Tag: Epimetheus

Saturn's shadow slices the rings

By Phil Plait | August 29, 2012 6:30 am

There is a whole lot of awesome in a picture of Saturn and its rings just released from the Cassini spacecraft. Check this out:

Mmmmm, ringalicious.

Cassini was about 2 million kilometers (1.2 million miles) from Saturn when it took this picture, so we’re seeing a decently wide-angle view. At the time, the spacecraft was below the plane of the rings, looking north (up, if you like). The Sun is off mostly to the left and up a bit.

The first cool thing is obviously the shadow of the planet itself cast on the rings. It cuts across like a black scythe! As I looked at the picture my eyes and brain kept trying to fill in the missing arc of rings, which was amplified by a slight afterimage as my eyes moved around. It’s a difficult illusion to ignore.

Second, I love how you can see all the different rings in the picture, including the thin, lumpy F-ring outside the main band. The big gap is called the Cassini Division; it’s not really an empty space since there are many faint thin rings inside it. They’re just hard to see here. The Cassini Division is fairly easy to spot even through a small telescope, looking from Earth like someone took a knife to the rings and sliced them.

Third, you can see the tiny moons Janus (below the rings on the left) and Epimetheus (above the rings on the left) as well. I wonder how hard it is to get a picture like this without seeing any moons in it? Saturn has quite the fleet of them.

Fourth, look to the left, just where the inner arc of the rings cuts across Saturn. You can see the planet right through the rings! The rings aren’t solid; they’re composed of gazillions of particles of nearly pure water ice. There are spaces between the particles, so we can partially see through them, like looking through a screened window.

Fifth, and perhaps most cool of all: the part of Saturn we’re seeing here is the night side, entirely unlit by the Sun. The bottom (southern) part of Saturn is only noticeable by its absence! But what’s that glow in the north?

That, my friends, is ringshine! Although this part of Saturn is in nighttime, the Sun is still shining on the rings (wherever you don’t see Saturn’s shadow across them). The ring particles are very bright and shiny. They reflect the sunlight, which then illuminates the northern hemisphere of Saturn. The southern half is still dark because the ice particles tend to reflect light back up, like a mirror. Since the Sun is coming from the north, that’s the way the light gets reflected. I’ll note that most of the light gets reflected away from Saturn, to the upper right in this picture, but enough is reflected back to make the cloud tops glow softly.

This happens on Earth too, when sunlight reflects off the Earth and illuminates the dark part of the Moon. This is called Earthshine, also poetically called "the old Moon in the new Moon’s arms." It’s quite lovely.

And it’s science! Which is lovely, too.

Image credit: NASA/JPL-Caltech/Space Science Institute


Related Posts:

- Lightning strikes in a storm bigger than worlds
- Saturn eclipses an icy moon
- An unusual view of the Death Star moon
- Saturn, raw (this one’s a jaw-dropper)

CATEGORIZED UNDER: Astronomy, Pretty pictures

The ups and downs of Saturn pictures

By Phil Plait | June 11, 2012 1:00 pm

When I look at Cassini images of Saturn — with its multitude of rings and fleet of moons — I am inspired, moved, and even awed.

And sometimes I laugh. When I saw this image, for example, I actually chuckled to myself. Why?

[Click to encronosenate.]

This gorgeous shot was taken on December 30, 2011 and released just today as the Cassini Image of the Week. It shows Saturn’s gorgeous rings seen nearly edge on, and the tiny moon Epimetheus, only 113 kilometers in diameter, next to them.

It’s a lovely image to be sure, and my very first thought was; I wonder if Epimetheus is closer to us than the rings, or farther away? If we’re looking down on the rings, from the north, then Epimetheus is closer to us. But if we’re looking up from underneath the rings, Epimetheus is on the other side of the rings. I could mentally switch my perspective back and forth, but I couldn’t tell which view is correct! This prompted my chuckle, as I wryly smiled at my brain’s confusion (I love optical illusions).

So take another look: are we looking down on the rings, or up? Hint: the Sun is shining from the north, down on the rings.

