Hi Pete,

As I write this, I see that Clay has directed you to the file
on the MPC site. Be advised: this will work if you aren't using the
updated Guide that I posted yesterday. That update will simply
download and use the required files when you use Extras...
Asteroid/Comet Options... Add MPC Comets/Asteroids... Click to
download updated comet data and add it to Guide.

I'd recommend just going with the revised Guide.

> I am usually very confused with MPC's website and had been accessing their
> http://www.minorplanetcenter.net/iau/MPCORB/CometEls.txt data from the comet
> link on their http://www.minorplanetcenter.net/iau/MPCORB.html page.
> Why would they have two different datasets?

For various historical reasons (I think), MPC has a whole slew of
different orbital element formats. There's an MPCORB.DAT format, which
works only for heliocentric elliptical orbits (i.e., no comets, and a
serious loss of precision for near-parabolic objects). There's an
eight-line format that is more human-readable and which can store
parabolic, hyperbolic, and non-heliocentric orbits. And an 80-column
orbit used on Daily Orbit Update MPECs. And there's the format used
in CometEls.txt... that's the one that isn't readable by Guide.

MPC has also kindly provided elements in the format used by Guide
and other desktop planetarium programs. At the time, I very much
appreciated their efforts in this area. Now, though, I wish I'd
suggested a single format that would cover all reasonable cases, to
avoid the current situation of bazillions of orbital element formats.
Though quite likely, a new format would have led to this situation:

https://xkcd.com/927/

-- Bill