The Apollo Lunar Orbit Rendezvous was a big win because they did not need to land their return vehicle or fuel on the moon, just the lander. With a rotating tether we could win even more by only having a small scoop at the end of the tether touch the moon.
Since there is no landing vehicle, we can also use a high ISP ion drive the whole time. By lifting a small scoop of regolith (probably under 10 Kg, maybe under 1 Kg) many times we could lift a reasonable total mass of lunar regolith using a small tether. For the 1.6 km/sec tip speed of a tether for lunar pickup, the tether only needs to be like 3 to 10 times the payload mass. The ion drive has to replace the momentum before the next pickup.
The Dnepr at $10 to $13 mil for 9920 lbs (4500 Kg) to LEO seems like a good deal.
Starting with a 4500 Kg vehicle in LEO we use an ion-drive to go to the moon, spin up a tether (probably winching in and out 2 tethers), and start picking up samples when the end of the rotating tether touches the moon. In order to avoid spinning too fast when you are winching a sample in on one tether, you let the other tether out on the other side at the same time. This stores up the angular momentum and gets the other tether ready to pickup a sample. After picking up enough that it is running low on fuel, it could head back to earth and have a capsule reenter with the samples.
The question of how many Kg of lunar sample you could bring back depends on a lot of things, the ISP of thruster, watts/Kg for solar, ratio of capsule mass to payload mass, how long the mission can be, etc. You can also trade off time and mass returned to some extent (higher ISP can bring back more but takes longer). Our initial guesstimate is that you could return between 1,000 Kg and 10,000 Kg in something like 1.5 to 4 years.
All of the Apollo missions combined returned 381.7 Kg. The Apollo costs have been estimated at $100 billion in 1994 dollars. If this project can be done for $50 million (not by NASA for sure) then this would be like 2000 times cheaper and return 2.5 to 25 times as much.
In a more fair comparison, the Artemis people were going to start with 44,000 lbs in LEO (so 4.4 times as much as us). Artemis planned to return between 200 lbs (95 Kg) and (227 Kg) of lunar material (we return ~4 to ~100 times as much).
If you think of the investment cost as scaling with the lbs to LEO (a reasonable first approximation) and the financial return value as the Kg of Lunar material returned (not totally fair as the price goes down with a bigger supply), then the tether method is 16 to 400 times better. Artemis was sort of marginal as an investment, but the tether approach could be a reasonable investment.
It is of course hard to estimate what people would pay for lunar regolith once there was a real supply. If it was $100/gram and you had 2,000 Kg, that would be $200 mil. Probably be hard to get that much, but it could really be a good return on investment.
Additional missions would cost much less than the first, since you would not have the development costs again. You could even design the vehicle to be resupplied for a new mission (more xenon, new reentry capsule, etc).
This would be able to pick up samples from many parts of the Moon, any part that passed under the orbit. If it was in a polar orbit it would even be possible to get a sample from a dark crater at the North or South pole to see if there was water ice.
Most of the regolith is very fine dust. You might get more money selling lunar rocks. It might be possible to have a computer guided harpoon on the end that could target small rocks.
Another interesting option is to use the tether to toss small reentry capsules from the Moon in such a way that they fall back to Earth. One nice thing about this is that you could start selling your product much sooner. The other nice thing is that if at some point there was a catastrophic failure you would at least have what had been returned so far. People bidding on what you had so far would not know how much more would be coming. So concern that your vehicle might fail at any time might keep the price of Lunar material high longer.
The idea of a "Tether Assisted Planetoid Sampler" was looked at by Forward and Hoyt. They proposed using a sampling penetrator on the end of a rotating tether to obtain a sample of some airless planetoid during a flyby trajectory.
The rotational momentum of even 1 Kg at the end of a 100,000 meter long tether is so huge that no momentum wheel will have any impact on it. However, you can easily control a tether's rotational momentum, when near a gravitational body, by winching the tether in and out as it is going up or down relative to the body. If you want to rotate faster you winch in when the tether is vertical and let out when it is horizontal. So rotational momentum is an easily solved problem.
