March 18, 2010
One Year Until Mercury Orbit Insertion
One year from today "” starting at 12:45 a.m. UTC "” MESSENGER will transition from orbiting the Sun to being the first spacecraft ever to orbit the planet Mercury.
"We are finally closing in on the most intense phase of the mission," says MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. "MESSENGER's six and a half years of interplanetary flight are a long warm-up for the main event, when we are in orbit about Mercury. The final year of that flight will be a busy time for the team, as we review orbital operation plans for all spacecraft subsystems."
Entering orbit about Mercury will require the probe to perform the largest propulsive maneuver of the entire mission. For Mercury orbit insertion (MOI), MESSENGER will point its largest thruster very close to the direction of travel and fire that thruster for nearly 14 minutes as well as other thrusters for an additional minute, slowing the spacecraft by 862 meters per second (1,929 miles per hour) and consuming 31% of the propellant that the spacecraft carried at launch.
For an animation of the orbit insertion maneuver and initial orbit of Mercury, see http://messenger.jhuapl.edu/the_mission/gallery.html. Two animations and various view perspectives of the orbit insertion maneuver and initial orbit of Mercury are available here.
"Less than 9.5% 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 future orbit correction maneuvers," says MESSENGER Mission Design Engineer Jim McAdams of the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md.
MESSENGER engineers recently tweaked the strategy for entering into orbit about Mercury. "MESSENGER's propulsion system has consistently performed with high accuracy," McAdams explains. "We replaced an MOI clean-up maneuver with a placeholder for a contingency clean-up maneuver, which reduces risk by simplifying the orbit insertion process."
Another change affecting the orbit insertion is a shift in the spacecraft orbit's tilt relative to Mercury's equator plane, from 80.0° to 82.5°, "a carefully studied change that will improve overall science data returned during the Mercury orbital phase," McAdams says.
The orbit insertion will place the spacecraft into an initial orbit about Mercury that has a 200 kilometer (124 mile) minimum altitude and a period of 12 hours. At the time of orbit insertion, MESSENGER will be 46.14 million kilometers (28.67 million miles) from the Sun and 155.06 million kilometers (96.35 million miles) from Earth