So close and still so far

Twenty-seven years ago, NASA's Viking 1 lander -- the first spacecraft ever to touch down on another planet -- transmitted dramatic photographs of an alien landscape, a barren red plain flecked with rocks.

Six years ago, when Sojourner became the first moving vehicle on Mars, it also revealed a barren plain flecked with rocks. And when the Spirit rover touched down last month, the landscape looked much the same.

Satellite pictures show that Mars has the highest known volcano in the solar system and a valley that reaches more than five times the depth of the Grand Canyon. It has a familiar-looking delta and intriguing gullies that could have been formed by water. But because of the dangers of the planet and the limits of space engineering, those sites all remain out of reach.

As President Bush calls for a long, expensive campaign to send humans on the perilous journey to Mars, the lander program is highlighting a difficult truth of space travel: Nearly three decades after the first Mars landing, scientists are still much better at observing the planet's tantalizing geological features from afar than landing anywhere near them.

Mars has destroyed two out of every three probes sent to the planet, including the European probe Beagle 2 in December. The only successes -- including the current $820 million Spirit and Opportunity missions -- have been carefully targeted at flat areas around the equator. Until the raw mechanics of landing on Mars improves, some scientists say that NASA's landing program, at least for now, is less about sophisticated science than about simply learning how to land probes safely on a planet infamous for wrecking them.

"I can tell you without any question we will never land on the top of Valles Marineris [the giant Mars Canyon] if we don't learn to land in the flat areas of the equators first," said Maria Zuber of the Massachusetts Institute of Technology's Earth, Atmospheric and Planetary Sciences Department.

Mars landers are also, in no small measure, parts of a public-relations mission.

"Right now we are getting much more information from the orbiters," said Mike Carr, a planetary geologist for the US Geological Survey who helped choose the landing sites for the two rovers now roaming Mars. "We could abandon surface exploration -- it is so difficult -- but how much interest does the public have in the [Mars Global Surveyor orbiter]? It's the sense of exploration and daring and awe on the ground."

It's not that scientists want to scrap robot probes to Mars. No matter how compelling the satellite pictures of the planet's dust-streaked plains and magnificent rises, scientists still need landers to get the kind of data they call "ground truth." Without it, Carr said, "you are not sure of yourself, of what you are seeing."

But to reach the most scientifically interesting sites on the planet, engineers must first develop more reliable ways to land.

Part of the challenge comes from landing precisely -- a difficult thing for an unmanned spacecraft to do after it hits the Martian atmosphere at 12,000 miles per hour. In the case of the Opportunity rover now on Mars, for example, engineers could whittle down its landing site only to an ellipse 37 miles long by 2 miles wide. With so much uncertainty, they knew it had to land in an area with a relatively smooth surface -- or else it could crash into a canyon wall or disappear into a hole.

Another limitation is the landing technique itself. The Viking missions of the 1970s largely used retro-rockets to bring the craft down softly onto the Martian surface, a technique that required a great deal of fuel, making the spacecraft heavier and thus more expensive to launch. As part of NASA's "better, faster, cheaper" philosophy, the most recent missions to Mars largely relied on airbag cushioning for a soft landing. The spacecrafts can't land anywhere known to have large rocks, which could tear the airbags. Steep slopes are also out, because the lander could smash into them -- and the small-wheeled rovers could get stuck trying to climb the slope. And, at least for this trip, scientists stuck the solar-powered rovers close to the equators, where sunlight is maximized.

NASA's one recent attempt to send a lander to a different part of the planet -- the Mars Polar Lander, in 1999 -- was lost because it was operating in metric and scientists were giving it commands in nonmetric units.

In many ways, the evolution of Mars probes mimics that of the Apollo moon program: Start in the safest areas and then branch out. The first manned moon landing, Apollo 11, landed on an extremely flat part of the moon, while Apollo 14 perfected a precise landing and went to more-challenging terrain. By Apollo 16 and 17, astronauts were in even rougher areas.

"You have to build confidence," said James Head, a planetary geologist at Brown University who worked on Apollo.

NASA's future plans count on this strategy working. The Phoenix Scout, scheduled to be launched in 2007, will attempt a more precise landing in a polar ice region. Two years later, the Mars Science Laboratory will set out to land even more precisely, partly navigating its own course through the Martian atmosphere and then traveling around the planet. And sometime between 2011 and 2015, scientists will try to launch a mission to bring back rocks and soil from Mars.

If a manned mission ever occurs, it is likely to touch down in a flat area for safety, but scientists expect that the astronauts would be able to use a vehicle to rove the surface.

Until landing craft reach that level of sophistication, the best scientific information will come from the two NASA orbiters and the one European orbiter currently circling the planet.

"The Mars orbiters have been fantastic -- a much richer source of information than the landers," said Paul Hoffman, a Harvard geologist. But for many scientists, it's all a matter of perspective. Spirit and Opportunity may be examining only a tiny swath of the planet, but then anything they can study on Mars has scientific value.

And if Opportunity's limited landscape in Meridiani Planum is any indication, a new kind of landscape just might be shaking up those flat rocky red pictures of years past. The six-wheeled rover landed in a shallow crater that has bedrock nearby.

"Meridiani is not what we expected," said Matt Golombek, cochairman of the site selection committee for the rovers on Mars. "It's not red, it's not dusty, and it's not really rocky in the same way the other sites are. Really, you just don't know what's there unless you go down and do [the experiments]."