Celestia Forums

Well the next flyby event is for sure going to be interesting.

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The Enceladus flyby of Cassini at only 500 km distance in 6 days, 12 hours 59 mins ...
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Our (Jestr's ) Cassini orbit is always good for a pleasant surprise:


Time of closest approach: 9:08:00 UTC
http://saturn.jpl.nasa.gov/operations/saturn-tour-dates-05.cfm

Closest approach in Celestia: 9:08 UTC!

However the closest distance is incorrect by about a factor of two: ~1200 km instead of 500 km.

Here is the actual image, displaying the data of closest approach:



Bye Fridger

t00fri wrote:However the closest distance is incorrect by about a factor of two: ~1200 km instead of 500 km.

I've been playing around with Saturn's orbit and noticed a discrepancy in position of up to 4,000 km between Celestia's "vsop87-saturn" and a Sun-centered XYZ plot (1-day steps) for Saturn generated by JPL Horizons. This is far less than the radius of Saturn itself, but most moons are even smaller than that and may easily be displaced in their orbits in comparison to the Sun-centered Cassini trajectory.

I made more detailed Sun-centered XYZ plots for Saturn, Enceladus and Cassini on 2005-Mar-09 (using 9-minute steps), and the centers of the two instances of Saturn appear to be around 1,000 km away from each other on that day. For Enceladus, the separation is about the same, and Cassini does seem to fly by at an approximate altitude of 500 km above the JPL Enceladus.

I suppose this is a known inaccuracy in the "vsop87-saturn" implementation that may be difficult to do anything about. However, it helps not mixing spacecrafts and moons using entirely different coordinate systems during close encounters. I'm working on a Saturn-centered Cassini trajectory, but I'm concerned there may be some other discrepancy in the orientation of Saturn's equatorial plane making this a little difficult.

Thanks for the explanation, Anders; I had always wondered precisely what the difficulty was.

Interesting about the orbit discussion, would be nice to see the best info available for Celestia.

By the way I've updated my Enceladus texture with a reasonable synopsis of the February flyby images, just in time for comparison with the new batch we anticipate from next week. You can check the link out at the www icon.

Latest update - I've now added three images from the E04 flyby to my Enceladus map. The updated result is on my website at:

http://laps.fsl.noaa.gov/albers/sos/sos.html#ENCELADUS

When there is more information available, it looks like we may need some atmosphere settings for Enceladus?

http://saturn.jpl.nasa.gov/news/press-r ... newsID=553