From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.science
Subject: Re: Effect of Asteroids On Trajectories
Date: Mon, 18 Mar 1996 16:58:38 GMT

In article <826870169snz@jk-o.demon.co.uk> Comp2002@jk-o.demon.co.uk writes:
>  ...When a clever slingshot type of trajectory is calculated
>  for a mission to Jupiter or beyond, to what extent does the Asteroid
>  Belt come into the equation?

Not at all, except insofar as one often looks for asteroids near the
trajectory, in hopes that a small expenditure of fuel could add an
asteroid flyby to the mission.

>  Obviously a calculated orbit must be checked as a matter of routine 
>  against those of X thousand asteroids held on computer!

Nope.  No need to bother.  The asteroid belt is 99.999...% empty space.
On a completely random trajectory through it, the chances of hitting
anything large are miniscule.  Asteroid encounters have to be arranged
deliberately.

There was some anxiety, during mission planning for Pioneers 10 and 11,
about the uncertain density of smaller particles.  In fact, the Pioneers
carried scattered-light instruments to detect dust grains and larger
particles passing nearby, in hopes of getting the first solid data about
the density of small stuff.  There was some concern that the spacecraft
might be damaged if there were a lot more small particles than expected. 
As it happens, there was no damage, and the instruments reported that the
actual density of small particles was less than expected. 

>  ...Or are space probes
>  carefully directed through Kirkwood gaps to minimize the risk of 
>  collision? 

The Kirkwood gaps are rings at specific radii from the Sun -- you can't
fly outward (or inward) along them. 
-- 
Americans proved to be more bureaucratic           |       Henry Spencer
than I ever thought.  --Valery Ryumin, RKK Energia |   henry@zoo.toronto.edu