On Sunday May 11th, at 6:53 PM, looking from my backyard, the sun will still be appealingly gleaming above the western horizon, with almost an hour to go before it sets. Almost straight overhead the almost-first quarter moon will be waiting to steal the show as soon as the sun is gone. But I won’t be looking at either one. My eyes will be focused just above the eastern horizon where my current favorite outer solar system object – 2003 EL61, better known as Santa – will just be rising. OK, so I won’t see anything but blue sky; even at night Santa is about 10,000 times too faint to see with the naked eye. But I’ll be looking that direction thinking about the fact that at that moment the Hubble Space Telescope will be joining our hunt for moon shadows. On May 11th and then four other days over the following two week period, the telescope will come around the earth, swing towards Santa, and snap a quartet of pictures to help us determine precisely where the small satellite (aka Blitzen) is.
This is good news! Without the Hubble we feared that it would be another year or two before we figured out the orbit well, and in that time it was quite possible that shadows of Blitzen would no longer be falling on Santa. The case that we made in our emergency plea to use the telescope must have been compelling; within two days of sending in the proposal we had heard back that we had been approved. But there was some bad news, too. Hubble is approaching two decades in space, so things sometimes fail. Visits by the space shuttle continually fix the Hubble back up and add new capabilities, but with the space shuttle fleet itself barely limping along, Hubble has gone without a visit now for more than six years (a new visit is scheduled for late summer). In that time some of its gyroscopes have failed.
Gyroscopes are critical on a spacecraft like the Hubble, because they keep track of which direction is which. They work just like a spinning top works. As long as the top stays spinning fast it stays pointed in the same direction (in the case of a top that would be up); as the spinning slows the top starts to wobble and finally falls down. In space, with no gravity, the top would just keep spinning in whichever direction it was originally pointed. If the spacecraft does some maneuver to point in a different direction, the top still stays fixed pointing in whatever direction it started. Tops – which is all that gyroscopes really are – are great for space, because, with no gravity and no compass, there are not many other ways to figure out which direction you’re pointing.
If the Hubble had no gyroscopes left it couldn’t do anything. Luckily, three still survive. With three gyroscopes you can point anywhere in space at anytime. Wisely, though, the people who run Hubble decided that it was better to keep one in reserve in case one of these last three fails. So Hubble operates with two gyros. With only two you can still point to anywhere in the sky, but not at anytime. And this where the bad news comes in. After about noon on May 24th Hubble can’t observe Santa again for a few months.
The people at Hubble wanted to know: was it still worth doing the observations? We had to ponder. We think Blitzen takes about 19 days to go around Santa. From May 11th (which was the soonest we could get on the telescope) until May 24th is only 13 days. So we won’t see the complete orbit, but we think we’ll see enough to be able to calculate where Blitzen is the rest of the time. “Proceed!” we said.
But then there was worse news. Hubble uses the gyros for course pointing, but for keeping the telescope absolutely still during the course of the observations it also tracks a pair of bright stars close to the target. And, by bad luck, there aren’t enough of them close to Santa. A single star is available up until the 19th, and then absolutely nothing. We can do the observations, slightly degraded, with a single guide star, but there is nothing to be done after the 19th. So now we were crammed into May 11th through the 19th, an eight day window, when we really had hoped for a full nineteen day window. They asked again: is it still worth it?
By luck, if you only had eight days out of nineteen, these might be precisely the eight days you would want. They are when Blitzen is closest to Santa, which is the part of the orbit we need to know best. But still, it’s going to make our lives even harder than before. Will it still work? We did some quick calculations and decided, once again, we could do it. So we’re on for our eight days in May.
Now I’m nervous. We promised a pretty spectacular result to the people at Hubble. We need to deliver. There is always the chance that the new data will show that the shadows just finished happening and we’re too late. That would be bad luck, but we could at least hold our heads high and say we figured it out, just a little late. No, what makes me nervous is the possibility that we will get the data and still not be able to figure it out. People will say: OK, what’s the answer? And we will have to say. Well, um, we still can’t quite calculate the orbit. We can’t tell you when there will be moon shadows. Wait until next year.
I don’t think this will happen, so mostly it’s just paranoia. And I always have it. Every single evening when I am sitting at big telescope and the sun goes down and the dome shutters open I get similarly nervous. What if we did something wrong and all of our careful calculations about what we are going to look at and what we might discover were wrong? What if we forgot to take something into account? What if there is a better way to be using the telescope? What if…
And then the sky darkens and our first targets appear on the screen and I forget all of the nervousness and worry and get to work.
The same thing will happen, I hope, with this project. I won’t be at the telescope this time. I’ll be sitting at my desk sometime a few days after May 11th, when the data finally get transmitted and processed and downloaded onto my computer, and I’ll pull up the first image and forget all of the nervousness and worry and get to work.