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The Planetary Society BlogBy Emily LakdawallaNew Horizons at Jupiter: Dreams and RealitySep. 13, 2006 | 14:39 PDT | 21:39 UTC
by John Spencer
We have our final meeting tomorrow morning for the Jupiter planning team -- it's only half a day, but my friend Jeff Moore and I are in charge of this part, and we have some tough work to do. We've been working for several years on a plan for making the most of New Horizons' flight past Jupiter. For our prime mission, Jupiter's really just a stepping stone, a gravitational convenience, giving us a boost that shaves a few precious years off our flight time to Pluto. But it's also one of the most amazing neighborhoods in the solar system, a place full of wonders, and although we're the eighth spacecraft to Jupiter, the payload that we're carrying to Pluto is well suited for some unique Jupiter science, too. Since we started work on this mission in earnest in 2001, we've been aware of how much we stand to learn from a Jupiter flyby, and we've been developing a wish list of observations of the Jupiter system that we'd like to make. Some of these use New Horizons' unique capabilities to fill gaps in our knowledge -- for instance we don't yet have a good handle on the atmospheres of Jupiter's four big "Galilean" moons, and our ultraviolet instrument, Alice, can help with that by watching stars as they pass behind the moons. Other observations are similar to what's been done before, but are worth repeating because of the dynamism of the Jupiter system. Jupiter's weather changes from year to year, and Io's frantically volcanic surface, described by Rosaly last week, will likely look quite different from the face it presented to the Galileo spacecraft during that mission's final observations in 2001. To come up with our list, we've supplemented the patchy Jovian expertise of the science team with invaluable help from Jupiter experts outside the team. Once we launched last January and knew for sure that we would have a Jupiter flyby (if we had launched too late, we'd have had to skip Jupiter and take the long road to Pluto), our plans began to solidify. We soon figured out exactly where Jupiter's moons would be as we flew past, so we could plan exactly when to observe them (to our mild annoyance, they tend to cluster shyly on the far side of Jupiter near closest approach). We worked with the mission operations team to determine when we needed to interrupt our observations to downlink data to Earth, how much of our precious fuel it would take for all the fancy maneuvers we wanted to make, how much data we could afford to store on board, and so on. We worked with the instrument teams to determine exposure times, which way we need to tilt the spacecraft to get the best data and so on. For the last eight months we've debated and compromised and e-mailed and teleconferenced and simulated and drawn up our spreadsheets and polished them till they are things of beauty, and now we have a wonderfully detailed blow-by-blow account of exactly how we'd like to spend those precious weeks when Jupiter glides magnificently past our sensors. I've posted a description of our plan, mostly for the benefit of astronomers who'd like to help us out with supporting telescopic observations.
But there's a catch. New Horizons doesn't understand our spreadsheets. Everything we've done has to be translated into commands written in the spacecraft's exacting language, and then it has to be tested on our simulators, which include a rack of electronics at the Applied Physics Laboratory that reproduces the electronics on board the spacecraft, and speaks the same language. If our commands upset the spacecraft in some way, it's a lot easier to fix the code and try again on the simulator than it would be on the spacecraft. There are innumerable "gotchas" that must be avoided, some already discovered the hard way on the real spacecraft. Voltages must be set correctly, commands must be timed so they don't overlap, memory must not be overloaded, temperatures must remain within prescribed limits, and so on. All this work must be done by a handful of people on the spacecraft operations team who have been incredibly busy up till now just learning how to fly the spacecraft, and planning the vitally important initial calibration observations as the various instruments are switched on and tested for the first time. Only now, with that work largely behind them, can they turn their attention to Jupiter, and we have only a few months left as Jupiter bears inexorably down upon us. So tomorrow's meeting is all about how we reconcile our desires with the engineering realities. It already looks like we won't get everything we want -- there just isn't enough development and testing time. How much we *will* get remains to be seen. ----------- Postscript. The Jupiter planning meeting just ended, and I feel both more optimistic about how much of our plan we might be able to accomplish, and less optimistic about the amount of free time I'll have in the next few weeks. The long mountain hike I'd planned for tomorrow, as reward for surviving the last four days, ain't gonna happen. We spent a lot of time discussing ways to simplify the implementation process by standardizing and repeating sequences. We'd like to make each observation perfect, for instance by increasing the exposure times as we swing from the day side to the night side of the moons. But if we keep exposures the same, and accept that some images will be slightly underexposed, we'll be better off than if we lost the images entirely. We can shorten some observations, and eliminate some low-priority ones, to give us some breathing space in the timeline. We'll make it work. We'll still have a great encounter.
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