Re: Please check my math

Wasn't it James Nicoll who wrote:
Situation: you spot a comet nuclear headed for Earth. If left
alone, it will impact in 12 months. Now, you don't necessarily need
to destroy it to remove the risk. You could deflect it slightly, so
the Earth merely suffers a near miss. If, say, you accelerated it
slightly for the entire 12 months, you could shift where it passed
by 100,000 km, say, with an acceleration of about 1x10^-7 m/s/s.

Unfortunately, getting most propulsion devices to a comet
is something of a challenge, particularly if you need to do it fast
enough to use most of the year to alter the comet's path. There is
something we could get there fairly quickly, something from an old
thread on space-warfare: an x-ray laser beam could reach the comet
in the time it takes light to cross the system.

If all we need is a 300 m/s stream of material and if the
comet is a trillion tonnes, then I _think_ being able to deliver
20 gigawatts to the surface should do the job. Of course, once the
comet's coma forms, x-rays will be absorbed short of the target
and since no process is 100% efficient, we'll need a larger
energy source than 20 GW.

Can someone who remembers the numbers from that old thread
tell me if it's reasonable to cast a spot 10 km or so wide at 10 AU?

I've not checked your math, but something else to consider is that our
atmosphere is fairly opaque to x-rays. So you'd want to place the laser
above the atmosphere. But then you've got another two things to think
about - how to get 20GW of continuous power to the laser, and what to do
about the recoil.

--
Mike Williams
Gentleman of Leisure
.

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