past Earth, Venus, and Mercury. These flybys can utilize the force of gravity to alter the future trajectory of the spacecraft, allowing it to swing from planet to planet before eventually entering orbit around Mercury.

"One flyby down, five more to go," said Mark Holdridge, mission operations manager for MESSENGER at the Johns Hopkins University's Applied Physics Laboratory. "Now the mission begins."

MESSENGER -- or the Mercury Surface, Space Environment, Geochemistry, and Ranging mission -- was launched one year ago Wednesday aboard a beefed-up Delta 2 Heavy booster from Cape Canaveral. The August 3 launch last year deployed the 2,400-pound craft into a path similar to Earth's orbit around the Sun.

The probe approached Earth over the past few months as ground controllers commanded hydrazine-fueled thrusters to fire in several burns to precisely target MESSENGER for its close encounter. As both the planet and the spacecraft once again arrived near the same spot, Earth's gravity was used to kick MESSENGER into a different orbit to intercept Venus late next year.

"This Earth flyby is the first of a number of critical mission milestones during MESSENGER's circuitous journey toward Mercury orbit insertion," said mission principal investigator Sean Solomon from the Carnegie Institute.

MESSENGER snapped this view of Earth on July 30 as the spacecraft approached its home planet for a gravity-assist flyby. The Narrow Angle Camera in MESSENGER's Mercury Dual Imaging System took the image, showing clear morning skies over Australia, when the spacecraft was about 655,570 miles from Earth. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Northwestern University

The mission's complex suite of science instruments was switched on as it neared Earth to conduct calibrations and test observations to help gauge the operability of the payload before arriving at Mercury. Optical cameras captured images of the Earth and the Moon beginning several days before the flyby, and color pictures were taken throughout Tuesday to be combined into a time lapse movie.

"Not only did it help the spacecraft sharpen its aim toward our next maneuver, it presented a special opportunity to calibrate several of our science instruments," Solomon added.

The probe's atmospheric and surface spectrometers made observations of the Moon, while the energetic particle and plasma spectrometer instrument studied Earth's magnetosphere starting about six hours prior to closest approach.

Over the coming weeks, scientists will continue other measurements by MESSENGER's magnetometer, and solar wind observations are planned for this weekend. Spectral data from Earth's hydrogen corona will also be gathered.

Plans then call for a likely engine burn known as a deep space maneuver in mid-December to accurately set up for the Venus encounter in a little over fourteen months.

NASA says the craft has traveled 930 million miles over the past year on the first leg of a 7.9 billion mile voyage leading to a final arrival at Mercury in March of 2011. In its series of fleeting planetary visits, MESSENGER will gradually increase its orbital speed around Sun by over eleven miles per second.

Next up for MESSENGER is a pair of swings past Venus on October 24, 2006, and June 5, 2007, to further refine its trajectory toward the inner solar system.

More science observations are in the offing at Venus, with a series of instrument tests and data collections of upper atmosphere cloud layers, solar wind particles, and electrical storms with lightning on the night side of Earth's sister planet.

Mercury itself will then provide extra assists in three more flybys in January and October of 2008, followed by a final encounter in September 2009. During these flybys, MESSENGER will map most of the planet and determine surface and atmospheric compositions, greatly building upon information from the earlier Mariner 10 mission that rapidly flew past Mercury three times in 1974 and 1975.

The new MESSENGER data will be used to help plan priorities for the orbital mission, which begins when the probe conducts an engine firing -- burning one-third of the craft's total fuel -- as it is captured in orbit around Mercury. The nominal mission in orbit will last about one Earth year, or four Mercury years.