Enceladus Plume Search, Nov. 27 |
Enceladus Plume Search, Nov. 27 |
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![]() Senior Member ![]() ![]() ![]() ![]() Group: Members Posts: 1465 Joined: 9-February 04 From: Columbus OH USA Member No.: 13 ![]() |
Interesting item in the science plan kernel (S16) just released to the NAIF website:
OBSERVATION_ID: S1629 SEQUENCE: S16 OBSERVATION_TITLE: Plume Search SCIENCE_OBJECTIVE: Hope to detect/observe plumes, whether from volcanic activity or geysers. OBS_DESCRIPTION: Point and stare. SUBSYSTEM: ISS PRIMARY_POINTING: ISS_NAC to Enceladus (0.0,5.0,0.0 deg. offset) REQUEST_ID: ISS_018EN_PLUMES001_PRIME REQUEST_TITLE: ENCELADUS Geyser/Plume Search REQ_DESCRIPTION: 1;ENCELADUS Geyser/Plume Search 1x1xNPp -- 3 different exposures BEGIN_TIME: 2005 NOV 27 19:00:00 UTC END_TIME: 2005 NOV 27 20:00:00 UTC -------------------- |
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#2
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Senior Member ![]() ![]() ![]() ![]() Group: Members Posts: 1870 Joined: 20-February 05 Member No.: 174 ![]() |
The Voyager estimates of masses and densities for some of the moons were very marginal. The actual dataset used was the tracking data from 2 flybys, plus the rather noisy Pioneer 11 flyby a year before Voyager 1. (Pioneer was very close to Solar Conjunction and the signals were going through the solar corona.)
Added to the spacecraft tracking (and position location in images) was the historical record of optical observations of the moons. Precision is low, but the long time base gives moons time to show subtle orbital interactions due to resonances. Without checking old xeroxes, I think Dione's mass was rather well determined from it's resonant orbital mechanics effect on Enceladus. The larger inner moons masses were relatively well determined, the smaller ones were more marginally measured. Cut a moon's diameter in half and it's mass is 1/8'th the original. Iapetus, Hyperion and Phoebe were so far out they interact little and they were all far from Voyager's trajectories. I don't know how the final pre-Cassini mass estimates compared with analyses from 15-20 years earlier, but in all of those cases, to some extent, the error bars on mass estimates are educated guesses, based on adding random noise to models to judge sensativities for each case, and based on in part very educated guesses on the level of systematic errors in the data. Cassini's long orbital tour gives more ability to separate weak gravitational effects on the trajectory and better separate different satellites masses, giving on it's own a better set of data for moon mass estimates. And for any satellite, a really close flyby gives the first chance for a really accurate mass, and for all but the outer sats, a chance to crudely estimate the difference between the oblate and tidally elongated shape of the satellite and of it's gravity field. A moon that never differentiated and has "rock" and "ice" uniformly distributed from core to surface will have a gravity field that matches it's shape. A moon that fully melted and has all the "rock" as a well defined core will have it's physical shape more flattened and stretched than the gravity field, as the deeply buried rocky core's gravity will be nearly spherical. Local gravity anomalies, if found, are "icing on the cake", datawise. But the main point here is that searching for any deep meaning in differences between pre-Cassini moon masses and the new results is an effort in futility. |
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