MESSENGER

Historic First: A Spacecraft Orbits Mercury

March 18, 2011: NASA's MESSENGER spacecraft successfully achieved orbit around Mercury at approximately 9 p.m. EDT on Thursday, March 17. This marks the first time a spacecraft has accomplished this engineering and scientific milestone at our solar system's innermost planet.

"This mission will continue to revolutionize our understanding of Mercury during the coming year," said NASA Administrator Charles Bolden, who was at MESSENGER mission control at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., as engineers received telemetry data confirming orbit insertion. "NASA science is rewriting text books. MESSENGER is a great example of how our scientists are innovating to push the envelope of human knowledge."

An artist's concept of MESSENGER orbiting Mercury. [more]

At 9:10 p.m. EDT, engineers Operations Center, received the anticipated radiometric signals confirming nominal burn shutdown and successful insertion of the MESSENGER probe into orbit around the planet Mercury. NASA's MErcury Surface, Space ENvironment, Geochemistry, and Ranging, or MESSENGER, rotated back to the Earth by 9:45 p.m. EDT, and started transmitting data. Upon review of the data, the engineering and operations teams confirmed the burn executed nominally with all subsystems reporting a clean burn and no logged errors.

MESSENGER's main thruster fired for approximately 15 minutes at 8:45 p.m., slowing the spacecraft by 1,929 miles per hour and easing it into the planned orbit about Mercury. The rendezvous took place about 96 million miles from Earth.

"Achieving Mercury orbit was by far the biggest milestone since MESSENGER was launched more than six and a half years ago," said Peter Bedini, MESSENGER project manager of the Applied Physics Laboratory (APL). "This accomplishment is the fruit of a tremendous amount of labor on the part of the navigation, guidance-and-control, and mission operations teams, who shepherded the spacecraft through its 4.9-billion-mile journey."

For the next several weeks, APL engineers will be focused on ensuring the spacecraft's systems are all working well in Mercury's harsh thermal environment. Starting on March 23, the instruments will be turned on and checked out, and on April 4 the mission's primary science phase will begin.

"Despite its proximity to Earth, the planet Mercury has for decades been comparatively unexplored," said Sean Solomon, MESSENGER principal investigator of the Carnegie Institution of Washington. "For the first time in history, a scientific observatory is in orbit about our solar system's innermost planet. Mercury's secrets, and the implications they hold for the formation and evolution of Earth-like planets, are about to be revealed."

APL designed and built the spacecraft. The lab manages and operates the mission for NASA's Science Mission Directorate in Washington.

Production Editor: Dr. Tony Phillips | Credit: Science@NASA

The first probe to ever orbit Mercury occurred Thursday evening 2011 March 17.

The gravity well of the sun is so steep at Mercury that it takes more energy to reach it than Jupiter and required six gravity assists.

The following is from the MESSENGER website:

On March 17 at 8:45 p.m. EDT, MESSENGER -- having pointed its largest thruster very close to the direction of travel -- will fire that thruster for nearly 14 minutes, with other thrusters firing for an additional minute, slowing the spacecraft by 862 meters per second (1,929 mph) and consuming 31 percent of the propellant that the spacecraft carried at launch. Less than 9.5 percent of the usable propellant at the start of the mission will remain after completing the orbit insertion maneuver, but the spacecraft will still have plenty of propellant for future orbit correction maneuvers.

The orbit insertion will place the spacecraft into a 12 hour orbit about Mercury with a 200 kilometer (124 mile) minimum altitude. At the time of orbit insertion, MESSENGER will be 46.14 million kilometers (28.67 million miles) from the sun and 155.06 million kilometers (96.35 million miles) from Earth.

MESSENGER has been on a six year mission to become the first spacecraft to orbit Mercury. The spacecraft followed a path through the inner solar system, including one flyby of Earth, two flybys of Venus, and three flybys of Mercury. This impressive journey is returning the first new spacecraft data from Mercury since the Mariner 10 mission over 30 years ago.

