In this question:

Where to Anchor My Space Elevator

We get a new material:

Thanks to the recent discovery of handwavium filaments that can in principle be extruded to any length, we will be able to build space elevators sooner than most people would expect.

In my answer to that question, it becomes apparent that we'd prefer to go to orbit for free than use a space elevator. We'd use a rotating skyhook to do so.

From the wikipedia page:

A rotating tether is a type of cable that would orbit the Earth with a tip speed equal to its orbital speed (around 7-8 km/s). The tip would rotate down, moving in the direction of Earth's rotation. It would enter the atmosphere at low speed and pick up a payload from the ground or the atmosphere. It then carries it up into space.

However, a Boeing study in 2000 assessed that "Trying to lower the tether tip speed to 4.0 km/s (13 000 ft/s or Mach 13) would require a skyhook tether mass greater than 200 times the payload mass.

I'm unclear if 'low speed' means Mach 13 (when it hits the atmosphere) or not. In other literature (heh, wikipedia is literature), they extend the tether so that payload/attachment point comes sooner, and stays on the ground allowing time to process/move things into the container, lengthen the skyhook allowing more time on the ground, then a gentle lift away (although, looking at stuff some more, that example was on the Moon, with no air resistance at the surface).

But in any case, I suspect this will have some type of effect in/on the atmosphere/tether if it's anywhere above Mach 1.

Would a rotating skyhook hit the speed of sound in the upper atmosphere? Or worse yet, lower down in the thicker part of the atmosphere? How far do sonic booms propagate? What types of atmospheric disruption (and other issues) would my skyhook make? At Mach 13, I'd also expect a bunch of heating (How much? Enough to vaporize a nanotube?), as well as a trailing curve; which would reduce the time on the ground/at the attachment point.

  • $\begingroup$ Small comment: The "78km/s" should be "7-8 km/s" as described in the wikipedia page. I was wondering why something orbiting the Earth had that kind of velocity. $\endgroup$
    – March Ho
    Jan 21, 2015 at 8:04
  • $\begingroup$ They likely used an m-dash on the wikipedia page, which doesn't cut-n-paste worth a damn (which is why I never use m-dashes). $\endgroup$
    – user3082
    Jan 21, 2015 at 9:59

1 Answer 1


The point of a rotating Skyhook is actually that it doesn't hit any speed and it doesn't enter the atmosphere. The lower end would be nearly geostationary at its lowest point in the rotation, the upper end would be at orbital velocity.

It's physically impossible to build a rotating skyhook that goes through the atmosphere, the drag involved would quickly stall the rotation out so you lose the rotation, and on top of that it would almost certainly make the orbit unstable.

When you come up through the atmosphere you hook onto the lower end and that then pulls you out into a higher orbit and gives you an orbital speed at which point it releases you. You need to remember that actually climbing to LEO is only part of the problem, accelerating to the point that you don't fall back down again takes a lot of energy too.

The skyhook would generally not enter the atmosphere at any point though, you would need to climb out of the atmosphere on your own. The skyhook helps by giving you orbital velocity and a bit of altitude once you've already done most of the climbing yourself.

  • 1
    $\begingroup$ I think you missed the part that says "from the ground". Please try reading the question. Yes, there are options which don't go to the ground. Cool. $\endgroup$
    – user3082
    Jan 21, 2015 at 9:55
  • $\begingroup$ @user3082 I did read it. Rotating skyhooks don't go through the atmosphere though. If they did then they would very quickly stop rotating. I've added another paragraph to make that clearer. $\endgroup$
    – Tim B
    Jan 21, 2015 at 9:58
  • $\begingroup$ Excellent. Please edit the wikipedia page and tell them that it's impossible for a skyhook to go through the atmosphere. They'll probably want you to cite a source. $\endgroup$
    – user3082
    Jan 21, 2015 at 10:01
  • $\begingroup$ "The version with best potential involve a much shorter tether in low Earth orbit which rotates in its orbital plane and whose ends brush the upper Earth atmosphere, with the rotational motion cancelling the orbital motion at ground level." - this part of the Wikipedia article seems reasonable. $\endgroup$
    – Tim B
    Jan 21, 2015 at 10:06
  • 1
    $\begingroup$ "The tip would rotate down, moving in the direction of Earth's rotation. It would enter the atmosphere at low speed and pick up a payload from the ground or the atmosphere." - this part does not. I'd really like to see a source because the only tether I'm aware of that reaches the ground is a space elevator. (And if you were able to build a rotating skyhook that reaches the ground then you'd just build a space elevator in the first place as its much more useful and actually easier to build). $\endgroup$
    – Tim B
    Jan 21, 2015 at 10:07

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .