# Is there an elegant method to stop an asteroid's spin?

There have been proposals to mine asteroids, perhaps after “tugging” them to a more accessible location. As well, proposals have been made to move asteroids which threaten collision with earth.

Asteroids would be much easier to deal with if they were not spinning. But the chance of a given asteroid having exactly zero angular momentum is, well, exactly zero. How to stop it spinning?

Brute force could be used with chemical rockets. But this involves transporting the mass of fuel for energy and reaction mass. Launches typically require 10 times the payload mass in fuel. That’s 10 tons of launch fuel for every ton of fuel that arrives on site at the asteroid. Yet there is already energy (sunlight) and reaction mass (rock) on site.

There is a similar problem if the reaction mass is in the form of a reaction wheel. A huge, high precision machine needs to be transported. Once angular momentum is transferred to the wheel, how is it dispersed? Using rockets gets us back to the problem of transporting fuel mass, as well as the mass of the reaction wheel.

Is there a more elegant way? Momentum shedding weights on wires? Light sails in the shape of a pinwheel? Giant radiometer vanes?

One method could be to use light sails:

A pair of spacecraft, joined by two tethers, encircles the asteroid with a snare. After carefully aligning themselves with the rotational axis, the snare is closed. The spacecraft extend masts. at the end of each mast a light sail is deployed, mirrored on one side and black on the other. The light sails are angled to slow rotation. Once rotation is stopped, the light sails are "feathered" to have no effect. Or they could be used to deliberately rotate the asteroid to sun synchronize or align to an incoming spacecraft or for temperature control.

@ontrack and @Ferrybig made a good point about using a yo-yo mechanism. These are commonly used to de-spin spacecraft which have been spin-stabilized. A yo-yo is very effective since the spin rate is decreased with the square of the “string” length. A plain-Jane yo-yo maneuver will always leave residual spin which will need to be handled with another mechanism, but this residual spin can be reduced to any arbitrary rate with a long enough string.

However, with orchestrated timing of the string feed-out, the residual spin can be used to wind the string around the spinning object, yo-yo style. Rapid retrieval of the string by the yo-yo mass could use the inertia of the yo-yo mass to halt residual rotation.

On-track’s suggestion of using mined asteroid material as the yo-yo mass gets full marks for elegance. However, that necessitates setting up a mining operation on a spinning asteroid. An alternative is to use the mass of the spacecraft's spent booster as the yo-yo mass.

The cable, winch and batteries are all located in the booster to maximize the yo-yo mass.

• Why would it be much easier to deal with if they were not spinning ? Oct 25, 2022 at 7:25
• How about giant nukyular bombs!
– GdD
Oct 25, 2022 at 7:59
• Is the root of your question specifically how to do it to mine an asteroid? Or just in general, even if it isn't needed, how would we stop an asteroid? Oct 25, 2022 at 14:15
• May I ask how did you create such beautiful illustrations? :) Oct 27, 2022 at 10:21
• @Kartoos ... Corel Draw. Glad you liked it. Oct 27, 2022 at 14:18

I found at least 1 paper proposing enclosing the asteroid in a bag filled with gas.

https://www.researchgate.net/publication/273913280_SHEPHERD_A_Concept_for_Gentle_Asteroid_Retrieval_with_a_Gas-Filled_Enclosure

This would allow a probes ion engines or other propulsion to keep the bag from rotating and the asteroid to slow down from friction against the gas.

• OR, you could build a DART clone and blast it in the other direction.... Oct 25, 2022 at 6:58
• @ColonelCornieliusCornwall Not as effective as it sounds: 1) it would need to aim at the asteroid body (= short lever), 2) the gravel explosion would not amplify the desired effect, it would simply give the asteroid an additional unintended acceleration, 2) if it hits too shallow, the directed ejecta will carry away much of the angular momentum of the impactor (problem with rubble pile asteroids). I think, you'd be much better of with placing a probe with an ion thruster at the tip of a long, telescopic pole on the equator of the asteroid. Oct 25, 2022 at 19:11
• @cmaster-reinstatemonica ok. A DART clone isn't really the best option for much of anything except for when you'd actually need to defend earth from something (AKA an asteroid/comet) in this case, it's just a test to see if you can stop a spinning asteroid. Oct 25, 2022 at 19:13
• @lijat... This paper describes a way to transfer angular momentum (very gently) to the spacecraft. It still requires the same amount of fuel to actually stop the rotation. Oct 26, 2022 at 2:47

Look at how satellites are commonly de-spinned, and apply it on a larger scale.

