"Twists and Turns in Scheduling a Look at Pluto" Marc Buie - January 31, 1996 Planet Advocate for Pluto |
The observing plan is in at STScI and its planning and scheduling is proceeding.
If the plan I submitted is good, then I would not expect to hear anything until the
observations are actually firmly scheduled on calendar. Once scheduled, I should get
notice of when the observations will take place. So, what did happen? Well, I didn't hear
much back at all and I took that to mean the observations were no problem to schedule and
everything was working smoothly. Unfortunately, that wasn't quite the case. Oh, it's
nothing really terrible and we will get our pictures of Pluto but several peculiar twists
came out of the attempts to schedule the observations.
If you remember my description of the observing plan, I figured out the times when
Pluto would present the same hemisphere as seen during the previous set of observations.
I also computed how long the window of opportunity was for each time. Then, I prioritized
them according to which hemisphere I thought would be most interesting. This is a sound
plan and would have worked just fine but for one minor flaw. The window of opportunity I
gave was about 2 hours long. In 2 hours, Pluto rotates about 5 degrees of longitude. I
figured this was long enough a window to permit scheduling the observation while
minimizing the amount of rotation possible between the new and old pictures. It turns out
that this duration is just a little too short. You may remember, an orbit of HST takes
about 94 minutes during which Pluto can be seen during about 45 minutes. Here's a crude
graph that should illustrate the problem:
Remember that the observation must fit in one orbit of HST. Each orbit has a time
where the object can be seen (o's above). That must then fit within my window. An orbit
lasts 94 minutes and a viewing window lasts 47 minutes. That puts a minimum length of 141
minutes on a window to make sure it will fit. My 120 minute window in this case starts
too late for the first orbit and finishes too early for the second orbit in the worst
case. You can see that if the window slides to the right or left then there can be
instances where this will work, but it won't always work.
This sort of oversight is easy to make and normally I would get feedback from STScI
that I goofed. At this point, the processing of this observation began to depart from
normal operating procedures. Instead of getting a message that I goofed, window #2 on my
list (March 4) was chosen for HST even though it didn't fit. Why #2? Well, it took quite
a few e-mail messages back and forth with Tony Roman and Alex Storrs to get to the bottom
of the problem. I thought that #2 was picked because #1 didn't work. As it turns out, #1
didn't work but neither did #2. They should have continued down the list until they found
one that would work.
What happened? Well, this observation is being planned specially for the Live live
broadcast to experience our first look at the data. Can you imagine what would happen if
for some reason the observation failed and there was no new picture of Pluto? There are
lots of ways that an observation can fail and it does happen more often than you would
think. The scheduling committee at STScI decided (without consulting me) that they would
put the observation as early as possible so that there would be time to repeat the
observation if the first try failed. The first possible day was March 4 and I happened to
have a window on that day so it was chosen.
After all this had already happened and the observation was scheduled, I was contacted
to see if the mismatch between my window and the Pluto observing window would harm the
observation. Unfortunately, this notification happened too late to make any changes to
the schedule. Anyway, I've since figured that the slight mismatch probably won't hurt our
observations and I really should have made my 2 hour window just a little bit longer. In
the end, we should get what we wanted but it happened in a strange way.
These experiences I've just outlined detail all of the discussions I had with STScI
about the planning and scheduling of the Pluto picture. You might be wondering what else
has been going on. After all, it's taken me far too long to get around to writing this
stuff down. That's the subject for my next journal entry.
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