It’s a bit of a conundrum, isn’t it? Just by looking it’s almost impossible to figure out! If you’re familiar with Cassini pictures, the rings look subtly different if they are illuminated from above and you’re looking at them from underneath, and vice-versa. But it’s hard to tell. And to be honest, I wouldn’t have known without reading the caption for the image.

The answer is we’re looking up. The Sun is shining down on the top of the rings, and we’re looking up from underneath, putting wee Epimetheus about 1.5 million kilometers (900,000 miles) from Cassini when this picture was taken. If it helps, hold up something round like a DVD and look at it from underneath. As another helpful guide: in the image above, the part of the rings at the top of the picture are closest to you, the bottom farther away, and Epimetheus father still.

And I bet that even knowing that, some of you are having a hard time picturing it. Our brains are funny things, easily fooled when there’s symmetry in a picture, especially when that picture shows an unfamiliar object. I’m sure Carolyn Porco can just glance at something like this and figure out everything she needs to understand the geometry! I’m not so sure I could’ve.

Remember: seeing isn’t always believing. It’s easy to fool our eyes and brain, but in the end the Universe knows what it’s doing.

Image credit: NASA/JPL-Caltech/Space Science Institute


Related Posts:

- Saturn brain bender
- For moons, size does matter
- A little weekend Saturn awesomeness
- The tiny moon with the long reach

For moons, size does matter

By Phil Plait | January 20, 2012 7:00 am

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!


Related posts:

- Cassini gives Dione a close shave
- The scale of Saturn
- The scale of Saturn, redux
- Cassini’s Pentaverate
- A panoply of moons and rings

CATEGORIZED UNDER: Astronomy, Pretty pictures, Science

The scale of Saturn

By Phil Plait | December 19, 2011 10:45 am

With the Cassini spacecraft orbiting Saturn and making frequent fly-bys of all the weird moons there, it’s easy to post one incredible close-up after another. But sometimes, you have to take a step back and get some context, see the bigger picture.

Cassini can do that, too. And when it does, the beauty and scale of the Saturn system is simply breathtaking:

[Click to encronosate.]

This image shows, of course, the ringed planet itself, with the rings seen edge-on and their shadow cast across the planet’s southern hemisphere cloud tops. But look to the left, just below the rings; see that half-lit disk? That’s Enceladus, an icy moon of Saturn. It’s about 500 km (310 miles) across, which may start to give you an idea of how much area this picture covers. Even though it’s as big as my home state of Colorado, it’s positively dwarfed by the looming presence of Saturn behind it… and we’re not even seeing very much of the planet here! Saturn is over 120,000 km (75,000 miles) across, nine times the diameter of Earth.

Saturn is big.

To pound this home, look even farther to the left of Enceladus. See that black speck? I’ve enlarged the picture and annotated it here; the arrow points to Epimetheus, a lumpy gray potato moon of Saturn. It’s about 113 km (70 miles) long. That’s small for a moon, perhaps, but on a human scale it’s a huge rock, more than ten times the height of Mt. Everest.

Yet it’s a speck in this picture, easily missed if you didn’t know it was there. But I guess that’s not surprising; Cassini was 1.2 million km from Saturn when it took this shot, three times the distance from the Earth to the Moon!

Sometimes people ask me, what’s the one thing you wish people understood better about the Universe? And if I had to pick just one only, it would be this: scale.

The Universe is huge, and we’ve barely dipped our toes into it.

Image credit: NASA/JPL-Caltech/Space Science Institute. Tip o’ the meterstick to Carolyn Porco.


Related posts:

- An icy Titanic encounter
- Enceladus fires on Alderaan
- Saturn weather forecast: rings, with light rain from Enceladus
- Enceladus sprays anew!

CATEGORIZED UNDER: Astronomy, Pretty pictures

Cassini gives Dione a close shave

By Phil Plait | December 13, 2011 6:00 am

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


Related posts:

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

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

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

Perspective on four moons

By Phil Plait | September 8, 2010 7:00 am

The Cassini probe orbiting Saturn returned an interesting picture yesterday. It shows four tiny moons and the rings seen nearly edge-on. Take a look:

cassini_4moons_rings

[Click to enjovianate.]

