Phobos, Mars's largest moon, gets closer to the planet every year and will eventually be torn apart.
Why it is true
Phobos orbits so close to Mars that gravity is slowly braking it: it drops about 1.8 metres per century. In 30 to 50 million years, tidal forces are expected to break it apart, forming a ring around Mars, or it will crash into the planet.
Source: NASA Science
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