Tag: Rhea

A trillion and five moons

By Phil Plait | September 13, 2011 9:45 am

On July 29, 2011, the unending dance of Saturn’s moons lined them up perfectly for a stunning view by the Cassini spacecraft: five moons arrayed for your pleasure:

[Click to embiggen.]

From left to right that’s Janus, Pandora (in the rings), Enceladus, Mimas, and Rhea. Perspective plays a role here; Rhea is three times bigger than Enceladus, but was much closer to Cassini when this picture was taken, so it looks even bigger.

But the moons themselves are so different from each other! Janus is a lump, too small to have enough gravity to crush itself into a sphere. Enceladus is mostly ice, so it appears very bright in this image compared to its rocky siblings. You can just barely see part of the monster crater Herschel peeking out of the dark side of Mimas, while Rhea is peppered with smaller craters. And Pandora orbits inside Saturn’s rings themselves, its meager gravity enough to entrain the particles in the thin F ring and keep it in place.

And, of course, the rings themselves, composed of countless tiny ice crystals. Over millions of years, collisions have ground them into pieces ranging in size from barely big enough to see to perhaps 10 meters across, the volume of a roomy two-car garage.

Amazing. And this vista was taken just a couple of weeks after Cassini’s seventh anniversary in orbit around Saturn. Even after all that time, and tens of thousands of images, it still has the capability to take our breath away.


Related posts:

- Cassini’s Pentaverate
- Cassini’s slant on the rings
- A little weekend Saturn awesomeoness
- The real Pandora, and two mooning brothers

CATEGORIZED UNDER: Astronomy, Pretty pictures

Saturn's biggest kids play peek-a-boo

By Phil Plait | July 1, 2011 11:30 am

I spent all day yesterday writing a 2000-word article for a print venue to be named later, and the weather outside is sunny and delightful and begging to be biked in, so I am disinclined to write something deep and philosophical today. So instead here is just a simply way-cool picture from Cassini taken in 2009, showing the Saturnian moon Rhea peeking out from behind the much larger Titan:

[Click to eneldergodenate.]

[UPDATE: I messed up here. In the original post I misread Titan's radius when I looked it up, and was comparing it to Rhea's diameter. This changes my numbers enough that I have simply corrected everything below; otherwise it would be too confusing to read. Thanks to the commenters for pointing this out!]

Rhea is a little over 1500 km (900 miles) across, and Titan 5150 km (3100 miles). However, in this shot, Rhea was almost two and a half times farther away than her big sister, so it looks smaller than it really is. Titan has a thick atmosphere, which is pretty obvious in the picture, while Rhea is basically a ginormous iceball.

Still, hmmm. Titan and Rhea are the two largest moons of Saturn, but to be honest Titan really is a lot bigger than Rhea, more than 3 times wider. Why such a big gap in sizes? Jupiter’s two largest moons, Ganymede and Callisto, are much closer together in size (5260 and 4820 km, respectively), and Ganymede is only 1.7 times bigger than Jupiter’s fourth largest moon, Europa. After that, though the rest are far smaller.

It seems to me that Saturn and Jupiter are telling us something about the physics of the way their moons formed. But what could it be? Titan orbits well over twice as far from Saturn as Rhea, while Ganymede is actually closer to Jupiter than Callisto. Is that important? Did those moons form at other distances and get their orbits jostled through gravitational interactions over billions of years, maybe even switching positions?

These are pretty basic questions, but it’s questions like these that lead to basic insights on how our solar system formed and changed as time went on.

And dangit! I guess I did get a little deep and philosophical here. Ah well, what can I say? Images like this are so pretty and so interesting to look at, they spark all kinds of thought processes in my head. And the more I do that, the more I want to do that. Science is like that: addictive, but in a good way.

Damn! Did it again.

Image Credit: NASA/JPL/Space Science Institute


Related posts:

- Cassini’s pentaverate
- Peeking past Rhea
- Dione and Rhea, sitting in a tree
- Titanic slice

Cassini's pentaverate

By Phil Plait | June 2, 2011 9:30 am

I haven’t posted a Cassini picture in quite some time. To make up for that, here’s a stunner of a family portrait showing five worlds!

[Click to enchronosate.]

This shot shows Saturn’s rings nearly edge-on, but dominating the scene is Rhea, 1500 km (950 miles) in diameter, seen here 61,000 km (38,000 miles) distant. Below it is Dione, to the right and just above the rings is Epimetheus, and Tethys is all the way on the right, below the rings.

So what’s the fifth moon? Look to the right of Dione, right at the rings. See that tiny bump? That’s dinky Prometheus, all of 119 km (71 miles) along its longest dimension — it’s basically a spud orbiting Saturn. Prometheus, along with its sister moon Pandora, act like shepherds, keeping Saturn’s F-ring particles entrained.

Saturn is a weird, weird place, and it’s orbited by a diverse collection of weird, weird moons. I forget that sometimes, but images like this really drive it home.

… on the other hand, as we discover more planets orbiting other stars, we see lots of them with masses like Saturn’s. Of course, low-mass planets like Earth are much harder to find, but still. Who knows? It may turn out Saturn’s normal, and we’re the weird ones.


… and if you’re wondering about the post title, this may help. Whoa, man.


Related posts:

- Dr. Tongue’s 3D House of Prometheus
- Perspective on four moons
- Cassini’s slant on the rings
- The bringer of fire, hiding in the rings
- The real Pandora and two mooning brothers

CATEGORIZED UNDER: Astronomy, Pretty pictures

Peeking past Rhea

By Phil Plait | February 28, 2011 10:00 am

If I had to pick a single word to describe the system of moons swarming around Saturn as seen by Cassini, it would be "bizarre", "amazing", "exquisite", "jaw-dropping", and "Holy Haleakala!"

