Bucket loop between Earth and Moon?

Question

Please don't bite my head off. If this is so stupid that it deserves to be squashed, I'm quite happy to delete the question. I mainly would like to know whether anyone's ever suggested this (I searched).

Supposing you have a long piece of string... I mean really long, 500 thousand miles in fact, and you make it into a loop and you attach buckets at intervals along its length.

Then you have a couple of fixed spindles, one on Earth and one on the Moon, and you start pulling on the string: if you wanted to get stuff (people, machines, commodities, etc.) down from the Moon simultaneously with stuff up from the Earth, wouldn't you in fact balance out the effects of the gravity wells (both Earth's and the Moon's)?

I feel sure that someone's going to point out the rather large strains that would be experienced throughout the length of this piece of "string" as the pulling took place*. Naturally I'm suggesting that instead of string it should be made from some suitable 22nd Century technology: maybe as well as being miraculous, the constituent material would also have to expend energy in some way (using solar panels presumably) in order to function.

Compared to the space elevator idea, OK, it's a bit longer. But the space elevator idea faces the challenge that, up to geostationary orbit height, the entire structure has to be supported from beneath, in the very bottom of Earth's gravity well. The engineering specifications of the material of this string would be challenging in a different way.

NB I'm aware that, in practical terms, rockets are in fact a pretty cheap and cheerful solution for getting stuff out of or into Earth's gravity well, particularly if you can make them re-usable. Talk about building a space elevator, space guns or "orbital tethers" etc. still continues though.

Edit

Jcaron's comment about the Moon not being geostationary made me think: of course it is primarily the spin of Earth which is the problem here, rather than the orbit of the Moon. This also indicates that, unlike with a space elevator, you would not want to have your maritime tether platform anywhere near the Equator. Instead you would want it to be as close as possible to one of the Poles: my knowledge of the trigonometry involved here is a bit lacking: feasibility would depend on factors like the tilt of Earth, the fact of the Moon's orbit being, unfortunately, inclined 5 degrees relative to the Earth's ecliptic (not equatorial) plane, etc. With the tilt being in the "wrong" position completely relative to the Moon's position once per month, I rather doubt whether you could site the platform actually stationary, AT the North or South Pole.

Instead this platform would probably have to travel at a constant several hundred km per hour, along a latitude line close to that of the Antarctic Circle, where there is less land than with the Artic Circle, doing one circuit per 24 h (length a technically challenging 16,000 km = some 670 km/h!). Although there is little land there, there is the pesky business of ice. Lots of ice. This may disappear in the near future of course.

Another possibility is to station your Earth tether at the South Pole, make it stationary, but disconnect it for maybe half the days in any given month, when a line between the Earth tether point and the Moon would pass through the mass of the Earth ... but ... at that point in the month the North Pole would be workable... so, yes, you have TWO polar stationary tether points, and you switch the Earth end of the loop between them every two weeks - problem solved!

Fortunately humankind has always relished a challenge.

Edit 2

I've done a bit of thinking about this since I posted this. The super crucial thing to bear in mind is that each link must be "smart". As a first hypothesis, each link might be 10 m in length (requiring approx. 80 million of them), and the loop would loop at a speed of 100 m/s. By my calculations, this means it would take about 45 days to transport something to or from the Moon. The "turnstiles" at each tether point might be 1 km in diameter or so.

Each link contains two crucial things: a solar array, which deploys only outside Earth's atmosphere, and a set of gears. The gears are powered by the array. The gears have two functions, without which this space loop could never work.

Firstly, the gears are responsible for driving the loop: throughout the length of the loop, in space, the "up" strand rubs up against the "down" strand, and the gears are therefore responsible for driving the loop mechanically. On this subject, it might be worth wondering what forces would actually conspire to slow the loop once set in motion. Friction between the links? I think the amount of power needed would turn out to be quite minimal in fact, relative to the potential solar power captured along 2 x nearly 400,000 km of links.

Secondly, and more controversially, the gears would be responsible for countering Earth gravity near Earth. At the South/North Pole, where we have our Earth tether point, the loop is stretching off towards the horizon, horizontally. This is determined by the nature of the Moon's orbit and the Earth's axis relative to it, and there is no getting around it. Unlike with a space elevator, we are not exploiting centrifugal force in any way. So a legitimate question is: "why doesn't the loop just fall down?".

The answer is not something to do with tension (some unvarying, dynamic tension might exist, between adjacent links, but nothing like enough to pull the chain "taut": motion of the chain would be the result of the powered gears but above all momentum), but instead that the loop is using the immense amount of electrical power generated constantly by the solar arrays along its length to apply a dynamic "curving force", to "curve" the loop away from the Earth at the most gravitationally difficult point of the loop, i.e. the Earth tether point. This means that, as it heads to or from Earth, in proximity to Earth, at 100 m/s, each link is applying a non-negligible force to its adjacent links, using its gearing, to act against and neutralise the effect of Earth gravity.

At the lunar tether point there are no problems of this kind: firstly, since the loop's attachment to the lunar tether point is vertical, but also because the gravity is much lower.

Since the solar arrays don't deploy in the Earth's atmosphere, you need to get the power transferred from the links currently in space.

