Hi James,

Quick fix first: download this file to your Guide folder

http://www.projectpluto.com/comets.dat

(about 160 KBytes), overwriting the copy currently there.
This will simultaneously give you up-to-date comet information,
including some not available through MPC; _and_ it will clean
out any NEOs or critical-list objects or other asteroids that
are lurking in your current 'comets.dat', masquerading as comets.

That's the fix. Now to describe why this bug is happening :

This is an issue that has arisen because the MPC's list of
NEOs and critical list objects have grown a _lot_. In the past,
Guide did a passable job of downloading NEOs and critical-list
objects, then merging them with the general asteroid data. I've
modified the "add MPC comet/asteroid" dialog to include a disclaimer
that you almost certainly _should not_ download NEOs and critical
list objects. (The better solution is to use MPCORB.DAT; this
replaces _all_ asteroids at once, so you don't get the weirdness
that inevitably comes of trying to mix heterogeneous datasets.)

The fix I hope to offer shortly is to skip merging entirely.
Guide will instead have options to download NEOs and critical-list
and distant objects, and then display asteroids using those lists...
_and not showing the remaining asteroids_. That is, if you tell
Guide to show the current MPC's list of NEOs, you'll get NEOs,
not NEOs plus everything else.

I really should have done this from the beginning. Among other
benefits (such as working properly and not leading to bugs such as
the one you ran into), it will cause Guide to run faster. There
are far fewer NEOs than there are asteroids in general; if Guide
is just showing NEOs (or critical-list or distant objects), the
display time will improve by a lot.

-- Bill