[ExI] (NASA.gov) NASA to chronicle close Earth flyby of asteroid (fwd)

spike spike66 at att.net
Sat Feb 16 22:17:02 UTC 2013

-----Original Message-----
From: extropy-chat-bounces at lists.extropy.org
[mailto:extropy-chat-bounces at lists.extropy.org] On Behalf Of Anders Sandberg
Sent: Saturday, February 16, 2013 11:58 AM
To: extropy-chat at lists.extropy.org
Subject: Re: [ExI] (NASA.gov) NASA to chronicle close Earth flyby of
asteroid (fwd)

On 16/02/2013 16:32, spike wrote:
>> ...OK, assume it into existence.  For the sake of thought experiments, 
> assume any advance warning you wish, and any existing or feasible 
> future rocket you want.

>...Any advance warning? Now you are making things too easy, Spike!...

Hmmm, I would cheerfully have it so, sir.  Read on please.

>...You just need to nudge the asteroid away from a collision keyhole, and
that can be done with a mere kick if you have enough time...

I agree you need only a small delta v.  The challenge is in getting even
that.  But don't give up:

>...The displacement goes up linearly with time, except for near Earth
passes where it can be amplified a lot...

Agreed, however...

>...So have your rocket just hit the asteroid straight...

Consider the path an incoming asteroid and the path of any earth-based
interceptor that is at any realistic distance from earth.  The impact you
really need is side-on, but the one available to us is almost straight
head-on collision.  If you can hit it far enough out there, I agree you can
get some side momentum, but any scenario I have been able to derive provides
very little momentum perpendicular to the path of travel, single-digit

(Keyholes are fun. They are the pre-images of Earth along a transversal
manifold: go through one, and your orbit will hit it next time. Besides the
1-orbit keyholes there are pre-images of pre-images, producing smaller and
distorted keyholes for more remote collisions. They form a Cantor-set like
fractal along a curve in the manifold. Aim your kick so that you miss the
fractal - possibly making use of the infinite subset of repelling periodic
points embedded in its border - and you are safe. )

Looking at the graph on
suggests that if you only have a few days, then either you must already have
nuclear defenses in place in space, or you need to do duck and cover civil
defense. Figure 2.2 in
http://isulibrary.isunet.edu/opac/doc_num.php?explnum_id=142 has more
detail, and doesn't assume an impressive space shield (I love the option
"continental relocation")

>>    ...  If the nuke is more than a 
> few meters away at detonation, the rock would scarcely notice, and 
> even if we manage a perfect detonation, it isn't clear to me it would
break up the meteoroid.

>...Yes, nukes in space are surprisingly wimpy. But I think you are too
pessimistic about detonation precision: typical space velocities are on the
order of several kilometers per second. So a millisecond error is a few
millikilometers - a few meters...

Doh!  Ja you are right.  I am remembering now where my intuition came from
on that.  Robert Bradbury and I were really pondering this about a dozen
years ago, back in the timeframe when you were out here for I think Extro4.
I need to consult my green notebooks to find when we were doing that.  We
did figure out that uncertainty of a few meters, but I wasn't sure that
would be good enough, since it isn't entirely clear to me how you would
detect when you are getting close to the target.  Any ideas on that?  Would
radar return be good enough?

Here's where I left it back when Robert and I did our brainstorming sesh: I
had a scheme whereby the device is detonated on impact by orienting a
trigger rod oriented forward which attempts to det the nuke upon physical
impact with the rock.  It isn't entirely clear this would work, but it would
be based on the notion of having a rod with an electric signal cable going
out to the end of the rod.  The device is armed by extending the rod when
impact is a few thousand milliseconds away, then as soon as the forward end
of the rod impacts the rock, the circuit's conductive properties suddenly
change (not necessarily open, since the impact site is a plasma region,
which is highly conductive.)  The change triggers the high explosive, which
triggers the plutonium core which triggers the tritium, which *might*
deflect the asteroid in a remarkably uncertain way. 

I can imagine a telescoping on-orbit deployed forward rod.  I don't know the
details, but I think that process from trigger to tritium ignition can all
occur inside a millisecond, so a 20 meter rod should be sufficient, and we
can do a 20 meter rod without too much trouble.  So now the real challenge
is in getting close enough so that there is no transverse corrections needed
in the last few thousand milliseconds, after the rod is deployed forward.

>... You don't want the detonation to be on the surface if you aim for heavy
deflection: the blast causes surface ablation that does the actual pushing.
Groundbursts apparently cause a jet of faster but lighter stuff, imparting
less momentum...

Ja, we are looking at a burst perhaps 10 meters from the surface.  There are
variations on the theme as well.  We could perhaps fire a projectile from
the device toward the rock (or even several projectiles) with a Don't
Explode signal radiating from it.  As soon as those signals stop, we know
the projectile has hit the rock.  We can calculate (very quickly) how long
it has been since the projectile was fired, and figure out how far the
device is from the rock.  This might give single-digit millisecond warning
of impact, which would allow detonation a few tens of meters away.

>...The big problem is that nobody knows how asteroids do plastic
deformation in the case of trying to nuclearly disrupt them: gravel piles
can just mould themselves into a new shape. And they can easily just turn
into a vast cloud of impactors that will now damage a large part of Earth
instead of a small corner. Some simulations I have seen produced very
impressive messes. -- Anders Sandberg,

Ja perhaps, but my intuition tells me a pile of rocks scattered far is
better than a single rock in every case regardless.  I could be wrong on
that, considering that most of the planet is wilderness and ocean.  The
chances of an asteroid hitting anything important is very low, but that bit
about the injury of a kilo-commie from broken windows from a sonic boom is
intriguing.  Consider that if this is a typical century-event, only in the
last couple centuries did humanity see anything like that; in all previous
history of mankind, a sonic boom would likely have been harmless to the


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