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How high must an object be to not be pulled back to Earth?

  1. Aug 30, 2012 #1
    My friend and I are using weather balloons and a rocket to send a camera into space. We need to know how far from earth we must be to not have the camera fall back. The edge of the atmosphere is around 13km and we can easily get past that. We have 2 weather balloons with a 100,000 ft breaking point and a pressure equalizing mechanism. We will have a small rocket launch just before the balloons break and possibly have it stay in space.
  2. jcsd
  3. Aug 30, 2012 #2


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    Staff: Mentor

    I'm sorry, but there is no such distance, there is only escape velocity. At 100,000 ft, escape velocity is only slightly lower than on the surface: 25,000 mph.

    Also, the atmosphere has no defined edge, but if it did it would be much, much higher than 13km. You're still in commercial airline territory at 13km! If one were forced to define it, we'd probably use 100 km: http://en.wikipedia.org/wiki/Kármán_Line
  4. Aug 30, 2012 #3
    I was told that it was about 13km, but thank you that helps a lot.
  5. Aug 31, 2012 #4
    Gravity falls off at as distance from earth increases, but it never actually fully disappears. So even if you're on the other side of the galaxy, eventually your camera will fall back down (assuming nothing else pulls it first).

    However, if your camera is moving fast enough, it will get slowed down by earth's gravity, but it will also keep getting further away from the earth (where gravity is weaker). Effectively, it can "outrun" the earth's gravity, and so gravity will never actually manage to stop it and pull it back down. This is escape velocity.

    Unfortunately, as russ waters pointed out, for objects within hundreds of miles from earth, this velocity is about 25,000 mph.
  6. Aug 31, 2012 #5
    I am curious, if you thought that there is no atmosphere past 13km, on what principle did you imagine your baloons were able to raise twice higher?
  7. Aug 31, 2012 #6
    DLHill, you might find this bried discussion of interest:


    As a balloon rises, lowering temperatures that would cause contraction of the gas and loss of lift is more than offset by reduced atmospheric pressure wihich causes balloons to expand,is also offset by reduced gravitational attraction.

    13km might have been someone's estimate for the altitude where typically 'normal' breathing is significantly diminished. In Denver, for example, the 'mile high city', MMA fighter commentaries often refer to who has trained at altitude and for how long.....and who hasn't....apparently even at that very modest altitude, it can make a difference in performance of highly conditioned athletes. In the Himilayas, at close to 9km altitude, climbers routinely use breathing apparatus for the strenuous effort required
  8. Aug 31, 2012 #7


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    Science Advisor

    I hope you have the necessary background education. It's not that easy to get something into orbit. It is rocket science, after all.
  9. Sep 1, 2012 #8
    I didn't think that the atmosphere just ended suddenly, I only thought that after around that height, an object is basically considered to be in space.
  10. Sep 1, 2012 #9


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    Staff: Mentor

    Is it difficult to obtain clearance from air traffic control for such a venture?
  11. Sep 1, 2012 #10
    Unfortunately, as russ waters pointed out, for objects within hundreds of miles from earth, this velocity is about 25,000 mph.

    Not on ring world it ain't:smile:

    Also for objects between Earth and the Moon at a point where the gravity is equal on an object, the escape velocity could be 1mph which is considerably greater distance than hundreds of miles and a lot less than 25,000 mph.:cool:
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