Learn about how MESSENGER utilized events such as planetary flybys and trajectory correction maneuvers to enter an orbit around Mercury, the fastest planet in our solar system. MESSENGER used gravity assists from Earth, Venus and Mercury to lower its speed relative to Mercury at orbit insertion.

In a gravity assist, a spacecraft flies close by a planet and picks up (or loses) a tiny amount of the planet's angular momentum around the Sun. The planet is so massive (compared with the spacecraft) that its orbit does not change. But each gravity assist changed the shape, size and tilt of MESSENGER's orbit until the onboard propellant was sufficient to insert the spacecraft into its planned scientific orbit around Mercury. "Mercury orbit insertion" is the mission planners' term for the maneuver that moved MESSENGER from an orbit around the Sun to an orbit around Mercury.

Launch and Cruise

MESSENGER launched from Cape Canaveral Air Force Station, Fla., on August 3, 2004. It returned to Earth for a gravity boost on August 2, 2005, then it flew past Venus twice, in October 2006 and June 2007. The spacecraft used the tug of Venus’ gravity to resize and rotate its trajectory closer to Mercury’s orbit.

MESSENGER Trajectory

Click the options below to highlight different portions of MESSENGER's trajectory.
Full Trajectory Earth to Venus Venus to Mercury Mercury Flybys Mercury Orbit

The mission began with launch from Cape Canaveral Air Force Station, Fla., on August 3, 2004. After returning to Earth for a gravity assist on August 2, 2005, MESSENGER headed toward the first of two Venus flybys. The first occurred on October 24, 2006, when the spacecraft approached the planet from its dayside. MESSENGER again flew past a mostly sunlit Venus on June 5, 2007.

The Mercury flybys on January 14, 2008, October 6, 2008, and September 29, 2009, provided the first close-up look at Mercury in more than 30 years. On all three flybys, the spacecraft departed with sunlit views of the planet, taking pictures of the regions not seen by the first spacecraft to visit Mercury - Mariner 10. This early science return was invaluable in planning observation strategies for MESSENGER's historic yearlong orbit mission, which began in March 2011.

On the way, MESSENGER also performed several trajectory correction maneuvers, thruster burns that adjust its path to Mercury.

Three Mercury flybys, each followed about two months later by a course correction maneuver, put MESSENGER in position to enter Mercury orbit in March 2011. During the flybys – which occurred in January 2008, October 2008 and September 2009 – MESSENGER mapped nearly the entire planet in color, imaged most of the areas unseen by Mariner 10, and measured the composition of the surface, atmosphere and magnetosphere. The three successful flybys of Mercury by the MESSENGER spacecraft returned the first new data from Mercury in more than 30 years. This data was invaluable as plans were made for MESSENGER’s yearlong orbital mission.

During the 4.9-billion (4,900,000,000) mile (about 7.9-billion kilometers) journey from launch to Mercury orbit, the spacecraft experienced a range of extreme conditions. One such extreme included repeated passes at a distance 70% closer to the Sun than the Earth's average distance from the Sun. At this distance, the spacecraft received about 11 times more of the Sun's thermal energy than Earth-orbiting satellites. Shortly after each of the first two Mercury flybys, the spacecraft approached the ultimate speed record for all spacecraft up to that point by achieving speeds of over 140,000 miles per hour (over 225,300 kilometers per hour) relative to the Sun - about eight times faster than the speed of NASA's Space Shuttle in low-Earth orbit. The Helios-2 spacecraft achieved at least 150,000 miles per hour relative to the Sun on April 17, 1976.

The final maneuver dates and velocity change (ΔV) values for the mission’s deep space maneuvers (DSMs) and trajectory correction maneuvers (TCMs) that took place during the cruise phase are shown in the table below. The DSM designation identifies the TCMs with the largest velocity change, which include all but one of the maneuvers (TCM-3) required in the pre-launch planned cruise-phase trajectory. Contingency maneuvers TCM-4, TCM-7, TCM-8, TCM-14, and TCM-17 were never implemented due to the achievement of sufficient accuracy with the prior TCM. After TCM-19 only DSMs were required before Mercury orbit insertion, based on the effective implementation of a strategy to orient the spacecraft sunshade and solar panel  for sufficient durations to use sunlight particles to correct small TCM implementation errors (known as “solar sailing”),

