AFTER AN 18-HOUR DAY AND NIGHT at the office, Brian Cooper returned to his home in Altadena, Calif., Monday morning in a near-vegetative state. Completely spent, he had trouble just watching TV. "He was so tired," says his wife, Lynne, "he couldn't even operate the remote control."

Well, maybe not that remote control. Cooper, 37, is the designated driver of Sojourner, the little six-wheeled robot that is exploring the desertlike surface of Mars, 119 million miles from Earth, for NASA's Mars Pathfinder Mission. He is living out the ultimate Nintendo fantasy. "This is what I wanted to do since I came to the Jet Propulsion Laboratory," says Cooper, who has been with NASA for 12 years. "I've always had a fascination for things you can control." Says Lynne, 36, also an engineer at JPL: "He basically built his own video game. Now he's playing it."

In fact the game would be maddeningly frustrating if Cooper didn't know just how to play it. Because of the 11-minute lag between the time a signal is sent from the JPL, in Pasadena, and the time it arrives on Mars, "If I were to see a cliff on my computer screen and tell the rover to stop, it would have fallen off by the time the signal got there," he explains. Instead instructions are sent and executed in stages. A camera on the Pathfinder landing vehicle takes a panoramic picture of the rover and the surrounding terrain and sends that image back to Earth. Cooper, wearing special goggles, studies the Martian landscape at the monitor on his desk. "I can see the surface of Mars as if I'm standing there," he says. "I can sense depth. I can see how far away the rocks are."

Then, in collaboration with his colleagues, Cooper identifies a target—a boulder or a geologically interesting stone. But before beaming instructions to Sojourner, he tries out different routes with a virtual rover at JPL. Only then is the vehicle told on Mars, via a set of computer codes, where to go. Creeping along at just two feet per minute, Sojourner will spend the day executing these commands. The arrangement makes for a convenient division of labor. "I'll be performing my functions while the rover is asleep," says Cooper. "Then I'll come home and sleep while the rover is driving."

The son of an Air Force master sergeant and his wife, Cooper has been smitten with robots since he was a kid growing up in Torrance, Calif., outside L.A. A science-fiction buff, he was captivated by the voice of HAL 9000, the computer in 2001: A Space Odyssey, and the robot stories of Isaac Asimov. "I built my first robot in college," says Cooper, who got his degree in electrical engineering from the University of California at Irvine in 1981. "I used it to explore my living room."

Cooper arrived at the JPL in 1985 after four years in the Air Force, during which he earned a master's in computer engineering. For 12 years he has been working on a prototype driver system for Sojourner. Indeed, while Pathfinder was making its seven-month journey to Mars, Cooper was driving a Sojourner clone around a room called the sandbox, complete with Marslike rocks and soil.

For a brief moment it looked as if he wouldn't get to drive the real Sojourner at all. Although the lander bounced safely onto the ancient Martian flood-plain of Ares Vallis on July 4, the rover ran into some early problems, including a communications glitch. "Then I saw the rover on the soil," says Cooper. "I started whooping it up!"

First, Cooper and his team had Sojourner check out a multicolored rock christened Barnacle Bill, assessing its composition with an X-ray spectrometer. Next the robot approached another, bear-shaped boulder dubbed Yogi. By midweek, Sojourner had gathered data to bolster speculation that Mars was flooded with water at least a billion years ago—an indicator that it might have supported life. For his part, Cooper is happy to be there—if only by remote control—and thankful there are no other moving vehicles on Mars, so he doesn't have to parallel park. That wouldn't be easy, even in stages.