MORE ON THE ORBITAL INSERTION DIRECTLY FROM THE MESSENGER WEBSITE:

Mercury Orbit Insertion & Station Keeping

Getting into Mercury orbit...

On March 16 at 11:40 a.m. EDT, just over 33 hours before the main Mercury orbit insertion event, two antennas from the Deep Space Network - one main antenna and one backup - will begin to track the MESSENGER spacecraft continuously. Nearly thirty-one hours later, at 6:30 p.m. EDT on March 17, the number of antennas tracking MESSENGER will increase to five – four of these are arrayed together in order to enhance the signal coming from the spacecraft, and the fifth for backup in case there is a problem.

About two and a half hours later, at 8:00 p.m., the solar arrays, telecommunications, attitude control, and autonomy systems will all be configured for the main thruster firing (known as a “burn”), and the spacecraft will be turned into the correct orientation for MESSENGER’s Mercury orbit insertion maneuver.

In order to slow the spacecraft down sufficiently so that it can be captured into orbit around Mercury, the main thruster will begin firing at 8:45 p.m. and will continue for 15 minutes until 9:00 pm. About 31% of the spacecraft's original allotment of propellant is required for Mercury orbit insertion, and MESSENGER's thrusters must slow the spacecraft by just over 0.86 kilometers (0.53 miles) per second. As the spacecraft approaches Mercury, the largest thruster must be pointed close to the forward velocity direction of the spacecraft. Ten minutes after the thruster has finished firing, at 9:10 p.m., the spacecraft will be turned toward Earth, and data from the maneuver will be sent back to the Deep Space Network antennas. This is the time when the MESSENGER team will be able to determine that everything has gone according to plan, and that the orbit insertion maneuver has been successful. It will take about 9 minutes for the data to be transmitted back to Earth, and additional time will be needed for the DSN station to process it and send it the Mission Operations Center at APL. It is expected that by about 11:00 p.m. EDT, the Mission Operations Team will be able to confirm that MESSENGER has successfully been captured into orbit around Mercury.

At the same time as the MOI data are being sent back to Earth, the spacecraft systems will be reconfigured into their normal post-maneuver operations mode, and just under an hour later, the Deep Space Network burn coverage will be stepped back to two stations. At 2.48 a.m. EDT on March 18, the spacecraft will begin its first full orbit around Mercury (as measured from the highest point in the orbit). At 12:00 p.m., the Deep Space Network coverage will be further reduced to continuous coverage with only one station.

The MESSENGER spacecraft will continue to orbit Mercury once every twelve hours for the duration of its primary mission. The first few days after orbit insertion will be focused on ensuring that the spacecraft systems are all working well in the harsh thermal environment of orbit; this interval is known as the orbital commissioning phase. On March 24 the instruments will be turned on and checked out, and on April 4 the science phase of the mission will begin and the first orbital science data from Mercury will be returned.

• Ground Receipt Time adjusted for one-way light time, which gradually decreases through reporting period.

• Events without specific execution times are 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 are approximately 9 minutes after a maneuver is executed on the spacecraft.

Three views of Mercury orbit insertion 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 maneuver is shown in light blue in the figures and places 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 arrives in the primary science orbit, small forces, such as solar gravity – the gravitational attraction of the Sun - slowly change the spacecraft's orbit. Although these small forces have little effect on MESSENGER's 12-hour orbit period, they can increase 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 prevent satisfactory completion of several science goals.

To keep the spacecraft’s minimum altitude below 500 kilometers (310 miles), propulsive maneuvers must occur at least once every Mercury year - one complete revolution around the Sun, or 88 Earth days. The first, third, and fifth maneuvers after Mercury orbit insertion will occur at the farthest orbital distance from Mercury where a minimum amount of propellant will 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 has an unavoidable side effect of also lowering orbit period by 13-15 minutes. The second and fourth maneuvers after orbit insertion will increase the orbit period back to about 12 hours by speeding up the spacecraft near its closest distance from Mercury. Because the sunshade must protect the main part of the spacecraft from direct sunlight during propulsive maneuvers, the timing of these maneuvers is limited to a few days when Mercury is either near the same point in its orbit as it was during Mercury orbit insertion or near the point where Mercury is on the opposite side of the Sun from that for orbit insertion.

