Here's a look at what you need to know about Mars exploration. The United States and the Soviet Union are the only two countries to land a spacecraft on Mars.
Facts: There have been seven successful U.S. Mars landings: Viking 1 and Viking 2 (both 1976), Pathfinder (1997), Spirit and Opportunity (both 2004), Phoenix (2008) and Curiosity (2012).
55 percent of NASA missions to Mars have ended in failure.
The only other country to land a spacecraft on Mars is the Soviet Union, in 1971.
Timeline: 1965 - Mariner 4 passes within 6,000 miles (about 9650 kilometers) of Mars and takes the first close-up photos of the planet's surface.
1969 - Mariners 6 and 7 pass within about 2,500 miles (4,000 kilometers) of the planet and transmit information about the planet's surface and atmosphere.
November 3, 1971 - Mariner 9 launches. It reaches orbit on November 24, becoming the first U.S. spacecraft to orbit a planet other than Earth.
December 2, 1971 - The U.S.S.R.'s Mars 3 lander makes the first successful landing on the planet's surface. It transmits data for 20 seconds before failing.
1975 - Viking 1 and Viking 2 are launched.
July 20, 1976 - Viking 1's lander reaches the surface of Mars.
September 3, 1976 - Viking 2's lander reaches Mars.
September 25, 1992 - The U.S. launches the Mars Observer. Contact is permanently lost in August 1993.
November 1996 - The Mars Global Surveyor is launched. It reaches Mars in September 1997 and begins its orbit. It is last heard from on November 2, 2006.
July 4, 1997 - The Mars Pathfinder, after six months of travel, lands on Mars. For four months the rover Sojourner explores the planet's surface and returns photos to Earth.
December 11, 1998 - The Mars Climate Orbiter is launched, but is lost upon arrival in September 1999.
January 3, 1999 - The Mars Polar Lander is launched. On December 3, it fails to make contact with ground control. NASA believes the lander was destroyed upon impact with Mars.
April 7, 2001 - The Mars Odyssey orbiter is launched, and reaches Mars on October 24.
June 2, 2003 - The European Space Agency launches the Mars Express Orbiter and the Beagle 2 lander. The Beagle 2 is scheduled to land on Mars on December 25, 2003, but never makes contact.
June 10, 2003 - The rover nicknamed Spirit is launched.
July 7, 2003 - The rover nicknamed Opportunity is launched from Cape Canaveral. Both Spirit and Opportunity are part of NASA's Mars Exploration Rover Mission.
January 3, 2004 - At approximately 11:35pm EST, Spirit lands on Mars and immediately starts to send back pictures from the surface.
January 15, 2004 - The Mars rover Spirit rolls off its lander and begins the next phase of its mission to study the rocks and soil of the Martian surface for signs of water
January 24-25, 2004 - The rover Opportunity lands on Mars. (Sources conflict on the date. NASA uses both Universal/EST and PST which changes the date from 9:05pm, January 24 to 12:05am, January 25)
February 2, 2004 - For the first time, both Spirit and Opportunity are fully operational. The rovers, 6,600 miles apart, collect soil samples and taking photographs of the Martian surface. Both rovers find evidence of ancient Martian environments that were intermittently wet and habitable.
August 12, 2005 - The Mars Reconnaissance Orbiter launches. It will scan the planet for more signs of water.