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SYFY WIRE Bad Astronomy

Cassini's (first) deep dive above Saturn's clouds

By Phil Plait

Wednesday night, the Cassini Saturn probe risked life and limb to get closer to the ringed planet than any spacecraft ever has: It took a dip down toward Saturn, skimming a mere 3000 kilometers above the cloud tops, and then passing a razor’s margin of just 300 kilometers inside the innermost visible edge of the rings!

It’s hard to convey just how risky this maneuver was. At the speed it was traveling —124,000 kilometers per hour, fast enough to cross the entire United States in just two and half minutes! — a stray ice particle the size of a snowflake could have put a hole in the spacecraft. Because of this, engineers turned Cassini so its big antenna dish was pointed into the direction of flight, to act like a windshield and protect the more delicate machinery behind it. But that also meant that the probe went “dark” for many hours after the pass, until it could reorient itself to send a message back to Earth.

The images and data it took are still being sent to us, but what we’ve seen so far is already spectacular! The image at the top of this post shows a circular hurricane precisely at Saturn’s north pole, a storm about 2000 kilometers across! Wind speeds inside it have been clocked at 300 kph. The picture is a "natural color" combination of three images taken using blue, green, and red filters; the blue color in the storm is real! That's due to scattering of sunlight, the same reason the Earth's sky is blue. The image was put together by my friend and astronomer Sophia Nasr, who graciously let me use it here on the blog. She used a tutorial written by another astronomer (well, planetary scientist) friend of mine, Emily Lakdawalla of The Planetary Society.

The storm looks pretty different when a clear filter was used:

Saturn north pole

It looks dark here because the image was exposed to show the brighter clouds around it; those may be composed of ammonia ice, which is an excellent reflector of visible light.

That permanent hurricane sits inside the much larger northern polar vortex, a huge hexagonal wind pattern 20,000 kilometers in diameter, caused by winds blowing around the planet. Cassini was too close to the cloud tops to be able to get wide shots of that on this pass.

clouds above Saturn's north pole

This shot shows some of those clouds near the pole; you can also see thin band of gas swirling around the storm. These images are small and haven’t been properly processed, yet (they have been archived “raw”, meaning right off the camera, so they still have blips in the pixels from cosmic rays — subatomic particles zipping through spac e— and other artifacts), but, by my eye, those bands are, at most, a few kilometers wide.

Mind you, Saturn is currently 1.4 billion kilometers away!

I’ll post more images as they come in. But bear in mind: This is the beginning of the end for Cassini. The reason mission planners have set up this risky trajectory is because the mission will end come September, when Cassini will fall into Saturn, itself, burning up as it violently compresses the atmospheric gas in front of it. Cassini is already on that path; a near flyby of the giant moon Titan a few days ago altered the spacecraft’s orbit, and some farther passes of the moon will gently nudge it this way and that, sending it on that final plunge.

However, there’s still more to come. This was the first dive of 22 total, so we’ll be seeing a lot more amazing images of the planet and its rings in the coming months! Stay tuned.

Image Credit: NASA/JPL-Caltech/Space Science Institute/Sophia Nasr