From: Doug Jones (random@qnet.com)
Date: Sun Jun 25 2000 - 00:07:30 MDT
"Terry W. Colvin" wrote:
>
> Science Frontiers, No. 127, Jan-Feb, 2000
> ...
> Not to worry, say the modern-day Technocrats, we will launch nuclear-armed
> rockets that will nudge such cosmic threats into harmless trajectories.
>
> These Pollyannas are presumptuous. They assume that asteroids are hard,
> cohesive objects that will be shoved aside by a few megatons of explosive
> energy. There are two things wrong with this idea, and these reveal how
> radically our ideas about the nature of asteroids have changed in just
> 10 years.
>
> To make matters worse, asteroids like Mathilde are stickier than a cloud
> of buckshot. This fact is deduced from photos of asteroids showing many
> to be marked by huge craters. (Mathilde has one 33 kilometers wide.)
> K.R. Housen et al, using laboratory tests and scaling data, argue that
> asteroid craters were not blasted out by collision. (Mathilde is not
> "shattered" as one would expect given such a huge crater.) Rather, the
> craters are "dents" instead of holes! Cosmic rubble piles are like
> sponges. Collisions with other rubble piles result in compression of the
> target surface and accretion of the smaller object. In effect, asteroids
> are energy absorbers and will hardly be fazed by a nuclear detonation.
True but irrelevant. The total delta-V needed to divert an object from
collision given 20-30 years advance notice is only a few millimeters per
second. This impulse would be delivered, not by a cratering surface or
subsurface explosion, but by a series of many blasts, each at an
altitude of .4 radius from the surface. The neutron and gamma radiation
would heat and spall off a 20 to 30 cm thick layer at modest velocity,
delivering a relatively gentle push to the asteroid. That a rubble pile
object absorbs energy is beside the point- the material ablated away by
the blast will impart *momentum*, which is the real purpose.
The author of this piece hasn't done his homework, and is engaging in
alarmist disinformation. The use of the words Technocrat (capitalized,
no less), pollyanna, and presumptuous is obviously pejorative. I wonder
at the editorial slant of this "Science Frontiers" publication, and I
give it little credence.
-- Doug Jones Rocket Plumber, XCOR Aerospace http://www.xcor-aerospace.com
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