Messenger

The MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) probe is a 485-kilogram (1067 pound) robotic American space probe in orbit around the planet Mercury. It was launched by NASA in August 2004 to study the chemical composition, geology, and the magnetic field of Mercury. It became the second mission to successfully reach Mercury when it made a flyby on January 14, 2008, followed by a second flyby on October 6, 2008,[2] and a third flyby on September 29, 2009.[3][4] (The first space probe to reach Mercury was Mariner 10 in 1975.) MESSENGER is the first spacecraft to orbit the planet Mercury. Orbiting Mercury is difficult because a satellite approaching on a direct path from Earth would be accelerated by the Sun's gravity and pass Mercury too quickly to orbit it.

The instruments carried by MESSENGER have performed well on previous flybys of Venus, Earth, and Mercury itself. MESSENGER successfully entered Mercury orbit on March 18, 2011 with no reported problems. The craft's science instruments will be reactivated March 23, and its formal science data collection mission is slated to begin April 4, 2011.

Mission background

History

In 1973 Mariner 10 was launched to make multiple flyby encounters of Venus and Mercury. Mariner 10 provided the first detailed data of Mercury, but only mapped 40-45% of the surface.[5][6] The final flyby of Mercury by Mariner 10 occurred on March 16, 1975, ending close-range observations of the planet for thirty years. As Mercury was the least explored terrestrial planet with no future planned mission, a study published in 1998 detailed a potential mission to send an orbiter to the planet. Since the Mariner 10 mission, subsequent mission proposals to revisit Mercury had appeared too costly, requiring large quantities of propellant and a heavy lift launch vehicle. However, using a trajectory designed by Chen-wan Yen in 1985, the study showed it was possible to seek a Discovery-class mission by using multiple, consecutive gravity assist, 'swingby' maneuvers around Venus and Mercury, in combination with minor propulsive trajectory corrections, to gradually slow the spacecraft thereby minimizing propellant needs.

The primary science objectives of the mission include:

    * determine accurately the surface composition of Mercury
    * characterize the geological history of the planet
    * determine the precise strength of the magnetic field and its variation with position and altitude
    * investigate the presence of a liquid outer core by measuring Mercury's libration
    * determine the nature of the radar reflective materials at Mercury’s poles
    * investigate the important volatile species and their sources and sinks on and near Mercury.

The contrived acronym MESSENGER was chosen because Mercury was the messenger of the gods according to Roman mythology.
Spacecraft design

The MESSENGER bus measures 1.85 meters (73 in) tall, 1.42 m (56 in) wide and 1.27 m (50 in) deep. The bus is primarily constructed with four graphite fiber / cyanate ester composite panels which support the propellant tanks, the LVA (large velocity adjust) thruster, attitude monitors and correction thrusters, antennas, the instrument pallet, and a large ceramic-cloth sunshade, measuring 2.5 m (8.2 ft) tall and 2 m (6.6 ft) wide, for passive thermal control.[8]
Attitude control and propulsion

    Main propulsion is via the 645 N, 317 sec. Isp bipropellant LVA thruster. Four 22 N (4.9 lbf) monopropellant thrusters provide spacecraft steering during main thruster burns, and ten 4 N (0.9 lbf) monopropellant thrusters are used for attitude control. For precision attitude control, a reaction wheel attitude control system was also included. Information for attitude control is provided by star trackers, an inertial measurement unit, and six sun sensors.

    The spacecraft is designed to carry 607.8 kilograms (1,340 lb) of propellant (hydrazine and nitrogen tetroxide) and pressurizer (helium).

Communications

    The probe includes two small deep space transponders for communications with the Deep Space Network and three kinds of antennas: a high gain phased array whose main beam can be electronically steered in one plane, a medium-gain “fan-beam” antenna and a low gain horn with a broad pattern. The high gain antenna is used as transmit-only at 8.4 GHz, the medium-gain and low gain antennas transmit at 8.4 GHz and receive at 7.2 GHz, and all three antennas operate with right-hand circularly polarized (RHCP) radiation. One of each of these antennas is mounted on the front of the probe facing the sun, and one of each is mounted to the back of the probe facing away from the sun.