Primary Maneuvers

Maneuver Alternate Name Calendar Date Delta-V (m/s) Purpose
DSM-1 TCM-9 12 Dec 2005 315.6 target the first Venus flyby
DSM-2 TCM-18 17 Oct 2007 227.4 target the first Mercury flyby and test the spacecraft configuration to be used in future maneuvers; component maneuver
DSM-3 TCM-23 19 Mar 2008 72.2 target the second Mercury flyby and test spacecraft rotation during a maneuver as required for Mercury orbit insertion
DSM-4 (part 1) TCM-29A 4 Dec 2008 222.1 target the third Mercury flyby
DSM-4 (part 2) TCM-29B 8 Dec 2008 24.7 target the third Mercury flyby and test spacecraft rotation during a maneuver
DSM-5 TCM-35 24 Nov 2009 177.75 target Mercury arrival for orbit insertion

Secondary Maneuvers

Maneuver Calendar Date Delta-V (m/s) Purpose
TCM-1 24 Aug 2004 17.9 correct most inaccuracies resulting from launch
TCM-2 24 Sep 2004 4.6 correct all remaining inaccuracies resulting from launch
TCM-3 18 Nov 2004 3.2 target the Earth flyby
TCM-5 23 Jun 2005 1.1 target the Earth flyby
TCM-6 21 Jul 2005 0.2 target the Earth flyby
TCM-10 22 Feb 2006 1.3 target the first Venus flyby
TCM-11 12 Sep 2006 1.7 target the first Venus flyby; component maneuver
TCM-12 5 Oct 2006 0.5 target the first Venus flyby
TCM-13 2 Dec 2006 25.6 target the second Venus flyby; component maneuver
TCM-15 25 Apr 2007 0.6 target the second Venus flyby
TCM-16 25 May 2007 0.2 target the second Venus flyby
TCM-19 19 Dec 2007 1.1 target the first Mercury flyby

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

Gravity Assists

The MESSENGER spacecraft greatly reduced its fuel load requirement by using the gravitational attraction of planets to bend, rotate, and shrink its orbit about the Sun. The more massive the planet and the closer the spacecraft passes above the planet, the greater the change in the spacecraft's orbit. For an August 2004 launch, the journey to Mercury had one Earth flyby, two Venus flybys, three Mercury flybys, and five major course-correction maneuvers prior to Mercury orbit insertion (also called "Mercury arrival"). Given the launch date and spacecraft operational rules (e.g., sunshade must protect the spacecraft from overheating; Earth-based antennas must monitor every course-correction maneuver without interference from the Sun; science goals and other factors limit the shape, orientation, and dimension of the orbit around Mercury; the spacecraft must carry enough fuel for all course-correction maneuvers), the path from Earth to Mercury required the lowest amount of fuel.

MESSENGER's flight to Mercury moved closer to the Sun as it caught up with the fastest planet in our solar system. Earth orbits the Sun every 365 days (Earth's "orbit period"), travels at an average speed of 29.8 kilometers per second (18.5 miles per second) relative to the Sun, and has an average distance from the Sun of 1 Astronomical Unit (an "AU" is the average Sun-Earth distance, about 150 million kilometers or 93 million miles).

Figure 1 presents the relative dimensions of and orientations of the orbits of Earth and Mercury. Table 1 shows how each gravity-assist flyby changed these orbit characteristics.

Figure 1. Relative dimensions and orientations of orbits of Earth and Mercury

Figure 2. MESSENGER mission timeline featuring major trajectory adjustments (DSM dates may change)

Planetary Flyby Plots

Trajectory Correction Maneuvers

the orientation soon after the start of orbit correction maneuver 14 (OCM-14) Click on the image above for a larger version

This view of MESSENGER shows the orientation at the start of orbit-correction maneuver 18 (OCM-18). The dark blue rectangles represent the front (sunlit) sides of the solar arrays. The large white feature is the spacecraft's sunshade, which pointed toward the Sun. Colored arrows indicate the directions of Earth, the Sun, the spacecraft's velocity with respect to Mercury, and the course-correction velocity change (delta-V or V). The "spacecraft +x axis" label identifies an axis direction in the local spacecraft body-fixed coordinate system. Above the MESSENGER spacecraft is a portion of Mercury’s northern hemisphere, with latitude lines at 15° increments and longitude lines at 30° increments. The curved purple line depicts MESSENGER’s orbit about Mercury. Active thruster plumes are no longer depicted, since such a depiction would not accurately portray the appearance of helium gas leaving the spacecraft via the primary thrusters for this maneuver.

