Space factPlanets
Mars’s moon Phobos circles the planet three times a day and moves about 1.8 meters (6 feet) closer every 100 years. NASA expects it to either crash into Mars or break up into a ring in about 50 million years.
Phobos orbits so close to Mars that from some places on the planet it is never visible. It circles faster than Mars spins, so tides drag it downward. The moon is only 27 by 22 by 18 kilometers (17 by 14 by 11 miles) across and weakly held together, so the stress may tear it apart before it hits the ground. If it breaks up, the pieces would spread into a ring around Mars.
~50 million years
Time until Phobos crashes or breaks up