Tag: Mimas

Looking up to Saturn

By Phil Plait | October 30, 2012 7:00 am

Just in case you’ve forgotten how brain-destroyingly big Saturn is:

[Click to encronosenate.]

This shot of the ringed wonder was taken by the Cassini spacecraft when it was well over 2 million kilometers from the planet. The spacecraft was south of the rings, looking "up" toward the north. The Sun is shining down on the rings from this perspective, so they look darker than you might expect, and the use of a near-infrared filter accentuates storms in the southern hemisphere cloudtops.

So why does this picture grind my mind to dust? Look at the the very top, near the center. Can you see that dot of light? You might need to click the picture to get the hi-res version to see it better; that’s how small it is.

Except it isn’t. That dot of light is Mimas, a moon of Saturn, and it’s 400 km – 250 miles – across! That’s roughly the size of the state of Missouri, and compared to Saturn it’s reduced to a mere pixel of light. And even then, Saturn’s rings are still too big to fit in this picture!

The scale of the solar system crushes me. And yet there we are, poking around and sticking our noses into it. We humans are pretty awesome.

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

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MORE ABOUT: Cassini, Mimas, Saturn

A moon casts a looooong shadow

By Phil Plait | April 10, 2012 12:00 pm

On January 21, 2012, the Cassini spacecraft was about 2.5 million km (1.6 million miles) from Saturn when it took this shot of the planet’s clouds:

That long, distorted black smudge is actually the shadow of Mimas, one of Saturn’s moons! Mimas was way off to the side when this picture was taken, so the shadow it cast was stretched out due to the curvature of the planet itself. Think of it this way: if Mimas had been directly between Saturn and the Sun, the shadow would be a nice circle right in the middle of the planet’s face. But because it was off to the side, the shadow happened to fall where the planet was curving away, so it got elongated.

This has been seen many times before by Cassini as Saturn’s fleet of moons dance around the giant planet. Here’s one from Titan, and another from tiny Epimetheus casting a shadow on the rings!

In the image I noticed a faint, circular feature above and to the right of the shadow that looked like either a storm or perhaps a camera defect — sometimes dust in the camera makes circular donut shapes in the pictures, which have to be corrected. So I went to the Cassini raw image database and found the shot — it’s real! It’s a storm of some sort, and it just didn’t show up well in the first picture (note I had to rotate the image to match the one that was released of Mimas’s shadow). What’s funny is, you can see one of those small donuts in the picture, off to the left!

Pretty cool. And I love that the raw images are so accessible, so if someone has a question like that, in many cases the answer is just a few clicks away.

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

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MORE ABOUT: Cassini, Mimas, shadow

Dione and Mimas have a mutual event

By Phil Plait | January 30, 2012 7:00 am

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.

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MORE ABOUT: Cassini, Dione, Mimas, Saturn

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.

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OMG! They killed Mimas!

By Phil Plait | April 14, 2011 12:28 pm

You know, over the past couple of years I’ve compared Saturn’s moon Mimas to the Death Star, an egg, Pac Man, and even now Rick Astley. But while I was prepping the image for that last one yesterday, it suddenly hit me that yet another comparison was in order.

I’ll just leave it here without comment…

… except to say that finding an image of Kenny from the side was almost impossible. At least one where he was still alive.

Side view of the Death Star moon

By Phil Plait | April 13, 2011 9:36 am

Saturn’s icy satellite Mimas is the Rick Astley of moons. It got one huge hit* and that’s all it’s been known for ever since.

But the Cassini Saturn probe sometimes sees things a little differently, and recently provided us with a sideways view of Mimas. Literally.

[Click to rickrollenate.]

On January 31, 2011, Cassini snapped this picture of the moon with the planet’s rings in the background. I really like this shot, since we see Mimas’s giant impact crater from the side. I don’t think I’ve ever seen it quite this way before.

A long time ago, Mimas got hit pretty hard with something pretty big. The apocalyptic impact carved a crater 130 km (80 miles) across in the moon, which we now call Herschel. In most pictures we see it from an angle and Mimas winds up looking an awful lot like the Death Star.

