|The Department Graduate Students Spacecraft Missions|
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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.
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