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Firing an object into orbit using a moon's gravity

  1. Apr 21, 2013 #1
    Let's say I'm on a planet and I have a space "gun" that can launch an object upwards at any speed in any direction.

    Typically with this, you could not launch an object without any rockets into orbit because the best you could do is make the object loop around the planet to exactly where it started.

    Now, introduce a moon orbiting the same planet. Using this moon's gravity, is it possible to construct a speed and direction for the object to be shot at so that it ends up in an orbit around the main planet?
  2. jcsd
  3. Apr 21, 2013 #2


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    Are you sure about that?
  4. Apr 21, 2013 #3
  5. Apr 21, 2013 #4


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    That's an orbit too. I guess you mean some other orbit, which doesn't return to the launch point. If you include air resitance in the atmosphere this is allready the case. You can fire it above escape velocity, but it will lose some speed in the atmosphere. But I'm not sure you could achieve a stable orbit above the atmosphere this way.

    Maybe if you launch it above the Planet's escape velocity, but below the Planet+Moon escape velocity?
  6. Apr 22, 2013 #5


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    Maybe not in principle but absolutely right if you are talking about a 'normal' orbit around the Earth. With a lot of care, you could organise a 'cannon' shot to pass close enough to the Moon to deflect the craft into an orbit that would not return to its launch point. I suspect that this orbit could give you trouble 'next time around' if it came close to the Moon again and it might not be too stable. You would be after an elliptical orbit with fairly low eccentricity, I presume?
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