But in the big picture above the crater was almost edge-on, and you can see how seriously it messed up the moon: a pretty hefty portion of the edge of Mimas looks flat where the rim of the crater distorts the horizon. An impact this size anywhere on Earth would be, well, bad. Very very very bad. And it’s not like Mimas hasn’t suffered enough, as you can see it’s been hit thousands of times; the surface is saturated with craters.

But that’s the way it is in the solar system. A lot of debris is floating out there, and over billions of years physics cannot be denied. After all… you know the rules and so do I. If you’re a moon, those small objects are gonna run around and hurt you.

* That link is safe. Seriously. I promise. Go ahead, click it. I dare you.

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The moon that almost wasn't

By Phil Plait | November 8, 2010 2:08 pm

Cassini images of Saturn and its environs never get tiresome. And in fact, they can be downright jolting… like this stunning shot of the icy moon Mimas.


Cassini was 100,000 km (60,000 miles) from the small moon when it captured this moody image. It shows a view we can never get from Earth: a crescent Mimas, with the Sun well off to the side.

The giant crater is called Herschel, and it’s a whopping 130 km (80 miles) across. Whatever hit Mimas eons past was huge, and had it been any bigger, or moving any faster, the moon itself may have shattered. In fact, there’s some thinking that this happens to some smaller bodies in the solar system; they can get hit so hard they do shatter, and if the event isn’t too energetic the pieces can recoalesce, reintegrate. This may be why some asteroids are so low in density; they’re essentially rubble piles, like bags of glass.

Mimas was spared that fate those many, many years ago. What was left was an icy moon with a single brooding eye… reminding us that when we stare into the abyss, sometimes the abyss stares back.

Image credit: NASA/JPL/Space Science Institute

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Herschel, Mimas, Saturn

The raw face of the Death Star moon

By Phil Plait | February 15, 2010 10:45 am

Of all the moons in the solar system, Mimas is one of the most recognizable. And new pictures from Cassini show us again just why.

On Saturday, February 13, the Saturn probe dipped low over Mimas, sliding past the small moon at a distance of just 15,000 kilometers (9000 miles). For comparison, the Earth is about 13,000 km (8000 miles) across, so Cassini really threaded the needle with this pass.

On its way out from the encounter, when it was about 70,000 km (44,000 miles) from the moon, it snapped this astonishing shot:


Yegads. Note that this image is raw and unprocessed — it’s basically straight off the camera (and converted to JPG). But holy cow, there’s a lot to see. The giant crater Herschel is pretty obvious. It’s about 130 km (80 miles) across (compare that to Mimas itself, which is 400 km in diameter!), with a central peak characteristic of large impact basins. I think that’s where the main weapon is located.

cassini_mimas_herschelMimas has clearly had an interesting past: it’s battered beyond belief, loaded with impact craters. In fact, one thing scientists hope to learn from this pass of the moon is a bit of its timeline. By counting up the number of craters inside Herschel, and comparing that to the crater counts outside of it, they can estimate its age. Fewer craters inside of Herschel means it’s younger than the surrounding surface, for example. But how much younger? Maybe we’ll soon know. And they’ll be able to see craters that are pretty small; the resolution in the image here is about 200 meters (1/8 mile) per pixel. That’s about the same as we can do on our own Moon from Earth!

Sometimes with raw images like this, the background can be a little screwed up due to artifacts in the camera. But in this case, the glow on the left hand side of the big image is quite real: it’s the face of Saturn itself! The geometry of the shot was just right to capture a bit of the cloudtops of the ringed world. Very cool.

These are gorgeous pictures, and it’ll be nice to see them once they’re completely processed, too. We’ll learn a lot about the moon from them… and as far as Mimas goes, I just hope it’s not fully armed and operational. I suppose I shouldn’t be too worried though. After all, I used to bullseye womp rats in my T-16 telescope, and they’re not too much bigger than Mimas.

Image credit: NASA/JPL/Space Science Institute

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, craters, Herschel, Mimas

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