Behind the scenes @ Cassini – A guided tour with Sarah Milkovich
I have a multiplanetary working life. It’s gotten rather complicated.
Half of my time, I am a science planning engineer on Cassini, and the other half of my time I am the Investigation Scientist for the HiRISE camera on the Mars Reconnaissance Orbiter. (Sometimes it seems like the hardest part of operating spacecraft is getting the work done in between all the meetings.)
Today I’m going to talk to you a little bit about some of my work for Cassini. I hope you’ve been following along with us last year as we observed the Saturn equinox (check out all our great images on the Astronomy Photo of the Day archives!) and had some fabulous moon flybys.
Cassini is a hugely complicated spacecraft to operate, and there are a large number of people who work behind the scenes to get the stunning data that you see online. I want to give you a taste of the amount of effort, and the many decisions, that get made by a lot of people who you rarely ever hear about in the press releases.
When I’m being a science planner, I am not affiliated with any particular instrument; my job is to work to get the most scientific bang for our buck, within spacecraft health and safety constraints. This includes working with the engineers to ensure that when engineering decisions are made we still get the best possible science, and also explaining the realities of spacecraft operations to the scientists. A science planner works with scientists to develop a good science plan for each moment along Cassini’s trajectory, and then works with the instrument teams to get that plan implemented and through health and safety checks. The plan is then handed over to the uplink team, who are responsible for inserting spacecraft housekeeping commands and getting the whole thing onboard the spacecraft. We also have a group of engineers – the attitude and control systems folks – who keep a close eye on how Cassini’s aging reaction wheels are behaving, and how we have to adjust our plans to keep the wheels happy (more on that later).
One of the groups I am a member of is the one that plans the flybys of the icy moons of Saturn.
Cassini’s trajectory is worked out years in advance, because it is a careful balance between the scientific goals and the capabilities (especially the fuel reserves) of the mission. Once we have the trajectory, we then assign chunks of it to the five planning groups which are organized by scientific discipline – Saturn, Rings, Magnetospheres, Titan, Icy Satellites, and Cross-Disciplinary. (Realize that when I say “assigned,” I really mean “heavily fought over and carefully negotiated.”) Each group has representatives from the twelve scientific instruments, plus a number of science planners to shepherd the scientists through this stage of the planning process. Within each group, the chunk of time (several to tens of days) is divided up amongst the instruments, down to the minute. One instrument is “prime,” or in charge of where Cassini will be pointing during that time, and other instruments can “ride along” as power and data volume allows. The chunks of time from each planning group are then stitched together to make a five week long sequence, and the actual commands to carry out the science plan are worked out and uplinked.
When I joined Cassini in October 2008, the icy satellites group was planning the 7th flyby of the moon Enceladus since arriving at Saturn. I learned how to do this part of my job by watching another science planner work on Enceladus 7 (E7) flyby, and helped out with the plan for the 8th flyby of Enceladus (E8). I was then made the science planner for several other flybys on my own, including the 9th Enceladus flyby (E9).
This fall was very exciting for me, because I finally got to see the results of all this planning. E7 occurred at 11:40 pm PST, or 7:40 am UTC, on Nov 2. You can read about some of the details here and here. We flew 100 km above the surface of the south pole of the moon, right through the heart of the plume while our particles and fields instruments took measurements of the plume material. Our camera and spectrometers observed the plume and Enceladus’ surface as well. E8 occurred on Nov 21, and while we were much further from the moon during this flyby, that allowed us to take some spectacular optical data of the surface (you can see some of it here).
Perspective View of Baghdad Sulcus, Enceladus
Credit: NASA/JPL/Space Science Institute/Universities Space Research Association/Lunar & Planetary Institute
I was particularly awaiting the results of E7 because E9 (which occurs in April) has the same geometry as this one, and we needed to see how the spacecraft behaved during E7 to determine how to do E9, which I tend to think of as “my” flyby.
We control the pointing of Cassini in two ways, thrusters and reaction wheels (basically gyroscopes). Each has pros and cons – thrusters use up fuel, which is a limited resource, and cause a small amount of additional wiggling which adds noise to the data, but allow the spacecraft to turn faster. In addition, the wheels are getting a bit creaky and we need to be careful what stresses we put them under. E7 was on thrusters to make sure the spacecraft is safe as we fly through the plume. One of the concerns of the engineers was that the jets of the plume will push too hard on Cassini – they were confident that Cassini would be fine on thrusters (as we were for E7), but they were worried that the push would be too much for the reaction wheels to accommodate, and that one of the onboard programs (called the Control Authority, which really should be a villain in an old-school Star Trek or Doctor Who episode, with painted styrofoam) might get upset and put us into safe mode. The science teams want to be able to fly E9 on wheels in order to get gravity data from the radio science experiment (RSS). We’re hoping that by measuring gravity fluctuations near the south pole of Enceladus, we can build up a picture of the internal structure of the moon, and gain insight into the source of the plume. Since we measure gravity by observing small changes in Cassini’s trajectory, if we fly E9 on thrusters, we won’t know what variations in the RSS data is due to gravity anomalies and what is due to the thrusters.
The engineers in charge of the attitude and control systems on Cassini wanted to see how the spacecraft behaved during E7 before they would allow us to fly E9 on wheels. We had models based on an earlier flyby that went through the plume but at a greater distance from the moon, and we thought the flyby to be safe on wheels, but much as we really really want to get the gravity data in April, we want to keep Cassini flying for a long time even more, so we needed to wait for the E7 data. Unfortunately, because of the complexities of operating Cassini, we require building our science plans so far in advance that even though E9 doesn’t occur until April, by November we had to have a settled science plan and already be working on building the instrument commands. However, the icy satellites planning team felt so strongly about the importance of the gravity experiment that we convinced the project to let us go to great lengths to keep our options open. And so we signed ourselves up for extra work in order to try to get the best science no matter what.
I helped the scientists develop two plans, one with E9 on wheels and one with E9 on thrusters. Our first choice version on wheels was entirely turned over to RSS. We won’t turn the spacecraft at all to look at different features during the two days we spend near the moon; this prevents any additional stress on the wheels and allowed us to pick a spacecraft attitude that helped minimize the torque of the plume on the spacecraft. This also prevents the other instrument teams from having to spend their time making extra observation designs when they already are working very hard on other observations. The back-up version of the flyby on thrusters was very similar to what we did in E7 – we would make optical observations of the plume and the surface of Enceladus on approach and departure, and make particle and fields measurements during the passage through the plume. By keeping the plan similar to E7, we allowed the instrument teams to save time in the implementation stage by using observations similar to ones they had already designed for E7. All of these choices simplified the plan enough to allow us to move forward with two timelines – something that is hideously complicated (How do we keep track of which plan is which in the planning database? How do we ensure no one accidentally submits their design to the wrong plan? How do we make sure we quickly and easily swap to the back-up plan if necessary? We’ll need more DSN stations if we are doing a gravity experiment than if we are making optical measurements – how do we keep track, and how to we let the DSN schedulers know? All of these were things I had to think about.)
I am now the science planning lead of the sequence that E9 occurs in, where I shepherd the plan through the integration process and then hand it over to uplink, so I am seeing this flyby through to the end. I am delighted to say that E7 went smoothly, and the project gave us the go-ahead to fly E9 on wheels. I am now working hard to get the entire sequence – which includes a Dione flyby and a Titan flyby – put together properly and handed over to the uplink office.
I hope you found this little glimpse behind-the-scenes at Cassini interesting. To me, getting the commands up to Cassini is just as exciting as the data it sends back! I’ll report back in April to let you all know how E9 goes.