Re: 100 megaton bombs atop Saturn V rockets

From: John Schilling (schillin_at_spock.usc.edu)
Date: 07/28/04 

Date: 28 Jul 2004 16:12:45 -0700

henry@spsystems.net (Henry Spencer) writes:

>In article <cd1out$4jl$1@spock.usc.edu>,
>John Schilling <schillin@spock.usc.edu> wrote:
>>>of course, we don't *yet* have the near-Earth asteroids and short-period
>>>comets mapped well enough to be sure we'd have plenty of warning for them.

>>Is it even possible to map those well enough to have decades of warning
>>with enough precision to warrant and plan for a diversion effort?

>Yes, definitely. It may not be possible, decades in advance, to say
>firmly that an object *will* strike Earth, but the precision of tracking
>and orbit prediction is sufficient to say whether there's a significant
>probability of it. If I recall correctly, currently there is only one
>well-tracked object with any noticeable chance of an Earth impact, and
>that's a fairly small chance during an encounter eight or nine centuries
>from now.

Yes, Louis Scheffer cited a very good reference for that one.

But it's the opposite problem, adding nines to the certainty that an
object will *not* strike the Earth. It's not entirely clear what this
implies when we're looking at the middling-high end of the probability
range for actual impactors, and I'm looking for something better than
extrapolation from the "Asteroid X will almost certainly but not
absolutely certainly miss Earth" type analyses.

>>If you can't pin down the CEP to an Earth radius or so, any attempt at
>>diversion is as likely to cause an impact as prevent one...

>Only if the error ellipse of the encounter is so large that the diversion
>can't shift it much. If it's currently centered on Earth and you move the
>center off Earth by a fraction of the ellipse's width, then you haven't
>really improved things. But if you move it by double its width, then
>Earth is no longer inside the range of likely trajectories at all. And
>these ellipses are often quite long and narrow -- much of the uncertainty
>is usually on one axis -- so if you move it in the right direction, the
>move doesn't have to be large.

Right, I hadn't considered the case of arbitrarily skinny error ellipses.
That is an important insight.

OTOH, some of the proposed long-term diversion methods, such as "paint it
black/white and let radiation pressure/Yarkovski effect do its thing",
tend to generate diversions along the long aspect of the error ellipse.
Oops...

And while the perpendicular diversion at least negates the "might make
it worse" aspect of long-term diversion, there's still the troublesome
economic and political questions involved in getting commitment to a
major effort to deal with a low-probability, long-term threat.

>>And even with arbitrarily good data, computers, and models, there's
>>a chaotic element to perturbation effects on asteroid orbits that
>>will prevent you from predicting the trajectory to within 1 Re
>>arbitrarily far into the future.

>Correct, but at least for the currently known objects, the realm of
>reasonably accurate prediction -- given good tracking data -- extends out
>a century at least.

Hmm. The best guess I get, working backwards from the error budget in
the 1950DA paper that Scheffer cited, is that you can only get single-Re
prediction accuracy single-digit years into the future.

But there are a lot of reasons why simple interpolation from the long-term
1950DA prediction might not be the best model for this sort of thing.

>>...the question of whether one can then actually say, "We've
>>now undertaken further study of asteroid XYZ-123 and determined that yes,
>>it would be a Good Thing to nudge it by 1 Re thataway", reliably, twenty
>>years in advance, has not to the best of my knowledge been rigorously
>>addressed.
>>Have you seen anything in that regard?

>Not a general study, no (with the caveat that I haven't really gone
>looking).

That's what I was afraid of. People prefer to do their research on
real subjects, and all (known) real objects are of the "will almost
certainly miss the Earth" variety. So the research is on expanding
the timescale and increasing the precision of the miss predictions,
which isn't quite the same thing. 

-- 
*John Schilling                    * "Anything worth doing,         *
*Member:AIAA,NRA,ACLU,SAS,LP       *  is worth doing for money"     *
*Chief Scientist & General Partner *    -13th Rule of Acquisition   *
*White Elephant Research, LLC      * "There is no substitute        *
*schillin@spock.usc.edu            *  for success"                  *
*661-718-0955 or 661-275-6795      *    -58th Rule of Acquisition   *

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