Last Night I Talked to a Man from Outer Space
Only briefly, of course. He was probably on his way between planets. I caught him after a lecture he was giving at the Rose Center for Earth and Space.
I walked up to him and asked a question which went something like, "Would the ion engines have to be firing continually for the entire year?" He answered in some detail. I imagine people from Outer Space always answer you in some detail, on account of overlooked details probably kill you when you come from Outer Space.
He was there to tell us about his Secret Intergalactic Plan of Action for towing asteroids that were going to hit the Earth.
Some people think that you could blow up an asteroid that was heading for Earth, but the Man from Outer Space said that would be silly -- not unlike turning a bullet that was heading for you into a shotgun blast that was heading for you.
Other people think that you could attach a rocket engine to an asteroid and push it away, but the problem with that (according to the Man from Outer Space) is that asteroids aren't really one big rock the way most of us Earthlings think they are. They are actually more like piles of rubble held together by a relatively weak internal gravity. If you start pushing on them with an attached rocket engine, you might end up just breaking the pile of rubble apart instead of pushing it out of the way. Another problem is that most asteroids are spinning so if you are trying to push it out of the way with a rocket engine, you'd have to figure out a way to have the engine fire at only one very specific point in the asteroid's "spin cycle". The Man from Outer Space said this would be a very difficult engineering task.
Yet other people like what seems at first a rather elegant solution... attaching a space sail to the asteroid and letting the solar wind carry the asteroid away. This isn't quite as elegant as it seems, of course, since launching the materials for a big enough sail, and assembling it in space, is not so easy. And there is the bigger problem of the asteroid's "spin cycle". How do you attach the space sail to a spinning pile of rubble?
Then of course there are those who want to paint the asteroid white. The Man from Outer Space laughed, merely.
Our suggested alternative is to have the spacecraft simply hover above the surface of the asteroid. The spacecraft tows it without physical attachment by using gravity as a towline. The thrusters must be canted outboard to keep them from blasting the surface (which would reduce the net towing force and stir up unwanted dust and ions).
This scheme is insensitive to the poorly understood surface properties, internal structures and rotation states of asteroids. A spacecraft needs only to keep its position in the direction of towing while the target asteroid rotates beneath it. The engines must be actively throttled to control the vertical position as the equilibrium hover point is unstable. The horizontal position is controlled by differential throttling of engines on opposite sides of the spacecraft. The spacecraft can be made stable in attitude by designing it like a pendulum, with the heaviest components hanging closest to the asteroid and the engines farther away.
I'm with the Man from Outer Space. Whatever he says goes, as far as I'm concerned. First, cause he's the Man from Outer Space and second, his secret plan of action makes sense to me. It has the elegance I demand from all my planet saving schemes. Elegance is the sine qua non of all your really high-class planet saving ideas, see. Ask any Man from Outer Space. He'll tell you.