This planet is at some risk for the impact of an asteroid or comet. The last time a "big one" hit us, it was extinction for the dinosaurs.
I will be attending the First IAA International Planetary Defense Conference in Granada, Spain, the last week in April. That conference will look at ways and means to defend ourselves from this threat.
I am going because my paper from last fall was accepted for poster presentation. It concerns the potential benefits of adding electrostatic attraction to the "gravity tractor" concept, one of several possibilities.
For asteroid deflection defense overall, there are three serious ("killer") concerns as I see it:
(1) adequate warning time is beyond our technical means for the more numerous smaller bodies,
(2) no adequate propulsion exists to get out there quick enough to do any good,
(3) most of the few objects sampled so far appear to be loose rubble piles, not solid objects capable being pushed by most of the deflection force concepts.
Item (3) - the rubble-pile problem:
The gravity tractor is attractive for item (3) because it is a body force, and could "tow" a sand pile with the force of gravity as the tow cable. We already have the micro-thrust ion and plasma thrusters that could do this job.
The gravity tractor requires long lead times because the forces are so small. It takes a long time to build up deflection effects, which is why items (1) and (2) are such conceptual "killers".
My electrostatic upgrade to the gravity tractor provides a "tow cable" that might be 4 orders of magnitude stronger, for very little added equipment to the spacecraft. Getting more deflection faster eases somewhat "killer" items (1) and (2), and, it just might make possible a second attempt if the first fails.
Item (2) - the response-time problem:
In my opinion, item (2) can be solved by dusting off the old 1950's "Project Orion" concept for nuclear explosion propulsion. That's the nearest-term super-powerful atomic rocket concept available, and we did everything but put one together and test it, back then.
There is also the solid core nuclear thermal rocket, which we actually did build and test quite successfully about 1959-1972. It is a lot less powerful than the explosion approach, but still far better than the best chemical rockets.
The solid core nuclear rocket has a more powerful cousin, gas core nuclear, which was well-studied experimentally, but never built and tested as a rocket engine, about 1965-1972. With any of these, the idea is to get out there very quickly, with multiple options for deflection forces, and do whatever is needed, based on what you find when you get there.
Item (1) - the distant-detection problem:
We need some observation equipment dispersed out there in space, all around the solar system, to find and characterize these things. Immensely-powerful propulsion would make this possible in a realistic fashion.
The key to all of this, and much more, is therefore immensely-powerful propulsion:
We need that nuclear explosion drive, and the gas core nuclear thermal rocket, for a variety of compelling reasons besides asteroid defense. Our past history with solid core nuclear thermal rockets says that neither of these is safe to do, down here on Earth.
Space stations are no help. You cannot develop rocket engines hanging weightless in space, where every test is a flight test.
But, as if made-to-order by God for us, the moon waits, only 240,000 miles from home. Airless, waterless, and uninhabited, it truly is a safe place to test dangerous things.
I cannot think of a better reason to go back to the moon, than to test and develop those immensely-powerful atomic rockets.
G.W. Johnson 4-21-09