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B Skydiving from Space -- maximum altitude?

  1. Jan 11, 2019 at 11:09 AM #1
    Hi everyone. I'm a skydiver. i would like to know the maximum altitude from where i can jump from the space and to be able to free fall back to earth without geting lost in space. i think the actual record is 41425m. here is a brief story about world record spacedive https://en.wikipedia.org/wiki/Space_diving
    Thank's
     
  2. jcsd
  3. Jan 11, 2019 at 11:35 AM #2

    russ_watters

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    Welcome to PF!

    Strictly speaking there is no such maximum altitude. Practicality says that if you start from too far away other things (like the moon) might get in the way and you'll have to deal with re-entry, but that's about it.

    You may have a misconception based on sci fi that once in space you are free to move around (or stop) as you please, but that isn't true. Almost every probe we've ever sent in space was in orbit around *something*. And if not in orbit, an object would drop straight into the nearest gravity well.
     
  4. Jan 11, 2019 at 12:22 PM #3

    A.T.

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    What do you mean by that? Getting lost like in the woods, or what?
     
  5. Jan 11, 2019 at 1:54 PM #4

    CWatters

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    Regarding getting "lost". The earths gravity extends out into space a long way. So unless you are travelling quite fast you won't escape the earths gravity and drift away to get "lost".

    The problem with skydiving from space is finding/getting something to jump from. You can't jump from the ISS because its orbiting the earth fast enough that the earths gravity is "used up" providing the required centripetal force. So if you step out you would float away slowly. Even geostationary satellites orbit the earth. They just take 24 hours to do so.

    If descending from anything like this i believe conservation of angular momentum will increase your tangential velocity causing a re-entry heat problem.

    I might be wrong but i think what you would need is something like a space elevator to get you high but not so high that conservation of angular momentum causes a very high entry velocity.

    https://en.m.wikipedia.org/wiki/Space_elevator
     
  6. Jan 11, 2019 at 3:09 PM #5

    jbriggs444

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    If you want to skydive from an airplane, you have the Karman line to worry about. Though I suppose that one could fire a rocket on a near-vertical (even slightly retrograde) ballistic trajectory and get to an arbitrarily high altitude.
     
  7. Jan 11, 2019 at 5:01 PM #6

    cjl

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    There are a couple limits for high altitude skydiving. The first, as already mentioned, is finding something to jump from. Balloons can get you to 150k feet or so, above that and you'd need a suborbital rocket (which complicates things immensely). The second problem is that freefalling in the thin air up there is very different than you're used to. People in the past have ended up in a high speed spin for part of the fall, which is obviously a potential problem. Also, due to the thin air, a freefaller will actually hit supersonic speeds in the highest altitude portions of the fall before slowing down as they reenter the thicker portions of the atmosphere. Fundamentally, I don't see any reason why someone couldn't add another 10km or so to the current record so long as they had a suitable jumping platform, beyond that I'd be curious if the supersonic effects would start to become a problem. You definitely wouldn't have to worry about getting lost in space though - even if you jumped from the altitude of the moon, you'd end up falling back to earth eventually (unless you also had orbital velocity sideways in addition to just the altitude).
     
  8. Jan 15, 2019 at 10:20 AM #7
     
  9. Jan 15, 2019 at 11:02 AM #8
    Go for Lagrange point L2 (1,500,000 km). Just push off the Webb telescope and try to avoid the moon or burning up when you hit the atmosphere. Calculating your velocity when you do hit earth's stratosphere is beyond my math skills.
     
  10. Jan 15, 2019 at 11:16 AM #9

    russ_watters

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    The tangential speed won't contribute much to re-entry velocity, but if you go too high, when you jump off you'll be in orbit!
     
  11. Jan 15, 2019 at 11:21 AM #10

    jbriggs444

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    It's the same as escape velocity to a reasonable accuracy.
     
  12. Jan 15, 2019 at 1:35 PM #11
    I asked about a figure skater arms in vs arms out velocity.
     
  13. Jan 15, 2019 at 1:44 PM #12

    russ_watters

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    It's not related. @CWatters was just saying that for the same rotation rate (1 rev/day) a higher platform has a higher tangential speed.
     
  14. Jan 15, 2019 at 1:52 PM #13

    jbriggs444

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    My understanding was different -- that regardless of the nature of the central force, conservation of angular momentum (aka equal areas in equal times) assures us that tangential velocity of the diver at low altitude will be higher than he started with at high altitude.
     
  15. Jan 15, 2019 at 2:09 PM #14

    russ_watters

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    You're talking about Kepler's Law? [googles] Hmm, I didn't realize that was about angular momentum, I thought it was conservation of energy!

    Either way, my thought on the broader issue was that atmospheric heating on re-entry becomes significant at a far lower altitude than where the tangential velocity at the top of the tower starts to throw the skydiver into an orbit. E.G., the tangential velocity at 200 miles altitude is only 544mph, a speed you reach vertically in 15 seconds after jumping.

    Maybe this is a standard problem, but I suppose it would be interesting to figure out how high the tower needs to be for the skydiver to achieve orbit.

    Edit: Yep, it's been done:

    wCXBu.png

    https://space.stackexchange.com/que...-space-elevator-to-get-a-stable-elliptical-or
     
    Last edited: Jan 15, 2019 at 2:22 PM
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