The best way to deflect an asteroid
Most contingency situations are rife with options. Even an inbound asteroid can be dealt with, if detected early enough. Unfortunately, as NASA's Near-Earth Object Program has shown in recent years, fast iron-bombs have a way of appearing out of the black with scant weeks or months to spare. Not years. Not decades.
Over the years, many methods have been proposed for deflecting would-be planet killers. They range from the benign - coat the asteroid with white chalk and let the sun's reflected light push it off course - to the blockbuster - atomic weapons used to vaporize (or at the very least aggressively ablate) the mo-fo. The first and the last do have potential, provided the asteroid fits within very specific bounds, both in terms of velocity and mass. In between these extremes are a number of other proposals, which generally involve landing or otherwise attaching some sort of thruster or solar sail to the traveling body.
One noteworthy exception is to pull an asteroid with the gravitational forces of an orbiting satellite. This so called gravitational tractor method can, in theory, be scaled to work for a wide variety of object masses and velocities. The chief advantage this method has over the others is that geology is not really a factory. It comes down to mass and velocity. The other thing that is not a factor is spinning. Not only do most things from out there travel at hundreds or thousands of kilometers per second (yikes!), they are usually also spinning. This makes it very difficult to push or pull them in any direct sense. Oh, and they are often extremely dense - especially if they are of the primordial nickel-iron variety we seem to get in this solar system. The gravitational method provides a good theoretical method, but to put it in practice against very massive objects requires scaling the mass of the satellite for scaled effectiveness. Mass is the most expensive thing to put into space.
Let's just pause this train of thought and consider what we really want from a method for repelling asteroids. Here's a short list of qualities:
- Effectiveness can be scaled for wide range of asteroid mass/velocity situations.
- Does not have to physically attach to the (probably spinning) asteroid.
- Is not prohibitively difficult to get into space near the asteroid
- Is not just a barrage of nukes - which will break one threat into several thousand of them
Fortunately, just such a technology exists! It is the VASIMR engine being developed by Texas based aerospace company Ad Astra. VASIMR stands for variable specific impulse magnetoplasma rocket; basically it is an electric rocket that uses an advanced microwave oven to heat propellant gasses to in excess of 1 million degrees C (1.8 million F). The propellant exits via a magnetic field at something like 180,000 km/hr (112,000 mph). These engines provide moderate thrust when compared to chemical rockets, but can operate for hours, days, and weeks at a time, as compared to seconds or minutes. Since they use extremely high velocities, the mass they eject can be low for a given amount of thrust - that is to say, the rockets can be lighter weight for their performance because they are more efficient with their propellant. Even with a nuclear reactor or giant solar collector in tow, VASIMR engines provide a massive advantage.
Pushing a rock with a rocket
The engine will let you put a spacecraft in the vicinity of an inbound asteroid in record time. Now what? A would-be planetoid pusher would be built with a second rocket in its nosecone, which it could shine like a flashlight at its target. This flashlight (actually a beam of high velocity charged particles) would impart momentum upon the asteroid, regardless of the spin. Carefully aiming the system at the center of mass would provide the most effective push. The two-ended rocket system could be carefully held in position at the optimal pushing distance by balancing the thrust and vectoring of the engines, which work against each other and, in so doing, work against the asteroid. In theory, a double-rocket approach such as this can greatly outperform that of a gravitational tractor. The system can deliver continuous force from one side, rather than orbital pulsations of force over an elliptical path. It can also deliver impulses far in excess of any force due to the meager gravitational attraction between a rock and a spacecraft. This is especially true as the system is scaled.
How much better is it? Maybe you are a physics student or motivated physicist looking for a good evening project to chew on. I would love to link to some of your figures and calculations. Mine are messy and off the cuff. Please feel free to write! Anyway, happy simulating, and good night.