A satellite orbiting Mars failed to pick up a signal today from a spacecraft missing on the surface, forcing ground controllers to confront the painful probability that they had lost an entire phase of their long-term campaign to study the Red Planet.

In what officials had described as the best chance yet to locate NASA's Mars Polar Lander, the orbiting Mars Global Surveyor passed 200 miles over the gently rolling terrain near the Martian south pole where the lander is assumed to be resting, in the hopes of hearing from the errant craft.

In previous tries to contact the lander, engineers had assumed it was using its main antenna--a medium-speed dish--to contact Earth. Today's attempt for the first time used a totally independent system--the lander's smaller ultra-high-frequency antenna, presumably circumventing any problems associated with the dish. Engineers compared the move to switching from a cell phone to a pay phone.

But the attempt, which began at 1:50 p.m. EST, produced nothing but more silence and deepening disappointment. Another window of opportunity was to come early Monday EST.

"Certainly, the team is getting more frustrated and more tense about all this," said project manager Richard Cook of the Jet Propulsion Laboratory (JPL) here, which manages the Mars program for NASA.

If the silence continues through the next attempt overnight Monday into Tuesday, several scientists said, their expectations for rescuing the $165 million lander mission falls off a cliff. Managers of future Mars missions are also reassessing their designs in order to prevent a recurrence of the ongoing nightmare of uncertainty.

Engineers will continue to work their way down through the complex branches of an engineering "fault tree" for at least a few more days, Cook said. But he added, "As we work our way through all these possibilities, it lessens our confidence . . . that we successfully landed on the surface. . . . After Tuesday, I think you really have to question whether or not the entry and descent occurred successfully."

This process is rendered confoundingly complex because, as an economy measure, the lander was not equipped to provide any data on its condition during the tricky descent to the surface Friday. As a result, engineers have to make a series of assumptions about the robot's status, take an action to counter that specific problem and, getting no response, check that assumption off their list.

They are also being careful to avoid creating any new problems. These steps have resulted in a series of attempts to get the spacecraft's main antenna to carry out systematic grid searches for Earth in the Martian skies, as well as today's attempt that used a different communications system.

Between midnight Monday and dawn Tuesday EST--which is daytime at the Martian south pole--the team hopes to command the largest sweep yet of the lander's medium-speed dish antenna, as well as another attempt to relay a signal from the independent UHF antenna on the lander through the orbiting surveyor.

Jacques Blamont, of the French Space Agency, a veteran Mars scientist whose radio aboard the orbiter provides the relay, said, "I think I will be very pessimistic if the next pass [overnight Monday] is not successful."

"Even I will lose confidence if we miss that one," said JPL scientist David Crisp.

In today's exercise, the surveyor craft passed above where the lander is assumed to be resting but failed to capture a signal. Unlike the lander's main antenna, the UHF system used today is omni-directional and should have worked if the spacecraft was in a generally operational condition, engineers said. Its failure to respond indicated that the craft is in some type of protective state known as a safe mode.

On Monday night's try with the UHF antenna, unlike today's, controllers will have sent commands that would bring the spacecraft partially out of its self-protective safe mode--if that's what the problem was.

The team is also considering various combinations of multiple problems that might exist.

"It's hard enough to repair your car if it's in the garage right in front of you," Crisp said. "Imagine what it would be like if it were 157 million miles away"--the current distance between Mars and Earth--"and you didn't know what was wrong with it."

The process is being prolonged by the absence of the Mars Climate Orbiter, which was designed to facilitate communications with the lander but was lost in September due to a contractor's failure to convert from English units to metric, and JPL's failure to catch the error.

The bad news extended to the two companion microprobes that rode along on the lander's 416 million-mile journey and were to have separated just before entry into the Martian atmosphere. No signal from them has been detected since they were supposed to have taken a ballistic plunge into the landscape at 400 mph. Project manager Sarah Gavit said late today that time is running out on their power supply--and any hope of hearing from them.

Some scientists have taken the silence of all three probes to be an ominous sign that something catastrophic might have befallen the spacecraft and its passengers before they were scheduled to separate, minutes before they entered the Martian atmosphere. Among the lessons learned from the experience, said flight operations manager Sam Thurman, "I think, we have to go back and rethink the whole test and verification plan for entry, descent and landing."

Gavit had reported earlier that the basketball-size microprobes had apparently landed right in or on the rim of a huge crater, increasing the likelihood of a disastrous crash on a hard-rock surface, a tumble down a steep cliff faces or a sinking in soft sand dunes, instead of burying themselves into the surface as intended. She said the extreme cold might also be a factor, affecting the tiny batteries.

An ongoing theme of Mars missions is to study how the planet's climate has changed through its history and, in particular, what happened to the water that apparently once flowed in abundance there. Scientists believe that clues may be locked in the enigmatic layered terrain near the southern pole, whose alternating bands of color, like growth rings in trees, could yield clues to 100,000 years of Martian history.