🚀 go-pugleaf

In article <STEINLY.92Oct27103531@topaz.ucsc.edu> steinly@topaz.ucsc.edu (Steinn Sigurdsson) writes:

   [about Swift-Tuttle vs. other comets, stuff deleted]

   Probably can, but most will be dominated by Jupiter perturbations,
   Swift-Tuttle doesn't look like it's going anywhere (caveat: this is
   guesstimate from eyeballing the orbit parameters, somebody will
   actually have to find out for real bu integrating the sucker),
   ie it will keep coming by until it hits as far as I can tell, but
   that's most probably a few million years off...

I keep reading this, and have a couple of questions to ask:

1. S-T crosses Earth orbit twice every 120 years. The risk of
encounter is something very small, like 1e-6. So the expected time
until impact is 6e7 years. So any concern about S-T should also apply
to the bodies in the ecliptic, which I assert have an (order of
magnitude) similar expected time before impact (because there are far
more of them).  Now obviously this calculation depends on my `1e6'
assertion, but I can defend that (brief handwave: in order to hit, S-T
has to be at the right angle and distance from the sun when it crosses
the ecliptic. One can visualize the possible positions of S-T (as
perturbed by outgassing) at that point as a long oval. The length can
be judged from the fact that S-T was 17 days late this time, so S-T
can be perturbed between orbits by times of 17 days. 17 days = 27
million km at Earth orbital speed, 73 million km at S-T orbital
speed).

2. On any encounter, it's much more likely that S-T will pass very
close to Earth (say, within 10 radii) than actually hit (could be a
cool display :->). Wouldn't such an encounter radically change S-T's
orbit?  Could it throw it into the ecliptic? OK, whatever orbit
results will be an earth-encounter one, but what are the chances that
it will also be, say, a Jupiter-encounter one (i.e. an ecliptic one),
throwing it into the planetary bar-billiards game (and therefore
equally to be accounted with the other ecliptic bodies)? A simplistic
momentum-conservation argument occurs to me, saying "no chance, S-T's
ecliptic-perpendicular momentum will be conserved," but I intuit that
that is flawed (but can't see why).  Anyone want to help me out with
more handwaving? Anyone got any PD software (for Unix) that lets me
play planetary bar-billiards?

Nick Haines nickh@cmu.edu