Which Moon Orbits Closest To Its Host Planet?
Among moons in our solar system Phobos, the largest and most innermost of Mars’ two moons, is quite a curiosity. Phobos orbits just a scant 3,700 miles above the surface of Mars; for comparison Earth’s moon has an average distance of 238,900 miles from Earth (or nearly 65 times the distance between Phobos and Mars).
The tiny distance between Mars and its largest moon would be more than enough to make it noteworthy but the close distance creates all sorts of equally as noteworthy elements of the Martian/lunar relationship. Because Phobos is so close to Mars it actually rotates the red planet faster than the planet itself rotates such that Mars experiences two moon rises and sets per day.
Further, mathematical models suggest that, based on the fact that Phobos is quietly drifting closer to Mars a couple meters every century, within 30-50 million years the moon will either smash into Mars or break apart and create a planetary ring like a smaller version of those seen around Saturn.