You are standing above a sweeping cinnamon plain, dotted with crescent dunes beneath a pink sky. A breeze stirs sand into dust devils. Craggy ridges far across what was once a carved river valley point at the rising potato shape of a small moon.

But all this you ignore, beautiful though it is, because halfway to the rosy horizon is a splash of yellow and brown. The possible site of an ancient thermal vent, or so the satellite pictures say.

With a touch of your thumb you send the robot rover ahead, while you climb back into your dune buggy. If the yellow stain signals sulfur, this might well be an entrance into the mysterious underground of Mars.

Once, lava and water probably spewed from that opening. Your expedition's geologist says this river valley once held a sprawling stream, when Mars was wet and warm. If life was here once, it might have retreated below, just as the non-oxygen breathers did on Earth long ago.

And the easiest way to find out was to descend through the tubes where lava once flowed, into the heart of the Red Planet. Eagerly you press on.

I used "you" in the above little scene because it could indeed be you standing on Mars, carrying out the most exciting and profound scientific expedition of the century to come. We can do such wonderful things in the lifetime of most people reading this.

This may seem a curious time to talk about such grand ambitions. We lost both this season's Mars missions, and NASA is under enormous pressure to drop the faster-cheaper-better mantra. Some say we should hold off on Mars generally.

Nonsense. Space is tricky and dangerous, but we knew that already. Instead of obsessing over the embarrassing smashup of an orbiter, plus the mysterious vanishing of the Polar Lander -- probably due to a faulty engineering design in the landing system -- we should look long. The purpose of these landers and orbiters is primarily to understand the Red Planet's deep past. But they can also give us a list of promising landing sites for humans.

The argument that people are essential to finding evidence of past (or present!) life is solid. Robots cannot crawl down thermal vents and take samples, assessing them on the spot. After dual exploratory expeditions in 2001, 2003 and 2005, NASA plans to return a small soil sample to Earth around 2007, using elementary robotic scavenging. (Even returning less than a kilogram of Martian soil will probably cost around a billion dollars, Jet Propulsion Lab scuttlebutt has it.)

After that, what? Settling the big scientific issue of life on Mars will demand a crewed expedition. Maybe we could go to Mars by offering an international prize, an idea I used in a recent novel, The Martian Race. But funding is only a side issue, really.

Right now, the entire space community should demand that present operations work with this goal in mind: human exploration before 2020.We're spending many billions on the space station, which NASA advertises as telling us how to "live in space" for "eventual planetary voyages."

But mostly it will prove what the Russians' Mir experience underlined: there's not a lot of science to do there, and zero-g is very bad for you.

So, we should demand that the space station do something really useful for a Mars flight.

Do the Math

For creating rotational gravity aboard a habitat bound for Mars, the likely counterweight would be the last stage of a big booster, which has already injected the habitat into a long 6-month orbit for Mars, starting from Low EarthOrbit.    More Stories NASA Has Lost Its Nerve The acceleration (A) of such a spinning dynamics problem is A= S2 R/1000.

Here A is in units of one g, so for Mars it's 0.38. S is the spin in units of revolutions/minute, and R the length of the cable in meters. For a spin rate of 3 rev/minute, R=9 meters. This is too short, for several reasons -- but then, I've left out a detail. The habitat rotates around the system's center of mass -- the length of the cable divided by (1+m/M), where m is the habitat mass and M the counterweight mass.

A plausible ratio m/M is 4, so the whole cable length L for the above case would be about 5R, or 50 meters. I include this level of techie detail to suggest the considerations we must work out to design a scheme to get comfortably to Mars. Some 1960s NASA ground experiments suggested that spinning does not produce dizziness until S=6 rev/minute,so there's room to maneuver.

First, make it the first centrifugal gravity experiment ever. It's shocking that no one has ever done this simple, crucial task, and making gravity by rotating a habitat suspended by a cable to a counterweight is probably the easiest way to go to Mars.

But we really don't know anything about compensating for zero-g in space itself. A basic theorem: Theory is no substitute for experience.

Kubrick's 2001: A Space Odyssey showed us a classic space station like a revolving bicycle wheel. It's time to mount such an experiment, maybe a habitat on a cable with a counterweight, next to the International Space Station.

Use the station as a construction shack. Only by direct experience will we learn.

And then there's the whole problem of a truly cyclic environment. We'll need to reprocess wastes in air and urine to survive on dry Mars. NASA is studying this, but how does such a system work in 0.38 g, the Mars standard? We could do a true dry run by running a truly closed environment on a rotating centrifugal gravity habitat, in Low Earth Orbit, next to the space station.

This is an exciting program, with real work for the astronauts, and -- undoubtedly! -- plenty of unforeseen problems. Real work, not just camping out in space, as the Russians spent decades doing, fruitlessly.

Years of experience doing this will give us a team of tough, experienced astronauts, ready to go to Mars. They're alive right now. Maybe you could be one.

How to get to Mars? Start now.

You are edging carefully down the slope of the thermal vent, peering into the throat that once belched hot mud and scalding water -- once a super-Yellowstone, billions of years ago.

Your team partner carries the 3D-TV camera on his shoulder. You're the biologist on this expedition, so you have to carry the sample pouches, added mass. And it looks like you're going to be needing them.

Knowing that all of humanity will hang upon your words, when you transmit to Earth at the end of this day, you say carefully, "Looks like some mottled growth on the rocks below. Discoloration, brown and gray ... like an algae mat."

Your heart pounds. It does indeed look...different.

You're only twenty meters below the deserts of Mars. There might be anything below, caverns to be searched, secrets uncovered.

"I'm going further in."