MESSENGER

[2] The probe's second extended mission lasted for over two years, but as its low orbit degraded, it required reboosts to avoid impact.The scientific objectives of the mission were:[27][28] The MESSENGER spacecraft was designed and built at the Johns Hopkins University Applied Physics Laboratory.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] 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.[29] The spacecraft's onboard computer system was contained in an Integrated Electronics Module (IEM), a device that combined core avionics into a single box.[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.[34][35] Objectives:[34] 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] Objectives:[37] Mapped mineral composition within the top millimeter of the surface on Mercury by detecting X-ray spectral lines from magnesium, aluminum, sulphur, calcium, titanium, and iron, in the 1–10 keV range.[60] MESSENGER performed an Earth flyby one 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.Ensuring that the instruments functioned correctly at such an early stage in the mission allowed opportunity for multiple minor errors to be dealt with.During the encounter, MESSENGER passed behind Venus and entered superior conjunction, a period when Earth was on the exact opposite side of the Solar System, with the Sun inhibiting radio contact.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.[64] On June 5, 2007, at 23:08 UTC, MESSENGER performed a second flyby of Venus at an altitude of 338 km (210 mi), for the greatest velocity reduction of the mission.Although this had no effect on the trajectory necessary for later orbit insertion, it resulted in the loss of science data and images that were planned for the outbound leg of the fly-by.[33] Principal Investigator Sean Solomon, then of the Carnegie Institution of Washington, said: "With the beginning today of the primary science phase of the mission, we will be making nearly continuous observations that will allow us to gain the first global perspective on the innermost planet."[33] On October 5, 2011, the scientific results obtained by MESSENGER during its first six terrestrial months in Mercury's orbit were presented in a series of papers at the European Planetary Science Congress in Nantes, France.[71] In November 2012, NASA reported that MESSENGER had discovered a possibility of both water ice and organic compounds in permanently shadowed craters in Mercury's north pole.[76][77][78] As its orbit began to decay in early 2015, MESSENGER was able to take highly detailed close-up photographs of ice-filled craters and other landforms at Mercury's north pole.[80] On July 3, 2008, the MESSENGER team announced that the probe had discovered large amounts of water present in Mercury's exosphere, which was an unexpected finding.[83] The probe also constructed the most detailed and accurate maps of Mercury to date, and furthermore discovered carbon-containing organic compounds and water ice inside permanently shadowed craters near the north pole.The spacecraft's operation was extended by several weeks by exploiting its remaining supply of helium gas, which was used to pressurize its propellant tanks, as reaction mass.