Why Don't Moons Have moons?

  
    Astronomers  can  say with  near certainty that there are no moons with moons in our  system. But that doesn't   mean   it's   physically impossible. After all, NASA has successfully put spacecraft into orbit around our moon.

      Although astronomers have spotted some asteroids with moons, a parent planet's strong gravitational tug would make it hard for a moon to keep control of its own natural satellite, says Seth Shostak, a senior astronomer at the nonprofit Search for Extraterrestrial    Intelligence    (SETI) Institute. "You would need to have a wide space between the moon and planet," he says. Orbiting far from its parent planet, a relatively massive moon might be able to hold onto a moon of its own.    Condiuons like these might exist in far- off solar systems, but while hundreds of exoplanets (planets outside of our solar system) have been detected, there's almost no chance we'll be able to spot exomoons, much less moons of exomoons, for decades to come. Most planet-hunting methods- such as spotting one as it passes a large star -lend themselves  to  detecting huge, Jupiter-Iike planets, or sometimes Earth- sized, rocky planets, but not their moons.   

    

     Even if astronomers spot a moon with a moon, it probably won't last long. "Tidal forces from the parent planet will tend, over time, to destabilize the orbit of the moon's moon, eventually pulling it out of orbit," says Webster Cash, a professor at the University of Colorado's Center for Astrophysics and Space Astronomy. "A moon's moon will tend to be a short-lived rfienomenon."