[X] My Assistant

[dvandorn]

OK -- before y'all get all het up over the topic title, let me emphasize that this is *my* new idea for asteroid defense. I want to know what people think.

My idea deals with the subset of NEOs that are rubble piles. I'm assuming that a rubble pile is made up of numerous small bodies ranging from sub-micron size up to pieces of solid rock as large as 20 or 30 meters across.

My idea is based on the concept that the Earth's atmosphere can handle the impact, over a period of days and weeks, of thousands of tons of meteorites without generating catastrophic atmospheric heating. The reason the entire mass of an asteroid will cook you whether it comes in intact or in millions of pieces is based on the concept that the entire mass enters the atmosphere within a very short time frame.

So -- if you can bust a rubble pile apart such that the rubble enters the atmosphere over a period of days, or weeks, and if you can push the larger frags away from impacting trajectories, you'd be reducing the overall impact of even a large-ish rubble pile. Depending on how much mass is in the entire pile, you could reduce the overall impact of the event to eliminate any serious threat to life on Earth.

So -- the idea is to choose a point in the asteroid's orbit where you can maximize the spread of the rubble into the largest ellipse possible prior to its impacting the Earth. You use whatever means is most efficient to effect a *relatively slow* disassembly of the rubble pile into this disperse ellipse. And here's the point that I don't think I've read or heard anyone come up with before -- you attach propulsion and attitude control systems to the largest remaining chunks and steer them into trajectories that are designed to 1) disperse the remaining rubble even further, and 2) push them onto trajectories that don't impact Earth.

This is why you want the *relatively* slow initial breakup speed. You use the gravity interactions between the large chunks in their planned traverses of the rubble to spread it all out to your specifications.

If you have a good decade to plan and implement such a defense to a given body, I think it might be one of the few strategies that could be done within our current technologies. It would be expensive -- you'd have to jet around within the initial debris field, attaching propulsion modules to the biggest chunks, and you wouldn't be able to design your large-chunk trajectories until after the breakup was effected. It would take a lot of energy for the maneuvering, and you'd have to have rather massive armor to jet around within the rubble field. But it's do-able with current technologies, if not easily.

-the other Doug

[JRehling]

It seems to me that given enough lead-time, the best approach is to nudge the thing off course so that it just misses the Earth. If you only have to nudge it by 4,000 km and you have lead-time of a year, then naively you only have to impart a delta-v of 0.13 m/s.

Naturally, for larger bodies, small delta-vs still translate into a lot of kinetic energy, but then, so does blasting an object apart. I'm sure that less kinetic energy is involved in a nudge than an explosion (which would naturally accelerate the individual pieces by much more than 0.13 m/s). A nuclear weapon might end up doing more nudging than blowing-apart regardless of our intentions.

For really long lead time (multiple orbits), I think the desirable situation is to determine on which upcoming pass the object would come nearest the Moon and then just steer it right into Luna, eliminating that particular threat permanently.

[nprev]

For really long lead time (multiple orbits), I think the desirable situation is to determine on which upcoming pass the object would come nearest the Moon and then just steer it right into Luna, eliminating that particular threat permanently.

Wouldn't that be a bit more difficult to accomplish in terms of guidance requirements given that the Moon's orbit is geo- rather than heliocentric? Seems as if the delta-V would have to be extremely precise, and I don't know of a way to make large changes in velocity with great accuracy. (Not so bad with mass drivers or ion thrusters, but of course these take considerable time).

My take on the whole thing is that, generally, any delta-V that takes a threatening asteroid out of an uncomfortably close encounter ellipse (guess "prolate spheroid with semimajor axis roughly aligned along the object's net orbital motion vector" would be more accurate ) is all that's needed. We can get fancy later when it's time to start mining the things.

BTW, that brings up a poser: What do we do if we find a threatening object that turns out to be very rich in metals or volatiles (long-period comets excluded because, uh, they'd exclude us in any case)? I'm thinking that we'd want to do more than just deflect it or destroy it, we should think about putting it in an accessible orbit for later use.

[Del Palmer]

BTW, that brings up a poser: What do we do if we find a threatening object that turns out to be very rich in metals or volatiles (long-period comets excluded because, uh, they'd exclude us in any case)? I'm thinking that we'd want to do more than just deflect it or destroy it, we should think about putting it in an accessible orbit for later use.

Good thinking, although it would be nice to study it up-close before we mine the heck out of it...

Regarding disposal a la luna, I don't think that would be compatible with expected future uses of the Moon.