From left to right the moons are Epimetheus (113 km/70 miles across), Janus (179 km/111miles), Prometheus (86 km/53 miles) and Atlas (30 km/19 miles). Like I said, tiny.

When I see images like this I like to amuse myself by fiddling in my mind with their perspective. For example, is Epimetheus closer to us (well, to Cassini when the picture was taken) than Janus was? Even more interestingly, are we looking down on, or up at the rings?

Images like this don’t give us the clues we usually get here on Earth to figure out distance. Look at the picture: the rings make a tight curve across the field. We know we’re seeing a circular ring nearly edge-on… but are we looking down on it, so that the top of the curve is farther away, or are we looking up at it, so that the bottom of the curve is farther away?

For example, take a DVD and hold it so that you’re looking at it almost edge-on. Tilt the near edge down a bit so you’re looking down on the top side. Now tilt the near side up so you’re looking up on the bottom side. See the issue? Without lighting, focus, or other cues, it’s hard to tell which way you’re seeing an object.

So for the Saturn picture, which is it? I’ll tell you below, but see if you can figure it out.
Read More

CATEGORIZED UNDER: Pretty pictures, Science

Don't have gravity? Take your lumps.

By Phil Plait | May 21, 2010 9:12 am

It might seem like a tautology — and that’s because it is — but sometimes the only word you can use to describe an image from the Cassini Saturn probe is otherwordly:

cassini_rhea_epimetheus

[Click to engasgiantize.]

This otherworldy picture was taken on March 24, 2010. The big moon is Rhea, seen from 1.2 million kilometers (750,000 miles) away, and the little one below it is Epimetheus, from 1.6 million km (990,000 miles) away. Perspective makes them look right next to each other, but in reality the distance between them is the same as the Moon from the Earth! Saturn and its rings provide the backdrop for this stunning alien portrait.
Read More

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

The real Pandora, and two mooning brothers

By Phil Plait | May 6, 2010 11:45 am

Cassini continues making loop-de-loops around Saturn, returning tens of thousands of way cool pictures. Like this one:

cassini_epimetheus_pandora

From 1.3 million kilometers (800,000 miles) away — 3 times as far as the Moon is from the Earth — Cassini spied this pretty scene. It shows, obviously, Saturn’s rings to the right. The very thin ring extending to the left is the F-ring; it’s very faint and wasn’t even discovered until 1979, when Pioneer 11 passed the planet.

The two moons are Pandora (the flying saucer-shaped one) on the left, and Epimetheus on the right. Usually, in pictures like this, perspective is a problem; one moon is much farther away than the other, so your sense of scale gets a bit bollixed. But in this case, both moons are about the same distance from Cassini! Pandora is about 114 x 84 x 63 km (68 x 50 x 38 miles) in size, and Epimetheus is a bit heftier at about 144 x 108 x 98 km (86 x 64 x 58 miles). In this shot, the rings are in the background relative to the moons, and Pandora is just a hair closer to Cassini than Epimetheus.
Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures

Cassini eavesdrops on orbit-swapping moons

By Phil Plait | April 8, 2010 2:00 pm

The Cassini spacecraft just had a few close encounters with some of the odder moons in the Saturn system… and given how weird Saturn is, that’s saying something. I was particularly enthralled with these two small worlds:

cassini_janus_epimetheus

On the left is the moon Janus and on the right is Epimetheus. The scales are not quite the same; Janus is roughly half again as big as Epimetheus’ size of 135 x 110 x 105 km (81 x 66 x 63 miles). Cassini was a little over 100,000 km from Epimetheus and 75,000 km from Janus when these images a were taken.
Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures
NEW ON DISCOVER
OPEN
CITIZEN SCIENCE
ADVERTISEMENT

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

ADVERTISEMENT

See More

ADVERTISEMENT
Collapse bottom bar
+

Login to your Account

X
E-mail address:
Password:
Remember me
Forgot your password?
No problem. Click here to have it e-mailed to you.

Not Registered Yet?

Register now for FREE. Registration only takes a few minutes to complete. Register now »