As Cassini weaves its way around the multiple moons of Saturn, it’s not really a coincidence when one gets in the way of another. As a matter of fact, it’s a guarantee. These are called mutual events, and when Cassini dove past Dione, it saw this terrific view of Mimas peeking out from behind it:
Nifty, huh? [Click to encronosenate.]
Dione is nearly 3 times larger than Mimas (1100 versus 400 km wide), but Mimas was also more than 6 times farther away, making Dione loom nearly 20 times larger in this shot. I like how you can’t really see the unlit side of Dione, but Mimas marks it pretty well, sliced in half by the edge of the larger moon.
Funny, too: I was thinking to myself that if Cassini was in position to catch this shot, then it should have also caught Mimas when it was on the other side of Dione, the lit part. Well, seek and ye shall find: I searched the Cassini raw image archive and found it! I put a small version of it here; click to embiggen. You can just barely see a small segment of Saturn’s rings in the lower left corner, too.
Neat! I like it when stuff makes sense. While this alignment is rare to see from Earth — we’re a lot farther away, and the geometry has to be precise — we do see moons transiting across their parent planets, and, far less often moons in front of moons. But what’s rare to us is common to Cassini, with its front row seat to this amazing system of worlds.
One of these things is not like the others:
The Cassini spacecraft took this lovely image in December 2011, during a close pass of Saturn’s moon Dione. Ignoring Saturn’s rings slashing through the picture, we see, from left to right, the moons Dione, Prometheus, and Epimetheus. Which is the odd moon out?
Here’s a hint: Dione is 1100 km (700 miles) across, Prometheus 86 km (53 miles) along its longest axis, and Epimetheus 113 km (70 miles). Got it now?
Yeah, sure, Dione is far larger than the other two! But that’s not my point: Dione is round, while the other, smaller moons are lumpy and rather potato-shaped. Why?
Size matters. In this case, a bigger moon means more mass, and that means more gravity. In general, the force of gravity points toward the center of an object. As you add more mass to an object, gravity gets stronger. On a small moon, a big lump of rock like a mountain feels very little force downward, while on a more massive moon the force would be larger. If the moon has enough mass, and enough gravity, the force will be more than the internal strength of the rock itself, and the mountain crumbles.
So moons that are big and massive enough will tend to flatten their surface, or, more accurately, shape them into spheres. Dione is big enough to do that. Prometheus and Epimetheus are not. Dione is a big ball, the other two are spuds.
Note that gravity’s not the only thing that can make objects spherical. Water has surface tension, for example, caused by the electrostatic attraction between water molecules. In space, without gravity, drops of water are spherical. Random processes can generate round objects too: I bet if we could get a super-duper close look at Saturn’s rings, we’d see the trillions of chunks of ice that make up the rings are round too. But that’s from collisions; there are enough of those bits of ice that they smack into each other. Since they spin and tumble, over time any part of a chunk will have gotten hit by some other chunk, and that will tend to make them round.
So how big does an object have to be before it starts to become round via gravity? That’s complicated, and depends on its composition — a ball of ice the same size as a ball of iron will have far less gravity since it’s so much less dense, and will have lower mass. But for a ball of ice and rock — like Dione — that size is clearly no bigger than 1100 km across. And if you’re wondering how this might play into our concept of what a planet is, then you might want to read this. I’m way ahead of you!
The Cassini spacecraft has been touring Saturn and its moons for 7 years now, and yet still manages to send back images that are simply astonishing. Just yesterday, the probe swung past the icy moon Dione at a distance of just 99 kilometers (62 miles) over the surface! Compare that to the moon’s diameter of over 1100 km (670 miles) and you get an impression of how close that was.
The purpose of the pass was to get infrared spectra of the moon, so only a few visible light images were taken. But oh, what pictures they were! Check this out:
Wow! Dione dominates the view, its cratered surface of ice looking like a golf ball that had a dimpling machine accident. You can see Saturn’s rings on the left, nearly edge-on, and two more moons as well: the gray lumpy potato of Epimetheus, only about 130 km (70 miles) across, and Prometheus, also about 136 km along its long axis. My first guess is that Prometheus is farther away than Epimetheus in this shot, since it looks smaller (I wondered for a second if it’s possible we’re seeing it rotated a bit so it’s pole-on, but it’s a very elongated rock; so we’re definitely seeing it mostly from the side here).
