Wednesday, June 10, 2009

Cassini Tour Guide

Busy week, not much time for reading or blogging. One thing I noticed that is really cool is this guest post on the Planetary Society blog. John Smith is a planetary tour designer at JPL, one of just two or three people who work on designing the orbital journeys of the Cassini spacecraft since it arrived in Saturn orbit in 2004. His article is really fascinating, and it doesn't even delve into much in the way of orbital mechanics. It's amazing how many constraints they have to meet to try to achieve the many science goals (all within the constraints of orbital mechanics, i.e., physics).

Just as we optical engineers will say "it's all done with mirrors" (not really), orbital engineers will say, "it's all done with gravity," and of course it is. The "engine" that really allows Cassini to travel in the wide range of orbits needed to visit the various moons of Saturn is actually the gravity of Titan, Saturn's largest moon. Every time Cassini passes near Titan, its orbit can be slightly or dramatically changed, with full control depending on the location and closeness of the orbital path to Titan. Cassini has thrusters and a small fuel supply for attitude control and for minor adjustments to some orbits, but Titan's gravity is what does the "heavy lifting" when it comes to changing the orbit.

I've simulated a few close passes to Titan in Orbiter (with the Cassini add-on or other spacecraft, it doesn't really matter), usually when something interesting has happened recently with Cassini (like finding those methane/ethane lakes) and I want to show this in a space demo. I've always gotten specific orbit data from JPL's Horizon's database so I know it will work (then I play around with the views in Orbiter so it looks really cool in the demo, especially with time acceleration), but you can also use Orbiter to experiment with gravity-assist maneuvers on your own, using the scenario editor and navigation tools like IMFD.

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