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MESSENGER in Orbit (with Sun)Artist's impression of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit at Mercury. MESSENGER launched from Cape Canaveral Air Force Station, Fla., on Aug. 3, 2004, and will begin a yearlong orbital study of Mercury in March 2011. Though the Sun is up to 11 times brighter at Mercury than we see on Earth and surface temperatures can reach 450 degrees Celsius (about 840 degrees Fahrenheit), MESSENGER's instruments will operate at room temperature behind a sunshade of heat-resistant ceramic fabric. The spacecraft will also pass only briefly over the hottest parts of the surface, limiting exposure to heat reradiated from the planet.Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

An artist's impression of the MESSENGER spacecraft in orbit around Mercury.

Image courtesy NASA

Rachel Kaufman

for National Geographic News

Updated March 17, 2011

NASA made history tonight as the MESSENGER probe became the first spacecraft to orbit the tiny planet Mercury.

Launched in 2004, the MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission marks the first time a craft has gone near Mercury since 1975, when NASA's Mariner 10 probe conducted flybys. (Get MESSENGER facts and figures.)

For the past six and a half years MESSENGER has been maneuvering itself into an orbital path via so-called gravity assists, using the tugs from flybys of Earth, Venus, and Mercury itself to speed up and alter course.

At 8:45 p.m. ET, MESSENGER performed a "burn"—essentially "riding its brakes" by firing its main thruster—to slow the spacecraft enough to be captured by Mercury's gravity.

The mission control team at the Johns Hopkins Applied Physics Laboratory in Maryland was monitoring MESSENGER's progress from 96 million miles (155 million kilometers) away.

At 9:10 p.m. engineers confirmed that the burn had occurred. By 9:45 p.m. the probe had turned its antenna back toward Earth and began transmitting more detailed data showing that the 15-minute burn was "clean"—indicating that the probe has entered orbit.

As with burn sequences during the craft's previous flybys, the team had contingencies in place if MESSENGER had failed to enter orbit, Sean Solomon, principal investigator of MESSENGER's science mission, told reporters at a press briefing Tuesday.

But the backup plans, he said, didn't involve an immedite retry and would have substantially changed the time line of the mission.

(Related: "Japan Probe Missed Venus—Will Try Again in Six Years.")

Mercury Probe to Fill in Blanks

With orbital insertion complete, MESSENGER should start collecting science data by early April.

During the probe's year-long mission, it will orbit Mercury twice every 24 hours—conducting the equivalent of two flybys a day and sending back reams of data from a suite of onboard cameras and spectrographs.

For instance, being in orbit will allow the probe to take "ultra high-resolution images" of the planet's entire surface, said Solomon, of the Carnegie Institution of Washington in D.C. (See pictures: "Mercury Craft Shows 'Spider,' Asteroid Assaults.")

Mariner 10 captured just 45 percent of the cratered surface before moving on, and the three previous MESSENGER flybys didn't quite fill in the whole picture.

MESSENGER will also be studying Mercury's atmosphere and inner structure, as well as its magnetic environment, which changes rapidly due to the planet's close interaction with the sun.

(Related: "Final Mercury Flyby Reveals Huge Magnetic 'Power Surges.'")

"We will benefit tremendously from being there, rather than having to take drive-by snapshots," Solomon said.

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