Power

    The space probe is powered by a two-panel, Gallium Arsenide/Germanium (GaAs/Ge) solar array providing an average of 450 watts at Mercury. Each panel is rotatable and includes optical solar reflectors to balance the temperature of the array. Power is stored in a common-pressure-vessel, 23-ampere-hour nickel hydrogen battery, with 11 vessels and two cells per vessel.
Computer

    The computer system is based on the Integrated Electronics Module (IEM), a device which combines core avionics into a single box. The computer features two radiation-hardened IBM RAD6000, a 25 megahertz main processor and 10 MHz fault protection processor. For redundancy, the spacecraft carries a pair of identical IEM computers. For data storage, the spacecraft carries two solid-state recorders able to store up to one gigabyte each. The IBM RAD6000 main processor collects, compresses, and stores data from the MESSENGER instruments for later playback to Earth

Mission profile

The MESSENGER probe was launched on August 3, 2004 at 06:15:56 UTC by the National Aeronautics and Space Administration from Space Launch Complex 17B at the Cape Canaveral Air Force Station in Florida, aboard a Delta II 7925 launch vehicle. The complete burn sequence lasted 57 minutes bringing the spacecraft into a heliocentric orbit, with a final velocity of 10.68 km/s (6.64 miles/s) and sending the probe into a 7.9 billion-kilometer trajectory that took 6 years, 7 months and 16 days before its orbital insertion on March 18, 2011.[8]

Traveling to Mercury requires an extremely large velocity change (see delta-v) because the relative closeness of Mercury's orbit places the planet deeper in the Sun's gravity well. As MESSENGER traveled to Mercury, it was constantly being accelerated as it fell toward the Sun. For planets such as Venus and Mars, a technique known as aerobraking is used to slow a spacecraft to orbital speed. However, the tenuous atmosphere of Mercury is far too thin for this maneuver to work for MESSENGER. Instead MESSENGER made extensive use of gravity assist maneuvers to gradually slow the spacecraft. This process greatly reduced the amount of propellant necessary to slow the spacecraft, but at the cost of prolonging the trip by many years and to a total distance of 5.9 billion miles. To further minimize the amount of necessary propellant, the spacecraft orbital insertion targeted a highly elliptical orbit around Mercury. The elongated orbit has two other benefits. It allows the spacecraft time to cool after the times it is sandwiched between the hot surface and the sun, and it allows the spacecraft to measure the effects of solar wind and the magnetic fields of the planet at various distances while still allowing close-up measurements and photographs of the surface and exosphere.

Encounter with Earth

MESSENGER performed a successful Earth swing-by a year after launch, on August 2, 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.[31]

During the Earth flyby, the MESSENGER team imaged the Earth and Moon using MDIS and checked the status of several other instruments observing the atmospheric and surface compositions and testing the magnetosphere and determining that all instruments tested were working as expected. This calibration period would be useful for ensuring accurate interpretation of data as the spacecraft encounters Mercury

Encounter with Venus

On October 24, 2006 at 08:34 UTC, MESSENGER encountered Venus at an altitude of 2,992 kilometers (1,859 mi). During the encounter, MESSENGER passed behind Venus entering superior conjunction, a period when Earth was on the exact opposite side of the Solar System, with the Sun inhibiting radio contact. For this reason, no scientific observations were conducted during the flyby. Communication with the spacecraft was reestablished in late November and performed a deep space maneuver on December 12, to correct the trajectory to encounter Venus in a second flyby.[33]

On June 5, 2007, at 23:08 UTC, MESSENGER performed a second encounter of Venus at an altitude of 338 km (210 mi), reducing the velocity of MESSENGER the greatest during the mission. During the encounter, all instruments were used to observe Venus and prepare for the following Mercury encounters. The encounter provided visible and near-infrared imaging data of the upper atmosphere of Venus. Ultraviolet and x-ray spectrometry of the upper atmosphere were also recorded, to characterize the composition. The ESA's Venus Express was also orbiting during the encounter, providing the first opportunity for simultaneous measurement of particle-and-field characteristics of the planet.

Encounter with Mercury

MESSENGER made a flyby of Mercury on January 14, 2008 (closest approach 200 km above surface of Mercury at 19:04:39 UTC), followed by a second flyby on October 6, 2008.[35] MESSENGER executed one last flyby on September 29, 2009, that further slowed down the spacecraft.[36][37] 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.[38] 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

Orbital encounter with Mercury

The thruster maneuver to insert the craft into orbit began at 12:45 AM (00:45 hours) UTC on March 18, 2011. It lasted about 15 minutes, with confirmation that the craft was in Mercury orbit received at 1:10 AM UTC, March 18 (9:10 PM EDT, March 17, U.S.).Mission lead engineer Eric Finnegan indicated that the spacecraft achieved a near perfect orbit. MESSENGER's orbit is highly elliptical, taking it within 200 kilometers (120 mi) of Mercury's surface and then 15,000 kilometres (9,300 mi) away from it every twelve-hours. This orbit was chosen to shield the probe from Mercury's highest temperatures.

The primary mission is expected to begin April 4, orbiting once every twelve hours, for the duration of twelve Gregorian months, an equivalent of two solar days on Mercury. Until scientific observations begin, supervising personnel will be switching on and testing science instruments on the craft to ensure they made the journey without damage