Earth and Moon as Seen by MESSENGER

from 114 Million Miles

Release Date: August 17, 2010

Topics: Earth, Stars, WAC

Date Acquired: May 6, 2010

Image Mission Elapsed Time (MET): 181616382

Instrument: Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS)

WAC Filter: 2 (clear filter)

Field of View: The WAC has a 10.5° field of view

Of Interest: In the lower left portion of this image, the Earth can be seen, as well as the much smaller Moon to Earth's right. When MESSENGER took this image, a distance of 183 million kilometers (114 million miles) separated the spacecraft and Earth. To provide context for this distance, the average separation between the Earth and the Sun is about 150 million kilometers (93 million miles). Though it is a beautiful, thought-provoking picture, viewing our planet from far away was not the main reason that the mission team planned the collection of this image. Instead, this image was acquired as part of MESSENGER's campaign to search for vulcanoids, small rocky objects that have been postulated to exist in orbits between Mercury and the Sun. Though no vulcanoids have yet been detected, the MESSENGER spacecraft is in a unique position to look for smaller and fainter vulcanoids than has ever before been possible. MESSENGER's vulcanoid searches occur near perihelion passages, when the spacecraft's orbit brings it closest to the Sun. Today is another such perihelion, and MESSENGER is taking a new set of images to search for tiny asteroids lurking close to the Sun.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

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FEATURED RELEASE

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November 23, 2010

Mercury's Vast Expanses of Smooth Plains

Travel to Mercury

The Launch of MESSENGER.

The Boeing Delta II rocket carrying MESSENGER lifted off from Cape Canaveral Air Force Station, Florida at 02:15:56 EDT on August 3, 2004. An hour later, NASA confirmed that MESSENGER had successfully separated from the third stage booster and commenced its roundabout route to Mercury.

Travel to Mercury requires an extremely large velocity change, or delta-v, because Mercury lies deeper in the Sun's gravity well; a spacecraft traveling to Mercury is greatly accelerated as it falls toward the Sun, so there must be a mechanism to slow it. Mercury does not have an atmosphere thick enough to aerobrake on arrival. To make the trip feasible, MESSENGER makes extensive use of gravity assist maneuvers. These reduce the amount of rocket fuel needed to slow down, but greatly prolong the trip. For additional fuel savings, the thrust used for insertion into orbit about Mercury will be minimized, resulting in a notably elliptical orbit. Besides the advantage of saving fuel, such an orbit allows the spacecraft to measure solar wind and magnetic fields at a variety of distances from the planet, yet still get close-up measurements and photographs of the surface.

MESSENGER performed a successful Earth swing-by a year after launch, on 2 August 2005, with the closest approach at 19:13 UTC at an altitude of 2,347 kilometers (1,458 statute miles) over central Mongolia. On December 12, 2005, a 524 second long burn (Deep-Space Maneuver or DSM-1) of the large thruster adjusted the trajectory for the upcoming Venus swing-by.^[1]

MESSENGER's Trajectory Above

A view of Earth from MESSENGER during its Earth swing-by.

MESSENGER made its first flyby of Venus at 08:34 UTC on October 24, 2006 at an altitude of 2,992 kilometers (1,859 mi). A second flyby of Venus was made at 23:08 UTC on June 5, 2007 at an altitude of 338 kilometers (210 mi). On October 17, 2007, Deep-Space Maneuver 2 or DSM-2' was executed successfully, putting MESSENGER on target for its first flyby of Mercury.^[2] MESSENGER made a flyby of Mercury on 14 January 2008 (closest approach 200 km above surface of Mercury at 19:04:39 UTC), followed by a second flyby on October 6, 2008.^[3] MESSENGER executed one last flyby on September 29, 2009, that further slowed down the spacecraft. Both the second and third flybys were preceded by DSM-3 on 19 March 2008 at 19:30 UTC and DSM-4 on 04 Dececember, 2008 at 20:30 UTC to adjust the velocity of the spacecraft.^[4][5] One last deep space maneuver, DSM-5 was executed on November 24, 2009 at 22:45 UTC to provide the required velocity change for the scheduled Mercury orbit insertion on March 18, 2011, marking the beginning of a year-long orbital mission.^[6]

