Spirit has taken its first sample of Martian dust, and soon it will start roving around for evidence of what used to be running water. In three days the next rover, Opportunity, will land, unfold, reach out and finally start moving around–this time on the opposite side of the planet. NASA has made a detailed computer simulation of how the rover navigates. (The more technical stuff comes in the second half.)

Time is critical. Martian dust will slowly build up on the rover’s solar arrays. It will eventually block the energy needed to recharge the batteries that keep the rover alive. The rover has to move as quickly as possible to interesting places without the need to wait for orders from Earth. The trick is to make it move automatically. And the best way to do that is not to try to plan a long journey across the Martian surface all at once, but rather to analyze just a small portion of the landscape ahead. NASA provides frequent updates of the rover’s status.