The nineteenth OCM since Mercury orbit insertion, OCM-18 raised the spacecraft’s minimum altitude above Mercury from 5.3 kilometers (3.3 miles) to 6.3 kilometers (3.9 miles), ensuring impact onto Mercury during the desired orbit. During OCM-18, a ΔV of 0.45 meters per second (1.00 miles per hour) was imparted by releasing helium gas pressurant through the four largest monopropellant thrusters. Implemented when the spacecraft was close to the farthest point in its orbit from Mercury, OCM-18 increased the spacecraft’s speed relative to Mercury. OCM-18 also increased the spacecraft’s orbit period to 8h 21.3m. During OCM-18, the sunshade protected heat-sensitive parts from direct sunlight. OCM-18 was the fourth MESSENGER course-correction maneuver to use helium gas pressurant to change the spacecraft’s orbit. The final OCM in the MESSENGER low-altitude hover campaign, also called the extension of the Second Extended Mission, OCM-18 used the four medium thrusters that point in about the same direction as the large bi-propellant thruster.

Mission controllers at The Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, verified the start of the maneuver about 8.6 minutes after the start of OCM-18, at 5:28 pm Eastern Daylight Time on April 28, 2015, when the first signals indicating spacecraft thruster activity reached NASA’s Deep Space Network (DSN) tracking station in Goldstone, California. The 3.02-minute-long maneuver began at about 5:20 pm Eastern Daylight Time.

Choose a date from the dropdown menu to view more information, as written on the selected date:

Working from Orbit


Click either image above to enlarge.

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.

After MESSENGER arrived in the primary science orbit, small forces – such as the gravitational attraction of the Sun – could slowly change the spacecraft's orbit. Although these small forces had little effect on MESSENGER's 8- to 12-hour orbit period, they could have changed the spacecraft's minimum altitude, orbit inclination, and latitude of the surface point below MESSENGER's minimum altitude. The change in these orbit characteristics was 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.

Click either image above to enlarge.

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 occured when the minimum altitude was lowered to 200 km.

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 protected the main part of the spacecraft from direct sunlight during propulsive maneuvers, the timing of these orbit correction maneuvers 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 where Mercury was 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

Three perspectives of the spacecraft orbit at Mercury

Three perspectives of the spacecraft orbit at Mercury

Click the image above to enlarge.

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 measured 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 covered an additional 4½ Mercury solar days in orbit, providing opportunity for further global surface measurement products. The second extended mission 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 included a low-altitude campaign that provided 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).

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

Three perspectives of the spacecraft orbit at Mercury

Click the image above to enlarge.

Throughout the orbital mission, the orbital line of apsides, which connects the orbit's lowest and highest points, rotated 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 acted on the spacecraft's trajectory, those due to Mercury and the Sun, increased the spacecraft's minimum altitude and shifted 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 decreased until the spacecraft impacted Mercury's surface 30 April 2015.

Extended Mission

First Extended Mission

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 was 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 protected 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.

Maneuver Calendar Date Delta-V (m/s) Purpose
OCM-7 April 16, 2012 53.3 Decrease orbit period to 9.1 hours; deplete oxidizer
OCM-8 April 20, 2012 31.5 Decrease orbit period to 8 hours

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

Second Extended Mission

This final phase of flight operations, which began on 18 March 2013, include such mission "firsts" as the observations of two comets and a low-altitude campaign, and concluded with a high-speed Mercury surface impact on 30 April 2015. The lowest-altitude (periapsis) portion of each orbit descended throughout the second extended mission (XM2) except during mission-extending orbit-correction maneuvers (OCMs) that raised periapsis altitude and during two times per 88-day Mercury year when the change in periapsis altitude was 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 were 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 occured 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 have remained 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 limited 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.

solar incidence graph

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.

view from sun to mercury

Maneuver Calendar Date ΔV (m/s) Purpose and Result
OCM-9 17 June 2014 5.0 Target 25-km minimum altitude on 12 September 2014
Met 25.2-km minimum altitude on 12 September 2014
OCM-10 12 September 2014 8.6 Target 25-km minimum altitude on 24 October 2014
Met 25.4-km minimum altitude on 24 October 2014
OCM-11 24 October 2014 19.3 Target 25-km minimum altitude on 21 January 2015
Met 25.7-km minimum altitude on 21 January 2015
OCM-12 21 January 2015 9.7 Target 15-km minimum altitude on 1 March 2015
Met 14.9-km minimum altitude on 1 March 2015

Final Extended Mission

Final Extended Mission

During the final six weeks of MESSENGER's orbital mission (often referred to as XM2', for a 6-week addition to MESSENGER's second extended mission) the spacecraft maintained a low periapsis altitude between 5 and 35 km above Mercury's surface. The figure below shows minimum altitude above Mercury's terrain and the timing of orbit correction maneuvers (OCMs) 13 to 18 over these final six weeks. Occurring near apoapsis, each OCM increased the periapsis (minimum) altitude in order to delay the spacecraft's impact onto Mercury's surface. The net effect of OCMs 13 to 18 also increased the orbit period from 8h 17m to 8h 21m.

