A NASA Discovery mission to conduct the first orbital study
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Mission Design
Launch and Cruise  |  Gravity Assists  |  Trajectory Correction Maneuvers  |  Working from Orbit  |  Extended Mission

Working from Orbit

MESSENGER's trajectory about Mercury began as a highly eccentric (egg-shaped) orbit, about 200 kilometers (124 miles) above the surface at the lowest point and more than 15,193 kilometers (9,420 miles) at the highest. At the outset of the orbital phase of the mission, the plane of the spacecraft's orbit was inclined 82.5° to Mercury's equator, and the low point in the orbit was reached at a latitude of 60° N. The low-altitude segments of the orbit over the northern hemisphere allowed MESSENGER to conduct a detailed investigation of the geology and composition of Mercury's giant Caloris impact basin - the planet's largest known surface feature.

About 31% of the spacecraft's propellant was required for Mercury orbit insertion (MOI) – the process of placing the spacecraft into its primary science orbit around Mercury. 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 was pointed close to the forward velocity direction of the spacecraft. Three views of Mercury orbit insertion are shown below; 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. The maneuver, which lasted about 15 minutes and is shown in light blue in the figures, placed the spacecraft into the primary science orbit, which is shown in dark blue in the figures. The bright areas near the poles indicate portions of the surface not imaged by either Mariner 10 or MESSENGER during their respective flybys.

Click either image above to enlarge.

After MESSENGER arrived in the primary science orbit, small forces – such as the gravitational attraction of the Sun – slowly change the spacecraft's orbit. Although these small forces have little effect on MESSENGER's 8- to 12-hour orbit period, they can change the spacecraft's minimum altitude, orbit inclination, and latitude of the surface point below MESSENGER's minimum altitude. The change in these orbit characteristics is an increase from one orbit to the next during the first two years after MOI, and a decrease during all subsequent years of the mission. Left uncorrected, the increase in the spacecraft's minimum altitude would have prevented satisfactory completion of several science goals.

To keep this minimum altitude below 500 kilometers (310 miles) during the first year after MOI, propulsive maneuvers that lowered the minimum altitude were completed once every Mercury year - one complete revolution around the Sun, or 88 Earth days. The first, third, fifth, and sixth maneuvers after MOI were at the farthest distance from Mercury where an orbit-correction maneuver (OCM) slowed the spacecraft just enough to lower the minimum altitude to 200 kilometers (124 miles). The image below depicts the first of these orbit-correction maneuvers as seen from the direction of the Sun and as seen from the direction of the Earth. The spacecraft orbit during the orbit insertion maneuver is shown in blue, the orbit before the maneuver is shown in light purple, and the orbit after the maneuver appears in light green. A reduction in spacecraft orbit period of about 15 minutes occurs when the minimum altitude is lowered to 200 km.

Click either image above to enlarge.

The second and fourth maneuvers after MOI increased the orbit period to about 12 hours by speeding up the spacecraft near its closest distance from Mercury. These maneuvers occurred approximately 44 days after the first and third orbit correction maneuvers. In the figure above and on the right side, the orbit just before the second maneuver is shown in light green, the orbit during the maneuver is shown in blue, and the orbit after the maneuver is shown in dark red. Because the sunshade must protect the main part of the spacecraft from direct sunlight during propulsive maneuvers, the timing of these orbit correction maneuvers is limited to two periods during Mercury's 88-day orbit around the Sun. These two times when it is safe to adjust the orbit include a few days when Mercury is either near the location of Mercury orbit insertion or near where Mercury is on the opposite side of the Sun from the side it was on for Mercury orbit insertion. The final maneuver dates and velocity change (delta-V or ΔV) values are shown in the table below.

Maneuver Calendar Date Delta-V (m/s) Purpose
MOI 18 Mar 2011 861.7 insert spacecraft into orbit around Mercury
OCM-1 15 Jun 2011 27.8 lower minimum altitude to 200 kilometers
OCM-2 26 Jul 2011 4.1 increase orbit period to 12 hours
OCM-3 07 Sep 2011 25.0 lower minimum altitude to 200 kilometers
OCM-4 24 Oct 2011 4.2 increase orbit period to 12 hours
OCM-5 05 Dec 2011 22.2 lower minimum altitude to 200 kilometers
OCM-6 03 Mar 2012 19.2 lower minimum altitude to 200 kilometers

Click here for detailed information on all of MESSENGER's propulsive activity from launch to the present.

MESSENGER's 12-month first extended mission covered a third and fourth Mercury solar day in orbit. The initial year of the first extended mission reaped the benefit from increased solar activity as solar maximum neared for instruments that measure surface composition and fields and particles in the vicinity of Mercury. The first extended mission enabled coverage of small gaps in imagery near Mercury's north pole and provided opportunity for new global surface measurement products.

MESSENGER's 26-month second extended mission covers an additional 4½ Mercury solar days in orbit, providing opportunity for further global surface measurement products. The second extended mission has enabled coverage of permanently shadowed craters near Mercury's north pole and refinement of Mercury’s gravity field, as well as improvement in numerous global and north-polar region image products and surface composition maps. The later portion of this mission phase includes a low-altitude campaign that provides opportunities for measurements and images at extraordinarily high resolution.

The spacecraft orbit views below include descriptions such as "dawn-dusk" and "noon-midnight" that indicate the surface lighting conditions directly beneath the orbit near both Mercury equator crossings. The "north" direction corresponds to the direction of Mercury's axis of rotation (from Mercury's center toward the north pole).

Three perspectives of the spacecraft orbit at Mercury Click the image on the left to enlarge.

Three perspectives of the spacecraft orbit at Mercury

Throughout the orbital mission, the orbital line of apsides, which connects the orbit's lowest and highest points, rotates clockwise around Mercury. The argument of periapsis (ω) is the angle between the northerly crossing of Mercury's equatorial plane and the minimum-altitude end of the line of apsides, as measured in the spacecraft's orbit plane. When ω is greater than 90°, the strongest gravitational forces that act on the spacecraft's trajectory, those due to Mercury and the Sun, increase the spacecraft's minimum altitude and shift the tilt (inclination) of the orbit closer to passing over the north and south poles. The argument of periapsis reached 90° on 6 March 2013, just prior to the end of MESSENGER’s first extended mission. As the argument of periapsis decreased below 90°, the spacecraft orbit's inclination decreased (moved toward the equatorial plane) and the minimum altitude decreased. After the final planned orbit-correction maneuver, OCM-17 on 24 April 2015, the minimum altitude will decrease until the spacecraft impacts Mercury’s surface no later than 30 April 2015.


Three perspectives of the spacecraft orbit at Mercury Click the image on the left to enlarge.

Rotation of orbit line of apsides from orbit insertion through the projected date of Mercury impact

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