All along the way, numerous trajectory corrections were made to MESSENGER's course. The corrections numbered 35 as of 24, November 2009 and are referred to as TCM or Trajectory Correction Maneuver. TCM which use the large bi-propellant thrusters are also referred to as DSM or Deep Space Maneuver. DSM generally concern major adjustments to the spacecraft's velocity while TCM usually deal with modifying the craft's orientation with respect to the sun (crucial for thermal management) and targeting aim points for flybys of planets.^[7]

During the Earth flyby, MESSENGER imaged the Earth and Moon and used its atmospheric and surface composition spectrometer to look at the Moon. The particle and magnetic field instruments investigated the Earth's magnetosphere.

The spacecraft was originally scheduled to launch during a 12-day window that opened May 11, 2004, but on March 26, 2004, NASA announced that a later launch window starting at July 30, 2004 with a length of 15 days would be used. This was to allow more time for testing and spacecraft processing.^[8] This change significantly altered the trajectory of the mission and delayed the arrival at Mercury by two years. The original plan called for three fly-by maneuvers past Venus, with Mercury orbit insertion scheduled for 2009. The new trajectory features one Earth flyby, two Venus flybys, and three Mercury flybys before orbit insertion on March 17, 2011 - 8:45 PM EDT.

The navigation team is led by KinetX, Inc. of Tempe, AZ. KinetX is the first private company to be responsible for navigation of a NASA deep space mission. In that role, they are responsible for determining all trajectory adjustments throughout the probe's flight through the inner solar system ensuring that MESSENGER arrives at Mercury with the proper velocity for orbit insertion.

Mercury Observation Plan

MESSENGER's first image of the side of Mercury which was never seen by Mariner 10, from a distance of about 17,000 miles (27,000 km).

An unidentified patch of black on Mercury.

The nominal orbit has a periapsis of 200 km (120 mi) at 60 degrees N latitude, and an apoapsis of 15,193 km (9,440 mi), a period of 12 hours and an inclination of 82.5 degrees. The periapsis will slowly rise due to solar perturbations to over 400 km (250 mi) at the end of 88 days (one Mercury year) at which point it will be readjusted to a 200 km (120 mi), 12 hour orbit via a two burn sequence.^[9] Data will be collected from orbit for one Earth year, the nominal end of the primary mission. Global stereo image coverage at 250 meters/pixel resolution is expected. The mission should also yield global composition maps, a 3-D model of Mercury's magnetosphere, topographic profiles of the northern hemisphere, gravity field to degree and order 16, altitude profiles of elemental species, and a characterization of the volatiles in permanently shadowed craters at the poles.

Once there, scientists hope to test a theory that the planet is shrinking, contracting on itself as its core slowly freezes. The probe will look for signs of surface buckling on Mercury's unobserved hemisphere, as well as collect surface composition data on material that may have once spewed out of the planet's interior. The idea that Mercury's surface was somehow shrinking arose when Mariner 10 returned images of great scarps biting deep into the planet's surface. One such scarp, Discovery Rupes, cuts 1.6 km (1 mi) into Mercury's crust.

Spacecraft and Subsystems

MESSENGER assembly installation of solar panels Astrotech.

