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Set on course by rocket firings last week, Opportunity is scheduled to arrive on Mars at 12:05 a.m. ET, promising plenty of landing drama, slow-motion unpacking and views of a grayish plain "fundamentally different" from anything glimpsed by previous Mars explorers, says the Jet Propulsion Laboratory's Matthew Golombek.

Opportunity's landing target is Meridiani Planum, an equatorial plain laced with veins of gray hematite, an iron-oxide mineral associated with hot springs on Earth. It lies on the opposite side of Mars from Gusev Crater, where the rover Spirit landed Jan. 3.

As of Thursday, however, the Spirit stopped transmitting data for more than 24 hours, said NASA managers, who call the situation an "extremely serious anomaly."

Public interest in Mars remains high. JPL officials in Pasadena, Calif., home of NASA's rover missions, reports 2.5 billion hits on the lab's Web page since Spirit's landing.

And with President Bush's call this month for more exploration of the moon and Mars, the space agency hopes Opportunity garners the same attention as Spirit.

The mission of the $820 million Mars Exploration Rover is to "follow the water." Pictures and instruments suggest that water — considered a fundamental requirement for life — once carved Mars' surface, perhaps within the past few million years, and still lurks below the ground. To test these notions, Opportunity will rove over what is suspected to be a 3.8-billion-year-old seabed.

Compared with the red deserts seen by every previous lander, "Meridiani is going to have a completely different look and feel," Golombek says. It's smoother than Gusev Crater's interior with no hills and a few "fossil" craters, ancient features with sanded-off rims.

That smooth surface and less windy conditions make Meridiani a relatively safe bet for a landing, mission controllers say, but they still warn that a bad bounce could be trouble. "The question," JPL project manager Pete Theisinger says, "is always what you hit and how hard you hit it."

With minor modifications, the descent will follow the heat shield, parachute, braking rocket and airbag landing plan carried out by Spirit.

Landmarks in Mars exploration Mariner 4, USA, July 14, 1965 Passed within 9,920 kilometers of the planet's surface. It returned 22 close-up photos showing a cratered surface. Mars 3, U.S.S.R., Dec. 2, 1971 Released a probe that became the first successful landing on Mars. It relayed 20 seconds of video data to the orbiter; the orbiter returned data until August 1972. Mariner 9, USA, Nov. 1971 The first U.S. spacecraft to enter an orbit around a planet other than the Earth's moon. Viking 1, USA, June/July, 1976 Entered orbit around Mars on June 19. The lander touched down July 20. Orbiter was deactivated on Aug. 7, 1980, when it ran out of altitude-control propellant. The lander was accidentally shut down Nov. 13, 1982. Viking 2, USA, July/August, 1976 Entered orbit around Mars on July 24. The lander touched down Aug. 7. Both Viking landers provided detailed views of the terrain and monitored the Martian weather. Mars Global Surveyor, USA, Sept. 12, 1997 Inserted into an elliptical orbit, it collects data on the surface topography, composition, gravity, atmospheric dynamics and magnetic field. Mars Pathfinder, USA, July 4, 1997 Hit planet at about 40 mph, bouncing to 50-foot heights. Rover Sojourner rolled onto the surface July 6. Pathfinder returned 2.6 billion bits of information. 2001 Mars Odyssey, USA, April 7, 2001 Orbiter will circle Mars for three years to help determine whether the environment was ever conducive to life and to study potential radiation hazards to future missions. Mars Express orbiter/ Beagle 2 lander, European Space Association, Dec. 25, 2003 Orbiter is circling Mars. There has been no word from the lander; it is presumed lost. Mars Exploration Rover Spirit, USA, Jan. 3 First of NASA's two rovers reached the surface and began transmitting photos shortly after arrival; Spirit rolled off the lander Jan. 15. Source: NASA

On impact, Spirit bounced 28 times to nearly 30-foot heights and traveled about 1,000 feet from its impact point in Gusev Crater.

Remote sensing instruments suggest Meridiani is only about 5% covered with rocks, but the possibility that a rock could puncture a bouncing airbag remains a worry.

At best, orbiters can pick up surface details only 6 feet to 30 feet across, which leaves some uncertainty about the landing site. And random bounces aren't events designed for an engineer's comfort.

"You never know. You shouldn't be overconfident," says Rob Manning, JPL's entry, landing and descent manager. If Spirit had landed on the now-famous hills about 2 miles east of its resting spot, it might have been a disaster, he says. "It doesn't take too many bounces and you fall off a cliff."

