[ExI] Asteroid on track for possible Mars hit

[Eugen Leitl]

On Sun, Dec 23, 2007 at 09:13:38AM -0800, spike wrote:

> > Doesn't it depend quite a lot on the composition of the mass?...
> 
> Dr. Graps might be able to answer this.  

A very interesting problem, actually. Would a rubble pile do worse than
a solid silicate or FeNi rock? What about ice with organic/silicate muck
on it? Nobody has nuked a rock in space yet. Hell, we should do it, just
for the sake of science.
 
> Hmmm, these are two very different things.  After doing a few BOTECs, I
> convinced myself that it would be impractical to try to match the velocity
> of an incoming rock: to get out to it, then stop, turn around etc, would

I don't see many reasons to do so. There are some suggestions to land
a ion or nuclear drive on a rock, but that'd take years for forewarning.

> required way more delta Vee than can be practically carried.  (Perhaps 30
> km/sec, which is crazy hard to do, don't know if it has ever been done.) It
> might be worth a trade study of 30 km/sec delta Vee with a relatively small
> fission device as Gene suggested, or 10 km/sec carrying the biggest boomer

I haven't done a BOTEC, but you'd almost certainly need several of them.
Maybe hundreds. That's quite good, since you wouldn't want to put all your
nukes in one basket, and it keeps the payload at 100 kg or below.

> we can make.  The payload capacity of a rocket heading to 30 is less than 1
> percent of the payload capacity of one heading to 10.  
> 
> So that means exploding the device, (which needs to be truly enormous) as

Not at all, you'd just need several of the small 50-500 kT ones (these
are tiny to small as fusion weapons go, though the higher-yield ones
might be not COTS in the arsenal).

> the object and the nuke pass at very high speeds.  What I don't know is how
> much time it takes for a nuclear explosion to develop, or to convert most of

The nuclear reaction is complete within a microsecond, or so. What we'd want
is xray hitting the surface from one side, causing an asymmetric ablation.
The whole assembly is a nuclear-powered rocket, with asteroid material for
working fluid.

> the energy from the fusion device into photons.  Is anyone here up to speed
> on that, or know how to estimate it?  Does that reaction develop in a
> millisecond?  10? 

I think the xray-bright plasma ball in a vacuum should be well developed within a ms, or, so.
This is pulled completely out of /dev/ass, since atmospheric tests
don't really apply, and I'm not familiar without out-of-atmosphere tests.
 
> 
> >   I would think this is even more
> > likely the less uniformly dense the object is and partially depending
> > on its primary composition.  A comet would be the most fragile of all. -
> samantha
> 
> Ja, a sometimes comet is described as a dirty snowball (Sagan).  I can
> imagine a scenario where a nuclear flash causes some of the photons to
> penetrate the surface and vaporizes the ice below the surface, pushing the
> material above it (on the nuke side) spaceward.  That would only apply to a

It doesn't have to be ice. Any material will turn to plasma if hit with
an xray flash.

> translucent object like an icy comet, methinks, so if the object is a rock,
> I don't see that it would break up, or if so, why or how.

The only danger if you're hitting the object with a stray fast nuke, especially
a large one. 

-- 
Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
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