Someday soon astronomers may discover a large asteroid or comet on a collision course with Earth. Small asteroids strike Earth on a daily basis, burning up harmlessly in the atmosphere. The consequences of the impact of a large asteroid (diameter of 1 km or larger) vary, depending on it's size, from tsunamis that could disrupt shipping and devastate fishing communities, to impacts comparable to the one that killed the dinosaurs (10 km object) -- causing the extinction of the human race and all large animals.

How could we protect the Earth and ourselves from such an impact? Nuke it? That's one way, but it begs the question of how to nuke the asteroid or comet. There are a few ways nuclear weapons could be used in the defense of Earth from such objects, and several other methods as well.

Nuke It!

The most obvious idea it seems, would be to launch a multitude of US and Russian nuclear ICBMs at the NEO (near Earth object) and blow it out of the sky. There are a few fairly obvious problems with this approach however.

First, are the ICBMs currently maintained in the US and Russian nuclear arsenals capable of achieving escape velocity in order to rendezvous with the NEO? Remember, they were designed to follow a parabolic, sub-orbital course to strike targets on the other side of the planet -- not to achieve Earth orbit, and definitely not to leave the gravitational influence of Earth all together! So, just getting our existing nukes to the NEO is a problem.

Second, gigatons of explosive force would be needed for an object one kilometer in diameter or larger to even notice you did anything to it. The yield of the most powerful nuclear weapons in the world today is measured in megatons, so we'd need to use a lot of them, perhaps more than we currently have.

Last and most significantly, the goal of this approach is to break up the NEO into millions of tiny pieces that would burn up harmlessly in a spectacular meteor shower when they finally did hit Earth. The problem is that having sent probes to only as many comets and asteroids as you can count on one hand, we currently know very little about the physical structure of such objects, and even less about the dynamics of explosions in low gravity environments. We could easily make the situation much worse if we miscalculated the explosive energy needed, and simply broke one large object into several smaller, but still devastatingly large objects -- all of which are still headed for Earth. As David Morrison, Chief of Space Science and NASA's Ames Research Center put it, "If you talk about blowing something up and don't have any idea what the size distribution of the fragments would be, you could be worse off. You don't want to turn the asteroid from a cannonball into a cluster bomb."

Nuclear Stand-off Explosion

The idea here is not to destroy the NEO, but just to push it a little, into a trajectory that will not intersect that of the Earth. The explosion to nudge the NEO out of the way would take place at a distance of about a third of the diameter of the object. Heat from the explosion causes the surface of the object to fracture and split, blowing off chunks and creating a natural thruster on the comet or asteroid, changing it's course by a few precious fractions of a degree.

Again, the first two problems mentioned above apply, getting the nukes to the NEO, and getting enough of them there to have any effect. There's also a third problem. The splitting of the object's surface is analogous to the way a cold glass would shatter when boiling water is poured in. But what happens when you pour the same boiling water onto cold sand? If the NEO has a loose, dusty surface, this approach could have no effect whatsoever.

Put Something in it's Way

With this approach you essentially put something in the NEO's path for it to hit before it hits Earth. Enough relatively stationary matter in it's path would break up the object, again resulting in a spectacular meteor shower when the debris finally met Earth. The questions are what to use and how much. One possibility might be compressed air or water. It's plentiful, not very dense, and might not be too cost prohibitive to launch in sufficient quantities -- especially given the alternative. Another candidate might be dust gathered from the path of another comet. Another problem with this approach is precisely calculating the trajectory of the NEO in order to be sure you left the material in it's path. This could be more of a problem with a comet, where outgassing can alter the comet's own trajectory.

Automated Mining Operations

Establish an automated mining operation on the surface of the object where buckets on something like a "mass driver conveyor belt" repeatedly hurl matter off the surface, producing thrust to change the object's path. This idea seems pretty far fetched. While perhaps simple in concept, from a technical perspective nothing like the kind of "automated mining operation" that would be needed has ever been attempted, even on Earth! This idea would take quite a bit of research to develop. On the other hand, such technology may someday deliberately be developed specifically for the purpose of mining asteroids for precious minerals, after which point it would be a proven technology which would then only need to be adapted to this purpose.

Solar Heating

Jay Melosh, Professor of Planetary Science at the University of Arizona, has the idea to employ a space-borne solar collector and a series of mirrors to focus the sun's energy on the object, vaporizing the surface to produce thrust to change it's trajectory. The problem is that fragments blown off the object come back and damage the mirrors. The solution to this is to spin the mirrors and run clear silicone oil over the surface to wash off the debris. While this is another idea that will take considerable research, Melosh estimates that a three kilometer solar collector would be able to deflect a ten kilometer object given a year of the sun working on it.

Put a Better Spin in It

Personally, I think this is by far the "coolest" solution. If you change the rotation of the object, you change it's trajectory. Physics to the rescue. A space craft attaches a cable to the object and allows it's rotation to wind the cable around it a few times. Then the space craft thrusts in the opposite direction, spinning the object just like a yo-yo. You'd need a very strong cable that can take the stress and also will not be cut by the surface of the object. The space craft also has to be able to produce a heck of a lot of thrust to be able to reverse the direction of spin of an object hundreds if not thousands of times it's own mass. Considering these factors, perhaps this idea seems to be the most beyond our current technology. Still, the mere fact that changing the direction of rotation of the object also changes it's trajectory, is a very elegant solution.

"Eight Ball in the Corner Pocket"

If the object has broken into a few pieces, a combination of the above techniques could be used to knock the smaller fragments into the larger ones like billiard balls. A stand-off explosion near a smaller fragment that causes it to impact a larger fragment, could change the larger fragment's course either by direct force of impact or by altering it's rotation. The results of this would be very unpredictable though. It would be extremely difficult, if not impossible, to calculate the resulting trajectories of both objects before the attempt is made. So, the likelihood that any resulting trajectory would also be on a collision course with Earth would need to be calculated, and then you'd still be playing the odds. Scientists and engineers don't like to gamble very much, which puts this idea pretty low on the list of possible solutions.