Last week, it was Pan. This week, it’s Tethys. The 15th ring-grazing orbit is just over a day away, and Cassini will pass within 4,900 miles of Saturn’s F ring. Onboard the spacecraft, the science instruments will be gathering as much as data as they can during the quick dive towards F ring.

The Ultraviolet Imaging Spectrograph (UVIS) will continue observations of Saturn’s northern aurora. The Ion and Neutral Spectrometer (INMS) along with the Cosmic Data Analyzer (CDA) will take another whiff of gases and particles near the plane of Saturn’s rings.

But it’s the work of Cassini’s visible light camera, the Imaging Science Subsystem (ISS), I want to talk about. This is the same camera that gave us last week’s incredible view of Pan.

Pan moon

This week, the ISS instrument will take a peek at Tethys. Specifically, the puzzling red streaks stretching for hundreds of miles across its surface. Here’s an image captured in April 2015.

Tethys red streaks

The enhanced-color image shows several arc-shaped streaks slashing across Tethys’ surface. Scientists have seen reddish features on moons before, most prominently on Jupiter’s moon Europa. But, besides a few small craters on Dione, reddish features are rare on the dozens of moons circling Saturn. What causes them? That’s a mystery for scientists back on Earth.

There are a couple of ideas floating around, though. It could be exposed ice with chemical impurities. Or, maybe outgassing from within Tethys. Fractures are another possibility, but the images we have right now just aren’t good enough to say for sure. With new images release daily, we should get the first RAW images of Tethys and its red streaks by late next week.

Countdown to Orbit 270

Cassini’s upcoming 15th ring-grazing orbit is also its 265th orbit overall. The 270th orbit marks the beginning of the end for the spacecraft. About one month from now, Cassini will make its final close flyby of Saturn’s largest moon Titan. This orbit will bring the spacecraft within 600 miles of the moon on April 22.

Cassini will use Titan’s gravity to change the shape of its orbit. This change puts the spacecraft on an inevitable collision course with Saturn. If the team on Earth called it quits after orbit 270, the spacecraft would eventually crash in Saturn’s atmosphere and burn up.

With Cassini’s fate sealed, the mission enters the Grand Finale. A harrowing set of flybys that will bring the spacecraft between Saturn’s upper atmosphere and its innermost rings. The images captured here will be breathtaking. We’ll see beautiful closeups of Saturn’s rings and atmosphere that will amaze for years.

After 22 of these close flybys, Cassini will slam into the upper atmosphere of Saturn and go out in a blaze of glory. September 15th will be the last day we talk to Cassini.

Image credits: NASA

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