See for yourself:

[Click to enchronosenate.]

Wow! I love these shots showing perspective! The moon at the top is Rhea, which is about 1500 km (950 miles) across. We’re looking past its south pole here. The moon farther away is Dione, which is 1100 km (700 miles) in size. And since Cassini was very nearly in the plane of Saturn’s equator, the rings are nearly edge-on. Note that Dione is on the other side of the rings as seen by Cassini, so the bottom of the moon is obscured by the rings. We can’t see Saturn itself, but it’s off to the left in this shot.

Rhea is only a little bigger than Dione, but is a lot closer in this shot: 61,000 km versus 924,000 for Dione! That’s why Dione looks so much smaller. As seen by Cassini in this shot, it’s actually more than twice as far as our Moon is from the Earth. Both moons are composed of mostly water ice, with some rock. Both have been heavily battered by impacts, as you can see.

What a gorgeous, spectacular picture. I never get tired of these.

Image credit: NASA/JPL/Space Science Institute

Tip o’ the rings to Carolyn Porco on Twitter.


Related posts:

- The more distant moon
- Dione and Rhea, sitting in a tree
- Two alien worlds, superposed
- Midnight on a ringed world

CATEGORIZED UNDER: Astronomy, Pretty pictures

Top Ten Astronomy Pictures of 2010 – Runners Up

By Phil Plait | December 20, 2010 6:30 am

When I made my Top 14 Astronomy Pictures of 2010, it was really tough cutting some out. This is a gallery of the images that, for whatever reasons, I decided to leave off. They’re still spectacular and gorgeous, though! Click on the thumbnail in the slider to go to an image, use the arrows to navigate back and forth, and click on the big image displayed below to get more info and a bigger version if available.


CATEGORIZED UNDER: Astronomy, Pretty pictures, Top Post

The Top 14 Astronomy Pictures of 2010

By Phil Plait | December 14, 2010 7:01 am

Use the thumbnails and arrows to browse the images, and click on the images themselves to go through to blog posts with more details and descriptions.


Dione and Rhea, sitting in a tree

By Phil Plait | September 24, 2010 9:23 am

I know, just a couple of days ago I was saying that I was surprised that Cassini could still, um, surprise me. But then it sends back a completely crazy picture like this!

cassini_figure8moons

HA! What an awesome shot! But what’s going on here?

OK, let me explain this gently. When two moons are in love, they…

No, wait. So, moons are really gigantic single-celled organisms, and when they reproduce, they fission…

No, wait. OK, seriously: the top moon there is Dione, and the bottom one is Rhea. As Cassini flew by them, Dione was closer (a little more than 1.1 million km or about 690,000 miles), and Rhea farther away (1.6 million km or 1 million miles). The angle of Cassini’s trajectory was just right such that Dione passed right in front of Rhea, and it snapped this image just as it happened.

Read More

MORE ABOUT: Cassini, Dione, Rhea, Saturn

Two alien worlds, superposed

By Phil Plait | September 22, 2010 7:00 am

Every time I think Cassini can’t possibly send back a more dramatic image from Saturn, it takes my preconceived notions and crushes them to dust.

cassini_titan_rhea

Oh. My.

That shows the moons Titan (in the background) and Rhea, the two largest of the gazillions of moons circling the ringed planet. Titan has an atmosphere, which is why it looks fuzzy.

This picture is crazy amazing! Rhea looks like it’s about to smash into Titan — if you’re curious, this is what my nightmares look like — but in fact they’re very far apart. Rhea was about 1.1 million kilometers (680,000 miles) away from Cassini when this was taken, and Titan was more than twice that. So they were actually separated by over a million kilometers, about three times the distance of the Moon from the Earth! Cassini used the narrow-angle camera (essentially a big telescope) to get this shot, so it looks foreshortened — Titan is actually more than three times wider than Rhea, but it looks smaller than that here because it’s twice as far away. The two moons are in no danger of collision.

What a stunning shot! And I love how these two worlds are so different. Icy Rhea is pitted and cratered, but Titan’s thick atmosphere smudges out all details in visible light. Together, they’re an excellent example of diversity in the Saturn system, and a reminder of just why we sent Cassini there in the first place.

Tip o’ the spacesuit visor to Carolyn Porco. Image credit: NASA/JPL/Space Science Institute

CATEGORIZED UNDER: Pretty pictures
MORE ABOUT: Cassini, Rhea, Saturn, Titan

The bringer of fire, hiding in the rings

By Phil Plait | June 23, 2010 12:04 pm

After yesterday’s depressing picture, how about one that will make you smile?

The ever-amazing Cassini spacecraft sent back this pretty nifty shot of Saturn’s icy moon Rhea playing peekaboo in the rings:

cassini_rhea_prometheus

Beautiful, isn’t it? You can see that Rhea was on the other side of the rings from Cassini when this image was taken, and that the spacecraft was almost, but not quite, in the plane of the rings, too.

But there’s more to this shot… Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures

A little weekend Saturn awesomeness

By Phil Plait | May 28, 2010 12:01 pm

If you need a little more awesome in your weekend, then try this:


cassini_rhea_janus

I know I’ve been posting quite a few Saturn images from Cassini, but I really like this one. It shows the big round moon Rhea, the lumpy small moon Janus, and a lovely view of the foreshortened rings. Both moons were on the far side of the rings, well over 1 million kilometers away, when Cassini took this shot. Rhea is over 1500 km (900 miles) in diameter, while Janus is only about 180 km (110 miles) across. For comparison, our own Moon’s diameter is almost 3500 km (2100 miles).
Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Janus, Rhea, Saturn
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