# [ExI] Asteroid on track for possible Mars hit

Eugen Leitl eugen at leitl.org
Sun Dec 23 21:16:45 UTC 2007

```On Sun, Dec 23, 2007 at 12:27:45PM -0800, spike wrote:

> Reasoning: the idea was, assuming we need to nudge the object a few m/sec to
> the east, to fly directly toward the object, then set off a nuclear blast
> while as close to and directly west of the object.  Then the west side of
> the rock would be hot and would ablate to space.  Conservation of momentum
> would push the object to the east.

Right.

> But then I realized this scenario makes some possibly unreasonable
> assumptions.  The most important assumption is that the rotation rate of the
> object is slow.  If this is not the case, then we cannot expect the object

The ablation event is less than second. Possibly, much less (10-100 ms).

> to be deflected east.  Assume we are able to guide the device spatially with

Plural: devices. A single nuke won't do a damn thing.

> perfect precision and detonate the fusion device temporally with perfect

We're talking about an object at least 100 m across, possibly ~km or more.
You don't want to hit it square, and especially detonate on surface.
Flyby at few 100 m height is desired. It's tough, but IMO doable. Timing
is completely trivial in comparison, at few 10 km/s a nanosecond or
two is not important (even at 1 ms at 10 km/s you've just traveled only
10 m).

> precision, the west side of the rock heats up as starts ablating violently,
> carrying momentum west.  One half a rotation later, which might be one
> second, that same hot side is facing east, pushing the object west.

The side is not hot at all (you could touch it with your bare hand, immediately
or very soon after the blast). The ablation is explosive, takes less than a second,
and carries away the momentum with gas. The surface cools down to ambient (either
stone cold or scorching hot) temperatures within less than a minute after the
event. You could hammer the thing every minute, or so, if that's what you
want.

> So now the object is following a decaying sinusoid.  The first wave is
> likely the largest, so it is deflected *slightly* east, but I thought of
> another scenario based on the reverse thermal wave model.  This might apply
> here, I don't know, but it could be that there are more volatiles under the
> surface of the rock than at the surface (perhaps sorta related to Amara's
> comment about comet P/Holmes brightening suddently.)  This could cause the
> *second* hump (the negative side) of the first sinusoid to be larger than
> the first, which could actually deflect the rock to the west.
>
> Oy.
>
> But I have another idea, which would intentionally break up the rock,
> without a nuke.

You'd need a nuke. Nothing else packs that much wallop in a small package,
and even then you'd need many of them, for a nontrivial sized rock.

> More later.  I need to ponder deeper and do some calcs.

Much more interesting: when you nuke a rock in space with a 0.5 MT device,
how much will you flash-vaporise, and just how much momentum does that carry,
assuming the stuff is 3-6 kK, and how much would that deflect
the impact, given few weeks until impact? Modelling the gravitation well
of the Earth for capture analytically sounds like quite a bitch. A simulation
is perhaps in order.

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