Dark and dusty, Phobos and Deimos orbit close to their parent planet. The surface of these tiny Martian moons are pitted with large craters and covered by a loose layer of broken rock, absorbing almost all sunlight that reaches them. The moons are probably small asteroids - cosmic debris left over from the formation of the solar system - that were later snared by Mars' gravitational pull. The trap is a lethal one for Phobos: Its orbit is decaying and it is on a collision course with the red planet.

Small, dark and fast-moving, the two Martian moons went unnoticed in the blackness of space until 1877, when U.S. astronomer Asaph Hall picked them out as mars made a particularly close approach to earth. For the time, it was quite an achievement. Phobos and Deimos are among the smallest known satellites in the solar system. To small to have formed in a regular spherical shape, they only measure about 16 by 12 miles and 10 by 6 miles respectively.

From earth, even the most powerful telescope can make out no more of the moons than dim spots. For the first detailed view of Phobos and Deimos, astronomers had to wait for the U.S. Mariner 9 probe in 1971. Mariner's pictures pictures were followed by more images taken by the Viking 1 Orbiter as it neared Mars in 1976 - the centennial of the moons' discovery.

Unlike Earth's moon - a chunk of metallic rock that was probably ripped from its parent planet early in its creation - the Martian moons seem to be adopted companions, stray asteroids that passed to close to Mars that they have been trapped by its gravity.

The surface of Phobos is more dramatic, testimony to a turbulent impact history. It is covered in deep ridges, steep hills and numerous craters, one of which - the six-mile-wide Stickney - stretches across almost half of the moon's diameter. Stickney would be visible from the surface of Mars.

Phobos orbits at a mean distance of only 5,830 miles from the planet's center, and appears in the sky as almost half the size of a full Moon seen from Earth - although it is much darker. Deimos, orbiting at 14,580 miles, shows up in the Martian sky as a tiny disk, only just discernible to the naked eye.

Phobos and Deimos both orbit east to west, but as seen from the surface of mars, they seem to travel in opposite directions. This apparent paradox is caused by the orbital periods of the two moons. Phobos whizzes around Mars in just 7 hours and 39 minutes, so it circles the planet more than four times in the course of the Martian day of 24 hours 37 minutes. But Deimos travels slower than Mars rotates. Left behind in the sky, it appears to move in a easternly direction

While Deimos orbits at a safe distance from Mars, Phobos is spiraling slowly toward eventual destruction. The planet's gravitational pull is reeling in the moon at a rate of 60 feet per century. But collision between Phobos and Mars may never occur - the moon may suffer the fate of being broken up by the planet's tidal forces.

As Phobos orbits around Mars, the side of the moon that faces the planet is subjected to a greater gravitational force than the opposite side. The difference between the forces puts the little moon under a constant strain that could be enough to tear it apart

Phobos orbits close to the Roche limit - the point where a planet's tidal force is powerful enough to destroy a moon. Whether Phobos will be demolished in this way depends upon its cohesive strength. The moon's density suggests that, like many asteroids, its probably consists of carboncaceous chondrite and pea-size nodules of various minerals. This composition would make the moon brittle and likely to crack and fracture.

The largest crater on Phobos, six mile wide Stickney Crater, may offer clues to the moon's cohesive strength. If the mood can withstand such a large impact, it may be able to resist Mars' tidal forces - at least for a long time to come. But Phobos' orbit will continue to decay under the influence of Mars' gravitational field. If Phobos remains in one piece, it may be pulled through the thin Martian atmosphere before finally crashing onto the planet's surface.

Alternately, the enormous force of the impact that produced Stickney may also have weakened Phobos, and the large grooves that run outward from the crater may be fractures. With so many splits, the moon may fall victim to Mars' tidal forces, ending up as a rocky ring of debris around the planet. Either way, astronomers have plenty of time to prepare for the dramatic event: Phobos is expected to stay in one piece for at least another 40 million years.