Soviet scientists have lost "stable radio contact" with a spacecraft scheduled to rendezvous with the Martian moon of Phobos late next week, an ominous development that could mean the most ambitious planetary program that the Soviets have attempted in the past decade may be in serious jeopardy.

The spacecraft, called Phobos 2, is the survivor of two unmanned craft launched toward Mars last July.

According to a report by the official news agency Tass, the spacecraft's "on-board control system" ordered the craft to turn Monday so that its camera could take pictures of the moon Phobos.

"After these operations were completed, the information was to be sent back to Earth, but mission control was unable to establish stable radio contact with the probe as scheduled," the report said. "The possible causes of the loss of contact are being analyzed, and efforts are continuing to regain it."

The first craft, Phobos 1, was lost last October when a ground controller sent it an erroneous computer message, disorienting it so that its antenna was no longer pointed toward Earth and thus could not receive commands. After trying unsuccessfully to re-establish contact, Soviet officials finally abandoned the effort.

The loss of the first craft was considered a major setback, because it had special instruments to study the sun that were not duplicated aboard its sister ship. But at the time, Soviet scientists could take heart in the fact that they had a second craft, launched just five days after the first, that could carry on the mission. Redundancy has been the hallmark of the Soviet space program.

But it is the second craft that is now in jeopardy.

The extent of the problem was not clear in the United States on Tuesday, largely because the Tass report included few details. The National Aeronautics and Space Administration, which has assisted the Soviets in the Mars mission through its deep space network of antennas, was scheduled to hold a teleconference with Soviet officials this morning to see if any further assistance is needed.

NASA would not speculate on the nature on the problem, and the space agency's scientists have been specifically prohibited by NASA from commenting on problems with Soviet spacecraft. The attitude in NASA is that bad news ought to be announced by the Soviets, not by NASA officials.

If the second craft is lost, the Phobos mission will be a total failure except for some data that was radioed back to Earth as the craft orbited Mars over the past few weeks.