Which of Mars' moons is slowly spiraling inward?

Phobos is the moon of Mars that is slowly spiraling inward. Its orbit is so close to Mars that it circles the planet in about 7 hours 39 minutes—faster than Mars rotates once. As a result, Phobos rises in the west and sets in the east, an unusual pattern for a natural satellite.

Mars’s gravity is gradually pulling Phobos closer because the moon orbits inside Mars’s synchronous orbit. Tidal interactions transfer orbital energy and cause its orbit to shrink by roughly 1.8 centimeters per year. Scientists estimate that Phobos may eventually break apart or collide with Mars in about 30–50 million years.

Phobos is often confused with Deimos, Mars’s other moon. Deimos is smaller and farther away, with an orbit that is slowly expanding rather than shrinking. Phobos is also not the same as Earth’s Moon: it is irregularly shaped, heavily cratered, and may be a captured asteroid or material from an ancient Martian impact. Its most famous feature is Stickney Crater, a huge crater nearly half the moon’s width.