Blogs / Bad Astronomy

Today (as I write this), the Cassini spacecraft passed just a hair under 100 km (62 miles) from the surface of Enceladus, an icy moon of Saturn. This little moon is scientifically incredibly interesting; there are geysers at the south pole that are spewing out water! The images are just now coming in, and have not been calibrated or processed yet, but they are still breathtaking. I particularly like this one:

[Click to embiggen, as usual.]

That, me droogs, is high art. Enceladus was about 190,000 km (118,000 miles) away from Cassini when that shot was taken, a little under half the distance of the Earth to the Moon. From this angle, Enceladus is lit in a gorgeous thin crescent, but we can see detail on the dark side, I suspect due to light reflecting off Saturn onto the moon. You can see ridges in the surface; the moon has a thick crust of ice presumably floating on an undersurface ocean of water (though there have been arguments about that), so the surface is a bit of a mess, looking for all the world(s) like ice floes seen at our own north pole.

The geysers are obvious too, blasts of light at the top of the moon’s limb as the water erupting from the south pole is lit by the Sun. Thumbing through the raw images is a delight (once there, set the target for Enceladus, choose both narrow and wide angle, and put in dates of October 30 through November 3 to narrow the search). You’ll see dramatic images of the moon, its limb, the geysers, and everything.

Stunning, and wondrous. And there’s better to come: as Carolyn Porco herself mentions on Twitter, the primary purpose of this flyby was not to get images; November 21st is the imaging flyby where we’ll see lots of spectacular shots of the moon. So stay tuned!

Image credit: NASA/JPL/Space Science Institute

Mike Brown, who goes by the name plutokiller on Twitter — for good reason — has written a lengthy but interesting post about Saturn’s moon Titan. Why do I like it? Because it involves people, and the people are important to the story. I could write about clouds on Titan here on the BABlog, but I don’t know the people involved. Mike does, and that’s why his post is fun to read.

Gemini/AURA/Henry Roe – Lowell Observatory/Emily Schaller – IfA-University of Hawaii

So you got a telescope for Christmas/Hanukkah/Newtonmass/whatever… or you’ve had one for awhile. Either way, you get a treat this week. Or a lack of one. Saturn’s rings are going away.

Hubble caught Saturn with edge-on rings in 1996. Image courtesy Erich Karkoschka (University of Arizona Lunar & Planetary Lab) and NASA/ESA.

Well, kinda. Saturn, like the Earth, is tipped a bit compared to the plane of its orbit; we’re canted at a 23.5 degree tilt, and Saturn is off from being vertical by about 26.7 degrees. Saturn’s magnificent rings are aligned with its equator, so that means that roughly twice every Saturn orbit we cross the "ring plane". In other words, from Earth we see them edge-on.

And the rings are thin. Incredibly thin. Despite being over 200,000 km across, the rings are typically at most only a few dozen meters thick. To scale, that’s far thinner than a piece of paper.

So when we pass through the plane of the rings, they practically disappear from sight. I’ve seen it once through a telescope, when we were near (but not quite at) that point, and Saturn looks pretty weird when it goes commando. We’re used to it wearing these big gaudy rings, and there it was, nearly nude. It’s maybe not the best time to show the planet off to friends and family, but it’s still pretty cool.

The Earth actually doesn’t pass through the ring plane until September 2009, but at that time Saturn will be on the other side of the Sun, and pretty much unobservable. You’d think that a month or two before then would be the best time to observe the narrowly thinning rings, but in fact the best time is right now! Due to the vagaries of our mutual orbits, the rings are actually at a minimum right now, the last week of 2008, when they are inclined just 0.8 degrees to our line of sight.

If you have a telescope, get out and take a look! Saturn will be nearly ringless for the next few months, and then the rings will start to open up once again. After that, you’ll have plenty of time to soak in the phenomenal view of the solar system’s best showpiece — the next ring plane crossing isn’t until March of 2025.

Right now, Saturn is in the constellation Leo and shines fairly brightly at about magnitude 1, about the same brightness as the star Regulus which marks the heart of the lion. It rises around midnight local time right now, and is high enough to observe a couple of hours later. You can find sky maps at Your Sky and Heavens Above, and you can read more about the ring plane crossing on the NASA news page, and on Alan Dyer’s astronomy page.

And I have to add: this isn’t merely a curiosity; there is scientific value to this event. Telescopes can focus on the planet and see things otherwise hidden in the glare of the very bright rings. Faint moons, the existence of material above and below the ring plane, features on Saturn itself: all these can be easier to see without the icy, reflective ring particles blasting out light. It’s funny. Saturn is the most beautiful planet in the solar system through a telescope because of those rings, but it may be the most scientifically interesting when we can’t see them at all.