NASA's Cassini spacecraft delivers stunning views from its new orbital perch.

Cassini final orbits
This diagram shows the ring-grazing orbits (gray) versus grand finale orbits (blue) and Cassini's final plunge (orange).

Welcome to the planet Saturn as you've never seen it before. NASA's Cassini spacecraft completed its first periapsis (closest approach) pass of this new phase of its mission phase. It crossed the plane of Saturn's rings this past Sunday at 8:09 a.m. EST (13:09 Universal Time). The passage was 57,000 miles (91,000 km) from Saturn's cloud tops and just 6,800 miles (11,000 km) from the center of Saturn's tenuous outer F ring.

“It's taken years of planning, but now that we're finally here, the whole Cassini team is excited to begin studying the data that come from these ring-grazing orbits,” says Cassini project scientist Linda Spilker (Jet Propulsion Laboratory) in a recent press release. “This is a remarkable time in what's already been a thrilling journey.”

The new orbit is part of Cassini's penultimate Ring Grazing Orbits mission. Cassini will perform 20 ring-grazing orbits in all, with the next one coming right up on Sunday, December 11th. Cassini also burned its main engine one final time for 6 seconds during the ring plane crossing.

Moonlet Madness to Come

A gibbous Tethys, imaged by Cassini on December 4th.
NASA / JPL / Space Science Institute

We've already seen some amazing new images, mostly from Cassini's pass over the north pole of Saturn about two days prior to its closest approach. Cassini also passed Tethys on December 4th, giving us more great views of the battered world.

This first ring-plane crossing was dedicated to the final main engine burn and radio occultation experiments, which aim to make accurate measurements of the overall density and mass of Saturn's ring system. Expect more views to come on successive ring-plane passes of the mysterious propeller systems embedded in the rings, as well as views of Saturn's inner moons.

Saturn's Polar Hexagon

The highlight of this first pass was new close-up views of Saturn's mysterious polar hexagon. First spied by Voyagers 1 and 2 in 1981 and 1982, this strange feature gracing Saturn's northern polar region is as beautiful for its symmetry as it is perplexing. The images below were taken using Cassini's wide-angle camera from a distance of 400,000 miles (640,000 km) — about 1.6 times the Earth-Moon distance — revealing features about 95 miles (153 km) across per pixel. Image filters covering the violet (420 nanometers) through near-infrared (939 nanometers) spectrum penetrate and reveal the structure of the hexagon through successive cloud layers.

Saturn Hexagon
A collage of Saturn's north polar hexagon as seen by Cassini on December 2nd, in (clockwise, from top left) violet, red, near-infrared, and infrared.
NASA / JPL / Space Science Institute

Located at latitude 78° north, the polar hexagon features six sides, each extending about 8,600 miles (13,800 km), longer than Earth is wide. One leading theory proposed by researchers at Oxford University in 2010 is that the strange hexagonal shape results from a steep latitudinal wind gradient at high latitudes. Saturn rotates once every 10 hours 33 minutes. Fluid dynamics studies carried out on gas-filled spheres have also managed to produce hexagonal, triangular and even octagonal features.

Saturn's "Hexagon"
This colorful view is the highest-resolution view of the unique six-sided jet stream at Saturn's north pole. More information about this image is here.
NASA / JPL / SSI / Hampton University

Clearly, something unique is going on here. How long-lived is the polar hexagon? Why don't we see such an enigmatic feature on other gas and ice giants, or even on the southern pole of Saturn?

Of course, there's a lot more to come. As we've mentioned on these pages, a final close pass by Saturn's giant shrouded moon Titan on April 22nd of next year will set Cassini up for its final 22 passes during its Grand Finale Orbits phase of the mission, plunging the spacecraft through the 1,500 mile-wide (2,400 kilometers) gap between the innermost rings and Saturn's cloud tops starting next year on April 26th.

This will also put Cassini on course for its final demise next year, with a fiery plunge into Saturn's atmosphere on September 15th, a protective measure to avoid potential future contamination of Saturn's moons.

Enjoy these final views over the next year of one of the most exotic and photogenic planets in the solar system.


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