Hi Kevin,

One problem you may have encountered is this: "probes"
are controlled as if they were "asteroids". That is, you
have to go to Display... Data Shown, and turn asteroids
On. (Or to a really, really faint magnitude limit, since
you're looking at really, really faint objects.)

Obviously, they ought to be controlled as "artificial
satellites". Something for me to fix... as I write this,
I see a possible way of fixing it that shouldn't be too
unpleasant to do.

Adding interplanetary probes requires doing essentially
what Jari described. I'd add only the following comments:

"...The other probes have additional information but
Guide does't seem to mind the truncated version." The
added data provides a magnitude and (optionally) the
ability to specify the time range of the elements. That
allows you to do things such as prevent the probe from
being "seen" before it was actually launched, or to keep
it from showing up after it hits a planet or comet or
some other solid object. Also, you can specify that the
elements are actually referenced relative to some other
object. (For example, there are some heliocentric
elements for Cassini prior to its arrival at Saturn.
Then it switches over to "Center: 6", i.e., Saturn-
centered elements.)

To get EPOXI/Deep Impact to appear over its entire life
span, from launch on 12 December 2005 to its arrival at
P/Hartley in 2010, you may want to download the following:

http://www.projectpluto.com/epoxi.zip

and unZIP it in your Guide folder. Then take the eight
lines from 'epoxi.txt' and append them to 'probes.dat'.
Add eight to the number at the top of 'probes.dat'.

This provides one long set of heliocentric ephemerides,
plus some geocentric ephemerides near launch and at the
times of the three Earth encounters. That's why eight
lines are required: there are seven time intervals
(launch, launch-to-first-encounter, first Earth
encounter, first-to-second-encounter, second Earth
encounter, second-to-third encounter, third Earth
encounter, and the entire period after that final
Earth encounter until it reaches P/Hartley.)

Getting the Horizons e-mail system to generate what
you want usually takes a bit of trial and error. But
it'll usually work, eventually!

-- Bill