Thursday, December 16, 2010

Non-Rotating Artifical Gravity

If you put a long boom on a satellite, it will align to the "local vertical". This was first demonstrated on the GOES 3 probe in 1978. It works because the forces of gravity and the "centrifugal force" balance at the center of mass.

Today, long life space tethers are available, using existing materials they are light enough for long lengths to be launched on a cheap launch vehicle like SpaceX's Falcon 9. A length of 500 km, with a lifetime of 10 years, would have mass of just 1275 kg. The tether would stretch from the altitude of the International Space Station (~340 km) up past the altitude of the Hubble Space Telescope (~595 km), out to where the polar orbiting satellites do their job (~700 km+). Within equal masses on each end of the tether, a force of 0.1g would be experienced, with no rotation of the structure required.

Of course, this only works while in orbit around a planetary body. Around the Sun, say for generating artificial gravity for a trip to Mars, a much longer tether would be required and it quickly becomes infeasible. So what good is it? Currently, we have very little data on how the human body responds to partial gravity. Sure, we know lots about how it responds to 1g, and we have some ideas about how it responds to "zero g", but we don't have any data at all about anything in-between. 0.1g is just enough gravity for a human to stand upright and walk around (although Joe Carroll recently suggested that 0.06g was sufficient). If we could put some astronauts on a station with this level of partial gravity for a few months it would give us some vital data for determining whether people can actually live on the Moon or Mars or even asteroids for long periods of time without serious health issues.

Having the low end of the tether at the same orbit as the ISS, and having it not rotating around at high speed would make getting to the experiment very convenient. The high end of the tether could simply be the upper stage of the launch vehicle. SpaceX has already demonstrated that their second stage can be relighted, and flying to a circular orbit of 800km seems well within their capabilities. A 500km tether reel could fit in the trunk of the Dragon, which can remain connected to the second stage after separation.

Considering the availability and low cost of the hardware, the low risk of non-spinning artificial gravity, and the massive scientific payoff, NASA should be pursing this.. now.


  1. Victor Moraes9:19 PM

    Dear friend Trent.

    Perhaps it is more difficult. First the great extension of the string (500Km!?) will require a huge rotating speed to achieve the 1G centrifugal force. In terms of spending more energy to achieve the optimal speed than if it were a short rope, which would reach the optimum speed in less time with less energy.

    A second factor that hinders the use of strings is close to the orbit. It should be also considered that a minimum difference between the terrestrial attraction on the body closest to the Earth and the body farthest from Earth, on the other end of the rope (on the chart you present, you admit that there is a variation of the gravitational force of Earth acting on the ends of the rope, more or less, depending on distance). This would cause a movement of bodies accelerate because when I was approaching the Earth and reduce your speed when you are away. In practice, the rope would not be straight. Perhaps a rigid frame ... anyway maybe turning should be done horizontally, quite far from Earth orbit, to suffer minimal effects.

    Another factor to be considered, which does not render the operation simple is the construction of specific modules for this purpose, where they can survive those who undergo testing. It is not cheap or easy.

    Finally, I would understand. For a string of 500Km?

  2. Victor Moraes10:22 PM


    Moreover, the chart you present, you're wrong. The centrifugal force does not vary, only the gravitational force (as variation of the distance from Earth) and is incorrect diagram-drawing. The only variation of centrifugal force, in practical terms is the acceleration of the module area when approaching the Earth and its deceleration when moving away from Earth. If we do not consider this variation caused by Earth's gravity, centrifugal force does not vary and is incorrect to say that it is lower when the strength of Earth's gravity is greater. Rather, it is certain to add strength. It is also incorrect to say that the centrifugal force is greater because it is smaller the force of gravity on Earth. Centrifugal force depends exclusively on the rotation. What will be, in the diagram, is an imbalance, causing everything to tend to the ground.

  3. Victor Moraes10:49 PM


    Ah! It is no rotation. : ~ (

    Now I understand. Forgive me. It is a rope straight without twisting. Sounds interesting, but produces only 0.1 G.

    Now I understood why 500Km. Excuse me. It really is a good idea. But it's too little. Sorry I did not understand his explanation. If you want to call me "slowness" may.

  4. Victor Moraes10:55 PM


    To my disgust is in the title: "Non rotating ..." But as I always took bad notes in the assessments of English language ... forgive me ...

  5. Yes, I agree that NASA needs to try this. Not only this, but other things as well. People can organize and put pressure on the politicians to fund these projects in order to make it happen. The space program doesn't get the support it needs. The time has come to change that.

  6. It seems funny to discuss this idea without explaining it in terms of tidal forces. For that matter, it seems like Larry Niven's stories such as the Integral Trees or Descent of the Anansi deserve a mention -- that's where a lot of us were first introduced to the idea.

  7. The title is misleading because in this sort of gravity gradient orientation the tether system rotates 360 degrees for each orbit around the earth. In fact, the diagram indicates the centrifugal force from this.

