Mars as it was observed shortly before opposition in 2016 by the NASA/ESA Hubble Space Telescope. Image via NASA, ESA, Hubble Heritage Team, J. Bell, M. Wolff.
Orbiting a mere 3,700 miles above the surface of Mars, Phobos is closer to its planet than any other moon in our solar system. Mars’ gravity is drawing in Phobos by about 6.6 feet every 100 years. Scientists expect Phobos to be pulled apart in 30-50 million years by tidal forces, the mutual gravitational pull of the planet and the moon. The same fate may await Neptune’s moon Triton, which is also slowly falling inward towards its planet. (NASA/JPL-Caltech/University of Arizona)