Tag: Prometheus

Con and Prometheus

By Phil Plait | June 18, 2012 1:39 pm

So a week or so ago I went to the movies and watched Prometheus. While I didn’t hate it unequivocally as a lot of people seem to, I didn’t love it without reservations either. On the Alien sliding scale, it was better than being attacked by a face hugger, but worse than listening to Hudson complaining about being a space marine.

I think there were some really good things about the movie, including many of the over-arching themes, but the problem wound up being in the details. And by "details" I mean "science", for the most part. At some point though, the details are the plot, especially when the movie revolves around those points.

So I wrote up some thoughts and sent them over to the good folks at Blastr, who added some pictures from the movie and let me rant. After my first draft, I edited it, and then again, and it was still 1300 words — it’s hard to do a good/bad analysis of plot points without racking up quadruple-digit essays, so I left a lot of stuff out that I thought was cool, as well as stuff that — haha — bugged me. Think of the article as an analytical sampling of the movie science.

Of course, there are spoilers flying as furiously as pressurized acidic blood there, so if you haven’t seen the movie, you might not want to read it. And if you have, feel free to leave comments there! The discussions I’m reading about the movie are actually pretty interesting, so I’m curious to know what everyone thought.


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MORE ABOUT: alen, Blastr, Prometheus

Worlds bigger than worlds

By Phil Plait | March 1, 2012 7:00 am

Because I love each and every one of you, here is a fantastic portrait of two worlds: Saturn and its ginormous moon Titan, courtesy of the Cassini spacecraft:

Isn’t that breathtaking? [Click to encronosenate.]

I love the panoply of shadows from the rings on the cloud tops of the gas giant planet, clearly showing Saturn has not one big ring, but thousands of thin ringlets. You can also see subtle patterns in the clouds as well. If you look very closely, you’ll see the shadow of the moon Prometheus on the left just below the ring shadows — the moon itself is the white speck just above the rings to the right, just to the right of Saturn’s limb — as well as the shadow of the moon Pandora on the right below the rings. Pandora itself is well outside the frame of this shot though.

Of course, fuzzy Titan looms of the planet’s edge on the right as well. Titan is huge, bigger than Mercury, and if Saturn weren’t there might be considered a planet in its own right. But definitions aside, Titan is a varied and complex place, worthy of intense study. It has weather, lakes of liquid methane, dunes blown and sculpted by wind, and boulders made of water ice harder then rock is on Earth.

Who wouldn’t want to take a closer look at a world like that?

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


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CATEGORIZED UNDER: Astronomy, Pretty pictures

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!


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CATEGORIZED UNDER: Astronomy, Pretty pictures, Science

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


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The real Pandora, and two mooning brothers
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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.


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Dr. Tongue’s 3D House of Prometheus
- Perspective on four moons
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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

Cassini's slant on the rings

By Phil Plait | July 2, 2010 9:40 am

Check out this latest image from Cassini at Saturn!

cassini_diagonalrings

[Click to embiggen.]

Oh, wow! This was taken when the spacecraft was almost in the plane of the rings, which are incredibly thin. You can see several different rings, including the broad A ring in the middle and the thin F ring on the outside. There are also two moons: Janus (the larger one, above) and Prometheus (smaller, below). Janus is about 180 km (110 miles) across, which isn’t terribly big, but Prometheus is even smaller: 120 km (75 miles).

cassini_epimetheus_pandoraThis picture made me smile not only because it is carved out of raw coolness, but also because it’s the complement of an earlier image from Cassini (to the right). The earlier shot is of the moon Epimetheus, which shares an orbit with Janus, and Pandora, which shares an orbit with Prometheus! So the two images go together like a pair of gloves, each showing one of a pair of orbit-sharing moons.
Read More

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Janus, Prometheus

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

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

Dr. Tongue's 3D House of Prometheus

By Phil Plait | February 19, 2010 4:30 pm

If you’ve ever wondered what it would be like to hang out near the Cassini Saturn spacecraft and get the same view it does, then put on your red/green glasses and check out this anaglyph of the moon Prometheus:

cassini_prometheus_3d

Mmmm, threedeealicious. Click to enjovianate.

Prometheus is a bit weird. OK, it’s a lot weird. It’s an irregularly-shaped elongated spud of a moon, measuring about 119 x 87 x 61 km (71 x 52 x 37 miles) in size. The long axis always points toward Saturn due to tides; basically the change in Saturn’s gravity from the front end of Prometheus to the back end acts like a stretching force on the moon, keeping it aligned. The tip on the right always points toward Saturn, and the long side we see in the image is the leading half of the moon, always facing ahead into the direction it orbits. Think of it as facing into the wind if that helps any.

Prometheus is a shepherd satellite, meaning its orbit gets it near Saturn’s F ring, where it helps keep the ring particles in place. It does this along with Pandora, another smallish moon. Prometheus orbits Saturn inside the F ring. When it gets close to the ring, it gives a little bit of its orbital energy to any ring particles that are on the inside edge of the ring, which boosts them to a slightly higher, slower orbit. Pandora does the opposite; it orbits outside the ring, and it steals energy from ring particles on the outside edge, which drops them into a slightly lower, faster orbit. Together, the two moons shepherd the F ring particles, corralling them and keeping the ring narrow. The animation shows the effects of Prometheus on the inner edge of the ring.

Just so you know, I think this is one of the coolest things ever. Shepherding moons were theoretically discussed for a long time, but we didn’t have any evidence of them until Voyager swept past Saturn a few decades ago, and now Cassini has the chance to study them in detail. It’s such a weird thing, and there it is playing out in the solar system for us to examine! It’s a good reminder that Nature is sneaky, and a lot more clever than we are. I’m glad we’re clever enough to catch up with it, too.

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures
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