# Can I spin 3753 Cruithne and keep it spinning?

I am planning to build a habitat with close to Earth gravity on asteroid 3753 Cruithne. I would dig a circular tunnel (or series of parallel tunnels) deep under the surface, with a radius of 4 kilometers and a tube diameter of 20 meters. This would give the habitat a length of around 13 kilometers.

I would then use mass drivers and mined material to spin the asteroid to a rate of 0.4728 rotations per minute. This would create earth-like gravity in the underground habitat.

I am curious whether it is possible to spin the asteroid to this extent without damaging it. I am also wondering how much effort it would take to maintain the spin. Could it be done using currently proposed propellantless propulsion methods, such as solar sails?

EDIT: Different site States Cruithnes diameter to only be 2.1 Kilometers, at this size, the diameter of habitat tubes would be only 1.8 kilometers, which would put desirable rate of spin to around 1 rotation per minute.

• A cursory search doesn't lead to any specific information about the structural integrity or exact shape, not even to a rough idea of the structure of similar bodies. Somewhat frustrating, but an opportunity perhaps to "make it up" to fit your story, perhaps adding that it's maybe a remnant of the giant-impact hypothesis and conveniently solid in structure. Jul 24, 2023 at 18:44

Having Earth like gravity inside the tunnel means that also the asteroid is experiencing a centrifugal force equivalent to that intensity.

Whether the asteroid is able to withstand it, it depends on its internal structure: if it is made of compact metal/rock, it might very well be capable of doing so, while loosely aggregated element the force will disintegrate it during the spin up.

Structural Coherence of Asteroids

Many asteroids are not solid blocks of rock, but rather, they are 'rubble piles'—collections of rocks and dust loosely held together by gravity. This poses a significant challenge when it comes to spinning up an asteroid to create artificial gravity. The forces involved would cause such a rubble pile asteroid to disintegrate if it spun faster than every 2.2 hours; Cruithne is 5km wide and the artificial gravity would be only 0.00016g.

In order to proceed with your plan, you'd first need to determine the structural integrity of 3753 Cruithne. If it's not a solid body, you might have to consider ways to solidify it, which could involve anything from compacting it under its own gravity to using some sort of binding agent—though these are speculative solutions and would present their own challenges.

Maintaining Spin

In the vacuum of space, a spinning object should, according to Newton's first law of motion, continue spinning indefinitely unless acted upon by an external force. However, in reality, several factors could slow down the spin over time.

One of these is the YORP effect, where uneven heating of the asteroid's surface by the sun causes a torque that can change the object's rotation rate and axis orientation. Gravitational influences from other nearby celestial bodies could also affect the spin.

To maintain the desired spin rate, you would need a method of propulsion that can apply corrective forces as needed. Solar sails, electromagnetic tethers, or other propellantless propulsion methods could theoretically be used for this purpose. However, the scale of the operation and the precision required would be substantial.

Living Conditions

Creating a habitat with Earth-like gravity is just one aspect of making an asteroid livable for humans. A host of other factors would need to be considered and engineered.

For example, the habitat would need a breathable atmosphere, which means not only having the right mix of gases (primarily nitrogen and oxygen), but also maintaining the right temperature and pressure.

Protection from radiation is another major concern. On Earth, our magnetic field and atmosphere shield us from most of the sun's harmful radiation. On an asteroid, you'd need to provide this protection artificially, perhaps by burying the habitat under several meters of rock or using some sort of magnetic shielding.

Lastly, resources such as water, food, and energy would need to be provided. While some of these could potentially be extracted from the asteroid itself or brought from Earth, sustainable methods of production would likely be necessary for a long-term habitat.

• Just to be clear, the YORP effect (and other similar effects transferring angular momentum to or from asteroids without a collision) is vanishingly small. It'll make no noticeable difference on timescales shorter than millennia. The only reason it matters at all is precisely because there's no friction or anything similar in space that would stop a spinning asteroid from spinning, which allows even the tiny incremental changes in angular momentum from things like the YORP effect to build up over thousands and millions of years. Jul 25, 2023 at 0:15
• Would the axis of spin be a problem? I could recon when moving material and people inside and maybe drilling, or space-ships docking... the center of mass of the asteroid will change and it will wobble? How big of a problem would this be for stable artificial gravity? Will it be make people dizzy, or will it not even be noticeable ? Jul 25, 2023 at 13:57

Working on the assumption the asteroid is solid enough to withstand the spin, or you provide some way to strengthen or fix the rock, you could spin it up using various propulsion units. Perhaps the engines from a rocket used to get workers, supplies or materials to the asteroid.

