NASA has published a new Tech Brief on TRESSA, the Three-Robot System for Traversing Steep Slopes (TB login needed to view full brief). TRESSA is a group of three autonomous robots that work together to climb steep slopes. And by steep we mean up to 90 degree slope angles. Two of the robots, called Anchorbots, remaining at the top and support the third robot, named Cliffbot, with tethers as it negotiates the rocks. The Anchorbots dynamically adjust the tension of the tethers, allowing Cliffbot to go up, down, or across the slope. This is more complicated than it sounds as the Anchorbots have to combine the amount of tether tension needed to offset gravitational forces with anticipation of desired motion of Cliffbot. The robots also monitor themselves and each other for faults and communicate any potentially unsafe conditions to each other. For more detail on the TRESSA project, see the 2007 JPL paper, TRESSA: Teamed Robots for Exploration and Science on Steep Areas (PDF format).