    I've long wondered about the advantages of such gravity gradient stations. Even if local gravity in the 0.1g range is insufficient to prevent all negative health effects, it should slow them and allow for longer stays of the crew. It also makes Life Support, gardening, and other systems design easier since fluids flow in expected directions.

    For tourists it should provide most of the fun of zero g but not be quite as difficult a transition. As Joe says, you can still do normal activities like walking and sitting and eating with knife and fork. A large window on the "bottom" of the lower would provide a wonderful continual panorama view of the earth.

    One challenge is docking with the system. The lower module is moving slower than orbital speed at that altitude, the upper module faster. So going directly to one of those modules would require a powered rendezvous or some sort of catcher or hook. Rendezvous at the CM would be easier but requires a elevator system of some sort to send people to the lower or upper modules.

    Note that the longer the tether, the slower the lower module moves. As noted with sky hook concepts, this reduces the requirements on the launcher and could make a single stage vehicle easier to build. If the tether CM was at GEO, suborbital vehicle could just launch straight up elevator style to a hook on the lower module. This would avoid the weather and other problems for space elevators that reach all the way to the ground.

  8. The only problem I have with long tethers, rotating or non, is the existing constellation of satellites. Something like a space elevator can set up waves in the string by moving the base to avoid collisions, but everywhere other than the center of mass you have potential high velocity collisions all the time. For this particular idea to work the structure might need to be in medium earth orbit.

    A big advantage of this very long tether is that you never need propellant to maintain the orbit. If the tether is conductive can borrow energy from earth's magnetic field to reboost.

    1. Actually, you can move a tether system as well. It's not that much different from moving a non tethered system. Easiest is to just boost it at the c of g in desired direction. But if you are really clever, you can do it while maintaining the same amount of C of G throughout so that passengers on the space station can't even feel any change while the boost is going on - if the boost is done off centre from the axis of the spin.

      That's an idea by Damon Landau

      I describe it briefly in my blog post

      Details described in this paper here

  9. Clark,

    Remember Larry Niven's 'The Integral Trees'? You dock at the Zero point and put the tourists on an elevator to one of the modules on the ends. ;)


    When you pull energy out of Earth's magnetic field with a tether, you slow the tether's orbital speed. You pump energy INTO the field with a tether based system to maintain or raise the structure's orbital velocity. Les Johnson, David Brin, and a double handful of other folks looked at this in the '90's and later for ISS re-boost and other apps. (stick the dedicated Solar Cell power system on a ~10m tower 'above' a/the station, the tether 'below' it, and you can maintain the extreme micro-gravity environment for on station science that was one of the major reasons for rejecting this method of re-boost for ISS. Although with Jupiter and it's freaking large and powerful magnetic field it was found that you could do Electrodynamic Propulsion AND supply ~1KW for the spacecraft and science instruments use.

  10. Sigh.. going up to 550km altitude to dock with a tether is not only dumb, it's impossible. The point of putting the lower habitation module at 300km is so you can use existing transfer vehicles to get to and from it easily. Yes, there will have to be a little software upgrade to the docking computer, so what? Say it with me, "everything is hard in space". Get used to it and solve the problems.

  11. Today, impossible, next year also. Two year? Gotta start slipping a little 'maybe' in there.

    Still, going to the CG point to dock lets you dodge some huge logistical and Orbital Mechanics issues since if you dock with the lower module, which is not 'in orbit' you instantly change the balance of the system and the whole structure would be dragged lower till it re-balances itself. To pull it back to where the lower module was at the same altitude when you dock with it you shift mass to the upper module (and reverse when you leave from the lower module to re-enter and land {unless the transfer mass is something you want to launch :) }).

    All of the above would take time, which translates into SpaceCom Traffic Control pulling even more of their hair out because the 520km x 20km x 20km box going around the Earth just slipped lower adding to the 4D 'keep clear' and 'dodge' headaches. ;)

    Yeah, I totally agree that "everything is hard in space", which is why you look for things to make it just a bit easier, even if you do things just a bit harder at the beginning.

  12. Sigh.. how do you dock with a string? What does that do for you?

  13. Hi there, just to say, I described this idea in my Science20 post here

    Can you check to make sure I describe it accurately. I can edit the post and update it and do corrections.

    I make the amount of artificial gravity at both ends 0.09 g for a 500 km tether, if I did the calculation right.

    I'm on David Livingston's space show tomorrow to talk about Artificial Gravity. I might well touch on this idea (depends on questions my main focus is to describe Joe Carroll's experiment).

    Sorry meant to post here about it before when I did the article but forgot.

  14. Another thing I just worked out - if you did this in free space, not near a planet, then 500 km tether, rotating once every 1.60553 hours means, radius of 250 km, and 0.0267588 rpm, and you get an artificial gravity of 0.2 g (using spincalc simple way to do the calc.)

    So - you get more than double the gravity in free space for same rpm. So - main advantage I see - if I've got that right, is that the gravity is constant 0.09 g instead of a continually varying gravity.

    But may be missing something, do say if you spot any mistakes and interested in the details of your calculations.