MESSENGER was designed and built by the Johns Hopkins University Applied Physics Laboratory (JHU/APL). It is a squat box (1.27 m × 1.42 m × 1.85 m) with a semi-cylindrical thermal shade for protection from the Sun and two solar panel wings extending radially. A 3.6 m (12 ft) magnetometer boom also extends from the craft. The total mass of the spacecraft is 1,093 kg (2,410 lb); 607.8 kg (1,340 lb) of this is propellant (hydrazine and nitrogen tetroxide) and helium. The structure is primarily graphite cyanate ester (GrCE) composite and consists of two vertical panels which support two large fuel tanks and two vertical panels which support the oxidizer tank and plumbing panel. The four vertical panels make up the center column and are bolted at their aft ends to an aluminum adapter. A single top deck panel mounts the LVA (large velocity adjust) thruster, small thrusters, helium and auxiliary fuel tanks, star trackers and battery.

Main propulsion is via the 645 N (145 lb_f), 317 s bipropellant LVA thruster. Four 22 N (4.9 lb_f) monopropellant thrusters provide spacecraft steering during main thruster burns, and ten 4 N (0.9 lb_f) monopropellant thrusters are used for attitude control. There is also a reaction wheel attitude control system. Information for attitude control is provided by star tracking cameras, an inertial measurement unit, and six solar sensors. Power is provided by solar panels which extend beyond the sunshade. They are rotatable to balance panel temperature and power generation and provide a nominal 450 watts in Mercury orbit. The panels are 70 percent optical solar reflectors and 30 percent GaAs/Ge cells. The power is stored in a common-pressure-vessel, 23-ampere-hour nickel hydrogen battery, with 11 vessels and two cells per vessel.

Communications uses two small deep space transponders (SDSTs) operating at X-band. Downlink is through two fixed phased array antenna clusters, and uplink and downlink through medium- and low-gain antennas on the forward and aft sides of the spacecraft. Passive thermal control, primarily a fixed opaque ceramic cloth sunshade, is utilized to maintain operating temperatures near the Sun. Radiators are built into the structure and the orbit is optimized to minimize infrared and visible light heating of the spacecraft from the surface of Mercury. Multilayer insulation, low conductivity couplings, and heaters are also used to maintain temperatures within operating limits.

Five science instruments are mounted externally on the bottom deck of the main body: the Mercury Dual Imaging System (MDIS), Gamma-Ray and Neutron Spectrometer (GRNS), X-ray Spectrometer (XRS), Mercury Laser Altimeter (MLA), and Atmospheric and Surface Composition Spectrometer (MASCS). The Energetic Particle and Plasma Spectrometer (EPPS) is mounted on the side and top deck and the magnetometer (MAG) is at the end of the 3.6 meter boom. Radio Science (RS) experiments will use the existing communications system (see:Radio Science Subsystem).

MESSENGER's onboard computer system is based on the Integrated Electronics Module (IEM), a device that combines core avionics in a single box. The spacecraft carries a pair of identical IEMs for backup purposes; both house a 25 megahertz main processor and 10 MHz fault protection processor. All four are radiation-hardened IBM RAD6000 processors, based on the IBM POWER1 CPU architecture (similar to that of older Macintoshes). The RAD computer is slow by current personal computer standards, but is capable of radiation tolerance required on the MESSENGER mission. For data storage, the spacecraft carries two solid-state recorders (one backup) able to store up to one gigabyte each. Its main processor collects, compresses, and stores on the recorder images and other data from MESSENGER's instruments, which can then be sent back to Earth.

Scientific Results

This article may need to be updated. Please update this article to reflect recent events or newly available information, and remove this template when finished. Please see the talk page for more information. (September 2009)

An image of part of the previously unseen side of the planet.

MESSENGER performed its first Mercury flyby successfully on 14 January 2008, and its second flyby on 6 October 2008, taking pictures with both the wide angle and narrow angle cameras as well as using some of its other sensors. Preliminary image results from this first pass can be viewed at JHUAPL's MESSENGER Science Photos page.

