Exploration of Mars
The Viking program launched "Viking 1" and "Viking 2" spacecraft to Mars in 1975; The program consisted of two orbiters and two landers – these were the first two spacecraft to successfully land and operate on Mars.
The primary scientific objectives of the lander mission were to search for biosignatures and observe meteorologic, seismic and magnetic properties of Mars. The results of the biological experiments on board the Viking landers remain inconclusive, with a reanalysis of the Viking data published in 2012 suggesting signs of microbial life on Mars.
The Viking orbiters revealed that large floods of water carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers. Areas of branched streams, in the southern hemisphere, suggest that rain once fell.
Opportunity mission timeline
Opportunity, MER-B (Mars Exploration Rover – B), is a robotic rover active on the planet Mars since 2004. Launched on July 7, 2003, "Opportunity" landed on Mars' Meridiani Planum on January 25, 2004 at 05:05 Ground UTC (about 13:15 Mars local time), three weeks after its twin "Spirit" (MER-A), also part of NASA's Mars Exploration Rover Mission, touched down on the other side of the planet. While "Spirit" became immobile in 2009 and ceased communications in 2010, "Opportunity" remains active as of December 11, 2017, having already exceeded its planned 90 sol (Martian days) duration of activity by (in Earth time). "Opportunity" has continued to move, gather scientific observations, and report back to Earth. What follows is a summary of events during its continuing mission.