Ground-Based Research, Page 2

Steve Larson, on the discovery of co-orbital satellites around Saturn

We were on the ground observing with the 61-inch telescope in the Catalinas, which was funded by NASA specifically for high-resolution imaging for the Moon and planets, as well as developing other instrumentation for infrared work.

After Kuiper had died, Brad Smith came over from New Mexico State University and he was Principal Investigator of the Voyager mission. He had been pushing for the application of charged-coupled device, CCD, cameras at the telescope. They were still very crude and under development, and they weren’t quite ready for flight time on spacecraft.

But we did get to use a prototype that had been developed for the Hubble Space Telescope, and observed Saturn during the 1980 ring plane crossing. It occurs roughly in 14-year intervals. If you’ve seen Saturn’s rings, they look tilted, but as the Earth goes around and Saturn goes around, they’ll be a period of time when we’re right in the plane, so the rings just look like a thin line. Because of that, there’s less light scattering, so we can look for inner moons and other phenomena that you wouldn’t normally see.

We were following up on what had been announced as the discovery of a satellite, based on data taken in 1966 with a 61-inch during a previous ring plane crossing. There was indication that there might be another satellite, very hard to see because it was faint. But we had looked at all the 1966 data and came to the conclusion that there was a new satellite orbiting very close to these rings.

Well, as it turned out Pioneer 11 flew by Saturn just about the same time we were observing it. Its camera was not capable of taking pictures nearby, because things were flying by so fast, but it sensed something that blocked some of the charged particle radiation that was in the magnetic field of Saturn. We didn’t know what it was at the time. Later it would establish that it was the wake of the satellite that we thought was there, and observed later in 1980. We were watching a satellite that we expected to see coming out of one side. It was called Janus.

And then we saw one on the other side come out.

It turned out, they had about the same orbital radius. This was the first existence, the circumstance of observing two satellites that were in essentially the same orbit. They’re called co-orbitals. We named the co-orbital Epimetheus.

It was a thrill to be at JPL during the Voyager encounter of Saturn. Because of our observations they had planned to make observations to include the satellite. Not only did they get Epimetheus, they got a series of pictures showing the shadow of the ring going across. It was neat to experience a world going from a little point of light to actually seeing it as a chunk with craters.

There’s still discoveries being made like that, with Cassini and whatnot, but those first were always unique experiences. Dynamists knew that such a co-orbital situation was possible but it had never been observed before. The orbits were just slightly different, but they were different by less than their radius. What happens is, as they go around, they revolve in the plane of one other satellite. The satellites will come close, but they’ll have mutual gravitational attraction, and they’ll pull each other, and one will be pulled into a lower orbit and one will be pulled into a higher orbit, and then it’ll go apart again, and they play this dance all over again. It hasn’t been conclusively determined how it got in that situation in the first place.

Robert McMillan, on the founding of Spacewatch

I think about the spring of 1980, Tom Gehrels had written a draft of a proposal to look for asteroids that might hit the Earth. He gave it to me just to criticize: “What do you think of this?”

I wrote back what I think looking back now was probably a pretty stinging criticism of it, because I thought the way that he was going to do it simply wouldn’t work. And I said so. I gave the reasons why, thinking, “Well, that’s the end of my job.” But I didn’t want to lead him on; I thought there were some real problems with the initial approach.

Instead of firing me he said, “Well, why don’t you help me do this project, because I think I need you.” So to my astonishment he made me the deputy investigator on Spacewatch and we wrote a proposal. I think the first proposal was in March of 1980.

Spacewatch is an exploration of the whole solar system for asteroids and comets, with an emphasis on potentially hazardous asteroids that might hit the Earth. In addition to finding a number of Potentially Hazardous Asteroids, PHAs, we’ve also discovered trans-Neptunian objects; we’ve discovered Centaurs that orbit in the outer solar system between the orbits of Jupiter and Neptune; and many comets. We’ve done scientific investigations of the statistics of asteroids in the main belt, and statistics of the asteroids in the near-Earth object region.

Over the last 26 years, Spacewatch has gone through a number of revolutionary phases in which we’ve had different kinds of technology and equipment. We’ve upgraded the telescopes from time to time; we built a new telescope so we now have two. We’re observing very intensively on Kitt Peak with both telescopes as I speak. We’re well-funded by NASA at least until spring of ’09. Indications are we’re going to continue to have a role in follow-up and discovery of asteroids, especially the ones that are possibly going to hit the Earth.

I’m quite proud of the accomplishments of Spacewatch. We have a certain niche in the world effort that nobody else is doing. It’s of course very hard work, observing on Kitt Peak, long hours, sixteen hour days, or nights if you like, and we all have to put in about eight nights a month, two telescopes with one person, so it’s a real handful. I’m one of the three observers. And I’m the Principal Investigator as of June of 1997, when Tom handed off the PI-ship to me. But he’s still associated with the project, he still goes to the telescope, and he offers advice and so on. His international reputation is still associated with Spacewatch; we benefit from that too.

I see Spacewatch as my life’s work in more than one way. I helped to invent it, and I’m doing a lot of observing with it, of course I’m responsible for it, getting the funding and so on. I am collaborating with people at JPL who are developing a couple of instruments, spacecraft, to detect asteroids from space. The reason that Spacewatch is relevant to that is that ground-based telescopes will be needed to follow up on discoveries that are made from spacecraft, and so the ground-based follow-up is an integral part of these new spacecraft missions.

I’ve steered Spacewatch in the last several years over toward following up objects after they get too faint for the survey telescopes to follow them. That’s our niche, doing faint follow-up, and that is ideally suited to collaborating with spacecraft missions. So I think I have a pretty decent future, a certain niche in planetary science. We have actually the largest telescope in the world that is dedicated full-time to searching for asteroids and following them up.