Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Skyhook - is this better then space elevator?

  1. Jun 27, 2011 #1
    Hi Physics Forum!

    I've been reading about the space elevator and the amazing feat of running a cable past geosynchronous orbit. It occurred to me that there might be an easier way. I may have heard about this somewhere ages ago, or maybe I made it up, but I haven't been able to find any info about it.

    What if instead of running the cable to geosynchronous orbit, you had one that was about 150-200 Km long, that ran from a heavy mass like a small captured asteroid, to a hook? An object at that altitude would be orbiting much faster then the rotation of the earth, so instead of letting the cable hang down, it would spin the hook around the asteroid in the opposite direction of the earth's rotation. Thus the speed of the hook could be adjusted so that when it enters the earth's atmosphere it is moving at roughly the speed of the earth's surface. (See illustration)

    When viewed from the Earth, the hook would appear to enter the atmosphere at a steep angle, come to a nearly complete stop at about 15 km altitude, and then rise back into space.

    (Is it proper to call the hook's path an "orbit"? It is more like a ball on a string, with centrifugal force pulling against the cable.)

    You could then use aircraft to fly to the hook and attach payloads to it. The hook would lift the payloads into space, and then release them at high speed and altitude. The payloads would still need rockets to reach a stable orbit, but they would not need remotely as much fuel as a liftoff from earth.

    Payloads returning to Earth could attach themselves to the hook, and be gently slowed down and lowered into the atmosphere. This would return kinetic energy to the asteroid, keeping it at the desired altitude.

    I think you'd need to lift the asteroid from time to time. But since the asteroid is already in space, solar sails, ion drive, or laser boosts from Earth might do the trick.

    Attached Files:

  2. jcsd
  3. Jun 27, 2011 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Skyhooks as you have described do already exist as theoretical ideas. An advantage over rocket launches is that instead of a non-reusable rocket a high altitude plane is needed and advantages over space elevators include being (potentially) cheaper and easier to avoid debris. However the disadvantages are that such a tether would only launch to low orbit (perhaps this could be mitigated by giving the payload an ion drive that boosts the orbit over a couple of months?), every time such a device boosts something into orbit it's own orbit would decrease and the tethers would produce (albeit small) drag slowing down the spin.

    The NASA Institute for Advanced Concepts have worked up an idea called http://www.spaceelevator.com/docs/355Bogar.pdf [Broken] that you may be interested to read about, though I warn you it is a pretty comprehensive and long document. For potentially advantages/disadvantages page A1-14 goes over some issues with the implementation.
    Last edited by a moderator: May 5, 2017
  4. Jun 27, 2011 #3
    Ak! Google has failed me! (I just got stuff about cell phones and software.) Oh well. The wikipedia article could use some better illustrations though. Maybe I'll tweak mine up. Any suggestions?
  5. Jun 27, 2011 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    I would suggest flicking through relevant sections of the NASA article linked above, they've done a lot of work there on the topic. I agree the wiki page needs updating. You might also be interested to read about Lofstrom Loops another megascale contender for launching to space.

    EDIT: A thought occurred to me that your design may have too many tethers, The one in the atmosphere will move slightly slower than the ones not in atmosphere (thanks to drag) and so may cause instabilities. This is probably why the NASA design has only one tether.
    Last edited: Jun 27, 2011
  6. Jun 27, 2011 #5
    Oh. I meant for that to be the same tether at different points in time. Maybe an animated gif would help.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook