MESSENGER Begins Historic Orbit Around Mercury

NASA’s MESSENGER spacecraft successfully achieved orbit around Mercury at approximately 9 p.m. EDT Thursday. This marks the first time a spacecraft has accomplished this engineering and scientific milestone at our solar system’s innermost planet.

For the next several weeks, APL engineers will be focused on ensuring the spacecraft’s systems are all working well in Mercury’s harsh thermal environment. Starting on March 23, the instruments will be turned on and checked out, and on April 4 the mission’s primary science phase will begin.

Getting into Mercury orbit…

On March 18, 2011 UTC (March 17, 2011 EDT), after almost five years in development and more than six and a half years in cruise toward its destination, NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft will execute a 15-minute maneuver that will place it into orbit about Mercury, making it the first craft ever to do so, and initiating a one-year science campaign to understand the innermost planet. The Mercury Orbit Insertion maneuver and subsequent orbital activities are described in the next few pages.

Just over 33 hours before the main Mercury orbit insertion event, two antennas from the Deep Space Network — one main antenna and one backup — will begin to track the MESSENGER spacecraft continuously. Nearly thirty-one hours later, at 6:30 p.m. EDT on March 17, 2011, the number of antennas tracking MESSENGER will increase to five — four of these are arrayed together in order to enhance the signal coming from the spacecraft, and a fifth will be used for backup.

About two and a half hours later, at 8:00 p.m. EDT, the solar arrays, telecommunications, attitude control, and autonomy systems will all be configured for the main thruster firing (known as a “burn”), and the spacecraft will be turned into the correct orientation for MESSENGER’s Mercury orbit insertion maneuver.

In order to slow the spacecraft down sufficiently so that it can be captured into orbit around Mercury, the main thruster will begin firing at 8:45 p.m. and will continue for 15 minutes until 9:00 p.m. About 31% of the spacecraft’s original allotment of propellant is required for Mercury orbit insertion, and MESSENGER’s thrusters must slow the spacecraft by just over 0.86 kilometers (0.53 miles) per second. As the spacecraft approaches Mercury, the largest thruster must fire close to the forward velocity direction of the spacecraft. After the thruster has finished firing, the spacecraft will be turned toward Earth and reconfigured for normal post-maneuver operations. Data will be collected by Deep Space Network antennas and transferred to the Mission Operations Center at APL to be analyzed. It is expected that by 10:00 p.m. EDT the Mission Operations Team will be able to confirm that MESSENGER has been successfully captured into orbit around Mercury.

Approximately one and a half hours after the maneuver is complete, the DSN coverage will be stepped back to two stations. At 2:47 a.m. EDT on March 18, the spacecraft will begin its first full orbit around Mercury (as measured from the highest point in the orbit). About 10 hours later, the Deep Space Network coverage will be further reduced to continuous coverage with only one station.

The MESSENGER spacecraft will continue to orbit Mercury once every twelve hours for the duration of its primary mission. The first two weeks from orbit insertion will be focused on ensuring that the spacecraft systems are all working well in the harsh thermal environment of orbit; this interval is known as the orbital commissioning phase. 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.

* Ground Receipt Time adjusted for one-way light time, which gradually decreases through reporting period.

— Events without specific execution times are initiated by direct commands from the ground.

This table summarizes the spacecraft events surrounding Mercury orbit insertion. Note that the times given in the first column are ground receipt times, which are approximately 9 minutes after a maneuver is executed on the spacecraft.

Three views of MESSENGER’s insertion into orbit about Mercury are shown above; they include a view from the direction of Earth, a view from the direction of the Sun, and a view from over Mercury’s north pole looking down toward the planet. Time is given in Coordinated Universal Time (UTC). The 15-minute orbital insertion maneuver is shown in light blue in the figures and places the spacecraft into the primary science orbit, which is shown in dark blue. The bright areas near the poles indicate portions of the surface not imaged by either Mariner 10 or MESSENGER during their respective flybys.

…and staying there

After MESSENGER arrives in its primary science orbit, small forces, such as solar gravity — the gravitational attraction of the Sun — slowly change the spacecraft’s orbit. Although these small forces have little effect on MESSENGER’s 12-hour orbit period, they can increase the spacecraft’s minimum altitude, orbit inclination, and latitude of the surface point below MESSENGER’s minimum altitude. Left uncorrected, the increase in the spacecraft’s minimum altitude would prevent satisfactory completion of several science goals.

To keep the spacecraft’s minimum altitude below 500 kilometers (310 miles), propulsive maneuvers must occur at least once every Mercury year — one complete revolution around the Sun, or 88 Earth days. The first, third, and fifth maneuvers after Mercury orbit insertion will occur at the farthest orbital distance from Mercury, where a minimum amount of propellant will be used to slow the spacecraft just enough to lower the minimum altitude to 200 kilometers (124 miles). The act of lowering the spacecraft’s altitude in this way has an unavoidable side effect of also lowering orbit period by about 15 minutes.

The second and fourth maneuvers after orbit insertion will increase the orbit period back to about 12 hours by speeding up the spacecraft around the time when it is closest to Mercury. Because the sunshade must protect the main part of the spacecraft from direct sunlight during propulsive maneuvers, the timing of these maneuvers is limited to a few days when Mercury is either near the same point in its orbit as it was during Mercury orbit insertion, or near the point where Mercury is on the opposite side of the Sun from that for orbit insertion.