Key figures: NASA, Johns Hopkins University Applied Physics Laboratory (JHU/APL), Sean Solomon (principal investigator)
Summary
On March 18, 2011 (01:00 UTC; the evening of March 17 in the United States), NASA’s MESSENGER spacecraft fired its main thruster to slow into orbit around Mercury, becoming the first spacecraft ever to orbit the innermost planet. The braking maneuver, executed after a journey of nearly seven years and roughly 7.9 billion kilometers, ended a mission-design challenge that had kept Mercury unvisited by an orbiter since the Mariner 10 flybys of the 1970s. MESSENGER was built and operated for NASA by the Johns Hopkins University Applied Physics Laboratory.
MESSENGER—an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, and a nod to the Roman messenger god Mercury—began formal science observations in early April 2011.
The Mission
MESSENGER launched on August 3, 2004, atop a Delta II rocket from Cape Canaveral Air Force Station in Florida. Reaching Mercury is difficult because a spacecraft falling toward the Sun gains enormous speed that must be shed to enter orbit. To manage this, MESSENGER followed a long, looping trajectory that used six planetary gravity-assist flybys to bleed off velocity: one of Earth (August 2005), two of Venus (October 2006 and June 2007), and three of Mercury itself (January 2008, October 2008, and September 2009) before the final orbit insertion in March 2011.
Scientific Findings
From Mercury orbit, MESSENGER transformed scientific understanding of the planet:
- Water ice at the poles: It confirmed that permanently shadowed craters near Mercury’s poles harbor water ice, along with dark organic-rich material—remarkable on a planet whose sunlit surface can exceed 400 °C.
- Volcanic history: It found extensive evidence of past volcanic activity, including vast smooth plains formed by ancient lava flows.
- Magnetic field: It mapped a global magnetic field whose center is offset far to the north of the planet’s center.
- Surface composition: It measured surprisingly high abundances of volatile elements such as sulfur and potassium, constraining theories of how Mercury formed.
End of Mission
MESSENGER far outlasted its planned one-year orbital mission, operating for more than four years around Mercury. Having exhausted its propellant, the spacecraft was allowed to descend and impacted Mercury’s surface on April 30, 2015, ending a mission that spanned more than a decade from launch.
Significance
MESSENGER’s 2011 orbit insertion was a landmark of planetary exploration: for the first time, a machine circled and systematically studied the least-explored of the inner planets. Its data resolved long-standing questions—most strikingly the presence of polar water ice on a scorched world—and laid the groundwork for the joint European–Japanese BepiColombo mission that followed. The achievement stood among 2011’s defining feats of space exploration, in a year that also saw the end of NASA’s Space Shuttle program and the launch of the Curiosity Mars rover.