— Ch. 1 · Mission Genesis And Design —
MESSENGER.
~6 min read · Ch. 1 of 5
the 3rd of August 2004 marked the launch of MESSENGER from Cape Canaveral Air Force Station in Florida. A Delta II rocket carried the spacecraft into a heliocentric orbit with a final velocity of 10.68 kilometers per second. The mission cost under US$450 million and was designed to study Mercury's chemical composition, geology, and magnetic field. Engineers at Johns Hopkins University Applied Physics Laboratory built the probe using four graphite fiber composite panels. These panels supported propellant tanks, thrusters, antennas, and a large ceramic-cloth sunshade measuring 2.7 meters tall and 1.9 meters wide. Sean Solomon served as principal investigator for science operations while mission control remained at JHU/APL. The spacecraft weighed approximately 1,000 kilograms at launch with its full load of propellant. Previous attempts to visit Mercury had failed or been too costly due to fuel requirements. Mariner 10 provided the first detailed data in 1975 but mapped only 40, 45% of the surface. No close-range observations occurred for more than three decades after that mission. Chen-wan Yen developed a trajectory design in 1985 showing how multiple gravity assists could slow a spacecraft without excessive fuel use. This approach allowed NASA to execute a Discovery-class mission rather than requiring a heavy lift vehicle. The probe included two small deep space transponders and three types of antennas for communication. A high gain phased array antenna transmitted at 8.4 GHz while other antennas operated at different frequencies. Power came from a two-panel gallium arsenide/germanium solar array providing an average of 450 watts in orbit. Each panel rotated and included optical solar reflectors to balance temperature. Data storage used two solid-state recorders capable of holding up to one gigabyte each. The onboard computer featured radiation-hardened IBM RAD6000 processors running at 25 megahertz and 10 megahertz speeds.
Gravity Assist Trajectory
The journey to Mercury took six years, seven months, and 16 days covering 7.9 billion kilometers. MESSENGER flew by Earth once on the 2nd of August 2005 at an altitude of 2,347 kilometers over central Mongolia. It encountered Venus twice during the cruise phase. The first Venus flyby occurred on the 24th of October 2006 at an unspecified altitude where the spacecraft passed behind Venus entering superior conjunction. Radio contact was inhibited because Earth sat on the opposite side of the Solar System with the Sun blocking signals. Communication resumed in late November allowing a deep space maneuver on the 12th of December 2006 to correct trajectory for a second encounter. The second Venus flyby happened on the 5th of June 2007 at an altitude that provided the greatest velocity reduction of the entire mission. All instruments observed Venus during this event producing visible and near-infrared imaging data of its upper atmosphere. Ultraviolet and X-ray spectrometry recorded atmospheric composition while ESA's Venus Express orbited simultaneously enabling joint measurements. Three Mercury flybys followed before orbital insertion. The first Mercury flyby took place on the 14th of January 2008 reaching closest approach of 200 kilometers above the surface at 19:04:39 UTC. A second flyby occurred on the 6th of October 2008 and a final one on the 29th of September 2009 further slowing the spacecraft. During the last flyby the probe entered safe mode losing planned science data but recovered within seven hours. One final deep space maneuver executed on the 24th of November 2009 provided required velocity change for orbit insertion scheduled the 18th of March 2011. This multi-flyby process greatly reduced propellant needs despite prolonging the trip by many years. Solar radiation pressure acting on MESSENGER's solar panels performed fine course adjustments eliminating unnecessary thruster firings.