With NASA’s twin Mars rovers closing fast on the red planet, agency scientists and engineers are bracing for what could be two agonizing nights of waiting to hear from the robotic explorers once they begin their harrowing descents to the martian surface.

First to take the plunge through the martian atmosphere is the rover dubbed Spirit, which will jettison its cruise stage and begin its descent Jan. 3. About three weeks later, on Jan. 25, Opportunity will attempt to land on the opposite side of the planet.

Both golf cart-sized rovers will investigate whether Mars might once have been capable of supporting life. But before Spirit and Opportunity can begin exploring two carefully chosen spots thought to have once held substantial quantities of water, each must safely make it to the martian surface.

NASA scientists and engineers have been waiting half a year for the two spacecraft to complete their more than 450 million-kilometer journeys to Mars. But perhaps the most nerve-wracking part of each trip will be the last 12 hours or so.

During their dives through the martian atmosphere, the rovers, tucked inside folded-up landers, will transmit crude telemetry, but communications are expected to be spotty at best. Adding to the heartburn factor, NASA controllers at the Jet Propulsion Laboratory, Pasadena, Calif., expect to lose contact with the rovers for the night about 10 minutes after each is scheduled to hit the surface and tumble to a standstill secured inside a cocoon of airbags. Unless the landers come to stop right side up -- a 50-50 prospect, according to NASA -- they will not have time to transmit even the briefest status report before losing contact with controllers back on Earth for the next 12 hours.

“The easy part is over,” Ed Weiler, NASA’s associate administrator for space science, told reporters here Dec. 2. “Just getting to Mars is hard, but landing is even harder.”

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More than half of all missions launched to Mars have failed. Of six Mars landers Russia has built and launched since 1971, just one actually reached the surface, only to shut down for good 20 seconds later. NASA’s track record is considerably better, with three successful landings in four attempts. But NASA’s last attempt to land on the red planet ended in failure. Just two months after losing a separate probe dubbed the Mars Climate Orbiter to a metric conversion mix up in September 1999, NASA officials found themselves helpless as the Mars Polar Lander failed to phone home after disappearing into the martian atmosphere.

Weiler said NASA has done everything humanly possible to ensure the success of Spirit and Opportunity, including adding $100 million in testing, analysis and design changes to the program. But even with all the due diligence heaped on this mission, there is no guarantee that Spirit and Opportunity will safely reach the martian surface.

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“All it will take in the last couple of seconds is a strong gust of wind -- stronger than we expected — and the mission is over,” Weiler said.

For Spirit and Opportunity, NASA abandoned the retro-rockets used on the Mars Polar Lander in favor of the airbag landing approach that the Mars Pathfinder mission employed with success on July 4, 1997. But even airbags cannot guarantee a soft landing. Weiler said the Mars Pathfinder craft could have been destroyed had it struck one of the large rock formations visible in the panoramic views it returned from its landing site.

Assuming Spirit and Opportunity make it to the ground in one piece, NASA will have at its disposal the two most sophisticated science rovers it has ever built.

After considering more than 150 possible landing sites, NASA decided to send Spirit to Gusev Crater, a suspected dry lake bed about the size of Connecticut located near the martian equator. Opportunity is bound for Meridiani Planum, another equatorial spot where mineral deposits suggest a wet past.

Steve Squyres, a Cornell University astronomy professor serving as principal investigator for the two missions, said once Spirit and Opportunity are safely on the ground, they will spend a martian day or two getting the lay of the land before rolling out for a closer look at objects of interest. Every movement the rovers make will be deliberate and slow, Squyres said. “It doesn’t zip,” he said. “This vehicle has about the same mass and top speed of a Galapagos tortoise.”

Before the solar-powered rovers inevitably succumb to martian dust and radiation, Spirit and Opportunity will use onboard spectrometers to scan soil and rock targets for signs of the past presence of water. Each rover will use its Rock Abrasion Tool to scrape away the outer layers of a rock, and then probe inside with a microscope and spectrometers designed to help decipher the martian past. A “really good rock,” Squyres said, might be worth two or three days of examination. Squyres and his colleagues envision a slow pace for rover activities. “This isn’t a sprint, it’s a marathon,” he said. “We are going to be operating these rovers for months.”

Although NASA expects stunning pictures from the rovers’ panoramic cameras in the first 24 hours of their missions, the important scientific discoveries will probably take more time. As Squyres put it, “the best stuff may come in February, March or April.”

NASA will have spent about $820 million on the two missions by the end of 2004. Spirit and Opportunity, which together cost about $645 million to build, were launched in June and July, respectively, on Delta 2 rockets.