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.
) Cassini orbit is always good for a pleasant surprise:
