MESSENGER Primed for Mercury Orbit

After more than a dozen laps through the inner solar system and six planetary flybys, NASA’s MESSENGER spacecraft will move into orbit around Mercury on at around 9 p.m. EDT on March 17, 2011. The durable spacecraft — carrying seven science instruments and fortified against the blistering environs near the Sun — will be the first to orbit the innermost planet.

“From the outset of this mission, our goal has been to gather the first global observations of Mercury from orbit,” says MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. “At the time of our launch more than six and a half years ago, that goal seemed but a distant dream. MESSENGER is now poised to turn that dream into reality.”

Just over 33 hours before the main Mercury orbit insertion maneuver, two antennas from NASA’s Deep Space Network — one main antenna and one backup — will begin to track the MESSENGER spacecraft continuously. At 6:30 p.m. EDT on March 17, the number of antennas tracking MESSENGER will increase to five — four of these will be arrayed together to enhance the signal from the spacecraft, and a fifth will be used for backup.

At about 8 p.m., the solar arrays, telecommunications, attitude control, and autonomy systems will be configured for the main thruster firing (known as a “burn”), and the spacecraft, operating on commands transmitted last week from Earth, will be turned to the correct orientation for MESSENGER’s Mercury orbit insertion maneuver.

To slow the spacecraft down sufficiently to be “captured” by Mercury, MESSENGER’s main thruster will fire for about 15 minutes beginning at 8:45 p.m. This burn will slow the spacecraft by 1,929 miles per hour (862 meters per second) and consume 31 percent of the propellant that the spacecraft carried at launch. Less than 9.5 percent of the usable propellant at the start of the mission will remain after completing the orbit insertion maneuver, but the spacecraft will still have plenty of propellant for orbit adjustments during its yearlong science campaign.

After the burn, the spacecraft will turn toward Earth and resume normal operations. Data will be collected by Deep Space Network antennas and transferred to the Mission Operations Center at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., to be analyzed. It is expected that by 10 p.m. EDT, mission operators will be able to confirm that MESSENGER has been successfully captured into orbit around Mercury.

The maneuver — which will be completed at a time that MESSENGER is more than 96 million miles from Earth — will place the probe into an orbit that brings it as close as 124 miles to Mercury’s surface. At 2:47 a.m. EDT on March 18, the spacecraft will begin its first full orbit around Mercury, and the probe will continue to orbit Mercury once every 12 hours for the duration of its primary mission.

“For the first two weeks of orbit, we’ll be focused on ensuring that the spacecraft systems are all working well in Mercury’s harsh thermal environment,” says APL’s Eric Finnegan, the MESSENGER Mission Systems Engineer. “Starting on March 23 the instruments will be turned on and checked out, and on April 4 the science phase of the mission will begin and the first orbital science data from Mercury will be returned.”

While in orbit, MESSENGER’s instruments will perform the first complete reconnaissance of the cratered planet’s geochemistry, geophysics, geological history, atmosphere, magnetosphere, and plasma environment.

“The marathon cruise phase of the MESSENGER mission is nearing the finish line,” says Solomon. “Like a seasoned runner, the MESSENGER team is positioned to break through the tape. We are extremely excited by the prospect that orbital operations will soon begin.”