Regolith Return for Profit

The idea is to use a rotating tether to pickup some Lunar samples, bring them back to Earth, and sell them.

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.


There are 2 types of momentum you need to keep under control. One is the rotational momentum around your own center of mass and the other is orbital momentum around the moon.

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.

Scaling Project Down

I am not sure how far you could scale this project down. I don't think it would be too hard to get by on the 1,300 lbs to LEO of Space-X. I expect you could even get down to 500 lbs.

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 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.

Tether Regolith Thruster

You can have a Lunar tether pickup reglolith, winch in to spin faster, and then throw some of the regolith backwards for thrust and keep the rest. This lets you pickup some regolith from the moon without needing rocket fuel.

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.

Simulation of Tether Thrusting

Our simulator sample 89 now has a tether pickup 4 kg from the moon, winch it in to increase the tip speed, toss it backwards, and then winch the tether back out. After this the ballast does have more orbital angular momentum relative to the moon. So we did get some thrust by throwing regolith backwards. So Newton was right! :-)

Waves on Tether

While a winch can cause waves, it can also help get rid of them. Also you can use it in such a way as to not cause a problem wave. If you are winching in you can slow down when the tension is high and speed up when the tether is more slack and thus get rid of some of a wave. If you are letting out then you let out when the tension is high and not when it is slack. This can be done with a maximum tension when winching in and a minimum when winching out. You can also specify a maximum winch power and since it takes more energy to pull in when the tension is high it will not pull in as much. A computer program that knew the full state of the tether should be able to do a good job of reducing waves.

Efficiency of using winch to control spin

From page 18 (or 20 by PDF count) in the Guidebook for Analysis of Tether Applicaitons it says: "As discussed under Tether Control Strategies, changing a tether's length in resonance with variations in tether tension allows pumping or damping of libration or even spin. Due to Coriolis forces, in-plane libration and spin cause far larger tension variations than out-of-plane libration or spin, so in-plane behavior is far easier to adjust than out-of-plane behavior. Neglecting any parasitic losses in tether hysteresis & the reel motor, the net energy needed to induce a given libration or spin is simply the system's spin kinetic energy relative to the local vertical, when the system passes through the vertical." So not only can we use a winch to pump the spin up, it is very efficient to do so.

Great First Step

The perception/reality that tethers are exotic and not yet used to do real work would totally change if a tether probe came back from the moon with more regolith than all rocket flights combined.

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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