You won't miss and definitely won't be among the stars. Set in late 22nd century A.D. we are constructing a space elevator that would physically connects Earth and the moon, the lift will be using electromagnets to ferry people between a airship floating in the sky and the lunar base station like a maglev train. Given that we have perfected asteroids mining technology and have near limitless resource to realize space elevator concept, how can we attach a cable that won't succumb to tension when the moon drift further away?

(Bonus) I'm thinking can a lift move along the cable using hall effect principle?

  • 2
    $\begingroup$ How do you plan to deal with the moon rotating? It's not at a fixed spot in space. $\endgroup$
    – Erik
    Aug 22 '15 at 10:39
  • 1
    $\begingroup$ @Erik fortunately we never see the dark side of the moon from Earth $\endgroup$
    – user6760
    Aug 22 '15 at 12:04
  • $\begingroup$ @Erik I've edited my question, I haven't thought of it earlier hee hee $\endgroup$
    – user6760
    Aug 22 '15 at 12:13
  • $\begingroup$ And don't forget about about orbit eccentricity (check out "visible librations") $\endgroup$ Aug 22 '15 at 13:04
  • $\begingroup$ @Free Consulting I've an idea using segments with both ends capped with magnet so the space elevator's cable isn't continuous and we can set how wide the gap is. $\endgroup$
    – user6760
    Aug 22 '15 at 13:17

At its closest point the moon is 220 thousand miles away. At its furthest it is 250 thousand miles away. That is a difference of 30 thousand miles!

Clearly your airship is going to plow into the ground or fly off into space if you are not careful, there is a way around this though. At around the L0 point (so the weight of the cable is not relevant) you have a winching station. That winching station would spool in and out the cable as needed to keep it at the correct length and the airship at the right height above the surface of the earth.

However there is a much bigger problem:

The moon orbits the earth in 27 days (approximately). The circumference of the earth is 25 thousand miles, 27 days each of 24 hours. This would pull your airship through the air at 39mph.

However, the earth is spinning once every day. This pulls the airship in the other direction at 1041mph. In other words your airship is being pulled through the air at a combined speed of 1003mph, or considerably faster than the speed of sound at 761mph. Not only is this creating a sonic boom for anyone underneath but the drag and forces exerted on both airship and cable will be incredible. The cable will also be pulled a long way from vertical and have to be much longer as a result.

  • $\begingroup$ @AdamNicholls Actually once you are in space the additional weight isn't a big deal so long as it's in free fall. $\endgroup$
    – Tim B
    Aug 22 '15 at 17:27
  • $\begingroup$ Yes but a structure that long would have to be going at different speeds of free fall along it length, in my mind it would start spinning. It idea really seems impractical. $\endgroup$
    – Necessity
    Aug 22 '15 at 17:36

It can't be done.

There are four problems, one of which I believe is a showstopper.

1) As others have said, the moon's orbital period doesn't match Earth's rotational velocity. This will require a cable with a slip joint between an orbiting ring and the cable heading to the moon.

2) As others have said, the distance to the moon isn't constant. You'll need two cables designed to slip within each other. While the two cables are actually in separate orbits and thus don't have special strength issues from this they will be forcing each other sideways as they won't agree on exactly where the orbit is. I'm not sure how great this force will be and what will be needed to keep the cables from damaging each other.

3) The moon doesn't hold exactly the same face towards Earth. You'll have to compensate for this wobble--just like with the Earth you'll need an orbital ring with a joint that can move.

4) Finally, the showstopper: The moon doesn't orbit exactly in the plane of Earth's equator. I see no possible joint to compensate for this.


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