HITTING THE MARTIAN HIGHWAY

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The going will be slow. Commands from Cooper's computer will take 11 minutes to travel from Pasadena to Mars; it will take another 11 minutes for the rover to acknowledge that it has received the instruction. To prevent Sojourner from blundering into a chasm or over a cliff, engineers designed it to move no faster than 1.3 ft. per minute. Onboard gyroscopes and lasers will help it feel for dangers the camera might have missed. If Sojourner spots an obstacle, it will try to avoid it or simply stop. "We'll give it a point to go to and an amount of time to get there," says Cooper. "If it doesn't, we'll find out what the problem was."

Painstaking as the rover's exploration of Ares Vallis will be, it should be worth the effort. The water that flooded the valley billions of years ago came from all over the planet, carrying all manner of rocks with it. Sojourner will pick through this geological boneyard, photographing the remains and using X-ray spectrometers to study their composition.

The rover will not operate for long--no more than a month--before Mars' punishingly cold climate (-15[degrees]F by day but plunging as low as -125[degrees]F at night) kills it. The Pathfinder lander, also able to take readings, could function for up to a year. No matter when the machines wink out, however, Mars is unlikely to remain unattended. On Sept. 12, Global Surveyor, another robot probe, will arrive at the planet, settle into a 250-mile-high orbit and begin two years of mapping the surface. A second lander-and-orbiter pair is set to be dispatched Marsward in 1998, with more to follow roughly every other year until 2004. Finally, in 2005, the program will culminate in a first-ever round trip: a probe that lands on Mars and flies back home, carrying a bit of local soil with it.

That, at least, is the way NASA planners hope things will go; so far it looks as if they'll get their wish. The ships that will be used for these ambitious missions are remarkably cheap ones, hammered together from available, off-the-shelf parts. While this makes for less elegant vehicles, it also makes for less pricey ones. They cost no more than $250 million, in contrast to the $1.48 billion it costs to build luxury liners like the still-to-be-launched Cassini Saturn probe. Cheap ships means more of them, and for space planners that is a good thing.

"If you only fly two missions a decade and you lose one of those two spacecraft," says NASA Administrator Daniel Goldin, "you're set back for a decade. By breaking the mission into smaller parts and spreading it out, you have a continual return of science."

Congress likes this kind of budgetary horse sense, and though Washington hasn't guaranteed NASA funds for even a stripped-down Mars program, the enthusiasm for Pathfinder on Capitol Hill bodes well. "An entire generation has grown up in the two decades since we last viewed the soil of Mars close up," says Congressman James Sensenbrenner, chairman of the House Committee on Science. "I can't wait for our kids to see those pictures." Whatever may happen to future missions, this week the kids should start to see plenty.

--Reported by Dan Cray/Pasadena and Joanna Downer/Washington