NASA's Messenger spacecraft swung into position around Mercury Thursday night, making it the first spacecraft ever to orbit the innermost planet.

Engineers at the Johns Hopkins University Applied Physics Laboratory in Maryland, 96 million miles from Mercury, received the signal confirming that Messenger (MErcury Surface, Space ENvironment, GEochemistry and Ranging) had completed its final maneuver at 9:10 pm EDT.

To slow down enough to get caught in Mercury's gravitational field, Messenger fired its main thruster for 15 minutes. The burn slowed the spacecraft by 1,929 mph and used up 31 percent of its original fuel supply.

After finishing the burn, Messenger rotated to face the Earth by 9:45 p.m., and started transmitting data. Engineering and operations teams confirmed the maneuver went according to plan.

The event marks the end of a 6½-year journey for Messenger, which has made 12 laps around the solar system, two flybys past Earth, one past Venus and three past Mercury since launching in August 2004.

Although engineers still need to do some analysis to figure out the spacecraft's exact orbit, they expect Messenger to swoop around Mercury in a highly elliptical orbit once every 12 hours. It will dip within 120 miles of Mercury's surface at its closest point, and go out to 9,320 miles at its farthest.

The orbit goes nearly pole-to-pole, offset by about 7 degrees. That slight tilt is to help get a handle on the planet's gravitational field, said principal investigator Sean Solomon, a planetary scientist at the Carnegie Institution of Washington, in a press conference March 15.

Measurements of the gravitational field "will tell us something about Mercury’s composition, the size of the core and the structure of that core," he said.

One of the mission's main objectives is to figure out why Mercury's core is so big compared to the cores of the other rocky planets. Another is to make high-resolution maps of the whole planet, some of which has still never been seen.

"Many on the science team have been involved from the very beginning," Solomon said. "We are extremely excited to begin that mapping."

Scientists also plan to search for water ice in craters at the poles which, despite Mercury's proximity to the sun and scorching daytime temperatures, are stuck in eternal freezing shadow.

The spacecraft's seven science instruments were turned off for orbit insertion, but they will reactivate March 23. The first orbital image, planned for March 29, will include some uncharted regions near Mercury's south pole.

The science phase of the mission will begin April 4. The Messenger team will release data to the science community at six-month intervals, but will release images at least once a day throughout the mission, Solomon said.

"In addition to the global imaging we’ll be doing, we’ve targeted more than 2,000 areas for ultra-high-res with our narrow-angle camera. Many of them were not discovered until flybys," he said. "We’ve got a long list."

Images: 1) Artist's conception of Messenger approaching Mercury. 2) The target area for Messenger's first image from orbit, including never-before-seen terrain. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

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