On January 4, 2004, after traveling 487 million kilometers (303 million mi) through space, a rover named Spirit bounced 28 times and landed within 300 meters (984 ft) of its target on the surface of Mars. Another rover, Opportunity, took a very similar trip and, on January 25, landed near its mark on the other side of the planet. With two successful landings, the Mars Exploration Rover mission defied the odds.

Historically, the majority of missions to Mars have been failures. Many failed at launch while others were lost in space. In its first attempted mission, the European Space Agency (ESA) scheduled its Beagle 2 to land just weeks before the Mars Exploration Rovers. Upon being released by the Mars Express orbiter, Beagle 2 was supposed to land and then establish radio contact. After months of failed attempts to communicate with the lander, the operations team had to concede that Beagle 2 presumably crashed, most likely due to an unknown technical malfunction, and would not be able to complete its mission.

NASA's Mars Exploration Rover mission has had a very different outcome. Not only did the rovers land safely, they have gone on to complete and surpass their mission goals. More than 100 experts worked together to choose sites that were both scientifically significant and operationally feasible. For example, the landing site needed to be low in elevation to give the parachutes enough time to slow the descent through the thin atmosphere. Despite the use of durable airbags to cushion the landers on impact, steep slopes and rocks would pose risks. High winds, dust, and proximity to the equator were additional risk factors.

Using data and images from past orbital missions, 155 possible sites were narrowed down to two: Meridiani Planum and Gusev Crater. Meridiani Planum was chosen because a previous orbiter mission had found it to contain a large expanse of exposed hematite, a mineral that often forms in the presence of water. As a flat open plain, it was a safe landing site for Opportunity. Gusev Crater appealed to scientists because of a dry riverbed running into it, which suggested that it may have once held a large lake. However, the strong winds around the crater made it a risky landing site. To increase the likelihood of a safe landing, the team used cameras and rockets onboard Spirit to calculate its horizontal velocity and stabilize its motion.

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