On July 3, 2008, MESSENGER team member Thomas Zurbuchen announced that the probe discovered large amounts of water present in Mercury's exosphere. "Nobody expected that. I don't know a single person that did. We were astonished, just astonished," Zurbuchen stated.^[10]

MESSENGER also provided visual evidence of volcanic activity on the surface of Mercury as well as evidence for a liquid planetary core.^[10]

MESSENGER performed its third and last Mercury flyby on September 29, 2009 with the spacecraft coming within 142 mi (229 km) of the planet's surface. The inbound portion of the fly-by seems to have gone as planned, however sometime during the closest approach the spacecraft entered safe mode. Although this had no effect on the trajectory necessary for later orbit insertion it may have resulted in the loss of science data and images that were planned for the outbound leg of the fly-by. The spacecraft had fully recovered by about 7 hours later.^[11]

References

1. ^ Johns Hopkins University (12 December 2005). "MESSENGER Engine Burn Puts Spacecraft on Track for Venus". Press release. http://www.spaceref.com/news/viewsr.html?pid=18956. Retrieved 2009-05-01.
2. ^ Johns Hopkins University (17 October 2007). "Critical Deep-Space Maneuver Targets MESSENGER for Its First Mercury Encounter". Press release. http://messenger.jhuapl.edu/news_room/2007/status_report_10_17_07.html. Retrieved 2009-05-01.
3. ^ Johns Hopkins University (14 January 2008). "Countdown to MESSENGER's Closest Approach with Mercury". Press release. http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=115. Retrieved 2009-05-01.
4. ^ Johns Hopkins University (19 March 2008). "Critical Deep-Space Maneuver Targets MESSENGER for Its Second Mercury Encounter". Press release. http://messenger.jhuapl.edu/news_room/details.php?id=96. Retrieved 2010-04-20.
5. ^ Johns Hopkins University (4 December 2008). "Deep-Space Maneuver Positions MESSENGER for Third Mercury Encounter". Press release. http://messenger.jhuapl.edu/news_room/details.php?id=116. Retrieved 2010-04-20.
6. ^ Johns Hopkins University (24 November 2009). "Deep-Space Maneuver Positions MESSENGER for Mercury Orbit Insertion". Press release. http://messenger.jhuapl.edu/news_room/details.php?id=140. Retrieved 2010-04-20.
7. ^ Mission Design: Trajectory Correction Maneuvers, Johns Hopkins University, http://messenger.jhuapl.edu/the_mission/maneuvers.html, retrieved 2010-04-20
8. ^ Johns Hopkins University (2004-03-24). "MESSENGER Launch Rescheduled.". Press release. http://messenger.jhuapl.edu/news_room/2004/status_report_03_24_04.html. Retrieved 2009-05-01.
9. ^ "MESSENGER Mission Design: Working from Orbit". Johns Hopkins University. http://messenger.jhuapl.edu/the_mission/orbit.html. Retrieved 2010-04-20.
10. ^ ^a ^b Emily Lakdawalla (3 July 2008). "MESSENGER Scientists 'Astonished' to Find Water in Mercury's Thin Atmosphere". The Planetary Society. http://www.planetary.org/news/2008/0703_MESSENGER_Scientists_Astonished_to.html. Retrieved 2009-05-01.
11. ^ "MESSENGER Gains Critical Gravity Assist for Mercury Orbital Observations". MESSENGER Mission News. September 30, 2009. http://messenger.jhuapl.edu/news_room/details.php?id=136. Retrieved 2009-09-30.

External Links

Wikimedia Commons has media related to: MESSENGER

- JHUAPL homepage - official site.
- MESSENGER Mission Page - official information regarding the mission on the nasa.gov website.
- MESSENGER Mission Profile by NASA's Solar System Exploration
- Mercury Flyby 1 Visualization Tool - see simulated views of the instrument observations planned during the flyby.
- Mercury Flyby 1 Actuals - compares the simulated views to the images actually acquired by MESSENGER during flyby 1
- Mercury Flyby 2 Visualization Tool - a simulation of what MESSENGER does on this historic flyby.
- Mercury Flyby 2 Actuals - compares the simulated views to the images actually acquired by MESSENGER during flyby 2
- MESSENGER Image Gallery - Check the latest Images from MESSENGER.

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