The MESSENGER spacecraft successfully completed its first orbit-correction maneuver to reset its periapsis altitude - the lowest point of MESSENGER's orbit about Mercury relative to the planet's surface - from 506 kilometers to approximately 200 kilometers.

MESSENGER was 198 million kilometers (123 million miles) from Earth when the maneuver began at 3:40 p.m. EDT. Mission controllers at The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., verified the start of the maneuver about 10 minutes, 58 seconds later, when the first signals indicating spacecraft thruster activity reached NASA's Deep Space Network tracking station outside Goldstone, California.

This is the first of five maneuvers planned for the primary orbital phase of the mission to keep orbital parameters within desired ranges for optimal science observations.

The spacecraft's main rocket engine fired for only 15 seconds of the total maneuver duration of 2 minutes and 52 seconds. MESSENGER's orbital velocity was changed by a total of 28 m/s to make the corrections essential for continuing the planned measurement campaigns.

"The orbit that the spacecraft follows around the planet slowly changes as time goes by," explained APL's James Hudson, lead guidance and control engineer for the MESSENGER mission operations team.

"Because of Mercury's proximity to the Sun and MESSENGER's highly eccentric orbit, solar gravity has a strong effect on the spacecraft's orbit, particularly periapsis altitude."

MESSENGER Mission Systems Engineer Eric Finnegan, of APL, said that the team was well prepared for the maneuver and everything proceeded as expected. "Initial data from the burn indicate nominal maneuver execution. MESSENGER's orbital trajectory around Mercury has now been reset to continue our in-depth exploration of the innermost planet."

MESSENGER Completes First Mercury Year in Orbit
Laurel MD (SPX) Jun 16 - On June 13, MESSENGER completed its first Mercury year (88 Earth days - the time it takes Mercury to make one revolution around the Sun) in orbit about the innermost planet. The spacecraft has three more Mercury years to go during the primary science phase of the mission.

The spacecraft celebrated this milestone at the tail end of a four-day superior solar conjunction - the tenth since launch - during which the spacecraft was on the opposite side of the Sun from Earth.

Throughout that time, reliable communication between the spacecraft and mission operators at APL was not possible because of interference from the Sun's hot plasma, but telemetry received once MESSENGER came out of conjunction on June 14 confirmed that the spacecraft and all of its systems continue to operate nominally.

MESSENGER's instruments are providing a wealth of new information about the planet closest to the Sun. Tens of thousands of images of major features on the planet - previously seen only at comparatively low resolution - are now available in sharp focus.

Measurements of the chemical composition of Mercury's surface are providing important clues to the origin of the planet and its geological history. Maps of the planet's topography and magnetic field are revealing new clues to Mercury's interior dynamical processes. And regular detections of energetic particles are providing insight into the workings of Mercury's magnetosphere.

MESSENGER team members will be discussing these new findings in a news conference at NASA Headquarters on June 16, 2011, at 1 p.m. EDT.

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