Planet Advocate Heidi Hammel's responses

Hi everyone,

I wanted to clarify a few things about looking at Neptune and its rings and satellites.

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 satellites 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 filters.

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 planet.

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!