I wanted to clarify a few things about looking at Neptune and its rings
About the Neptune pictures I got just last week: here is an example of one
of the challenges of using HST. I was assigned three orbits for Neptune.
My plan was this: use one orbit to take a series of pictures in different
filters (colors). Then wait about 6 hours. During that time, the planet
would rotate about a third of the way around. Then use another orbit to
take pictures through the same filters again. Then wait *another* 6 hours
for the planet to rotate again, and use the final orbit to take the final
series of pictures. This way, I could map out the entire atmosphere of the
- Right now Neptune is the most distant planet in the Solar System.
It is an easier target than closer-in Pluto simply because it is so
much bigger than Pluto.
- If we look at Neptune we WON'T see the storms that Voyager saw, like
the Great Dark Spot and the Scooter. You see, they are GONE now! That's
what makes Neptune a special and interesting planet to study - its clouds
change dramatically very quickly. Those old storms are gone and new
ones have appeared.
- When I looked at Neptune in 1994 with HST, there was a NEW Great Dark
Spot - this is a different one than Voyager saw in 1989. We know it's
new because it is in the Northern hemisphere of the planet, but the
Voyager Great Dark Spot was in the Southern hemisphere. Where did the
old Great Dark Spot go? When will this new Great Dark Spot disappear?
And if we look again, will we see any NEWER Great Dark Spots?
- In addition to the Dark Spots, which are tricky to find, Neptune has
LOTS of bright spots which are easy to see. These spots are like the
Scooter, but are new since Voyager. I have looked at Neptune with HST
just three times now (once in 1994, once in the Fall of 1995, and then
just a week ago! See below). Each time, I have seen new and different
cloud patterns! You would definitely discover new bright storms on Neptune,
even if you only used one orbit for it. Guaranteed.
- No one has tried yet to take images of the rings and satellites of
Neptune with HST - it is hard! If we want to target Neptune's rings
and satellites, we will have to make some careful decisions, since you
can't take images of both Neptune's clouds and its rings/satellites
at the same time. This is because the rings and satelites are MUCH fainter
than the planet: you have to take long exposures to see them. But if
you do that, you overexpose the planet. So it is a choice. It may be
possible to do both during one orbit (take some long exposures for satellites/rings,
and shorter exposures for the planet). But we would have think about
our observing strategy carefully, picking the right exposure times and
That was the PLAN. Here is what REALLY happened. Orbit 1 executed perfectly.
Gorgeous pictures. Orbit 2 - perfect again. A whole new set of clouds!
Orbit 3 - disaster. Halfway through the orbit, the guide star lock failed.
This means that for some reason, the HST lost track of where it was pointing
(normally it has sensors "locked" on nearby "guide stars" to know where
it is pointed) . This is a potentially dangerous situation for the telescope,
since without knowing where it is pointing, it could accidentally point
at some bright star or even the Moon or the Sun - and that would destroy
the sensitive light detectors! So to protect itself, if HST "loses lock"
it automatically closes down the shutter. End of data.
So halfway through the Neptune pictures in the third orbit, the shutter
closed, and I got pictures of nothing. It is not clear why the lock failed;
these things just happen. So I did not get a complete map of Neptune.
I may or may not be able to find a Great Dark Spot with this data. If
I was unlucky, one may have been on the part of planet I was imaging during
that third orbit. Maybe I was lucky, though. I don't know yet....
That is what I am working on today - looking at the new pictures from
the first two orbits to see what is in there. It is very exciting to get
brand new data! And that's something you will know about in the Spring,
when your data is taken, no matter which planet you choose!
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