A NASA space probe streaked toward the surface of Mars today but failed to send a signal to anxious handlers on Earth confirming that the craft had landed safely.

The Mars Polar Lander was on course and in good health as it sped toward its planned touchdown on gently rolling steppes near the Martian south pole at 3:01 p.m. EST. If it survived, it would be the first Earth craft ever to land in the forbidding polar latitudes of another world.

But the spacecraft failed to report in during its first three opportunities, leaving ground controllers scrambling to try to determine its fate.

If the $165 million Lander is lost, it would be an embarrassing setback, especially coming on the heels of the loss of a sister ship in September that was part of a long-term NASA effort to explore Mars.

However, spacecraft managers had cautioned in advance that the absence of an immediate signal could mean many things, not necessarily failure. The delay could have been caused by relatively benign problems involving the spacecraft's self-protection system, which might have caused it to go to sleep temporarily, or a telecommunications problem involving, for instance, the spacecraft antenna.

"I'm very confident the Lander survived the descent. . . . We don't plan on giving up anytime soon," said operations chief Richard Cook as the afternoon wore on. The search for a signal from the probe could continue into next week.

The last contact with the spacecraft occurred just before it hit the atmosphere. After that, as one distraught scientist said, any one of "a million things could have gone wrong" in the complex and tricky run-up to the landing, including problems with the parachute or the retro-rockets, or simply a bad tilt at landing. Only one thing is sure: The spacecraft and two probes it was carrying are on the surface.

The Lander's first opportunity to communicate from the Martian surface began at 3:39 p.m. As the minutes ticked by with no signal, flight controllers and top NASA officials crowded tensely around computer monitors in a control room here at the Jet Propulsion Laboratory, which manages the mission for NASA.

When the painful silence stretched to 20 minutes, during which the team seemed frozen like statues, managers suggested that everybody take a "leg stretch" and try again in about 45 minutes -- the next window for communications. No contact was received during that session either, which began at 5:24 p.m.

The Lander's whisper of a signal would be traveling at the speed of light across 157 million miles of space, taking 14 minutes to arrive at Earth. At 3:52 p.m., engineers reported that the Deep Space Network antenna located in the California desert might have detected the signal barely above the general static at an unexpected frequency, but that proved to be false encouragement.

The team had scheduled the third and final opportunity to hear from the Lander beginning at 11:08 p.m. after controllers sent commands instructing the spacecraft to orient its medium-speed radio antenna toward Earth by sweeping the sky. But still no signal had arrived by 12:32 a.m. Saturday, when the communications window closed.

David Crisp, who has spent 10 years working on instruments for the mission, acknowledged that he was somewhat punchy on adrenaline but added, "I have no reason to give up the hope I walked in with this morning. But this is a risky business. . . . Mars has not been very nice to us."

Of 26 Mars missions launched by the United States and Russia, 16 have failed completely or partially. In 1993, the last of NASA's big-budget Mars missions, the $1 billion Mars Observer, suffered a catastrophic fuel leak as it approached Mars. The Lander team has been feeling extra pressure because of the loss of the Mars Climate Orbiter in September. The spacecraft's destruction was the result of a navigation error caused by a failure to convert from metric measurement, compounded by managerial failures to catch it. In the aftermath, the Lander mission has been exposed to intense scrutiny by the world's leading experts in planetary missions.

"I spent 10 hours yesterday going over every aspect of the mission," said NASA chief scientist Edward Weiler. "I don't think there's anything more that humans or computers can do. It's in other hands now."

No one knows exactly what the terrain looks like at the landing site. Even the most detailed images taken recently from the orbiting Mars Global Surveyor do not show objects as small as rocks (scientists predict there will be none), or the consistency of the soil. "The material we're landing in is totally unknown to us," said Crisp. It "could be like baby powder," or it could be frozen crust that caused the spacecraft to break through and re-settle a bit deeper.

