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In a YouTube video Colonizing the Solar System, part 2: the Outer Solar System, Isaac Arthur who is a YouTuber and also a physicist says something like "you don't spin an asteroid for artificial gravity because it will cause it to fly away".

In more than few instances, he also explains how easy it is to dig an asteroid as it is more of a loosely held together dust and debris than something solid. So,

  1. Why doesn't an asteroid turn to dust as soon as it enters the earth's atmosphere?
  2. Can't we spin an asteroid in order to pulverize it?
  3. What about mechanically grinding it, if spinning is not an option? When it is so soft, would that mean we surely can send moderately small spacecraft that can grind few cubic kilometers in few years?
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    $\begingroup$ BTW, the asteroids usually break up when they enter the atmosphere - and often explode from extremely rapid heating. Still, forces that lead to ripping the asteroid apart, compared to drag and its kinetic energy are small enough that the asteroid turns into a relatively compact swarm of meteorites. $\endgroup$
    – SF.
    Commented Jan 29, 2018 at 17:53
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    $\begingroup$ It is thought most asteroids are rubble piles. But not all. $\endgroup$
    – HopDavid
    Commented Jan 29, 2018 at 21:49
  • $\begingroup$ Getting an asteroid to spin would be like getting a bowl of M&M's spinning, which M&M would you apply thrust to? $\endgroup$
    – Pieter B
    Commented Jan 30, 2018 at 7:44
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    $\begingroup$ You can in fact get the bowl of M&M's spinning faster by spray-painting a portion of the M&M's white. You select the side based on the current spin axis and where the Sun is. Small natural albedo variations can in fact result in asteroids breaking up. See the YORP effect. $\endgroup$
    – Mark Adler
    Commented Jan 30, 2018 at 8:08
  • $\begingroup$ @EdwinBuck One must not feed the trolls... $\endgroup$
    – Nikolai Frolov
    Commented Feb 11, 2018 at 2:05

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Not really. If an asteroid that was a loose collection of rubble was spinning, it would fall apart. The question is, how do you get it spinning? And would it do any good anyways?

To make it spin one would have to apply an asymmetric thrust. The problem is, most likely if you can do this, then it will actually end up only affecting the area you are thrusting, still leaving the main body of the asteroid intact.

An easier solution almost certainly would be the nuclear option. With a loose collection of rocks, digging into it should be easy enough, just place the nuclear weapon in the middle of the pile and it would scatter quite effectively. This would be quite a bit easier then trying to spin the rock, and impart more energy. The amount of energy required to blow something up is known as the gravitational binding energy. Suffieceth to say, a 100 MTon bomb could blow up a 10 km asteroid, barely. That is the largest nuclear weapon ever built.

And the more interesting question, would it do any good? That is difficult to tell really, but the answer is probably not. A single dense asteroid will hit the Earth mostly intact, which would cause local severe damage, but probably not global damage, unless the rock was huge. A loose collection of objects hitting the Earth all over, or even worse dust, could cause a different problem. If all of that dust hit the atmosphere, it would probably stay up there. It is very difficult to get things from the upper atmosphere down, thus any dust reaching that high could stay up there for years. This could cause a massive cooling event, similar to a volcanic winter. That could have global repercussions. Note that it would take a large amount of dust to cause this kind of problem, 60 tons of dust enters Earth's atmosphere every day. If, say, over a million tons of dust entered the atmosphere in one day, it would probably cause issues. Also, if instead you convert it into a number of smaller rocks, the area of affect can be greatly increased. Instead of one large event, you might have a dozen smaller events, but overall increase the area of destruction.

Bottom line is, the best thing is to get the entire rock out. If you can't do that, then unless you have enough time to allow for a wide separation of the rocks, on the order of a million or more miles, you are probably better off just letting the rock hit the Earth intact. If you have the opportunity to do something tens or even a hundred years in advance, then trying to get it to disperse might be feasible, but otherwise, moving the thing completely at once is your best bet.

