Mercury Orbit Insertion & Station Keeping

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 executed a 15-minute maneuver that placed it into orbit about Mercury, making it the first craft ever to do so, and initiating a multi-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 — began tracking the MESSENGER spacecraft continuously. Nearly 31 hours later, at 6:30 p.m. EDT on March 17, 2011, the number of antennas tracking MESSENGER increased to five — four of these were arrayed together in order to enhance the signal coming from the spacecraft, and a fifth was 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 were configured for the main thruster firing (known as a "burn"), and the spacecraft was turned into the correct orientation for MESSENGER's Mercury orbit insertion maneuver.

In order to slow the spacecraft down sufficiently so that it could be captured into orbit around Mercury, the main thruster began firing at 8:45 p.m. and continued for 15 minutes until 9:00 p.m. About 31% of the spacecraft's original allotment of propellant was required for Mercury orbit insertion, and MESSENGER's thrusters slowed the spacecraft by just over 0.86 kilometers (0.53 miles) per second. As the spacecraft approached Mercury, the largest thruster fired close to the forward velocity direction of the spacecraft. After the thruster finished firing, the spacecraft was turned toward Earth and reconfigured for normal post-maneuver operations. Data were collected by Deep Space Network antennas and transferred to the Mission Operations Center at APL to be analyzed. By 10:00 p.m. EDT the Mission Operations Team confirmed that MESSENGER was successfully captured into orbit around Mercury.

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

The MESSENGER spacecraft continued to orbit Mercury once every 12 hours for the duration of its primary mission. The first two weeks from orbit insertion were focused on ensuring that the spacecraft systems were all working well in the harsh thermal environment of orbit; this interval was known as the orbital commissioning phase. Starting on March 23 the instruments were turned on and checked out, and on April 4 the science phase of the mission began and the first orbital science data from Mercury were returned.

* Ground Receipt Time adjusted for one-way light time, which gradually decreased through the reporting period.
-- Events without specific execution times were 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 were approximately 9 minutes after a maneuver was 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 placed 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 arrived in its primary science orbit, small forces, such as solar gravity — the gravitational attraction of the Sun — slowly changed the spacecraft's orbit. Although these small forces had little effect on MESSENGER's 12-hour orbit period, they increased 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 have prevented satisfactory completion of several science goals.

To keep the spacecraft's minimum altitude below 500 kilometers (310 miles), propulsive maneuvers occurred at least once every Mercury year — one complete revolution around the Sun, or 88 Earth days. The first, third, fifth, and sixth orbit correction maneuvers (OCMs) after Mercury orbit insertion occurred at the farthest orbital distance from Mercury, where a minimum amount of propellant was 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 had an unavoidable side effect of also lowering orbit period by about 15 minutes. The second and fourth maneuvers after orbit insertion increased the orbit period back to about 12 hours by speeding up the spacecraft near the time when it was closest to Mercury.

The first and second of the orbit correction maneuvers as seen from the direction of the Sun and as seen from the direction of the Earth. The spacecraft orbit during each maneuver is shown in blue, the orbit before the first maneuver is shown in light purple, the orbit after the first maneuver and before the second maneuver appears in light green, and the orbit after the second maneuver is shown in red.

Less than five weeks after the start of MESSENGER's extended orbital mission on March 18, 2012, two orbit correction maneuvers lowered the spacecraft's orbit period from 11.6 hours to 8 hours. These seventh and eighth maneuvers after Mercury orbit insertion slowed the spacecraft while it was near its closest distance from Mercury.

The seventh and eighth orbit correction maneuvers as seen from the direction of the Sun. The initial 8-hour orbit remains highly eccentric, with MESSENGER travelling between 278 kilometers (172 miles) and 10,314 kilometers (6,409 miles) above Mercury's surface. The orbit colors are red before the seventh maneuver, tan during the seventh maneuver, purple after the seventh and before the eighth maneuver, light blue during the eighth maneuver, and green after the eighth maneuver.

Because the sunshade must protect the main part of the spacecraft from direct sunlight during propulsive maneuvers, the timing of all OCMs was limited to two periods during Mercury's 88-day orbit around the Sun. These two times when it was safe to adjust the orbit included a few days when Mercury was either near the location of Mercury orbit insertion or near the point at which Mercury was on the opposite side of the Sun from its position at Mercury orbit insertion.

The 8-hour orbit period not only increased the number of orbits per day by 50%, but also provided observation opportunities at maximum altitudes nearly one third lower than during the primary orbital mission. Also, late in the one-year extended orbital mission, the orbit inclination will reach a maximum of 84° and the sub-spacecraft latitude at minimum altitude will reach its most northerly extent at 84°N, allowing for closer study of permanently shadowed regions near Mercury's north pole. However, the absence of planned OCMs after the eighth maneuver results in a higher altitude of around 450 kilometers as the orbit's closest point passes nearest Mercury's north pole in early March 2013.