Messenger

NASA probe, launched in 2004 with the challenging mission of comprehensively mapping Mercury from orbit between March 2011 and March 2012. To achieve Mercury orbit using a conventional liquid rocket engine meant a seven year flight. During that time Messenger would make six flybys of Earth, Venus, and Mercury to lower the aphelion of its solar orbit and minimize the needed final engine burn to place the spacecraft into Mercury orbit. Even with these planetary gravitational assists, 55% of Messenger's launch mass was rocket propellant, and the scientific payload amounted to less than 5% of its launch mass.

Spacecraft description

To minimize empty mass, Messenger's structure was primarily composed of graphite epoxy material and its dual-mode, liquid chemical propulsion system was integrated into the spacecraft's structure. Two solar panels, charging a nickel-hydrogen battery, provided power. Due to the intense sunlight at Mercury, the panels were mirrored to reflect away 67% of the sunlight.

Redundant integrated electronics modules controlled the spacecraft, each with two processors -- a 25 MHz main processor and a 10-MHz fault-protection processor. Attitude was determined using star-tracking cameras and an Inertial Measurement Unit. Six Digital Solar Sensors provided a backup. Attitude control was accomplished using four reaction wheels. Messenger's 16 small thrusters were available if needed as a back-up or for coarse maneuvers. Communications was through circularly polarized X-band phased-array antennas. A conventional dish antenna could not be used due to the temperatures at Mercury.

For heat protection at Mercury, where the Sun was 11 times brighter than Earth, a sunshade made of heat-resistant ceramic cloth was used. At Mercury the sunshield would reach 370 deg C, while keeping the spacecraft below 20 deg C.

Scientific Payload - the demanding mission meant a net scientific payload of only 50 kg, less than 5% of the spacecraft mass. Nevertheless the miniaturized instrument suite included:

• Mercury Dual Imaging System (MDIS): This instrument consisted of wide-angle and narrow-angle imagers. A pivot platform was used for pointing.

• Gamma-Ray and Neutron Spectrometer (GRNS): This instrument was to detect gamma rays and neutrons emitted by Mercury's surface. It was to be used to map the relative abundances of different elements and was to determine if there was ice at Mercury's poles

• X-Ray spectrometer (XRS): XRS was to detect these X-rays emitted by the surface and measure the abundances of various elements in Mercury's crust.

• Magnetometer (MAG): This instrument was at the end of a 3.6 meter boom, and was to map Mercury's magnetic field and was to search for magnetized rocks in the crust.

• Mercury Laser Altimeter (MLA): This instrument was to map Mercury's topography with high accuracy.

• Mercury Atmospheric and Surface Composition Spectrometer (MASCS): This spectrometer was sensitive to light from the infrared to the ultraviolet and was to measure the abundances of atmospheric gases, as well as detect minerals on the surface.

• Energetic Particle and Plasma Spectrometer (EPPS): EPPS measured the composition, distribution, and energy of charged particles in Mercury's magnetosphere.

• Radio Science (RS): RS experiment used the Doppler effect to measure very slight changes in the spacecraft's velocity as it orbited Mercury. This was to allow scientists to study Mercury's mass distribution, including variations in the thickness of its crust.

Messenger was to use gravity assists from Earth, Venus and Mercury to lower its speed relative to Mercury at orbit insertion. It would flyby Earth in August 2005, then Venus twice, in October 2006 and June 2007. Three Mercury flybys, in January 2008, October 2008 and September 2009, would each be followed two months later by a course correction maneuver by Messenger's engine. The flybys would be used to map most of the planet and measure the composition of the surface, atmosphere and magnetosphere.

In a final encounter with the planet in March 2011. 33% of the spacecraft's propellant would be used to make a maneuver of 0.83 km/sec, putting the spacecraft into a near-polar 200 kilometer x 15,193 kilometer 12-hour orbit inclined 80° to Mercury's equator. The perigee was over the northern hemisphere to allow Messenger to conduct a detailed investigation of Mercury's geology and composition of the Caloris impact basin. The orbit was to be maintained by small engine corrections every 88 days (Mercury year) until the one earth-year survey of the planet was completed in March 2012.

Mass: 1,100 kg (2,400 lb). Payload: 50 kg (110 lb). RCS Coarse No x Thrust: 16 thrusters. RCS Propellants: N2O4/MMH. Main Engine Thrust: 666 N (149 lbf). Main Engine Propellants: N2O4/MMH. Main Engine Propellants: 600 kg (1,320 lb). Main Engine Isp: 290 sec. Spacecraft delta v: 2,300 m/s (7,500 ft/sec). Electrical System: 2 x mirrored solar arrays, each 1.5 m x 1.5 m.

• 2004 August 3 - Messenger - Program: Discovery. Launch Site: Cape Canaveral. Launch Complex: LC17B. Launch Vehicle: Delta. Mass: 1,066 kg (2,350 lb).
  The NASA Messenger probe to Mercury was was first placed into a parking orbit. The Delta booster second stage's second burn raised the orbit, then the PAM-D solid motor burned to put the probe on an escape trajectory into a 0.92 x 1.08 AU x 6.4 deg heliocentric orbit. Messenger (Mercury Surface, Space Environment, Geochemistry and Ranging) was to make an Earth flyby on August 1, 2005; Venus flybys in 2006 and 2007; and Mercury encounters in January and October 2008 , September 2009 and March 2011 . On this last encounter the Aerojet 660N engine was to fire to put Messenger into a 200 x 15,193 km x 80 deg orbit around Mercury. Launch delayed from March 10, May 11, August 2

• 2005 March 8 - Messenger, Magnetometer Deployment, Successful -
  

• 2005 August 2 - Messenger, Earth Flyby, Successful -
  

• 2006 October 24 - Messenger, first Venus Flyby -
  

• 2008 January 14 - First Messenger flyby of Mercury -
  The Messenger spacecraft flew by Mercury at 200 kilometers altitude at 19:04 GMT. This changed the craft's solar orbit to to 0.32 x 0.70 AU leading to further flybys and a final orbital capture in 2011.