On January 4, 2004, the first of two identical robotic exploratory rovers, NASA's Spirit, snapped this stunning 360 degree image of its surroundings, moments after setting down on Mars. In the years to follow, both Spirit and its sister Opportunity helped revolutionize our understanding of the Red Planet.
Spirit beat its sister to the surface by about three weeks and landed at a site known as Columbia Memorial Station—in honor of the astronauts killed in the Colombia disaster. Its mission: to find evidence of water, either currently or historically, in the Martian soil over the course of its 90-day mission.
According to NASA:
Spirit drove 4.8 miles (7.73 kilometers), more than 12 times the goal set for the mission. The drives crossed a plain to reach a distant range of hills that appeared as mere bumps on the horizon from the landing site; climbed slopes up to 30 degrees as Spirit became the first robot to summit a hill on another planet...The rover returned more than 124,000 images. It ground the surfaces off 15 rock targets and scoured 92 targets with a brush to prepare the targets for inspection with spectrometers and a microscopic imager.
Nearly six years later, Spirit had set records for rover longevity and covered nearly five miles of alien terrain before one of its six wheels became hopelessly immobilized by sand, relegating the rover to function as a stationary science platform.
Spirit continued to operate for another four years before it stopped communicating on March 22, 2010, never to be heard from again. Opportunity, however, is still going strong and continues to investigate the possibility of water—and with it, life—on Mars. [NASA - Wiki]