It depends on the size. For 100 km size objects there's on the order of 100,000. And then if you go to smaller and smaller sizes, it gets bigger and bigger, and by the time you get to say, one meter, or one km, there's 1011 or something. When you talk about Pluto class, there's about on the order of 10. When I was at Harvard in 1995, I gave a seminar titled "How Many Plutos Are There?" The answer was between five and 10. I still believe that number.
What do you think we'll learn about the Kuiper Belt in the near future?
I think the dynamics still holds a lot of surprises, because we still don't understand the origin of these weird orbits. We have ideas, like maybe there were close encounters with passing stars. If the sun formed in a cluster so that there were other stars relatively nearby, they would have had an influence. They could have stripped off the outer part of the Kuiper Belt – the Kuiper Belt population drops off drastically beyond about 50 astronomical units [one AU is the distance between the Earth and the Sun, about 93 million miles]. Some people call that the Kuiper Belt cliff. It's unlikely that the solar nebula, the disc that formed everything in the solar system, just got truncated sharply at 50 AU. I think dynamics is still an extremely rich area that holds lots of surprises.
[Studying their] physical properties, that still goes on. It's difficult, because they're so faint. There's a handful of telescopes that are capable of doing these things: Keck, Gemini, the big telescopes. Those big telescopes are rare and everybody wants to use them, so getting time on them is even more rare. So I think the dynamicists might be faster, they might be more efficient in coming up with new answers than the observers, simply because they don't need the telescopes. There's a lot of work to be done in both theory and observation, but I think theory is going to yield more, just because these things are so faint.