Dante conquers the crater, then stumbles.

The robot explorer Dante 2 discovered last week that the road home is often the roughest. After spending 8 days successfully navigating the steaming crater of Alaska's Mt. Spurr and enduring attacks by falling boudlers, the spiderlike machine flipped over while climbing out. The Dante crew will attempt to airlift the helpless robot this week.

Despite the late setback, engineers connected with the mission say Dante performed beyond expectations in a harsh environment. "From the science and technology side, it's a clear victory and a success," says William "Red" Whittaker, a principal investigator on the project and a robotics researcher at Carnegie Mellon University in Pittsburgh.

The eight-legged robot descended 660 feet into the crater of the active volcano, analyzed gases escaping from several fumeroles, and collected terrain information for mapping the interior. For parts of the trip, the robot controlled its own movement by sensing the environment, picking a route, and planning where to place its feet. At other times, the Dante team controlled aspects of the robot's walking from a command center 80 miles away in Anchorage and from NASA's Ames Research Center in Mountain View, Calif.

Dante was connected by a tether to an electric generator and a satellite dish on the rim of the crater. During the first part of its trek, the robot descended snowcovered slopes of 20[degress] to 30[degress]; it later encountered bare rock, mud, and fields of large boulders. At certain points, the 10-foot-long, 10-foot-high robot climbed over rocks larger than itself.

Mt. Spurr erupted in 1992 and let loose smaller blasts in 1993. But even more than the threat of eruption, the primary danger inside the crater came from falling rocks. As the walker neared the crater floor, its video cameras showed several refrigerator-size boulders bounce close by. A 1-foot-diameter rock knocked one of the legs out from under Dante but did not cause major damage.

While in the crater, the explorer used onboard analyzers to test for sulfur compounds in the volcanic gases. Those tests did not detect hydrogen sulfide or sulfur dioxide, which suggests that no fresh magma lies at a shallow depth beneath the volcano waiting to erupt.

The robot had climbed about one-third of the way out of the crater when it rolled over while crossing a 35[degress] slope and could not right itself. "We were just on terrain beyond our capability," explains Project Manager John E. Bares.

Dante 2 evolved out of a walking robot that Whittaker's team took to Antarctica's Mount Erebus in late 1992. This earlier version descended only 20 feet into the crater before broken connections in its tether ended the mission (SN: 1/9/93, p.22).

The Dante 2 project pushed the bounds of robotics by exposing a walking machine to a range of demanding conditions never before experienced. "It sets the state of the art in walking on rough terrain. It's a pretty impressive accomplishment," says Roger D. Quinn, who designs walking robots at Case Western Reserve University in Cleveland.

NASA, which funded the mission, is encouraging the development of autonomous robots for planetary exploration. "The moon is the place for the fulfillment of this work," says Whittaker.