While in orbit 170 miles above Earth, Navy Captain Bruce McCandless II becomes the first human being to perform an untethered spacewalk, when he exits the U.S. space shuttle Challenger and maneuvers freely, using a bulky white jet pack of his own design. McCandless orbited Earth in tangent with the shuttle at speeds greater than 17,500 miles per hour—the speed at which satellites normally orbit Earth—and flew up to 320 feet away from the Challenger. After an hour and a half of testing and flying the jet-powered backpack and admiring Earth, McCandless safely reentered the shuttle. Later that day, Army Lieutenant Colonel Robert Stewart tried out the jet pack, which was a device regarded as an important step toward future operations to repair and service orbiting satellites and to assemble and maintain large space stations. It was the fourth orbital mission of the space shuttle Challenger.