A commonly done method is the Yo-yo de-spin where you have long ropes with weights, and extending them slowy from the base. As the ropes are unwound, the angular momentum is transferred onto the weight at the ends. At maximum distance, the ropes are cut and fly away. A single step of this can remove 90%(1) of the angular momentum, so other systems have an easier time arresting the remaining of the spin.

(1): The spin of the Dawn spacecraft (weight 1420kg) was slowed down from a spin of 46RPM to 3RPM with just 3kg of weight on a 12 meter cable

• That's about 1:500. For a 1km asteroid with a mass of 10^12kg, you would need "weights" of around 2x10^9kg, or 2 million tonnes. Oct 28, 2022 at 7:09
• @MartinArgerami or a much much longer rope. Oct 28, 2022 at 7:21

You could land on the asteroid and use a cannons/slingshots/rail-guns to fire parts of the asteroid at as shallow elevation angles as possible. If you fire the same way the asteroid is spinning, you could eventually stop its spin.

This is called a Reaction Control System and it is sometimes used on spacecraft. Though, of course, on space-craft, gas thrusters are used. On an asteroid, you might want to use solids, gasses, or liquids.

The advantages of this approach are that it could be more controlled than bombardments and explosions. Disadvantages could be that you are adding more debris in various paths around the asteroid, and that you may be losing valuable material as reaction mass.

• One method for icy asteroids (which I've read about in sci-fi) is to use solar power to make steam, which can be used to move the asteroid &/or modify its rotation. But I have no idea how practical that is on a typical ice-bearing asteroid. Oct 26, 2022 at 7:56
• @PM2Ring I think steam with solar could work great. One would just have to ensure that it departs the asteroid at least at escape velocity. If you have a lot of lead time, you could install a "spin-stabilizing" system years or even decades before coming back to mine the asteroid. Oct 26, 2022 at 16:12
• @ConnorGarcia so, a gun that lands on an asteroid? That might work, but if I had the choice between using an orbiter spacecraft, a giant gun, or a large rocket engine, I think I'd take the rocket. Oct 28, 2022 at 19:18

One that is simple in concept, although subject to engineering limitations, is to transfer the angular momentum to a reaction wheel. Attach a heavy mass to the asteroid, and spin it in the same direction with a motor (so the reaction is opposite). Of course, if the asteroid is a million times the mass of the reaction wheel, it has to spin many times faster. (Shaping it as a giant ring enhances its efficiency, but you are still probably talking about a spin tens of thousands of times faster.)

Another nice idea is to use tides. The moon used to spin much faster, but slowed until it always shows the same face to Earth due to tides. Wrap the asteroid in flexible stretchy tubes of heavy viscous fluid. The tidal peak lags behind the direction gravity is pulling it, and the result is to exchange rotation for orbital angular momentum. If the asteroid is orbiting the sun, the orbit will widen or shrink, which is where the angular momentum goes to. The moon of course took millions of years to slow - you would need a 'sea' with a mass comparable to the asteroid and ideally a steep gravity gradient for strong tides.

Another option is to find another asteroid spinning in the opposite direction and merge them together. That does require the ability to move asteroids at will around the solar system, which of course is a huge engineering challenge of its own. Or you could spin up a reaction wheel on one of them, then transport it across to the other and reverse its spin, repeatedly.