Orbital momentum needs to be controlled by either leaving something on the surface of the moon of equal mass to what you are picking up, or using some kind of thruster. The thruster could be a conventional chemical rocket, an electric thruster, or a solar sail. All of these, including leaving something on the surface, can be looked at in terms of ISP or exhaust velocity.
|Source of Momentum||ISP||Exhaust velocity|
|Leaving mass on surface||163||1.6 km/sec|
|Chemical Rocket||400||3.9 km/sec|
|Hall Thruster||2,000||19.6 km/sec|
|Ion Drive||10,000||98 km/sec|
|Solar Sail||infinite||speed of light - solar photons|
|Regolith Thruster||320||Depends on how far winched in|
The Hall Thrusters and Ion Drives come in different ISPs, these are just some sample values. Note that ISP times 9.8 equals the exhaust velocity in meters/sec.
The ratio of lunar-pickup-mass/reaction-mass is the same as the reaction-mass-exhaust-velocity/lunar-orbital-speed so that momentum is conserved. The orbital speed you will be giving the regolith is about 1.6 km/sec. The higher the exhaust velocity the less reaction mass you need. In the solar sail case the reaction mass keeps coming to you from the sun, so it is sort of an infinite ISP.
We are used to needing lots of rocket fuel to lift a small payload, since launching from Earth you might use 30 to 100 times as much reaction mass as you get payload to orbit. With a 10,000 second ISP ion drive and a tether, we could lift 61 Kg of lunar regolith for every 1 Kg of reaction mass (98/1.6=61). This is so amazingly good that it takes awhile to sink in.
With a solar sail the only thing limiting how much you can lift is how long your system keeps working.
At some smallness it gets hard to make fault tolerant tethers, but I am not sure where that is exactly. I don't think space junk is nearly the issue around Luna as around Earth. There has not been the human stuff smashing into each other and making lots of orbiting junk. You can get fault tolerance by having spare tethers, as the tethers you need for the moon are not really too heavy. You can make your tether shorter than 100 km, say 5 km, since the non-human payload can tolerate high Gs. This also reduces the chance of collision. Very small diameter Spectra lines are available. And if your last tether did happen to break, you just head back to Earth with whatever regolith you have so far. So a very small tether is probably doable.
Scaling down solar power is easy. You can get small Hall Thrusters. At busek.com they have one that is just 900 grams.
It might really be possible to do this project in under 100 lbs, though that would be impressive.
When you picked up the regolith it was going backwards relative to your center of mass about 1.6 km/sec. So you have to throw it backwards faster than this to get any net thrust.
The second issue is that you can easily make all kinds of space junk that hits you or someone else on some future orbit. By picking your velocity to be something less than 3.2 km/sec backwards relative to the center of mass of the tether, then it will be less than 1.6 km/sec relative to the moon and fall to the moon. You might want to avoid hitting Apollo sites, or any man made objects on the moon.
Since you would not be limited by a fixed supply of xenon reaction mass, you could bring back more lunar regolith this way. It should take less in the way of solar power since the energy put into the reaction mass goes up with the velocity squared (1/2 m v^2). You save mass on solar and on the xenon. So you can put more mass into making a bigger reentry capsule and bring back more regolith. Starting with 4,200 Kg in LEO from a Falcon-5, it may be possible to bringing back a few times this mass in regolith.
Lets compare this to something NASA might do. NASA might spend nearly a billion dollars to send a rover to the moon that does a few tests on a few rocks as it drives over a 3 km stretch of moon. For 1/10th the cost we could get a few tons of samples from all over the moon and bring them back to Earth for extensive testing and sales. After such a contrast, tethers would be taken very seriously.
If the project makes lots of money, then money is available for getting tourists into space using tethers.
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