After seeing that picture, I excitedly grabbed the next one, and got confused for a moment:
Now, wait a sec. There’s Dione, the rings, and Epimetheus. See how before, Epimetheus was mostly above the rings, but now it’s mostly lower? That means Cassini moved up a little bit from the plane of the rings, so the little moon looks like it moved down. So then why did Prometheus move up?
Because it didn’t. That’s not Prometheus, it’s Pandora! A different moon, though they’re related: they are shepherd moons, which means they have very similar orbits, and occasionally swap places! It’s weird, but I’ve explained it before. Anyway, Pandora and Prometheus are almost exactly the same size, and both are elongated like an Idaho spud. So I’m not too surprised I was confused for a moment when I saw the second picture. When you look a little more closely you can see the shapes are different, though.
More pictures were returned from the pass (including a couple showing Mimas peeking out from behind Dione), so you should take a look. They’re pretty dramatic.
Image credit: NASA/JPL-Caltech/Space Science Institute
Take four moons, some rings, a schoolbus-sized spacecraft, and mix them together. What do you get?
That stunning shot is from the Cassini spacecraft orbiting Saturn. The big moon is Titan, and by big, I mean bigger than the planet Mercury. Big enough to have a thick nitrogen atmosphere, clearly visible in this picture. The bright moon superposed right on top of Titan is Dione, its icy surface shiny and white.
On the right, just outside the rings, is tiny, flying saucer-shaped Pandora. And the fourth moon? That’s Pan, the tiny white spot in the gap in the rings on the left, barely visible in this shot. But that’s understandable, since Pan is less than 30 km (18 miles) across, and this was taken from a distance of nearly 2 million kilometers (1.2 million miles) away!
I love pictures like this; they remind me that even after 7 years of Cassini touring around Saturn, there’s still much to see and much beauty to behold there.
Image credit: NASA/JPL-Caltech/SSI
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.
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!"
[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.
Quick! Which of these two moons was closer to the Cassini spacecraft when it took this image?
Hard to tell, isn’t it? Actually, it’s impossible to tell without knowing what’s what.
The gray moon to the upper right is Dione, and the blindingly white moon to the lower left is Enceladus (famed for its geysers of water erupting from its south pole). When I look at this picture, I can mentally swap the two moons in distance, making one seem farther away, then closer. It’s a bit like the Necker cube illusion.
But here’s a big hint: Dione is Saturn’s third largest moon at 1123 km (698 miles) across. Enceladus is comparatively small at 504 km (313 miles) in diameter. Since Dione is not twice the size of Enceladus in the picture, it must be farther away. In fact, Cassini was 510,000 km (317,000 miles) from Enceladus when it took this picture, but Dione was a more distant 830,000 km (516,000) — more than twice as far as our own Moon is from Earth.
Note that Dione is much duller and dimmer than Enceladus; its surface is rocky while that of Enceladus is highly reflective water ice. That’s why the smaller moon’s image is washed out; the exposure was set for darker objects. Both moons look slightly non-circular, but that’s because the Sun was very slightly off to the side when this picture was taken, so neither moon is quite "full".
Engineers and scientists know precisely where Cassini, Saturn, and its moons are at any given time by first knowing celestial mechanics: the math and physics of orbital motion. But they also get telemetry from Cassini that gives them its position. Physics is all well and good, but nothing beats a measurement in the field. And that’s how they know how distant those moons were when the shot was taken. Without that knowledge, these pictures would still be amazing and important to planetary astronomy, but would hold a lot less scientific value.
As usual, you have to be careful when looking at pictures of objects in space. Your brain is accustomed to all sorts of cues for judging size and distance like sunlight angle, amount of detail seen, the amount of haze in the air… all of which are totally absent in space. You might think you’re not easily fooled, but your brain has other ideas.
Image credit: NASA/JPL/Space Science Institute
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.
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.
When all is said and done, the outer solar system must be a pretty scary place to live. How’d you like to live in a location that has a battle-scarred history like this?
That’s Saturn’s moon Dione as seen in by the space probe Cassini in a recent pass of the small world. Earlier today (September 4, 2010), Cassini dipped to a height of less than 40,000 km (25,000 miles) above Dione’s surface — that’s about the same height as weather satellites above the surface of the Earth!
The landscape of Dione is shockingly battered. Craters are everywhere, indicating a fierce history of meteoric bombardment. It is also heavily littered with cliffs and chasms.
I learned something incredibly interesting looking up information about the moon: Read More