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MESSENGER was a NASA robotic space probe that orbited the planet Mercury between 2011 and 2015, studying Mercury's chemical composition, geology, and magnetic field.[9][10] The name is a backronym for "Mercury Surface, Space Environment, Geochemistry, and Ranging", and a reference to the messenger god Mercury from Roman mythology.
MESSENGER entered orbit around Mercury on March 18, 2011, becoming the first spacecraft to do so.[9] It successfully completed its primary mission in 2012.[2] Following two mission extensions, the spacecraft used the last of its maneuvering propellant to deorbit, impacting the surface of Mercury on April 30, 2015.[14]
MESSENGER's formal data collection mission began on April 4, 2011.[15] The primary mission was completed on March 17, 2012, having collected close to 100,000 images.[16] MESSENGER achieved 100% mapping of Mercury on March 6, 2013, and completed its first year-long extended mission on March 17, 2013.[2] The probe's second extended mission lasted for over two years, but as its low orbit degraded, it required reboosts to avoid impact. It conducted its final reboost burns on October 24, 2014, and January 21, 2015, before crashing into Mercury on April 30, 2015.[17][18][19]
During its stay in Mercury orbit, the probe's instruments yielded significant data, including a characterization of Mercury's magnetic field[20] and the discovery of water ice at the planet's north pole,[21][22] which had long been suspected on the basis of Earth-based radar data.[23]
In 1998, a study detailed a proposed mission to send an orbiting spacecraft to Mercury, as the planet was at that point the least-explored of the inner planets. In the years following 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. Moreover, inserting a spacecraft into orbit around Mercury is difficult, because a probe approaching on a direct path from Earth would be accelerated by the Sun's gravity and pass Mercury far too quickly to orbit it. However, using a trajectory designed by Chen-wan Yen[25] in 1985, the study showed it was possible to execute 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 and thereby minimize propellant needs.[26]
The MESSENGER spacecraft was designed and built at the Johns Hopkins University Applied Physics Laboratory. Science operations were managed by Sean Solomon as principal investigator, and mission operations were also conducted at JHU/APL.[29] The MESSENGER bus measured 1.85 meters (73 in) tall, 1.42 m (56 in) wide, and 1.27 m (50 in) deep. The bus was primarily constructed with four graphite fiber / cyanate ester composite panels that supported the propellant tanks, the large velocity adjust (LVA) thruster, attitude monitors and correction thrusters, the 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.[29] At launch, the spacecraft weighed approximately 1,100 kilograms (2,400 lb) with its full load of propellant.[30] MESSENGER's total mission cost, including the cost of the spacecraft's construction, was estimated at under US$450 million.[31]
Four 22 N (4.9 lbf) monopropellant thrusters provided spacecraft steering during main thruster burns, and twelve 4.4 N (1.0 lbf) monopropellant thrusters were used for attitude control. For precision attitude control, a reaction wheel attitude control system was also included.[29] Information for attitude control was provided by star trackers, an inertial measurement unit and six Sun sensors.[29]
The probe included 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 could 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 was 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 was mounted on the front of the probe facing the Sun, and one of each was mounted to the back of the probe facing away from the Sun.[32]
The spacecraft's onboard computer system was contained in an Integrated Electronics Module (IEM), a device that combined core avionics into a single box. The computer featured two radiation-hardened IBM RAD6000s, a 25 megahertz main processor, and a 10 MHz fault protection processor. For redundancy, the spacecraft carried a pair of identical IEMs. For data storage, the spacecraft carried two solid-state recorders able to store up to one gigabyte each. The IBM RAD6000 main processor collected, compressed, and stored data from MESSENGER's instruments for later playback to Earth.[29]
MESSENGER used a software suite called SciBox to simulate its orbit and instruments, in order to "choreograph the complicated process of maximizing the scientific return from the mission and minimizing conflicts between instrument observations, while at the same time meeting all spacecraft constraints on pointing, data downlink rates, and onboard data storage capacity."[33]
Measured gamma-ray emissions from the surface of Mercury to determine the planet's composition by detecting certain elements (oxygen, silicon, sulfur, iron, hydrogen, potassium, thorium, uranium) to a depth of 10 cm.[37][38]
Determined the characteristics of the tenuous atmosphere surrounding Mercury by measuring ultraviolet light emissions, and ascertained the prevalence of iron and titanium minerals on the surface by measuring the reflectance of infrared light.[45][46]
Measured the charged particles in the magnetosphere around Mercury using an energetic particle spectrometer (EPS) and the charged particles that come from the surface using a fast imaging plasma spectrometer (FIPS).[48][49]
Traveling to Mercury and entering orbit requires an extremely large velocity change (see delta-v) because Mercury's orbit is deep in the Sun's gravity well. On a direct course from Earth to Mercury, a spacecraft is constantly accelerated as it falls toward the Sun, and will arrive at Mercury with a velocity too high to achieve orbit without excessive use of fuel. For planets with an atmosphere, such as Venus and Mars, spacecraft can minimize their fuel consumption upon arrival by using friction with the atmosphere to enter orbit (aerocapture), or can briefly fire their rocket engines to enter into orbit followed by a reduction of the orbit by aerobraking. However, the tenuous atmosphere of Mercury is far too thin for these maneuvers. Instead, MESSENGER extensively used gravity assist maneuvers at Earth, Venus, and Mercury to reduce the speed relative to Mercury, then used its large rocket engine to enter into an elliptical orbit around the planet. The multi-flyby 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 7.9 billion kilometers (4.9 billion miles).
Several planned thruster firings en route to Mercury were unnecessary, because these fine course adjustments were performed using solar radiation pressure acting on MESSENGER's solar panels.[58] To further minimize the amount of necessary propellant, the spacecraft orbital insertion targeted a highly elliptical orbit around Mercury.
The elongated orbit had two other benefits: It allowed the spacecraft time to cool after the times it was between the hot surface of Mercury and the Sun, and also it allowed 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.The spacecraft was originally scheduled to launch during a 12-day window that beginning May 11, 2004. On March 26, 2004, NASA announced the launch would be moved to a later, 15-day launch window beginning July 30, 2004, to allow for further testing of the spacecraft.[59] 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 trajectory was changed to include one Earth flyby, two Venus flybys, and three Mercury flybys before orbit insertion on March 18, 2011.[60]
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 was intended to ensure accurate interpretation of data when the spacecraft entered orbit around Mercury. Ensuring that the instruments functioned correctly at such an early stage in the mission allowed opportunity for multiple minor errors to be dealt with.[62]
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