• ,,, I like the idea of using a reaction wheel to transfer momentum to another asteroid. Oct 26, 2022 at 1:39

Some (many? most?) asteroids contain some water and carbon. Proposals exist to use water as a propellant, either directly by electrically heating it so that it turns into steam which is the reaction mass, or by chemically turning water and optionally the carbon into propellants, for example methane and oxygen, which in turn can be used to fuel conventional rocket drives.

In fact, the chemical reaction path is lossy: The two main advantages of producing chemical rocket fuel first instead of using water directly as reaction mass are (1) the re-use of existing engines and (2) the high energy density of the fuel which allows higher-powered rocket motors. If the expedition to the asteroid can bring water powered engines like this one and is happy with the comparatively low output, direct water use is probably more efficient. One the other hand, for a manned expedition the ability to produce rocket fuel and oxygen through electrolysis is probably necessary and present in any case, so stopping the asteroid's rotation with conventional rocket motors would only use existing resources.

As an aside: If you are interested in large-scale scenarios that imagine asteroids as resources I recommend reading Neal Stephenson's Seveneves.

• great point about water as reaction mass. A solar powered steam rocket can be built with no moving parts, like a put-put boat. Thanks for the reading recommendation. I've been reading it today. Any other recommendations? Oct 28, 2022 at 3:57
• You read Seveneves in a day? Well, for some good Sci-Fi against a backdrop of political tensions between humans in Asteroid colonies, Mars colonies and good old Mother Earth you can watch Amazon's Expanse. Oct 28, 2022 at 6:14
• @Peter-ReinstateMonica try this, advancedsciencenews.com/…. admittedly, it's about a comet, but it should still be relevant. Oct 28, 2022 at 19:20

Assuming you already plan to move the asteroid, the most elegant solution imo would be gently bringing together two asteroids with complementary spin such that the coupled system has zero.

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– Community Bot
Oct 26, 2022 at 15:03
• @timeskull ... That would truly be elegant. But the problem is to remove spin so that a propulsive device can be attached. It is hard to move a spinning asteroid. If that was possible, it would not be necessary to stop the spin. Chicken/egg issue Oct 26, 2022 at 16:12

Light sails in the shape of a pinwheel? Giant radiometer vanes?

One method could be to use light sails:

A pair of spacecraft, joined by two tethers, encircles the asteroid with a snare. After carefully aligning themselves with the rotational axis, the snare is closed. The spacecraft extend masts. at the end of each mast a light sail is deployed, mirrored on one side and black on the other. The light sails are angled to slow rotation.

As suggested won't work nearly as well as you might think. The radiometer measures electromagnetic radiation but the motion is a result of the gas atoms trapped in the glass bulb. Without that gas it would just sit there motionless.

Why?

Both white and black sides receive the same "kick" from the incident photons, there's no difference there.

The white side diffusely scatters (re-radiates them nearly instantaneously if you want to get quantum mechanical) over a hemispherical distribution, producing a second kick.

The black side absorbs the photons' energy, then thermally radiates a lot more photons of lower energy. Since energy is conserved and the momentum of a photon is proportional to its energy ($$p = E/c$$) and the radiation will also have a hemispherical Lambertian distribution, to first order these suggest little or no net torque.

You would be better to maximize the force on one paddle by making it articulable mirrors that change their angle with respect to the paddle as it rotates, and minimize it on the other paddle by making them turn sideways so that the light just passes through, which is twice as good (on that side) as tilting them to reflect up and down (half-half) which still receives the input "kick".

