The outermost moon of Saturn, Phoebe, which was photographed by Voyager 2 last summer for the first time, is probably an asteroid caught by Saturn at the same time that many of the ringed planet's 16 other moons were being formed by collisions with each other.

"Phoebe may be the oldest object in the outer solar system beyond Mars to be imaged from a spacecraft," Voyager project scientist Edward C. Stone said yesterday from his office at the California Institute of Technology in Pasadena. "It may even be the most primitive object in the outer solar system to be seen and photographed with any kind of telescope."

In the current issue of Science magazine, the University of Arizona's Dr. Bradford C. Smith said that Phoebe's dark red color and spherical shape suggests "a class of asteroids apparently common in the outer solar system and believed to be of primitive composition," meaning that its color and appearance have not changed since the solar system was formed 4.6 billion years ago.

Photographed from a distance of 1.2 million miles as Voyager 2 flew past Saturn last September, Phoebe was found to be small, round and red. The most distant (8 million miles from the planet) of Saturn's 17 moons, Phoebe measures only 136 miles across, and appears to be the least cratered of the moons of Saturn, suggesting that it suffered the fewest impacts through time from collisions with other bodies.

Phoebe is moving in the opposite direction of Saturn's other moons, a strong suggestion that it was an asteroid caught by Saturn's gravity as it flew through space eons ago. The fact that it has few craters and is still there suggests that Phoebe was not captured by Saturn until a few hundred million years after the solar system was born.

"Had Phoebe been there earlier, when most planetary collisions were taking place," Stone said, "the moon would certainly have more craters, and there is a 50 percent chance that Phoebe would have been destroyed."

Phoebe's dark red color suggests that it is made of the same primitive carbon-bearing rocks found in many meteorites and in some asteroids that lie in the outermost part of the asteroid belt between Mars and Jupiter.