Hi Bernd,

Yes, this is definitely a bug; I was able to get the
same behavior. I eventually figured out that it happened
whenever I went to an asteroid, then a comet, then back
to the same asteroid; that last 'go to' would fail. But
if I asked for a _different_ asteroid, it would work.
I've fixed the problem, and that will be reflected the
next time I post an update. Thanks for the bug report.

-- Bill