Bill,

I think you might make this more complicated then needed.

Most people dealing with asteroid files other than the ones part of
Guide are aware how to download the MPC files.

Yes make the Comet update available as it is in Guide format.

Here my feel on this,

in Extras..

1. button "Update Comets" - this will download the comet file and
update Guide.
2. checkbox "Display MPC Asteroid"
3. button "Select MPC Asteroid File"

action 1. - if user select checkbox "Display MPC Asteroid" to true,
button "Select MPC Asteroid File" is enabled and Guide asteroids are
turn off.
action 2. - if user select checkbox "Display MPC Asteroid" to false,
button "Select MPC Asteroid File" is disabled and Guide asteroids are
turn on.

Guide retains the selections on shutdown.
On start if Guide, with "action 1" above active, does not find MPC file
then Guide displays nothing.
if action 2 on start and Guide Asteroids not installed, Guide displays
nothing.

To summarize, we have 2 choices, first one is to display Guide asteroids
from the DVD if installed or access from the DVD.
The second option is to use MPC asteroids from a downloaded file of our
choice (MPC format).
No other asteroids except those in the selected MPC file are displayed
in Guide.

As for rebuilding the asteroid cache, I don't mind waiting a few seconds
to rebuild the cache when a large MPC file is selected , i.e. "MPCORB.DAT".

Other comments welcome,


Denis