• The illustration is not a radiometer. It is a pair of light sails, functioning like an anemometer. A radiometer would rotate in the opposite direction: a radiometer rotates away from the black side. Lights sails create torque away from the mirror side. And the radiometer wouldn't work in a hard vacuum. It needs a low vacuum to work. Oct 27, 2022 at 3:16
• @Woody my answer already explains why it's not a radiometer, because there's no gas. I suggest that it pretty much won't rotate at all because the hot black surface reradiates just as much momentum in photons as the cool white surface reflects. What it is, what it does, and what someone says it is and what they say it does are four different things, though there can be plenty of overlap. Give it some louvers and articulable mirrors!
– uhoh
Oct 27, 2022 at 3:26
• Your “answer” does not follow SE format of question/answer. It is a valid “comment”, but should be submitted as such. Articulated mirrors are an interesting variation. Could you please delete the answer and instead re-post it as a comment under the OP? If you have an answer to “Is there an elegant method to stop an asteroid’s spin?” it would be welcome. Many thanks Oct 27, 2022 at 19:22
• I think your idea of rotating mirrors (shiny both sides, not just one) could close to double the light-induced torque. Does that agree with your figures? Oct 27, 2022 at 20:36
• @Woody I don't think it's my idea originally, I probably saw it somewhere a long time ago, perhaps several times. Yes at any moment the maximum torque in the "good direction" for a paddle moving towards the Sun will come from a mirror at the correct angle, and on the other side moving away from the Sun the minimum torque in the "bad" direction will come from no mirror, or something thin edge-on. There's no way to intercept photons and get attracted toward the Sun on that side as far as I know, so "at least do no harm" seems the best option.
– uhoh
Oct 27, 2022 at 20:55

Not a single person has mentioned orbital siphons. Assuming you plan to mine the asteroid anyway, the siphon is there… though of course the spinning doesn’t fall significantly until mining operations are already well underway.

• @calstrument ... great idea. Upvote! Certainly very elegant. And building on the scale of an asteroid, rather than a planet, would be within foreseeable materials technology. I was hoping to stop the spinning first, to made mining easier. But a siphon could make good use of angular momentum as long as the axis was oriented properly. Maybe light sails could tune the axis perpendicular to the ecliptic before siphoning started. Oct 27, 2022 at 14:31

SPACE ELEVATOR

Or, better yet to call it a centrifugal launcher.

On your garden variety asteroid, a space elevator is pretty much possible with modern materials. You can even build a rigid (or semi-rigid - with ropes anchored at few equatorial points) space elevator.

Just move the packs of the mined material UP and release them at a favorable angle towards the destination. If you are mining an asteroid, chances are you want the mined material out of the asteroid.

You will be losing angular momentum much quicker than you want.

You will also want to take off and land a great deal of space vehicles. If your landing site is at the counterweight, you can transfer momentum back to the asteroid instead of braking by engines. Much, much less delta-v required and you can recover some of the lost angular momentum.

• very elegant. Upvote ! I think this is the concept cainstrument was referring to as an orbital siphon. Oct 27, 2022 at 18:23

Un-boil the egg

A boiled egg spins well, losing very little angular velocity over time. An unboiled egg slows down rapidly, due to the viscosity of the yolk and egg white.

So you need to convert your hard crunchy asteroid to a viscous liquid.

Orbital bombardment might work. Or you could land a rock crusher to grind up the outer layer. You would need a large enough asteroid for gravity to prevent the resulting gravel from flying away. If necessary, you could go for a full "rock egg with shell": land a machine that digs down and breaks up the core into gravel, ejecting some percentage to the surface to leave room for the interior gravel to move around.

Perhaps use the first method for large asteroids, the second for smaller?

I have no idea how to compute how quickly this would work. I think the angular momentum is converted to heat.

You'd also need to protect your rock crusher from being destroyed by the gravel.

• I think you are wrong about the egg. True, if you flick a raw egg to spin it, it will rapidly slow because the liquid innards never picked up any spin (or momentum). But if you slowly spin it up so the liquid acquires the same speed and momentum as a hard boiled egg, it will take the same time to slow as the hard boiled egg. Try it with an egg on a 33rpm turntable. Give the raw egg 5 minutes to come up to speed..............Angular momentum is conserved. Viscosity can transfer momentum, but momentum never gets converted to heat. Energy and momentum are different things, like love and money. Oct 27, 2022 at 23:09
• You are destroying some of my long-cherished beliefs, based on nothing but my own suppositions and little external evidence. I don't like you. Now I'm going to have to find something to substitute for a turntable, evil @Woody construct. Oct 28, 2022 at 17:37
• ah, Grasshopper. It is time for you to leave. Oct 28, 2022 at 18:38