Note: I am posting my old blog entries from elsewhere on the internet. This entry was originally posted on 11/01/09.
A year and a month ago, I joined the Cassini Science Planning team. My job is to work to get the most scientific bang for our buck, within spacecraft health and safety constraints. 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. Each group has representatives from the twelve scientific instruments, plus a number of science planners to shepherd the scientists through 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, the icy satellites group was planning the 7th flyby of the moon Enceladus since arriving at Saturn in 2004. I learned how to do this part of my job by watching another science planner work on this flyby, and helped out with the plan for the 8th flyby of Enceladus (which occurs later this month). After that, I was put in charge of several other flybys on my own, including the 9th Enceladus flyby.
The E7 (as we call it) flyby occurs tonight at 11:40 pm PST, or 7:40 am UTC on Nov 2. You can read about some of the details here, in a writeup by one of my colleagues. We’re flying 100 km above the surface of the south pole of the moon, right through the heart of the plume of water that is shooting out of cracks in the moon’s surface. Our particles and fields instruments are going to measure the other particles in the plume as we dive through it.
I’m excited to finally see the results of all this planning. I am also eagerly awaiting the results of this flyby because we’re going to use them to determine how to do the 9th Enceladus flyby (which occurs in April). 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 and be overall more stable. Our April flyby has the same geometry as this one, and we want to be able to fly it on wheels to get gravity data from the radio science experiment. E7 is on thrusters to make sure the spacecraft is safe as we fly through the plume.
I helped the scientists develop two plans, one with E9 on wheels and one with E9 on thrusters. I am now in charge of the sequence that E9 occurs in, and am awaiting the analysis of the E7 data by our attitude and control systems engineers who will determine which version we can fly. We’ve gone to great lengths to keep our options open in the hope that we can fly E9 on wheels and get gravity data that will tell us about the subsurface underneath the plume area, and therefore what is the source of the plume. It is all up to Cassini now.
Question from a friend on the original post: Q: Ooh, right through the spray? Is it enough that it would potentially effect the trajectory of the craft itself? I suppose that’s one of the things you’re trying to find out!!
A: Yes, one of the concerns is that the jets will push too hard on Cassini – we’re confident that we can deal with it if we are on thrusters (as we are for E7), but we’re worried that the push is 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 Trek episode, with painted styrofoam) will get upset and put us into safe mode. We’ve got models based on an earlier flyby that went through the plume but at a greater distance from the moon, and we think we ought to be ok, but we’ll use the data from E7 – downlinking as I type this – to see if E9 is safe on wheels. Much as we really really want to get the gravity data in April, we want to keep Cassini flying until 2017 even more. (Of course, the woman from the radio science team whose observation this is might not agree with me! Part of my job is also explaining the realities of spacecraft operations to the scientists…)