Apart from cost, the biggest objection to this idea might be aesthetic: would we really want to look up into the night sky and see an unsightly chain stretched between Earth and Moon?

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* Maybe you might also find the Moon is being pulled out of orbit due to the strains involved, but you could always "correct" this by firing off thrusters stationed on the Moon (humanity might want to do this anyway in future, as the Moon is currently pulling away from Earth of course).

Answer 1

In addition to Mark Foskey's answer relating to the implausible strength required for this there are a number of other complications.

A traditional space elevator is placed in a circular orbit, the moon is not in a circular orbit so the system will need to change length by 42 800 km twice each month, which comes out to more than 100 kmh. Not something you do with a mechanical winch or similar.

A related problem is that the path traced across earth's surface is not along the equator, so during each day the ground end of the structure travels earth circumference every day, above 1000kmh/mach one, traversing substantial mountains. This also means the structure will have substantial drag forces that will need to be overcome in day to day operation, presumably with rocket thrust or risk the entire structure de-orbiting.

Trying to solve the problem by connecting to a pole turns the structure from a vertical tether into something akin to a bridge sticking out sideways involving some sort of foundation or balance. Note also that neither North (sea ice over ocean) nor South pole (moving ice Km thick) are good places for a mega structure in some form of artificial mountain.

In addition to needing to dodge obstacles at the earth end the LEO and GEO sections of the structure will be sweeping all the orbits and need either active mobility to avoid debris or carry substantial depth of armor.

Note that a failure mode for a debris strike or stability failure on this system will be absurdly strong material wrapping around the earth (potentially several times), striking the ground at or above orbital velocity.

The Lunar end is also not stationary which either induces motion or prevents that end being structurally supported.

The actual midpoints of this structure are also unstable, with tidal effects from the sun and variations of mass distribution in earth and the moon tending to set up wave motions along the length, that will probably need active thrust to damp out.

Mass movement along the length will also be a problem, since this is a suspended structure not a tower so if a mass is lifted up from earth to moon a similar mass needs to come down or the entire structure will shift in the earth direction and need thrust of some sort to compensate.

Answer 2

Consider, for comparison, the space elevator concept. It would extend from the surface of the Earth to a point past geostationary orbit, and weighted in such a way that geostationary orbit is where the center of mass is. This is actually conceptually very similar to your idea. For instance, it also is meant to benefit from descending loads balancing ascending loads.

Because the space elevator uses a much shorter cable than the one you propose, tensions will be lower and the demands on it will be less. However, it is still near the theoretical limit on materials strength sustained by chemical bonds. My understanding is that a cable with the tensile strength of a perfect carbon nanotube would work, but tensile strengths measured on molecular scales generally don't scale to larger objects. So I don't think there is a material strong enough to make the cable-to-the-moon proposal work. A better answer would actually compare the strength necessary to the strength of the strongest possible chemical bond, but my point is that there are physical limits on material strength. At some point, creating a material that strong is no more realistic than creating a wormhole between the earth and the moon, and I think this proposal is close to that level.

Answer 3

There is a misconception here:

[...] the space elevator idea faces the challenge that, up to geostationary orbit height, the entire structure has to be supported from beneath [...]

This is wrong. One can not simply support anything up to geostationary orbit, it's way too far out. Rock at the bottom of such a structure would behave more like a liquid, causing the entire tower to collapse in on itself. Instead, space elevators are suspended from a counterweight, their load is entirely tensile, just like the bucket chain.

Of course, one can use the moon as a counterweight. And yes, the part of the bucket chain between the L1 point of the earth-moon system and the moon would indeed also act as a partial counter weight for the part between earth and L1. And yes, the lower gravitational potential of the L1 point compared to other points at the same distance from earth would make things a bit easier than if one placed the bucket chain on the opposite side of the moon.

However, the key problem with this concept is that the bucket chain would be about ten times as long as a space elevator. The moon is really far out. If it weren't, geosynchronous orbits wouldn't be stable. Geosynchronous orbit is so far below the moon's orbit that the tug of its huge mass does not disturb our communication satellite's orbits very much. And, the distance to the Earth-Moon L1 point is much longer than geosynchronous height. As such, the bucket chain would be much harder to build, and it would need to endure much higher tensile stresses than a space elevator.

The cause for this discrepancy between elevator and bucket chain lengths, is that Earth spins much faster than the moon orbits the earth. It takes earth roughly 24h for one rotation, the moon takes roughly 26 days for an orbit. This faster spinning of earth synchronous stuff means that the centrifugal force equals the gravity acceleration much sooner. And this allows for space elevators which are less than 40,000 km long (assuming a serious counter weight).

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All of this is even before considering the problems of the end of the bucket chain moving relative to the Earth surface. Both in height (eccentricity of moon's orbit) and in horizontal movement (roughly 1667 km/h).

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That said, the idea of using a loop of moving "wire" instead of an unmoving structural wire is a good one. It would allow for the heavy lifting machinery to be located at one end, without the need for a serious power source in the lift cabins. It would also separate the descending cabins from the ascending cabins due to the Coriolis force acting in opposite directions on the two halves of the loop.