NASA's MESSENGER spacecraft maneuvered itself into an orbit closer to Mercury on Friday, putting the probe in position for more detailed measurements of the planet's surface composition, sharper images of its surface, and precise mapping of its terrain.

Artist's concept of MESSENGER at Mercury. Credit: NASA/JHUAPL
 
Now in an extended mission in orbit around Mercury, the MESSENGER spacecraft fired its main engine Monday in the first of two maneuvers to move its orbit closer to Mercury. Monday's burn exhausted the craft's oxidizer supply for its main engine.

Another burn Friday finished the orbit adjustment, placing MESSENGER in an orbit taking it around Mercury every 8 hours. Engineers programmed the spacecraft to target a new orbit between 172 miles and 6,409 miles above Mercury's surface.

MESSENGER was in a higher orbit with a period of 11.6 hours.

MESSENGER arrived at Mercury in March 2011, becoming the first spacecraft to orbit the solar system's innermost planet.

The spacecraft's imaging system collected more than 88,000 pictures of Mercury in the one-year primary mission. Another 80,000 images will be taken in the next year.

MESSENGER began a one-year extended mission last month.

Goals for the next year include tracing the history of Mercury's geology, estimating when volcanism ceased, reconstructing the planet's recent terrain changes, and probing Mercury's tenuous atmosphere and magnetosphere.

As the solar cycle ramps up, scientists will track the sun's changing effects on Mercury.

MESSENGER's new observing post closer to Mercury will allow scientists to gather better images of the planet's cratered surface and higher-resolution chemical maps of Mercury's composition.

"For instance, during the first year of orbital operations, MESSENGER's gamma-ray spectrometer and X-ray spectrometer provided the first measurements of the abundances of many elements on Mercury's surface, including magnesium, sulfur, calcium, and potassium," said Patrick Peplowski, a staff scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. "The eight-hour orbit gives us more observing time at low altitudes, which will permit measurements of variations in surface composition on shorter spatial scales. Such information will give us new insight into the chemical and geological processes by which Mercury's crust was formed."

APL built and operates the MESSENGER mission for NASA.

Ralph McNutt, MESSENGER's project scientst at APL, said the lower orbit will enable better topography measurements and geochemical analysis of Mercury.

"With our new orbit, it feels as though we're embarking on a new mission," said Sean Solomon, MESSENGER principal investigator from the Carnegie Institute of Washington.