Less than five weeks after the start of MESSENGER's first extended mission on 18 March 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 remained 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 first extended mission, the orbit inclination reached a maximum of 84° and the sub-spacecraft latitude at minimum altitude reached 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 resulted in a higher altitude of around 450 kilometers as the orbit's closest point passed 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 was 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.
This final phase of flight operations, which began on 18 March 2013, includes such mission “firsts” as the observations of two comets and a low-altitude campaign, and will conclude with a high-speed Mercury surface impact on or before 30 April 2015. The lowest-altitude (periapsis) portion of each orbit descends throughout the second extended mission (XM2) except during mission-extending orbit-correction maneuvers (OCMs) that raise periapsis altitude and during two times per 88-day Mercury year when the change in periapsis altitude is close to zero. Without these periapsis-raising OCMs, forces acting on the spacecraft would have caused the spacecraft to impact Mercury as early as 21 August 2014.
A most unusual convergence of events in November of 2013 gave MESSENGER scientists the opportunity to observe short-period comet 2P/Encke and hyperbolic-orbit comet 2012 S1 (ISON). The two comets were closest to the MESSENGER spacecraft in orbit around Mercury on 18 and 19 November 2013, respectively. The closest approach between MESSENGER and Encke of 0.0249 AU (astronomical units), which is only 9.7 times the average Earth-Moon distance, occurred a few days before Encke was closest to the Sun (perihelion), and less than 1.5 days before the 0.2420 AU closest approach between MESSENGER and ISON. The figure below depicts the cometary orbits and marks the comet positions at the time of Encke-MESSENGER closest approach. This figure also shows the trajectory that comet ISON would have taken had it remained intact after its perihelion at a solar distance of 0.0125 AU on 28 November 2013. Careful planning culminated in observations of both comets between 26 October and 4 December 2013, with 711 Mercury Dual Imaging System (MDIS) images of ISON (280) and Encke (431) acquired, along with data from MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer (MASCS) and X-Ray Spectrometer (XRS).
The figure below shows the trend of declining periapsis altitude and movement of the periapsis point southward from the north polar region throughout the first two years of XM2. The figure also highlights the locations of hot seasons, when components of the spacecraft sheltered from direct sunlight are subjected to excessive heating from Mercury’s sunlit surface at low altitude. In December 2014 the spacecraft experienced a prolonged period when the Sun’s presence between Earth and Mercury (called solar conjunction) degraded or prevented communication with the spacecraft. Although additional solar conjunctions occur during the final two years of flight operations, the December 2014 solar conjunction is shown since it prevented OCM-11 from targeting a period when minimum altitude would remain near 25 km. In the figure below, the red highlighted portions of each line, which denote solar incidence angle > 84°, indicate when the night side of Mercury limits opportunities to image much of Mercury’s surface. Labels such as “begin 3rd year” in the figure below measure the time elapsed, in Earth years, since the start of the spacecraft’s first orbit around Mercury.
The 26 months from OCM-8 on 20 April 2012 to OCM-9 on 17 June 2014 constituted the longest time between orbit-altering propulsive maneuvers during the MESSENGER orbital mission phase. Each of OCM-9 through OCM-12 not only raised periapsis altitude but also increased orbit period – such that the time for each orbit of Mercury increased from 8h 0m before OCM-9 to 8h 17m after OCM-12. Each of OCMs 9 through 12 targeted a minimum altitude relative to the highest surface feature of 25 km after OCM-9, -10, and -11 and 15 km after OCM-12. The propellant depletion strategy during these four OCMs included exhausting nearly all remaining usable propellant in the second main fuel tank (OCM-10) and drawing most of the estimated usable fuel from the auxiliary fuel tank (OCM-9, OCM-11, and OCM-12). As illustrated below, even the largest of those four OCMs (OCM-11) had little change on the overall size and orientation of the spacecraft’s orbit. Each OCM occurred at the farthest point from Mercury, called apoapsis, in order to most efficiently raise altitude near periapsis. The date, spacecraft Mercury-relative velocity change (ΔV), and purpose of each of OCMs 9 through 12 appear in the table below.
||Purpose and Result
||17 June 2014
||Target 25-km minimum altitude on 12 September 2014
Met 25.2-km minimum altitude on 12 September 2014
||12 September 2014
||Target 25-km minimum altitude on 24 October 2014
Met 25.4-km minimum altitude on 24 October 2014
||24 October 2014
||Target 25-km minimum altitude on 21 January 2015
Met 25.7-km minimum altitude on 21 January 2015
||21 January 2015
||Target 15-km minimum altitude on 1 March 2015
Met 14.9-km minimum altitude on 1 March 2015