Basically, slowly speed up the rotation until it reaches 'burst' speed.

I estimate that the surface would need to reach about 20 to 150m/s depending on whether it is loosely packed ice or a solid lump of cast iron.

Now, how to speed up the rotation?

I propose that a solar sail principle could be used, which admittedly imparts a very small force. However, if it is applied over many years the result would be achieved.

The force is exceedingly small...9N for 1 sq.km of total reflection sail. The object is prepared with a suitable coating by spraying from an angle, so the reflection varies according to the angle of the light from the Sun. Since one half of our object will be total reflection, and the other half with poor reflection, then we should expect nothing more than 4.5N per sq.km. If we are lucky, this is the torque applied to the NEO at the surface.

This is going to take a looong time... and is probably fine for those passing objects that might hit earth in the distant future.

For those other objects that come in out of the blue, the only adaptation is to strap a mass drive unit onto it and spin it up more rapidly. I'm not sure if the energy required to spin and explode is less than the energy required to shift the path.
-- Ling, Mar 02 2012

Lightmill http://en.wikipedia.../Crookes_radiometer
A.k.a. Crooke's Radiometer. [AntiQuark, Mar 02 2012]

If the object is loosely packed ice, or has some ice component to it, firing a laser at one point on the object will cause a jet of steam to come off it which will make it rotate.
-- hippo, Mar 02 2012


If you were to spin it using the principle of gyroscillation then very little energy would be required.
Placing the wave creating device near the center of the asteroid would speed the process up greatly.
-- 2 fries shy of a happy meal, Mar 02 2012


If you fire a laser at the equator, at what is to you the retreating horizon, the laser will mostly hit the sides of outcroppings that face away from the direction of rotation. The resulting spewings-forth of expanding bits will increase the speed of rotation.

You'll want to position your laser so the places where the laser hits level bits tends to steer the asteroid where you want it. You'll also want to claim the spewn-off bits toward mission accomplished.

(I am trying to type this on a wonky old laptop while lying on my back with a kitten on my chest. If she would lie down things would go a lot better. (But lying down with a kitten is actually things going pretty well.))
-- baconbrain, Mar 02 2012


I would suggest a solar-power-driven electric motor. The motor would drive a fairly simple lightweight drum, of a respectably large radius. The drum would have doors around the edges that can be opened during operation.

Have a robot harvest mass - whatever's available, or can be dug loose, on the surface of the asteroid. Feed it into the stationary drum, then spin the drum. When it gets to its maximum speed, open the doors to dump out the mass, then stop the drum. The angular momentum of the asteroid has now changed by an amount equal and opposite to that imparted to the dumped mass.

This should be able to rapidly impart the initial acceleration. The apparatus may become difficult to continue feeding and operating once a respectable velocity is achieved.
-- lurch, Mar 02 2012


[lurch], I like that.
-- baconbrain, Mar 02 2012


I think we should all come up with our own variants on the 'landing autonomous robots on Earth-threatening asteroids' idea. We've really got hold of something here.

I'm gonna go work on mine right now.
-- Alterother, Mar 02 2012


Do all the planet-killers come in on the plane of the ecliptic ? then blow up the moon and surround the planet with debris.
-- FlyingToaster, Mar 02 2012


I know I've seen that one baked in scifi somewhere...
-- Alterother, Mar 02 2012


All good ideas...but the simplest will win. The laser is good, but where will the energy come from, and from where will it be applied? Maybe it just needs a focussing lens which runs in front and directs the Suns radiation onto one part...only a little energy will be needed for manipulation.
-- Ling, Mar 03 2012


If you attach a solar sail to it, it would be much quicker to use the sail to change its orbit to avoid earth entirely. Although it would need adjusting to maintain the right angle, you could quite easily slow the asteroid down so it spirals into the sun.
-- mitxela, Mar 03 2012



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