Insider’s Cassini: How to Plan Your Flyby
Insider’s Cassini: How to Plan Your Flyby
Mar. 06, 2012
Cassini’s trajectory must be worked out years in advance because it is a careful balance between the scientific goals and the capabilities (especially the propellant reserves) of the mission. Once we have the trajectory we assign chunks of time (on the order of two to tens of days) to the six planning groups which are organized by scientific discipline – Saturn, Rings, Magnetospheres, Titan, Icy Satellites, and Cross-Disciplinary. These chunks of time - or "segments" - are hotly contested and carefully negotiated based on the highest scientific priorities as identified by the scientists from each discipline.
Within each planning group, segments are divided up amongst the instruments down to the minute. Each planning group has representatives from the 12 scientific instruments, plus a number of science planners to shepherd the scientists through this stage of the planning process. That's where I come in. My job is to work with the scientists in the group to develop a good science plan for each moment along Cassini's trajectory, one that fits within our data volume restrictions and doesn't point the spacecraft anywhere risky (like, say, aiming the camera optics at the sun).
I am on Cassini's icy satellites flyby planning group. I begin work on a segment by noting any high-priority science activities that were identified and agreed upon during the course of the segment negotiations with the other planning groups. I also look for times when the mission planning team has noted some sort of icy satellite event, such as a non-targeted flyby. These help me get a sense of where the scientists will want to point the spacecraft, so I can start looking for safe spacecraft attitudes. I take this draft plan, full of gaps, to the scientists who then begin to fill it in. For the most part, everyone shares pretty well - if one of our image science subsystem (ISS) representatives wants to take a rapid series of images with slight changes in position in order to build up a mosaic of a moon's surface as we approach, but our visible and infrared mapping spectrometer (VIMS) representative wants to sit in one position and stare at the surface to build up their signal-to-noise level, they will agree to have pauses for the stares in between each mosaic so everyone can get some good measurements.
Our segments can get complicated fast. Sometimes we want to look at too many things. Maybe there's a great opportunity for ISS over at Dione, but the composite infrared spectrometer (CIRS) would rather look at the night side of Mimas for some temperature mapping - objectives which are mutually incompatible. Frequently the teams are able to do some horse-trading: CIRS will only observe a portion of the time at Mimas, allowing ISS to swing the spacecraft around and get in some good shots of Dione. But sometimes the moons are too far apart (leading to large turns that eat up time and momentum) or we've already crammed in a lot of other observations. I then have to ask the scientists to decide which of the two observations is more important to the group as a whole. Sometimes all the instruments want to observe the same moon, but they physically can't. Our optical instruments (ISS, CIRS, UVIS [ultraviolet imaging spectrograph], and VIMS) all point along one axis, our particles and fields instruments point along another, and the RADAR and radio science subsystems point in a third. If we want to study the gravity field of Enceladus then we have to point one way as we fly past; if we want to measure dust particles in the plume of Enceladus we have to point another way. Sometimes we can find a spacecraft orientation that allows the optical instruments to sweep across the surface of Enceladus while the particles and fields instruments are perfectly positioned for their measurements, but this is unusual. Mostly we have to trade off who is in charge during closest approach at each flyby, so that some flybys are optimized for one set of instruments while other flybys are given to the others. Luckily many of our instruments can squeeze in some good science on either side of closest approach. All of these concerns end up making each icy satellite flyby unique and carefully crafted - and often a challenge to fit within spacecraft resources.
Once we have all agreed upon a plan, I have to make sure that we are able to fit all of our data on the solid state recorders and that we are able to downlink it all at the end of our segment. Usually this means going to the scientists and asking them to make cuts - maybe a few less images in a mosaic, not taking data in a particular channel; all of these are options that the scientists have to weigh. But if we can't get the data down onto the ground and into their hands, then there was no point in trying to take it! Once these painful choices have been made, I make some final checks and hand the segment over to the sequence leads. They will stitch it together with other segments to create a 10-week sequence and work with the instrument and spacecraft engineers to assemble the actual commands that carry out the science plan onboard Cassini. By the time the flyby actually occurs, I am usually elbows-deep into planning a flyby that occurs many months (sometimes over a year) later and seeing the data come down is a happy surprise!As I look back over this column and think about all of the exciting flybys of Enceladus, Rhea, and Dione that I have helped plan, my feelings are rather bittersweet. This is my last week on Cassini - I am heading back into the inner solar system and joining the science system engineering team for Mars Science Laboratory. But no matter what project I am working on, I have fond memories of the Cassini team and I look forward to seeing the results of all the upcoming icy flybys!