We use the gravitational effect of Titan almost exclusively to shape Cassini’s trajectory – the onboard propulsion system we use only for small navigational corrections – and so around November 2016 we’re planning a close fly-by of Titan that is targeted to bring periapsis down to just outside the F ring of Saturn. The F ring is a small ring that is just outside the main rings, and that’ll be the closest we’ve been to Saturn yet. We’ll spend about 20 orbits in this geometry. Then we’ll have another close fly-by of Titan around April 2017 that will change Cassini’s orbit enough that periapsis will take a kind of hop over the rings, passing inside of the rings at just above the atmosphere. There’s a gap there [between Saturn and its rings] of about 3,000 km, which the spacecraft will pass through on every orbit. This is obviously by far the closest we’ve ever been to Saturn and it gives us quite a number of unique science observing and data collection opportunities, things that just physically weren’t possible previously.
We’ll stay in that geometry for 23 orbits, and then there is a rather distant Titan fly-by. This encounter will perturb the orbit just enough that Cassini will enter Saturn’s atmosphere, where the spacecraft will vaporize. The primary reason for disposing of the spacecraft this way, in addition to the unique science data collection opportunities it will give us, is that it ensures that we will never have to worry about the spacecraft impacting a satellite or any target of biological interest – in particular, Enceladus. But the other thing is that by this time in the mission we’re going to be getting very low on propellant and would not be able to continue it for any appreciable amount of time beyond this current planned end anyway.
Cassini is well known for the tremendous images it returns. How does the Cassini team handle all of these images?
When the images first come down, they go to two places. One is a site we maintain here that has all of the unprocessed raw images – they come down to the ground and go straight to this site. At the same time they go to a server that’s maintained by the imaging team, so that the imaging team scientists all have access to them. These raw images, some of which are quite spectacular, all go to this publicly accessible site. And then, as the imaging team begins to process them, they will come up with significant, interesting or spectacular images; we release some of those to the news media and quite a lot of them are posted on our Photojournal site, which is also publicly accessible. And as the scientists work with the images and give them a first-level of processing and image calibration, they all are archived on the NASA Planetary Data System, used primarily by scientists, but available to anybody. That’s where they will be kept for long-term permanent storage and availability.
Do you have a personal favorite?
It would be hard to pick between two of them. One of them you’ve undoubtedly seen, it’s where the sun is on the other side of Saturn and you can see the sun’s rays coming through Saturn’s atmosphere and the rings. That’s probably about everybody’s favorite, and is just a spectacular image.
But the other is one that we took on approach to Saturn, when we were still about a month and a half out and looking at Saturn overtaking the spacecraft. It’s just a beautiful, global image; it shows the rings, their shadow on Saturn, and the shadow of Saturn on the rings. It is probably more meaningful to me just because of what it represented when we took it, “Wow, look at this, we’re just about there and this is what we’re going to get to deal with.”