[ExI] Asteroid on track for possible Mars hit

Eugen Leitl eugen at leitl.org
Mon Dec 24 10:52:33 UTC 2007

On Sun, Dec 23, 2007 at 05:11:52PM -0500, Gary Miller wrote:

> If we have weapons that are 200 times more powerful, why would we limit
> ourselves to a 0.5 MT device?

We want to approximate a nuclear rocket, without actually landing one on the
rock. To do so you need a series of small ablation events, not
one huge thump which is likely to dissipate into heat of deformation,
or worse, fragment your problem, becoming a control nightmare (imagine
trying to do precision shots to tens to hundreds of wildly tumbling
fragments, in a dust cloud).

Also, you want to make multiple small rockets, each with a 100 kg payload,
due to the physics of rocketry, and due to hedging your bet (not putting
all nukes into one expensive basket, which might break).

We want to approach the theoretical maximum yield/kg (6 kT/kg)
without having to launch a lot of interceptor mass -- many smaller
rockets are a lot better than one big one. Mass-production
reduces costs, and you might be able to do a lot with solid-fuel
rockets, which don't need to be refueled on short notice.
(That's probably not going to work, though, so we're back to
cryogenic fuels, which would add many hours to days of launch
> If a blast of this magnitude was to occur at surface level.  How much rock

You'd lose most of your energy to deformation and fragmentation. 
I'm sorry Dave, I'm afraid I can't do that.

> mass would be converted directly to plasma.

Much less than in a series of surface-only events. (I made one
mistake: forgot dust. There needs to be a clear line of sight to the
surface on order it to absorb the xrays, and ablate explosively).
I think we should most definitely try to nuke a small asteroid,
or a comet, or both. Much will be learned in the process.
> Another possibility to further reduce the mass and force smaller
> fragmentation would be to hit it from both sides at once with two
> synchronized maximum blasts.

If you did it to a 10 km rock, you'd suddenly have hundreds
of problems instead of one.
> Would such a blast have any effect on the forward momentum of the remaining
> mass?

Why are you trying to make things ridiculously hard? The more smarts
you use, the less muscle you need. And we don't have a lot of muscle
right now. Half a gigaton is nothing when it comes to mountains or small
worlds hurtling your way. 
> The Mars asteroid is estimated to be moving 8 miles a second.
> Assuming we had the same 2 week notice we had for Mars and it took us a week
> to put in place our defense, deflection would probably not be option.

It's a small rock. It wouldn't do a lot of damage, unless it'd hit a major
city -- probably not even then, given that our atmosphere is pretty dense.
What we'd like to do is is to put enough observation power to plot civilization-killers
and have an infrastructure in place in order to launch immediately,
which can have months to years of forewarning.
> Would hitting it head on repeatedly (assuming it didn't break up) negate
> enough of the mass and forward momentum to make it significantly less
> dangerous.   

No offense, but you don't really understand the problem space. Maybe a
refresher in highschool physics is in order.
> Also most websites simply state the size as similar to the one that hit
> Russia (coliseum sized).  But is that considered a small asteroid, medium,
> or large?

We're looking at things which are marginally probable. It's rather unlikely
that a 500 km rock tries to fall on top our heads.
> In short if we were to plan for a realistic planetary defense what quantity
> of mass should we be able to vaporize?

Depends on how early you can spot it, and start deflecting it. If you catch
it really early a few N of thrust would be enough.

Eugen* Leitl <a href="http://leitl.org">leitl</a> http://leitl.org
ICBM: 48.07100, 11.36820 http://www.ativel.com http://postbiota.org
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