(CNN) -- 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 and 1973.
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.
July and August 1973 - The U.S.S.R. successfully launches Mars 4, 5, 6 and 7. And each spacecraft takes about seven months to reach the planet's orbit. Only Mars 6 landed.
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.
March 10, 2006 - The Reconnaissance Orbiter reaches Mars and establishes its orbit. It carries the most powerful camera ever to leave Earth, called HiRise. The camera takes its first four pictures on March 23, while the orbiter circles Mars at a distance of between 600 and 2,500 miles (900 to 4,000 kilometers). This initial phase of exploration lasts two weeks.
January 24, 2007 - On the third anniversary of its landing, Opportunity continues its mission of exploring the Martian terrain - 33 months longer than originally intended.
August 4, 2007 - NASA's Phoenix Mars Lander is launched successfully. Its rocket launches at Cape Canaveral Air Force Station in Florida and takes about 10 months to reach Mars. Phoenix, part of NASA's Scout program for smaller and lower-cost spacecraft, is designed to analyze soil samples and scan the Martian atmosphere after landing near its northern pole.
May 25, 2008 - The Mars Phoenix Lander completes a 296-day, 422-million-mile journey, landing near the planet's North Pole.
November 2008 - The Mars Phoenix Lander ceases communications after completing its mission. Originally expected to last 90 Martian solar days the lander performs science for 149 of its 152 solar day lifetime.
March 22, 2010 - Last communication is heard from Spirit. The project continues to monitor for further communication. Opportunity is still operational.
September 10, 2010 - Scientists release data from the Mars Phoenix Lander suggesting that water has been weathering Mars' surface throughout history. Data also suggests that Mars' carbon dioxide atmosphere has been replenished by geologically recent volcanic eruptions providing evidence for what could be ongoing activity.
April 6, 2011 - The rover nicknamed Curiosity is unveiled. It is far larger than Spirit and Opportunity, weighing approximately 2,000 pounds (900 kilograms) and about the size of a Mini Cooper car.
May 25, 2011 - NASA scientists end attempts to regain contact with Spirit.
November 26, 2011 - The Curiosity rover launches from Cape Canaveral at 10:02am, aboard an Atlas V rocket bound for Mars. The journey is scheduled to last approximately 8.5 months.
August 6, 2012 - Curiosity successfully lands on Mars at 1:32am EDT.
September 26, 2013 - Journal Science publishes five articles regarding Mars, revealing raw data which indicates the Martian surface contains about two percent water by weight.