The agency's robotic emissaries have been studying the Red Planet nonstop since July 4, 1997, when the Pathfinder mission touched down. The landing marked NASA's first Mars success since the Viking 1 and Viking 2 orbiter-lander pairs arrived at the Red Planet in 1975.
Eight other robots have followed in Pathfinder's footsteps to date, including the Spirit, Opportunity and Curiosity rovers. [Occupy Mars: History of Robotic Red Planet Missions (Infographic)]
"Without Mars Pathfinder, there could not have been Spirit and Opportunity, and without Spirit and Opportunity, there could not have been Curiosity," Pathfinder project scientist Matt Golombek, of NASA's Jet Propulsion Laboratory in Pasadena, California, said in a statement.
Spirit and Opportunity landed a few weeks apart in January 2004, on three-month missions to search for signs that liquid water once flowed on Mars. Both six-wheeled robots found plenty of such evidence, then kept on rolling for years after their warranties expired. Spirit finally died in 2010, and Opportunity is still going strong.
The car-size Curiosity rover, meanwhile, has been cruising around Mars' 96-mile-wide (154 kilometers) Gale Crater since August 2012. Curiosity's observations suggest that Gale harbored a habitable lake-and-stream system for long stretches in the ancient past. The robot is currently climbing through the foothills of the 3-mile-high (5 km) Mount Sharp, looking for clues about how Mars shifted from a relatively warm and wet world long ago to the cold, dry planet it is today.
The other NASA robots that have made their way to Mars in the last 20 years are Mars Global Surveyor, Mars Odyssey, the Mars Reconnaissance Orbiter (MRO), the Phoenix lander and the Mars Atmosphere and Volatile EvolutioN Mission (MAVEN). (Two other spacecraft, the Mars Climate Orbiter and the Mars Polar Lander, launched toward the Red Planet during this two-decade stretch but failed to reach their destinations successfully.)
Mars Global Surveyor, Mars Odyssey, MRO and MAVEN are orbiters that reached the Red Planet in September 1997, October 2001, March 2006 and September 2014, respectively. Phoenix touched down in May 2008.
Odyssey, MRO and MAVEN (along with Opportunity and Curiosity) remain operational to this day, as do a handful of missions operated by other space agencies: Europe's Mars Express orbiter, India's Mars Orbiter Mission and the European/Russian ExoMars Trace Gas Orbiter.
NASA plans to launch two more Mars missions in the next few years — the InSight lander in 2018 and a Curiosity-like rover in 2020.
Editor's note: Video produced by Space.com's Steve Spaleta.
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