From: rclark@lpl.arizona.edu (Richard Clark x4971)
Newsgroups: rec.arts.sf.science
Subject: Re: asteroid impact
Date: 10 Oct 92 12:25:35 GMT
Organization: Lunar & Planetary Laboratory, Tucson AZ.

In article <tim.718642095@mik.uky.edu> tim@mik.uky.edu (timothy c huesman) writes:
>
>    Yeah, it's science fiction, and although possible, I wouldn't bet
> on the success of 'aiming' a missile from earth.  When Halley'd comet
> made its last pass, there was plenty of warning and they MISSED with
> flybys.  Hmm as one gets old the memory fades.  The comet was other
> than Halley's...not that it matters much, they still missed.  I'd
> put a better chance of success from a space-launched missile or 
> 'placed' nukes set in place by people in a vehicle matching velocities
> with said asteroid.
>
>--
>
>  Doc  Pan       tim@mik.uky.edu          Tim  Huesman


What do you mean by missed? There have been no attempts to impact or land a
probe on a cometary nucleus thusfar although such have been discussed.
The Europeans were careful NOT to fly Giotto to close to Halley--they hoped
it might survive the encounter.
There was some uncertainty about the exact location of the nucleus. This was
important both for camera pointing and for getting the probe as close as
possible while maintaining a reasonable probability of survival. The properties
of dust particles in the coma was also a bit uncertain.

Occasionally asteroids occult stars. Plans for groundbased observations of
these events start out with a lot of slop in the ground path. Continuing
astrometric measurements of cataloged asteriods allows the 'discovery' of
an upcomming event with 1-2 years advanced notice. Ground path errors are
on the order of continental. The occultation circumstances are refined as
the event nears until a couple of weeks before the big night (when the star
and asteroid can be photographed on the same plate) the uncertainty is down
to perhapse 100-200 km. This is as good as it gets with ground based optical
observations.

This is the uncertainty of the relative positions on the sky of the star and
the main belt asteroid. If the object were important enough to want the range
it might be determined from radar, someone else will have to take that one:
How far out can we get returns from a 10, 20, 100 km rock? snowball? Don't
need anything fancy like the radar studies of Venus' surface, just range and
v.

Of course a comet has that pesky coma which conceals the actual nucleus.
Our comet killer has to fly into a fog bank to find the nucleus on its own
and home in. Halley's nucleus wasn't detected photographically until Giotto
was within 1000 km or so. Of course it could have been detected by radar at
considerably greater distance but Giotto wasn't so equiped.

We would know the orbit of the Hammer better on the pre perihelion portion
than post perihelion because the active nucleus causes lots of non
gravitational perturbations. In any case suppose tha panic criterion is
achieved and General Sagan presses the button. The actual intercept is
a simpler than what's invisioned for the SDI kinetic kill vehicles.
Encounter velocity between probe and comet could be something like 50-60
km/s since we don't have the propulsion systems to do the intercept out
more than a few e6 km. Even with 3 months warning we can't intercept out as
far as 1.5 au (mars' orbit). So the probe's radar picks up the nucleus at
50000 km and has 10 minutes to home in. Since JPL is working with NORAD on
this one the cross range error shouldn't be over 100-500 km (the nucleus is
already quite active even if its pre perihelion so the opaque coma makes
radar necessary for terminal guidance--optical navigation during the day
prior to encounter doesn't help that much). If we can scale up a Brilliant
Pebble to carry a nuke on a short interplanetary mission it should be quite
simple to make the kill. There won't be much chance to go for the deflection
by optimal placement of the nuke but disruption of a comet nucleus should
be fairly easy.

The standard situation in which interception of an Earth threatening object
is actually feasible is when it is noticed that a recently discovered Earth
Crosser will come uncomfortably close in June of 2265. In this case the
deflection of a moderate size solid rock is quite possible. There is even
room to get fancy and assure that it WILL impact the moon! What a show that
would be!

Richard Clark
rclark@lpl.arizona.edu