Kudos for Cassini
The U.S./European Saturn mission takes home a trophy.
- By Heather Goss
- Air & Space magazine, March 2012
(Page 2 of 3)
Another thing that we do fairly regularly, about once a year, is a conjunction experiment when the spacecraft is on the opposite side of the sun as seen from Earth. When the radio signal comes from Cassini to Earth, that signal passes close by the sun, and the scientists analyze it to look for confirmation of the theory of relativity, which predicts that the signal should be affected by the sun’s gravity. And that, in fact, has worked and they have confirmed Einstein’s theory to a greater level of precision than had been done before.
We also had two Venus, one Earth, and one Jupiter flyby en route to Saturn. We didn’t do very much at Venus or Earth; we just didn’t have the resources to be able to, in addition to all the other preparation work we were doing for when we got to Saturn. And a lot of the software we would have had to have, including spacecraft flight software, was not complete and available at that time.
At Jupiter we did quite a bit: We ran a ‘full-up sequence,’ motivated partly by the dry-run opportunity to be ready to go when we got to Saturn, but we got quite a bit of scientific information at Jupiter, as well. The closest we got was about 10 million km, because that’s what the gravity assist requirement dictated. We had the Galileo spacecraft still functioning and orbiting inside of Jupiter’s magnetic field, and we had Cassini passing by outside of the magnetosphere, so the scientists were able to study how the solar wind variations as measured by Cassini affected the magnetosphere of Jupiter as measured by Galileo. That was a unique opportunity that had the scientists rather excited. And we got some global coverage of Jupiter that Galileo wasn’t able to do because of its antenna problem.
What has been the most difficult challenge in running the Cassini mission?
There really haven’t been any significant long-term challenges. It’s all gone remarkably well. The spacecraft has done excellently for us – a few glitches here and there, but nothing unexpected for a spacecraft that is this complex and operating for this long. Another part of it is that the team, which is made up of a few hundred people, is really just a great team and very capable.
Probably the single most challenging thing that I had to deal with was maintaining and defending our operating budget. We’re a rather large program, with a correspondingly large budget, so whenever NASA needed to find money, we were a very visible target. This was a bigger problem back during our cruise from Earth to Saturn because there had been, by design, a lot of development work postponed until after launch to be done during the cruise phase. And I think, particularly now in hindsight, that was a very good decision. The result of this was that we had a team that was fully on-board and trained and up-to-speed when we got to Saturn. The problem was that it was difficult to convince people of the scope of what had to be done. So I’d say preserving the budget during the cruise phase was probably the biggest challenge. But once we arrived at Saturn and we were getting science data back and everything was going remarkably well, and it was clear we were going to be successful, then things kind of let up and we’ve been adequately funded ever since.
With 16 European countries involved in the Cassini program, has that been difficult to coordinate?
I’ve been very happy with how it’s worked out. Although I should say that my role on the project started just shortly after launch; we had a different project manager for the pre-launch development phase, so I didn’t see some of the interactions there. But since launch, we’ve obviously had a lot of interactions with the Europeans.
We have about 250 or 260 scientists that were selected by NASA; about half are U.S. and half are European, and so we have a lot of interaction with them. They’re just a great group of people who get along together very well. One thing that helped is that there is no exchange of funds across the ocean, so there’s no issue of allocating money between the U.S. team members and the Europeans. Another thing that makes this not be a nationalistic issue is that the allocation of science resources – observing time, data memory allocation, data downlink time to Earth – is done among the instrument teams and not country to country. And the teams are all a mix of U.S. and European scientists, so there’s just not a distinction like that.
Can you explain ‘Solstice,’ the name of your second mission extension, and what you hope to achieve during this phase?
After the primary mission was done, which took four years, we had the Equinox mission; that was our first extension that went for 27 months. During that mission, in August 2009, the Saturn equinox crossing occurred, where the sun passed from below the equator to above the equator. At that time the sun was directly edge-on to the rings. That was a very interesting geometry for the scientists, since any little vertical displacement in the rings created a shadow, and those shadows told us a lot about the vertical structure of the rings.
For the Solstice mission, our current plan, which we have proposed to NASA and they have tentatively approved, is that Cassini will go to the northern hemisphere summer solstice that occurs in 2017. That will let us see seasonal variations, primarily on Saturn and Titan. Saturn’s seasonal variations get magnified by the rings, so that when it’s winter in one hemisphere, the atmosphere is colder just because it’s winter, but it’s even colder yet because the rings are creating very sizable shadows on that hemisphere. So one of the key things that we want to study in this second extended mission is the seasonal variations of Saturn and Titan, and to whatever extent there are variations on any of the other moons. And this is further opportunity to study things that we haven’t been able to get worked out prior to now in the mission. It sounds like a long time, but 13 years is not even quite half of one full Saturn year – one Saturn seasonal cycle – so there’s a lot to be learned, a lot to be studied at Saturn.
How many fly-bys will Cassini have made of Saturn, Titan and Enceladus by the time it’s finished?
We will have done 293 orbits and fly-bys around Saturn, 127 fly-bys of Titan, and 22 fly-bys of Enceladus. For Titan and Enceladus, these are the ones where we navigated to a specific target point, but there have been others, usually at greater distances, where we didn’t control the trajectory to a specific point. That would amount to tens more fly-bys that the scientists in fact use for observations, but we tend not to count them because we didn’t actively navigate to them.
Can you describe the mission’s planned end in 2017?