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  • $\begingroup$ I know all well how fracturing an asteroid is not a solution but consider we are talking about 50-200 years timeframe to deal with extinction-size asteroid (5 - 30km), and we will probably use something like ion thrusters or lasers evaporating the surface of the asteroid creating thrust, so really slow acceleration. I'm talking about an alternative to the gravity tractor and similar technologies $\endgroup$
    – Nikolai Frolov
    Commented Jan 29, 2018 at 18:20
  • $\begingroup$ "This could cause a massive cooling event, similar to a volcanic winter" - plus the cloud would be radioactive, thanks to our efforts. $\endgroup$
    – John Dvorak
    Commented Jan 29, 2018 at 19:16
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    $\begingroup$ I don't think you can spin it even slowly and carefully. Once it's spinning enough not to hold together, you have a spinning collection of rocks but they're still gravitationally bound. The individual pieces are themselves spinning so the laser thing won't work anymore and you can't spin it any faster. So they'll coalesce right back into an asteroid. $\endgroup$
    – Vectorjohn
    Commented Jan 29, 2018 at 20:28
  • $\begingroup$ "If all of that dust hit the atmosphere, it would probably stay up there": um, no. That dust hits the atmosphere at tens of km/s and will burn up. Meteor showers consist of dust-sized particles burning up. $\endgroup$
    – Hobbes
    Commented Jan 30, 2018 at 10:56
  • $\begingroup$ While that is true, the vaporized dust would still remain in the upper atmosphere, which if received in the massive amounts from a destroyed asteroid could cause issues. Small amounts aren't an issue, 60 tons of dust hits Earth every day. A large amount, however, could cause some fairly serious issues. $\endgroup$
    – PearsonArtPhoto ♦
    Commented Jan 30, 2018 at 12:42
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Actually yes an asteroid could (to an extent) be destroyed by spinning. I like the idea. It would at worst likely split in half and the two halves would go safely past either side of the target to be avoided (eg: Earth), or at best, pieces would keep coming off the periphery (and they should fly off well clear with centrifugal force).

The method of getting it to spin would be gyroscopic (a very fast spinning small weight forced to gradually change it's axis would cause an opposite spin on the asteroid if that's what the spinning weight's cradle was attached to. Theoretically, with not much power, the asteroid could be made to eventually reach a high RPM.

So what would be more difficult? Transporting an explosive to the surface of the asteroid, along with drilling equipment, or sending a gyroscope? The thing about the spinning destruction is it would be likely more reliable as explosives can be hit and miss. On the other hand, if the asteroid were iron, it would resist destruction either way. Another problem is if the asteroid is already spinning on an unfavourable axis....

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  • $\begingroup$ To let a very heavy object like an asteroid spin with high RPM would require a lot of energy. That is basic physics. To do it with not much power would require a very long time. $\endgroup$
    – Uwe
    Commented Jan 29, 2018 at 21:59
  • $\begingroup$ @Uwe, yes obviously depends on how big the asteroid is. However, the kinetic energy stored in a flywheel is enormous. Basic physics also dictates that one kilogram, on the edge of the flywheel, spinning at, for example, 500 mph (at the circumference) is a "massive" bullet. If you change the axis on which the flywheel rotates, all that kinetic energy has to go somewhere. So on that simple postulate, if your firing 1kg at 500mph repeatedly at the edge of an asteroid is not going to spin it, then you are right, you'll need a smaller asteroid! $\endgroup$
    – www-0av-Com
    Commented Jan 30, 2018 at 18:24
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Asteroids will be a collection of different sizes of objects, from dust up to perhaps very large chunks. Some may be solid, however they will be likely to have cracks, and won't have had any structural inspections. Thus "you don't spin an asteroid".

Spinning an asteroid may separate it into (variously sized) chunks, however, depending on your purpose, consider the effect of shotgun pellets vs. solid slugs. A single slug is more likely to miss.

Spinning requires energy. A LOT of energy, for asteroid-sized masses and any significant spin rate, in addition to any energy spent on deflection. It seems to me that if what you want is to deflect an asteroid, it's less expensive to do so as a whole.

Grinding requires a huge amount of energy. Ever put ice cubes in a blender and heard the motor lug down or had it stall? And that's just ice, that shatters. If you are dubious, put on some goggles, get a hammer and a sizeable rock, and try turning the rock into small pieces. Might want hearing protection too.

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