A Soviet space probe scheduled to drop into orbit around Mars in late January to explore Phobos, one of the red planet's two tiny moons, may be experiencing trouble with one or more onboard instruments, American observers said Saturday.

While space experts said rumors about problems with the Phobos 2 probe did not indicate the craft's showcase mission was in any immediate danger, they did indicate problems of some sort were being experienced by one or more instruments, onboard telemetry systems or both.Phobos 2 was launched July 12, one week after a sister ship was carried into orbit atop a powerful Proton booster. But Phobos 1 was declared a loss in September after incorrect commands were radioed to the spacecraft, leaving Phobos 2 on its own.

"We've heard some rumors (about problems with Phobos 2)," said Samuel Keller, deputy associate administrator for space science and applications at NASA. "What I am hearing is that they've had some instrument failure and that one of the spacecraft systems . . . is allegedly on a backup system.

"We've heard nothing authoritative from Moscow. It's kind of speculative."

But Keller said by telephone from his home in Washington that "there's certainly a reasonable question about them having some trouble. I suspect there's something to it."

Phobos 2 is scheduled to slip into orbit around Mars around Jan. 29 and the Soviets apparently are accelerating mission plans to gather valuable data as soon as possible.

Under the original flight plan, Keller said, Soviet ground controllers planned to spend several months slowly maneuvering the craft to as close as 150 feet from Phobos, a small potato-shaped moon that may be an asteroid captured by Mars' gravity.

A highlight of the Phobos mission will be the deployment of a small lander from the mothership to study the surface of the moon. The mothercraft, meanwhile, will fire laser and ion beams at the moon in experiments to study its chemical composition and structure.

"The original plan was to actually put their lander on Phobos three or four months after they got to the vicinity," Keller said. "What we're hearing now is, that's been speeded up."

The Phobos probes marked the first in a series of ambitious Soviet unmanned missions to Mars over the next decade expected to culminate in an automated soil sample return mission by the end of the 1990s.