Subject: Curious Cassini Conundrum
From: adamod <[email-masked]>
Date: Fri, 13 Mar 2009 09:44:08 -0500

ATIG Members-

In reading March 12's issue of Cassini Significant Events, a weekly JPL PAO
newsletter, I ran across the following riddle.

> Thursday, March 5 (DOY 064):

> How is it that at one moment Cassini can be found nearly at apoapsis, the
> farthest point from Saturn in an orbit, and within a day, be at periapsis, the
> closest point to Saturn in an orbit, when its orbital period is around 16
> days?  Well, it is happening at the end of March this year.  On DOY-86, Mar.
> 27, the Titan 51 targeted flyby will occur.  Just a few hours later, apoapsis
> and the start of orbit #107 would have occurred.  However, because of the
> gravitational influence of Titan during the flyby, Cassini's orbit is modified
> such that apoapsis on the new Saturn centered orbit occurs prior to the Titan
> encounter, on an orbit different from the one Cassini actually flew. For orbit
> 107, on DOY-87, Mar. 28, Cassini is now at the closest point to Saturn in that
> orbit.  The above scenario is unique in the history of the project and will
> not occur again before the end of the extended mission. The fact that
> Cassini's orbit about Saturn is very nearly circular at this time is what
> enables this rather curious sequence of events.

Saturn-centered geometry pertaining to Titan-51's 180-deg apsidal shift is
illustrated in the attached CassiniVsSaturn GIF.  The viewpoint for this
trajectory plot is nearly normal with respect to Cassini orbit planes before
and after the Titan-51 flyby.  Note Saturn's north pole, just visible from
this perspective and emerging from nearly 15 Earth years of winter darkness,
is pointed downward.

The attached CassiniVsTitan trajectory plot is also viewed from nearly
normal to the Cassini plane of motion, but in a Titan-centered reference
frame.  In this case, Titan's south pole is visible and pointed downward.
Periapsis height for this flyby is planned at +960 km.

If Outlook won't download these GIFs, remember that NASA WebMail will.

-Dan