Once it is spun up, it will stay spinning, but if you are moving people or materials in and out of the habitat, you may find you need small adjustments to the spin rate over time, so something like cold gas thrusters or even ion thrusters may be suited for that purpose.

Alternatively, if instead of placing your access at one of the poles of the spin, you could arrange it that incoming ships dock in a gantry at the equator, and their thrust is used to change spin. This is definitely not your best plan, but has some nice side effects - see the Ringworld novels for some ideas.

The shape of the asteroid would be important to whether or not you can spin it with easily predictable path.

One reason earth can spin easily predictable have to do with its shape and the source of the power.

Given that you are trying to do it manually, you have do a complete analysis on the shape and mass about the asteroid to calculate a perfect position to aquire force to start the spin.

And that also includes whether the whole rock is made up with approxmiately average mass with approxmiately average density with approxmiately average structure. So that it can hold on to its own after the force is implemented.

Then perhaps under these perfect condition you can spin with less required resources.

Honestly, it seems that the construction as described in your original post might work--but it would be very fragile against disturbances. @L.Dutch is right about the centrifugal force.

Your design would likely be vulnerable to sudden catastrophic failure, due to undetected faults in the asteroid, or semi-plastic flow of the asteroid's body over time. All in all, this seems very unsafe.

I would go with the notion (mentioned by @Uk rain troll and @Rory Alsop) that some kind of strengthening engineering would be the way to go. I do have a (possibly silly) suggestion to that end.

Suppose that the civilization of your projected future worked to set up space elevators to get off-planet. Suppose further that--for whatever reason--there was a surplus of tether material available when this asteroid construction took place.

It would not be too hard (assuming you have the engineering skills and resources to be doing this asteroid modification in the first place) to bring a goodly amount of space elevator tether cable up to the asteroid, and then wrap the outer surface with pieces of the cable, spaced fairly closely. You'd place the cables normal to the intended axis of rotation, so they would provide maximum counter-compression to the centrifugal force. You'd also want to keep them close to the asteroid's surface to avoid incidental damage.

It would be sort of fun to simply wind one continuous cable around the asteroid, in many many windings. Sadly, that would be vulnerable to being loosened by a break at any point along the single cable's length. So you would need to wind each section of cable independently, which requires a suitably strong means of joining the ends of each cable together.

Perhaps the most difficult aspect of this would be the need to support the cables by fairly closely spaced standoffs. (Something like the jackstand you put under the axle of your car when changing a tire?) The standoffs would need to spread the load over a wide area of the asteroid's surface; to prevent the cables from ever having to make too sharp a bend, in case the tether material would be subject to cracking if bent too far; to be stable against minor meteor impacts; and to be set to heights that would cause each cable to assume a strong shape (ellipse if not a circle) when under tension after the asteroid is spun up.

I'm not an engineer, so this might be nonsense. But it could be plausible nonsense, which might be close enough for science fiction. :-)

You don't actually need (or want) to spin your asteroid!

First ask yourself, why do you actually need your asteroid? For materials and as an environmental shield (protecting against impacts and radiation). None of those aplications would benefit from spinning.

What you should do instead is having a rotating habitat inside of a non-rotating shell. To prevent issues with angular momentum you can have your habitat be composed from an inner habitable cylinder and an outer shell that rotate in opposite directions. So outer-outer asteroid shell could be non-rotating.

Why would that solution be better than spinning the asteroid - even if it is reinforced? Because it takes less energy to rotate only your habitat without the asteroid shell (less mass -> less energy). The other reason is that it would be extremely difficult to reinforce the asteroid itself. Those things are generally not homogenous, so reinforcement would probably be more difficult from engineering point of view that building your habitat.