One of the worst-case scenarios involved a crater two-thirds of a mile deep that verges on the landing zone, engineers said. If its luck was extremely bad, the Lander could have tumbled off the edge or rolled down a steep slope.

The polar region, though treacherous for spacecraft survival, is scientifically alluring because it is intrinsically different from the rest of the planet, according to planetary researchers. Indeed, the mythical appeal of these climes was evident in the late 19th century, when students of Mars wrongly imagined the existence of canals built by an alien civilization to carry water from the poles to the temperate equatorial zones.

"My God, this is something special," said scientist Bruce Murray, a veteran of early U.S. Mars missions who has been studying the Martian polar regions since 1965. "I won't be alive, my children won't be alive, the next time there's a lander at the pole. These data are it. They will be Earth's knowledge of the polar region for many decades."

A goal of all three robots is to look for signs of subsurface ice, the frozen remnants of the vast flows of water that scientists believe once carved channels (not "canals") in the surface of the Red Planet. The fate of the missing water is a riddle that scientists hope they can solve at least in part by studying what they believe are its frozen remnants locked in the enigmatic layered terrain around the pole, where seasonal frosts advance and retreat.

The spidery 3.5-foot-tall landing craft, jammed with instruments, was scheduled to spend at least three months of the Martian summer on its research dig in this alien land of the midnight sun.

Equipped with a 6.5-foot-long robot arm, the 640-pound Lander was designed to dig shallow trenches, scooping up tiny soil samples for chemical analysis. Meteorology instruments would chart polar weather patterns while a Russian laser system would study the atmosphere over the landing site. A microphone funded privately by the Planetary Society was tucked within the Russian instrument to record and transmit the first sounds from another world.

Moments before the Lander entered the Martian atmosphere, it released two five-pound companion robots that then smashed into the surface at 400 mph, punching separate holes in the rusty dirt some 35 miles to the northwest of where the Lander was supposed to set down. The experimental microprobes, dubbed Amundsen and Scott, the first penetrators NASA has ever dispatched to another world, would complete their primary mission merely by sending a minimal radio signal to Earth that they had survived. The information had to be relayed through the orbiting Mars Global Surveyor. No signal was obtained in the first pass.

Mission scientist David Malin repeated the same disappointing news again about 1 a.m. as the second pass concluded.

The team was optimistic that subsequent passes, with the orbiter at a higher angle, would produce better results.

The probes' secondary goal was to complete novel excavations using a small power drill to draw dirt samples into small ovens, similar to larger systems aboard the Lander.

The landing mission and its recently destroyed sister craft, the Mars Climate Orbiter -- together a $357 million package -- represent the second phase of a planned long-term but relatively low-cost invasion of Mars that amounts to a permanent occupation by Earth robots. The program calls for pairs of spacecraft to depart every 26 months, when the planets are favorably aligned.

The continuing theme of the Mars campaign is to discover the history and fate of the water that once flowed on the planet. Scientists consider it a key to understanding the history of dramatic climate change there as well as the mystery of whether life ever evolved. Water in liquid form is essential for the development of all known forms of life and, while it either freezes or vaporizes in the low pressures and frigid temperatures of the Martian surface, it might exist at warmer depths within the planet.

But first, the Lander had to survive a fiery entry into the Martian atmosphere at 15,400 mph, followed by a tricky powered descent, a risky finale to its 470 million-mile journey from Earth.

An Unheard Voice

The Mars Polar Lander failed to send back radio signals immediately after its scheduled landing at 3:15 p.m. EST yesterday.

Earth-Mars distance at arrival: 157.2 million miles.

Radio signal transmission time: 14 minutes.

Dimensions: 3.5 feet tall by 12 feet wide.

Weight: 1,270 pounds.

Cost: $120 million (Lander only).

Science objectives

Search for near-surface ground ice in the polar regions.

Study geology and topography of the landing site.

Search for evidence related to ancient climates and more

recent climate change.

Study the current climate and seasonal change at high latitudes.