[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

[dvandorn]

That's exactly the point I was about to make, tty. My concern is more for getting extremely accurate predictions of the trajectories of bodies on parabolic (and heck, even hyperbolic) trajectories. The ability to predict a trajectory after perihelion is affected not only by even minute errors in our knowledge of the pre-perihelion trajectory, but also by myriad factors including gravitational perturbations from Mercury, Venus, and even Earth, as well as the current state of solar weather. An active Sun blows more mass out into near-solar space than a quiet Sun, thereby increasing (minutely) the drag such an object will encounter "coming 'round the horn".

It's also possible (if not likely) that a rather dim object, a km or more in longest dimension, could even now be wandering in on a relatively steep fall to Sol that is targeted for a pretty wide perihelion, a fairly minimal solar deflection, and a trajectory that would cross Earth's orbit (where Earth might happen to be located at the time) on its outbound leg. Such a body could be dim enough that it wouldn't be easily spotted if it sweeps in from a vector roughly opposite that of the Sun from Earth (i.e., spending a lot, though not all, of its of time hidden in the glare), and/or, even if it's spotted, not being noticed until it's too late to get a great hack on its trajectory prior to its disappearing completely behind the Sun.

Yes, Greg seems to be quite correct, that once we get some of these telescopes up and running we'll have enough observing time/power to find most outlying threats with lots of time to spare. There's a short window, though, in which an object could announce itself with only days or weeks to spare. (Of course, if something did happen in that window, it's not like we have the deployable technology at present to do anything about an object of any size, even with a few years of warning... *sigh*... )

-the other Doug