Minimum Alt Lat wrt terrain od429 OCM13 to impact

OCMs 13, 14, and 15 consumed nearly all remaining usable hydrazine from the auxiliary fuel tank prior to entering the 7-12 April 2015 solar conjunction, a period of limited or degraded ability for flight controllers to communicate with the spacecraft. This communications degradation limited the short-term ability to monitor precisely OCM-15, OCM-15A, and OCM-16, which were scheduled one day before and after the start of solar conjunction and two days after the end of solar conjunction, respectively. With all estimated usable fuel consumed earlier than expected during OCM-15, a new maneuver (OCM-15A) was designed and implemented using only helium gas pressurant within two days of OCM-15, thereby averting Mercury impact until after the next scheduled OCM. The final three OCMs also used gaseous helium pressurant from the main fuel tanks to accomplish the desired change in spacecraft velocity. These maneuvers followed the precedent set by OCM-15A as the first in-flight utilization of helium gas pressurant alone to prolong a mission in order to achieve final scientific observational goals. Another MESSENGER mission first involved determining the best orbit location and thruster orientation for OCMs with Mercury orbital positions and Sun-relative spacecraft orbit orientations far from the Mercury dawn-dusk orbit, in contrast with all 12 prior OCMs. Six of the final seven OCMs required this custom optimization to maximize orbit change per available propellant, particularly given further OCM timing restrictions that limited the maneuvers to specific time ranges within scheduled Deep Space Network (DSN) tracking station passes. Because the sunshade protected the main spacecraft from direct sunlight exposure, OCMs 14 and 15 used two small thrusters for which the nozzles protruded through the sunshade. These thrusters, which were used for the first time in 8 years 4 months, were exposed for the first time during operation to temperatures higher than that of an oven broiler. The date, Mercury-relative velocity change (ΔV), and purpose of OCMs 13 through 18 are given in the table below. Uncertainties in the effect of Mercury's gravity field on the spacecraft orbit at low altitude gave rise to the need for OCM-18 just two days prior to impact. This final maneuver raised the spacecraft's minimum altitude above Mercury just enough to ensure impact onto Mercury during an orbit for which coverage by one of DSN's large (70-m) antennas had been scheduled. Such a downlink arrangement enabled the transmission to Earth of nearly all images and science data remaining on the spacecraft recorder.

Maneuver Calendar Date ΔV (m/s) Purpose
OCM-13 18 March 2015 3.1 Target 5.7-km minimum altitude above terrain on 2 April 2015
OCM-14 2 April 2015 3.0 Target 13.1-km minimum altitude above terrain on 6 April 2015
OCM-15 6 April 2015 1.8 Target 6.9-km minimum altitude above terrain on 14 April 2015
OCM-15A 8 April 2015 1.9 Target 6.9-km minimum altitude above terrain on 14 April 2015
OCM-16 14 April 2015 1.0 Target 9.0-km minimum altitude above terrain on 24 April 2015
OCM-17 24 April 2015 1.5 Raise minimum altitude to delay Mercury impact to 30 April 2015
OCM-18 28 April 2015 0.45 Raise minimum altitude to delay Mercury impact to 30 April 2015 at 19:26:01 UTC

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

During the final 2.1 years of MESSENGER's 4.1-year Mercury orbit phase, trajectory perturbations dominated by the gravitational attraction of the Sun gradually pulled the spacecraft's orbital periapsis progressively closer to the surface of Mercury. The depletion of all usable propellant and most of the helium pressurant during the final OCMs delayed the high-speed 3.912 km/s (8,720 mi/hr) Mercury impact until Thursday, 30 April 2015. The first two figures below show the MESSENGER impact site, both in a close-up view of Mercury's surface and in a global perspective (the impact site is marked by an asterisk at the end of the final portion of the spacecraft's trajectory). The final two figures below show that the spacecraft impact location in late April 2015 was not visible from Earth. The hidden lower left corner of the letter "M" in the word "MESSENGER" in the lower figure indicates the location of MESSENGER, just behind the limb of Mercury, at the time of impact. Analysis of final MESSENGER spacecraft orbit data and the best available Mercury digital elevation model indicates that the best estimates of MESSENGER's impact time and location are 19:26:01.166 UTC on 30 April 2015 at 54.440° N latitude and 210.120° E longitude.