MESSENGER eases into Mercury’s orbit

Craft may help explain widespread volcanism on closest planet to the sun

NASA’s MESSENGER spacecraft has become the first probe ever to enter orbit around Mercury.

Having settled the craft into a path around the sun’s nearest neighbor on March 17, mission scientists plan to switch on the robotic craft’s suite of seven instruments March 24, and the first observations from orbit will be beamed to Earth on April 4.

To shield the craft from the planet’s highest temperatures, MESSENGER (short for Mercury Surface, Space Environment, Geochemistry and Ranging) takes a highly elliptical path about Mercury, swooping as close as 200 kilometers from the surface and then venturing more than 15,000 kilometers away every 12 hours.

MESSENGER had already visited the planet during three flybys in 2008 and 2009. During those passes, MESSENGER found evidence that Mercury’s surface has been sculpted by widespread volcanism (SN Online: 11/3/09). The discovery was a puzzle to scientists, who had thought any bubbling volatile gases would have long ago exited the hot planet or never been incorporated into Mercury in the first place.

From orbit, MESSENGER will look for the chemical signature of those volcanic gases and make observations that could explain why the planet’s core accounts for so much of its mass. The craft could also provide details about Mercury’s atmosphere and reveal the presence of water frozen in permanently shadowed craters near the poles.

Since its launch in 2004, MESSENGER has journeyed 7.9 billion kilometers and required gravity assists from Earth, Venus and Mercury in order to be captured by the innermost planet. The craft expended about one-third of its fuel during final maneuvers to reach orbit, using its thrusters to slow its velocity to less than 1 kilometer per second.

INTO ORBIT Mercury’s vast Caloris basin, 1,550 kilometers in diameter, is visible as a large circular orange feature at the center of this false-color picture. The region will be studied extensively by NASA’s MESSENGER spacecraft, which on March 17 became the first probe to ever orbit Mercury. NASA, Johns Hopkins Univ. Applied Physics Laboratory, Arizona State Univ., Carnegie Institution of Washington. Image reproduced courtesy of Science/AAAS

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