Cassini: Over the Top
This month Cassini passes the half-way mark of the “Titan 180 Transfer” portion of its four year nominal tour at Saturn. Because Cassini is a do-it-all mission with a dozen instruments on board, it has a broad set of goals that require a wide range of viewing geometries. The most obvious set of conflicting goals is that to study Saturn’s atmosphere equatorial orbits are best because then the rings, only a few tens of meters thick, virtually disappear and do not obstruct views of the planet, while on the other side, ring science prefers highly inclined orbits so that the rings can be easily seen at high resolution. The fields and particles instruments are there to sample the three-dimensional magnetosphere on all sides of Saturn relative to the Sun. Studies of icy satellites generally require equatorial orbits, while studies of Saturn’s aurora near the poles require inclined orbits. So Cassini has literally been flying around all over the place for the 2.5 years it’s been in orbit at Saturn.
This cartoon shows Cassini’s trajectory around Saturn in the 4 year nominal mission. Saturn is the small yellow circle in the center, surrounded by a slightly larger circle representing the main rings. The two white dotted lines show the orbits of Titan (inner) and Iapetus. The different colors represent different phases of the tour defined by geometry of the orbits. The initial orbits are depicted in white.
The side view shows that some orbits (such as the green phase, completed in mid-2006) are equatorial allowing unobstructed views of Saturn and frequent flybys of icy moons which orbit Saturn in its equatorial plane. Cassini is currently midway through the Titan-180 transfer orbits (blue).
During the tour design process in the late 90’s and early 00’s the clever folks at JPL hit upon an efficient mechanism to cover a lot of different geometries quickly: the Titan-180 transfer (now known as a “pi transfer” during extended mission discussions). Titan’s large size makes it the most effective satellite of Saturn to provide gravitational assists to Cassini and allow it to go onto a very different orbit of Saturn without using much fuel. The same process was used to accelerate Cassini to Saturn in the first place and is currently being used to nudge New Horizons on course to Pluto with a Jupiter flyby. Notice in the pictures above how Cassini’s orbit crosses Saturn’s equatorial plane at or near the orbit of Titan (inner dotted circle). Each Titan flyby allows the mission team to send Cassini onto a different orbit around Saturn. We are now roughly halfway through the Titan-180 transfer (the blue orbits above), so the inclination of Cassini’s orbit is decreasing toward the equator which it will reach this summer. The Titan-180 transfer changes the orientation of Cassini’s orbit, allowing magnetospheric studies to explore a large fraction of Saturn’s magnetosphere. The “transfer” of the title refers to flybys of Titan switching by 180 degrees (or pi radians) from inbound (flying by Titan as Cassini is heading in toward Saturn) to outbound (flying by Titan as Cassini is headed away from Saturn) or vice versa.
The final orbital phase for Cassini’s nominal mission (represented in red above) is a series of small, rapid orbits with increasing inclination. Both the Titan-180 and the final so-called “high inclination sequence” are particularly good for ring science, as well as polar atmospheric studies and magnetospheric studies. There is some exciting icy moon science in there as well, as a close (25 km!) flyby of Enceladus is anticipated for the high inclination sequence, giving us another close look at those intriguing geysers.