A NASA spacecraft failed again today to radio home from Mars, heightening concern about the fate of the highly anticipated mission to explore the Red Planet.

The Mars Polar Lander remained silent as another chance to communicate with Earth--the fourth since the robot craft plunged into the Martian atmosphere Friday--began at 11:30 p.m. EST.

"We missed it again," said David Crisp, a Mars mission scientist.

Though increasingly discouraged, ground controllers at the Jet Propulsion Laboratory were far from giving up on the $165 million Lander, despite two days of silence. Some engineers have been pinning their hopes all along on a Sunday attempt they consider the most promising opportunity so far.

"We're looking forward to tomorrow's transmission. Mars Lander, please phone home," Crisp said.

But if that attempt also fails, the prospects of recovering the craft will plummet, mission managers said. While they'll keep trying for at least several more days, the team faces the possibility that it may never know what happened, or what designs need fixing on future missions.

"Then we're into something much more serious," said project scientist Richard Zurek. "Right now, I'm just not going to think about that."

Working like airline crash investigators without the "black box" containing the crucial evidence, beleaguered flight operations managers were exploring potential failure scenarios, based on what hasn't worked so far, and plotting strategies for coming days.

By late today, there had also been no signal from two piggybacking microprobes that were supposedly ejected from the Lander just before it entered Mars's atmosphere. If they separated, the basketball-size probes were to smash directly into the Martian steppes at 400 mph. Their limited power supply could run out, under certain conditions, tonight, or they could last several more days. The two probes may have fallen into badlands around or inside a huge crater with high slopes and sand dunes--all treacherous for the devices, said Sarah Gavit, the project's manager.

The mission's goal is to look for signs of subsurface ice, the frozen remnants of the vast flows of water that scientists believe once carved channels in the surface of the Red Planet. The fate of Mars's water is a riddle that scientists hope they can solve in part by studying what they believe are its frozen remnants locked in the layered terrain around the southern pole, where seasonal frosts advance and retreat. This would be the first time any space probe has landed in the polar regions of another world.

The instrument-packed 3.5-foot-tall landing craft is scheduled to spend at least three months of the Martian summer on a research dig. 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 was tucked within the Russian instrument to record and transmit the first sounds from another world.

One asset ground controllers sorely miss, team members said, is the Mars Climate Orbiter, which was destroyed in the Martian atmosphere in September because of a navigation error caused by an embarrassing failure to convert from English to metric.

This sister ship of the Lander was to have served as its communications relay and would have been brought to bear immediately on the post-landing problem, Zurek said. "It could have flown over our site quickly," unlike the orbiting Mars Global Surveyor, already in the midst of its own mapping mission and not as well positioned to help.

Because of a planned communications blackout that began just before the Lander's complex and risky plunge into the atmosphere, however, mission managers said they are not even sure the spacecraft achieved the separation from its cruise stage, an event that would have been followed immediately by the release of the microprobes.

Among the most likely failures, flight managers say, are a bad tilt in the spacecraft or some other condition that has caused its main antenna, a medium speed dish, to be out of position, or any one of numerous types of "safe mode," which is a kind of defensive crouch the spacecraft assumes when it senses trouble.

The main attempt to contact the craft tonight assumes the Lander settled safely onto the surface but went into this protective sleep 20 minutes later. This could have been the result of a kind of temporary partial paralysis that engineers deliberately induced during the harrowing descent to the surface, a time when it was programmed not to respond to minor glitches or problems that could disrupt the more important landing operations. These could include a temperature fluctuation or any number of other things.

After landing, when the craft's ability to respond to such faults was restored, its memory of the recently ignored problem sat there "like a sort of time bomb." Now it was free to act to protect itself, which meant going to sleep while its batteries recharged--missing all the plaintive commands from Earth since Friday--and not waking up until it was noon on Mars, and the sun was at its highest angle over the polar spring.

That wake-up call was scheduled for 11:30 p.m. EST Saturday, at which time the Lander was to attempt to phone home for instructions. During this period, controllers also sent commands for it to scan with its main antenna, to search for Earth in the Martian skies. Depending on what mode the spacecraft was in, its response could have arrived at Earth as early as 11:30. p.m. But no signal came as the window opened.

Some members of the team were most optimistic about a totally different approach that will be tried on Sunday. This is the Lander's first opportunity to use the orbiting Surveyor as a communications relay. To transmit to the Surveyor, the Lander could rely on a smaller antenna, since it only has to reach Mars orbit and not 157 million miles to Earth. This antenna is omni-directional, covering the entire sky, in contrast to the main antenna, which has to be pointed with relative precision toward Earth. Any kinks or misalignments that may be plaguing the main antenna would be bypassed in the Sunday run, officials said.

If they get no results on Sunday, flight managers have a long series of other steps to pursue in their quest for their lost explorers.

"My mood is upbeat," said mission operations chief Richard Cook said before the communications opportunity Saturday night. "I think we have a long way to go before we're going to begin to get concerned."

But if the Lander crashed, for example, and nothing is heard from it, Zurek said, "We may never know" what became of it.

The prospect of all this uncertainty has already raised concerns for the next round of Mars flights, planned for launch in 2001 and already partially built. Their design has much in common with the missing Lander and the lost Orbiter. But if the fate of the Mars Lander remains unknown, engineers will be uncertain what to fix. "It's very sobering," Zurek said.

Some spacecraft, such as the Pathfinder craft that landed on Mars in 1997, emit a simple tone that gives clues to what's happening to it. But officials note that the missing Lander is even smaller and more efficiently packed than the diminutive Pathfinder. The lack of provision for information from the spacecraft on its descent to the surface was part of a general and inevitable trade-off of risks versus benefits, Zurek said. The decision was determined in part by mechanical and physical considerations, and "in part by economics. . . . Our emphasis was on 'let's make it work' rather than what if it fails.' " Ideally, he added, you do both, but that is not easy in the new era of tight budgets.

The problems with this year's Mars missions have prompted reassessment by some of NASA's new "faster, quicker, cheaper" approach to missions of all kinds. The approach calls for smaller, less expensive craft that can be produced more rapidly and whose failures would not be as devastating as the mysterious loss of the $1 billion Mars Observer in 1993. But critics say the tightening of the budget screws may have gone too far.

"We can't go back to the old days" of big budgets, Zurek said. "It may be that we've pushed the "quicker, cheaper" thing too far, asked too few people to do too much. But that's almost a discussion of the 1990s, isn't it. How do you get the most out of the system without breaking the system?"

In any case, he and others space professionals argue that the current system of multiple cheap missions is considerably more resilient than the old "all the eggs in one basket" approach.