Less than five weeks after the start of MESSENGER's extended orbital mission on March 18, 2012, two orbit-correction maneuvers (OCMs) lowered the spacecraft's orbit period from 11.6 hours to 8 hours. Although it would have been possible to complete this 3.6-hour orbit period reduction with a single maneuver, two OCMs minimized risk and enabled full use of the remaining accessible propellant. An initial maneuver, OCM-7, depleted all remaining usable oxidizer in a final firing of the large bi-propellant thruster. A second maneuver, OCM-8, was the last OCM to draw fuel from one of the two main fuel tanks. Both maneuvers slowed the spacecraft while it was near its closest distance from Mercury. Since the amount of usable oxidizer remaining onboard was not precisely known, the first maneuver accounted for an uncertain thruster-on time required to achieve the 53.3 m/s target velocity change and 9-hour, 5-minute orbit period. The uncertainty in usable oxidizer on MESSENGER meant that the spacecraft's large bipropellant thruster and all four of its largest monopropellant thrusters were prepared to operate between 0 and 29 seconds at almost eight times greater thrust level than is possible without the bi-propellant thruster. The result of OCM-7 was a highly accurate velocity change for the full 29-second maximum operation with the bi-propellant thruster. Four days after OCM-7, the clean-up maneuver, OCM-8, used fuel remaining in one of the two main fuel tanks as well as fuel in the auxiliary fuel tank to complete the spacecraft's transition to the 8-hour orbit. The deviation from the desired post-OCM-8 orbit period of 8 hours was less than 2 seconds.
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. In the figure below, the orbit colors are red before OCM-7, tan during OCM-7, purple after OCM-7 and before OCM-8, light blue during OCM-8, and green after OCM-8. 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 OCM-8 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.
Because the sunshade must protect the main part of the spacecraft from direct sunlight during propulsive maneuvers, the timing of these 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 is on the opposite side of the Sun from its position at Mercury orbit insertion. The maneuver dates and velocity change (delta-V or ΔV) values for OCM-7 and OCM-8 appear in the table below.
||April 16, 2012
||Decrease orbit period to 9.1 hours; deplete oxidizer
||April 20, 2012
||Decrease orbit period to 8 hours
Click here for detailed information on all of MESSENGER's propulsive activity from launch to the present.