What did you think when you discovered that first object in the belt?
We searched for five years before we found that one, so we were used to failure and we were very cautious. I remember Dave spotted it first; the two of us would go observing, and one person would run the instruments of the telescope and one person would look at the data as they came out.
The way you search for moving things is, you pick a spot in the sky where you're going to take a picture, then you take a picture of that spot and you wait a little while, and you come back and take a picture of the same piece of the sky, at least two or three times. The idea is that all the things like stars and galaxies don't move over time scales of minutes and hours, so anything that moves would be a solar system object. The speed at which these things move tells you how far away they are, so things that are closer would be moving much faster than something out at Pluto's distance.
We would take multiple images of a piece of the sky, separated by an hour or two, and then we could look at them sequentially to see if anything moved. The computer can do that very well. You just load the images into memory and it plays the images sequentially, very rapidly, like blinking. You remember when you were small and you had those little flip-books where, if you flip the pages very rapidly, it looked like things were moving? Same idea.
Dave was looking at two images, and he saw something move and said, "Hey, come take a look at this." It looked plausible, but it could have been an artifact – CCDs are very good at detecting cosmic rays. We would take a minimum of four images, so if you see something moving systematically in the same direction at the same speed in four images, then you can be pretty sure that it’s real. He spotted something after two images and I thought, Oh that's good, that's a good candidate, but… we didn't jump up and down because we'd been doing this for a long time. Then we took a third image and that object was still there, it was moving in the same direction, at the same speed. So then we were pretty excited. And we took a fourth image and it was still there. The next day we went back to the same field and it was still there. By the second night we knew that it was something real.
How many objects did you eventually find?
Many dozen, we lost track of them. Finding something is one thing, but if you don't follow up with more observations, you just lose them. Let's say you take 10 observations in two nights of some object. The only way to find that thing again is to fit an orbit [taking enough position measurements to extrapolate accurately] so that you can predict where it will be six months later, a year later, two years later. If you can't fit a good orbit, you'll never see it again.
We found lots of things, but to be able to fit an orbit so that these things are recoverable, that took time – time that we didn't have. We wanted to characterize the entire population, and we were just hoping that somebody would do the follow-up observations so we didn’t lose them.
What do we know about the Kuiper Belt now?
We now know Pluto is not alone. There is a huge population of objects beyond Neptune, and Pluto is just the largest one. This population is now called the Kuiper Belt. Not because [Dutch-American astronomer Gerard] Kuiper deserved the name. He wasn't the first or the only person to hypothesize this population, but somehow the name stuck.