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Emily LakdawallaJune 20, 2007

Fine-tuning Cassini's orbit

I found several items of interest in the latest "Cassini Significant Events" report. These reports are issued weekly by the Jet Propulsion Laboratory and constitute a blow-by-blow description of Cassini's activities, as well as the plans for future activities, much like the status updates that JPL issues for the rovers.

Here's the first interesting item:

Sunday, June 10 (DOY 161)

Orbit Trim Maneuver (OTM) #115 was performed today. This is the approach maneuver setting up for the Titan 32 encounter on June 13. The reaction control subsystem burn began at 6:30pm PDT. Telemetry immediately after the maneuver showed the burn duration was 25.5 seconds, giving a delta-V of 0.036 m/s. This maneuver was targeted to lower the upcoming T32 flyby altitude by 10 km, from 975 to 965 km. Navigation analysis showed that lowering the T32 closest approach would save about 2.5 m/sec in downstream maneuver sizes, and would bring the trajectory back closer to the reference trajectory. All subsystems reported nominal performance after the maneuver.

A "maneuver," in navigational parlance, is when a spacecraft fires its engines to adjust its trajectory. This update reports on a teeny maneuver and really brings home how precisely the navigators can steer this spacecraft. They fired small thrusters for 25 seconds, changing the spacecraft's velocity by 3.6 centimeters per second -- about 1.5 inches per second -- I'm trying to come up with an analogy, but am not doing well. Maybe some bugs walk at about that speed. It's a very small amount. The maneuver also produced a very small change in Cassini's trajectory, lowering its flyby altitude past Titan by only 10 kilometers (6 miles). Why bother with such a small adjustment? Because, as the report says, it means they can "save 2.5 meters per second in downstream maneuver sizes." Navigators tend to measure how much fuel the spacecraft has left in terms of how much velocity change using that fuel can produce. A savings of 2.5 meters per second is not chump change; it's quite a lot, actually, and will mean that much more fuel left for the extended mission.

Here's the next item, from the same day:

A new background sequence, S31, began execution on board the spacecraft today. The sequence will run for 33 days, concluding on July 14. During that time there will be two targeted encounters of Titan, twelve non-targeted flybys of Mimas, Atlas, Prometheus, Epimetheus, Methone, Tethys, Enceladus, and Rhea, and five maneuvers, numbers 116 through 120, are scheduled for execution.
We're in the ring plane again, so there's lots of encounters with moons now. Mimas, Tethys, Enceladus, and Rhea are all largish round moons; Atlas, Prometheus, Epimetheus, and Methone are lumpy little ringmoons.

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Emily Lakdawalla

Solar System Specialist for The Planetary Society
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