Taking the Kuiper Belt Census

The Kuiper Belt is a strange and mysterious place, where frozen asteroids roam, dwarf planets stalk and Neptune's gravitational influence dominates.

Discovery News Space partner Lowell Observatory is heavily involved in studying the Kuiper Belt and one of the prime objectives for the Discovery Channel Telescope (currently under construction at Lowell) is to probe deeper into the region than ever before. And for one astronomer, counting the population of Kuiper Belt Objects (KBOs) has become a career obsession.

Steele Wotkyns, Lowell Observatory's Public Relations Manager, managed to pull Lowell astronomer Larry Wasserman away from the telescope eyepiece for 5 minutes to ask him why KBOs are so important.

Steele Wotkyns: Tell us about your background. How did you get started in astronomy? What was some of your early work in astronomy and how long have you been working in the field?

Larry Wasserman: I sort of drifted into it. I started out in physics, and astronomy seemed like the most interesting part of physics, so I ended up in astronomy. I was a graduate student at Cornell working with Carl Sagan. But, Carl was pretty occupied so I also worked with Joe Veverka and others.

Sagan was doing a lot for planetary astronomy at the time; he brought on others in the field. So there was a large group working in planetary astronomy -- suddenly, a lot was being done at Cornell in this field that hadn't been done before.

I got into occultations at Cornell because that was my thesis topic. An occultation occurs when one body in the solar system passes in front of another and blocks our view of it. One particular occultation was the occultation of Beta Scorpii by Jupiter. That was my thesis topic.

So it's been a while. I've been working in astronomy since I got to Cornell in 1967. I came to Lowell Observatory in 1974 after I got my Ph.D. at Cornell.

SW: What got you started in Kuiper Belt research?

LW: Lowell astronomer Bob Millis had started a program called the Deep Ecliptic Survey with several colleagues. They were looking for Kuiper Belt objects (KBOs) and I joined that group. It took about seven years to find 500 objects -- that was the original goal of the project. When the project was first started in 1998 there were only a handful of KBOs known.

So, what was out there? The idea was to get a sense of the structure of the Kuiper Belt by looking at patterns in their orbits to see how the belt is structured. Eventually, we got long-term, guaranteed time at the 4-m Mayall Telescope at Kitt Peak and the 4-m Blanco Telescope in Chile at CTIO.

SW: Tell us a little more about the work to determine the structure of the Kuiper Belt?

LW: Once you have a large enough population of KBOs, you can start classifying them by type. One such type features resonant orbits with Neptune. For example, a 2:3 resonance KBO goes around the sun two times for every three times Neptune orbits the sun. So, there are objects in various resonances with Neptune. Some KBOs cleverly manage to put themselves in this dance with Neptune.

There are three basic classes of KBOs: resonant, perturbed by Neptune, and those that are unperturbed, the so-called "classical" objects. Classical objects have mostly low-inclination, low-eccentricity orbits and have probably been there since the beginning of the solar system.

At the time we started this none of these classes were known. The existence of these groups tells us something about the solar system and how the solar system evolved. So it turns out that by observing these objects we can learn what happened 4.5 billion years ago at the formation of the solar system.

SW: Describe a notable discovery you have been involved in during this KBO census.

LW: Well, in 2003 we announced the discovery of 2001 QR322. Its claim to fame is that it is the first Neptune Trojan asteroid discovered. It is in a relatively circular, low-inclination orbit. I was part of an observing team at the 4-m Blanco Telescope in Chile when we first got images of it in 2001. But, it wasn’t until we had follow-up observations over the next 16 months and had made orbit integrations before we could be certain that it was the first Neptune Trojan. I remember it was an interesting object but it was two years before we got an orbit to do something with. This is not an instant gratification game.

SW: So what are you working on now?

LW: I've been trying to stay on top of these objects, trying to make sure they get observed regularly so that when you go to observe them, you can find them where you think they are. The difficulty is they have orbital periods of 400 years or more. When you try to predict where they'll be in the future, you’re always extrapolating their orbits -- it's difficult to find them again. I’m using the 1.8-m Perkins Telescope [operated via a partnership with Lowell Observatory and Boston University] to make follow-up observations of the brighter KBOs.

I've also started a preliminary program to do basic photometry of KBOs to look at their colors. The most fundamental thing you can do is determine are they brighter in the blue end of the spectrum or in the red end. By studying their spectra I get clues as to where they came from -- how did these objects get there in the first place?

I have an interest in looking for high-inclination KBOs. They are high-inclination KBOs since their orbital plane is tilted to the plane of the solar system by more than 20-30 degrees. Nobody can really explain the large tilts. Where do the high inclinations come from? Also, the biggest KBOs all seem to have high inclinations. This is odd.