What the Jet Propulsion Laboratory refers to as ‘the first phase of the mission’s dramatic endgame’ begins tomorrow for the Cassini Saturn orbiter. Having given us an ocean within Enceladus and numerous images of Titan’s lakes and seas (not to mention ring imagery of spectacular beauty), Cassini now enters a phase in which it encounters the rings in a new way, diving past their outer edge every seven days in a series of 20 passes. The spacecraft will be in an elliptical orbit inclined some 60 degrees from the planet’s ring plane.

“We’re calling this phase of the mission Cassini’s Ring-Grazing Orbits, because we’ll be skimming past the outer edge of the rings,” said Linda Spilker, Cassini project scientist (JPL). “In addition, we have two instruments that can sample particles and gases as we cross the ring plane, so in a sense Cassini is also ‘grazing’ on the rings.”


Image: Cassini crosses Saturn’s F ring once on each of its 20 Ring-Grazing Orbits, shown here in tan and lasting from late November 2016 to April 2017. Blue represents the extended solstice mission orbits, which precede the ring-grazing phase. Credit: JPL.

That grazing will include two passes directly through a tenuous ring created by meteor strikes on the small moons Janus and Epimetheus. Each orbit will cross the ring plane just outside the F ring, considered to be the boundary of the main ring system, with Cassini actually moving through the outer edges of the F ring in April. Here the science should be particularly interesting — the 800-kilometer wide F ring is malleable, developing and dispersing filament-like structures, dark channels and streamers over short periods of time.

“Even though we’re flying closer to the F ring than we ever have, we’ll still be more than 4,850 miles (7,800 kilometers) distant. There’s very little concern over dust hazard at that range,” said Earl Maize, Cassini project manager at JPL.

A gravity assist by Titan on the T-125 flyby put the craft into its ring-grazing orbits. The new orbits (‘revs’ or revolutions in JPL parlance) begin when the spacecraft is at apoapse, its most distant position from Saturn, and the ring plane crossings happen when Cassini is at periapse, its closest approach to the planet during that orbit. The first ring-grazing orbit beginning on November 30 will see the ring plane crossing 5 days later. A final Titan flyby (T-126) in April sets up the last phase of the orbiter’s mission.

The new orbital configuration will allow Cassini to make close studies of the A and B rings as well as the F at a high level of detail. The A ring’s so-called ‘propellers’ — features that mark the location of tiny moonlets — should be seen in the best detail yet, offering researchers the chance to examine their structure. We’ll begin to see images from this phase of the mission in December as the spacecraft resolves details smaller than 1 kilometer per pixel.


Image: Saturn’s rings were named alphabetically in the order they were discovered. The narrow F ring marks the outer boundary of the main ring system. Credit: NASA/JPL-Caltech/Space Science Institute.

The distances involved in this phase of the mission are worth noting. The new orbits will take the craft within 90,000 kilometers of the planet’s cloud tops, but the Grand Finale phase, scheduled to begin next April, closes to within 1628 kilometers. This should be breathtaking, for the craft will move again and again through the gap between Saturn and the rings before making its final plunge into the atmosphere on September 15. Preparations for this final phase begin with a main engine burn on December 4. This is an engine that has served us well — this will be its 183rd burn — but the remainder of the mission will be handled with thrusters.

The spacecraft will be making a nine-hour movie of Saturn’s north pole with its Visual and Infrared Mapping Spectrometer, while also measuring (with other instruments) the boundaries of the planet’s upper atmosphere, which will be directly sampled in later orbits. You can find a complete list of Cassini’s ring-grazing orbits on this JPL reference page. Bear in mind as you look at it that there is a wonderful symmetry here. The level of detail we’ll see on these final orbits will be rivaled only by what Cassini saw upon its arrival at Saturn in 2004.