Sometime in the 2020s, NASA will launch the Asteroid Redirect Mission (ARM) towards a 30-meter space rock with the goal of picking a boulder up off of its surface and returning the rock to Earth for us to have a look at. NASA has to be very careful in deciding which asteroid to plunder for this mission, because the spacecraft the space agency plans to send won't have a good way of dealing with an asteroid that's spinning, which lots of asteroids are. And realistically, how the heck do you stop a giant space boulder from spinning, anyway? The answer is of course to use lasers, because, well, lasers solve everything.
To stop something from spinning in space, you need to impart a force on it that counteracts the spin. When we want to impart forces on things in space, we slap engines on them, which works pretty well. The most efficient engines that we have operate by using electrical energy (harvested for "free" from solar panels) to accelerate mass in one direction, pushing the spacecraft in another direction. We could use this technique for asteroids as well, except that we'd need to get the engines onto the asteroid somehow, and then top them up with an impractical amount of reaction mass.
At the University of California at Santa Barbara, astrophysicists have devised a better solution; and yes, it involves lasers. Their system, called DE-STAR (Directed Energy System for Targeting of Asteroids and, uh, exploRation) uses a laser to heat the surface of an asteroid to 3,000 degrees Kelvin, vaporizing it in a process called laser ablation. The vaporized material bubbles off of the surface of the asteroid, effectively acting like a very small rocket motor that's using the asteroid itself as a propellant. If you keep this up long enough, and your aim is good enough, you can get the asteroid to stop spinning, and even shove it around a bit. This isn't just a theory: the UCSB group has experimentally shown that it works. This video shows a 40-watt laser vaporizing the surface of a piece of asteroid-like basalt, and you can see the plume cloud that it generates, which is very cool:
Lab measurements have shown that in terms of thrust, the conversion of laser energy to force through this method is about 100 micronewtons per watt, which works out to 10 kilowatts per newton. A newton is not a lot of force (about what's required for you to hold half of a medium-sized apple up against gravity), and 10 kW is a lot of laser energy. So this doesn't sound like a particularly effective way of messing with ginormous rocks. Fortunately, as long as you're not in a hurry, the fact that you're trying to do this out in space (where there's no gravity or friction to speak of) makes it possible, even practical.
The amount of thrust that you need depends, of course, on where you want the object to go, how big it is, and how fast you want it to get there. In the case of an asteroid that you simply want to despin, stopping its rotation can happen pretty quickly if you have even a moderately powerful laser. For example, a 150-meter-wide asteroid with a density of 2,000 kilograms per cubic meter can be despun from 1 rotation per hour to zero by a 50 kW laser in just 11 days.
Here's a video showing a proof of concept demonstration showing a 40 watt laser despinning a small sample of basalt magnetically suspended in a vacuum chamber:
If you want to get a little more ambitious, this technique could also potentially be used for redirecting asteroids completely, not just spinning them. This concept has already been discussed (and will soon be tested) in the context of orbital debris mitigation—but only with objects smaller than a few centimeters. But what if we're interested in moving something a little bit larger—say, a 325-meter-diameter asteroid like 99942 Apophis, which, for a little while, appeared to have a not insignificant chance of impacting Earth within the next century? Or, if you'd like something more relevant, consider 2015 PU228, which is about the same size as Apophis and is currently the only object that NASA rates as worth paying attention to for a potential Earth impact (which would occur in 2081).
Anyway, to deflect the 325-meter asteroid of your choice by a comfortable 2 Earth radii, you'd need: a 1-megawatt laser firing at the asteroid for 2.5 years, which is not too bad. If you had more time, like 5 years, you'd only need a 260 kW laser, and over 15 years, you could make do with a 20-kW model. For smaller and harder to detect asteroids, like the one that exploded over Siberia in 1908 and flattened (literally) 2,000 square km of forest, we might have a lot less time to prepare, but a 1-MW laser could push one of those out of the way in a matter of months.
So, what would we need to actually pull this off? Nothing crazy, it turns out. The upcoming SLS Block 1 could launch a spacecraft equipped with 450 kW solar arrays, ion engines, and a steerable laser array able to operate anywhere from 1km to 100km from a target asteroid. It would look something like this:
The researchers point out that this system is more effective than using an impactor for the same amount of launch mass, and it's also more versatile, since you can also use it to despin asteroids, or potentially use it on multiple asteroids. In fact there, no reason not to launch it as soon as possible so that it's ready to go once we need it: in the mean time, it can hang out in Earth orbit and de-orbit space junk for us.