Meridiani is flatter than Gusev Crater and rests nearly one-third of a mile higher, so the air is thinner. Thin air also can affect the landing. Because dust storms thinned the air over Gusev, Spirit popped its parachute nearly 1 mile closer to the surface than expected. Meridiani looks less dusty, but if weather reports call for it, the team could direct Opportunity to pop its chute sooner, Theisinger says.

"The Spirit landing was very successful; it showed there was enough robustness in all the systems to handle conditions somewhat different from anticipated," says atmospheric scientist Paul Withers of Boston University.

Opportunity for science

Much of the excitement over Spirit seems derived from NASA having a success, especially after the British Beagle 2 Mars lander disappeared during Christmas, says astronomer Phil Plait of Sonoma State University in Rohnert Park, Calif. But many people who contact his badastronomy.com Web site, which is dedicated to correcting public misconceptions about astronomy, have genuine science questions about Mars.

"People do get that it's another planet, and that's intrinsically fascinating," he says.

Scientists are far from immune to that fascination. Viewed as a dry desert planet for decades, Mars has enjoyed a revival of interest.

In 2000, NASA's Mars Observer revealed signs of water-carved features. In 2002 another probe found evidence of enough water just under the surface to fill Lake Michigan twice, leading to the possibility that more could be hidden deeper.

"Both landing sites are about finding out whether the early environment on Mars kept liquid water stable," Golombek says. "We try not to have biases, but I'm very excited to see Meridiani."

Unlike Spirit, which scientists hope to drive about 800 feet to explore the rock layers within a small crater, Opportunity should land on top of its target, an ellipse 53 miles long and nearly 7 miles wide.

Gray hematite covers as much as 20% of Meridiani. Depending on Opportunity's findings:

• Layers of sedimentary rock may indicate that an iron-rich lake or ocean formed the mineral layer.

• Hot springs, which are considered a favorable habitat for microbes, might have bubbled up and deposited veins of hematite.

• Weathering of small amounts of water on iron-rich rocks can create a veneer of hematite, which the rover should reveal.

• Lava, indicated by volcanic rocks residing solely at the landing site, also can create hematite.

On the lava possibility, Golombek says that "the jury is still out" on the lake bed theory.

Others are more optimistic that the area is a lake bed.

William Hartmann of the Planetary Science Institute in Tucson suspects that Meridiani Planum is part of an antique region once covered with iron-rich waters that formed a layer of gray hematite and was exposed by wind erosion within the past 10 million years.

"Remember that we've discovered dry riverbeds all over Mars. There must have been occasional liquid water on the surface in ancient times," Hartmann says. "Apparently it soaked into the ground and froze, like in Canadian winters. ... There is probably much more underground ice on Mars today than had been thought."

That's promising, because on Earth many soils once thought to be sterile actually hide microbes. Examples are the Mars-like dry, cold areas of Antarctica and the Atacama Desert in Chile.

Long-term questions

Some of the interest in Mars springs from a 1996 report, which was widely doubted by scientists outside NASA, of fossilized Martian microbes discovered inside an Antarctic meteorite judged to be about 3.6 billion years old.

Perhaps similar microbes exist in rocks Opportunity will explore or in rocks examined by future explorers.

Golombek predicts both rovers will outlast their expected lifetimes of 90 days of battery power. The 1997 Sojourner lasted three times its design life.

"I expect we'll go at least twice as long this time."

OPPORTUNITY: THE SECOND ROVER Launched: July 7 from Cape Canaveral Air Force Station, Florida . Earth-Mars distance at launch: 48 million miles. ETA: Sunday, 12:05 a.m. ET. Earth-Mars distance on landing day: 123.5 million miles. Near-surface atmospheric temperature at landing site: Minus 148 F to 32 F.

That's good news for scientists, who view the rover missions as brief windows on a distant world.

And though they admire the "ground truth" collected by rover instruments, even the most hardheaded data geeks are mindful of the big questions about life in other places than Earth that Mars might be able to answer.

"If we do find life, it's the first time we know life can start on other planets and that we aren't alone," Hartmann says.

"Or, if we find no life ever started there, maybe we are more alone in the universe than we thought. Maybe there is something wrong with our theories about the origins of life. It's an exciting adventure for humanity either way."