Cassini continues to deliver. That astonishing image shows the rings of Saturn, with the giant moon Titan in the background, and the tiny moonlet Epimetheus in front.

What a vista! Look at the differences in scale: Titan, at 5150 km across, is bigger than Mercury. The rings are 250,000 km from edge to edge, and wee teeny Epimetheus is 115 km across (our own Moon is about 3500 km).

There’s just nothing like this that you can see from the Earth. Our Moon is pretty cool and all, but Titan has an atmosphere (twice as thick as Earth’s!) and is as big as a planet. Epimetheus is oblong and pitted and weird-looking. And geez, rings.

When I was a kid (okay, fine, even now) I read a lot of science fiction, and my favorite stories all took place in really weird places, alien planets, strange landscapes. But Nature is smarter and more clever than we are. Look how alien those worlds are! I wonder if any SF writer ever came up with anything that looked anything like that.

Remember a few days ago when I wrote about a comet that was not going to hit the Earth? Well, it turns out that even minus an impact, this is a pretty interesting object.

The comet’s full name (take a deep breath) is 73P/Schwassmann-Wachmann. It was discovered in 1930, lost for a while, then recovered in 1979. In 1995, it broke apart into several pieces.

Why?

The exact cause is a mystery, though there are plenty of reason why a comet would fall apart. Comets are made of rock and ice. When they get near the Sun, the ice sublimates– turns directly into a gas — and flows into space, which is why comets look fuzzy in pictures. The actual nucleus, the solid part, is very small, but the coma, the fuzzy part, can be thousands of miles across.

It makes sense that after repeated passes of the Sun, enough ice is lost to venting that the structure of the comet can be fragile, since the ice in a way is holding the comet nucleus together. Once enough ice is gone, a breakup could occur if the sublimating ice builds up enough pressure to disrupt the structure. But that is just one explanation. The famous comet Shoemaker-Levy 9 broke apart because of the immense tidal forces of Jupiter’s gravity. Most comets don’t break apart; look at comet Halley, which has been circling the Sun for a long, long time (it was seen in ancient times). It goes to show that some comets are very fragile, and some are not.

73p is a fragile one. It broke apart 11 years ago, and now it’s really disintegrating. The picture above is from Hubble, taken just last week, showing some of the small fragments peeling away from a bigger one, Fragment B. The whole thing busted into at least 40 pieces, and certainly there will be more. Here’s another Hubble image, of Fragment G:

(Clicking on the images will bring you to the Hubble page where you can get much larger versions.)

In my other blog entry, I talked about why we won’t get hit by this comet. It’s simply too far away to get any pieces of any real size to hit us. The nearest they’ll get, in mid-May, is about 9 million kilometers (5.5 million miles). I got an email from a reader who asked me why, then, does the JPL page which tracks Near Earth Objects give a miss distance of only about 1.5 million km (1 million miles) for fragment 3-AL? Well, conspiracy aside, that’s still a fairly comfortable margin — though I’d be happier with a bigger number! However, I am happier, because that is a statistical lower bound to the minimum distance. The far more likely miss distance is 9.4 million kilometers (about 6 million miles). We’re pretty safe.

Some of the comet fragments are visible in binoculars, and I intend of trying for them tonight. If you live in the northern hemisphere, the comet is in the constellation of Corona Borealis. Ian Musgrave, an amateur astronomer in Australia, has a blog with details on where to look. Unfortunately I can’t take any pictures, but I’m pretty sure I’ll be able to see it. I might even haul out my ’scope to take a peek too. This is a rare opportunity.

On Monday, I blogged about a mysterious sonic boom heard in San Diego (and made gentle — OK, not-so-gentle) fun at UFO proponents about it). No one knows the cause of these booms, though it’s safe to say an alien ship is pretty low on the list.

A followup story has been published in the same newspaper that had the first article about this. Scientists have traced the origin of the noise offshore, to a section of the Pacific known to have "military training exercises". That’s a logical place to continue looking for the source, of course.

However, even if it does explain the noises heard in San Diego, it does not account for similar booms heard over other states in early April. So there’s still more to this story. Stay Tuned.

As I sit here in my living room, laptop keeping me warm (and probably irradiating parts of me that are best left unirradiated), the sounds of "South Park" playing in the background, I feel pretty safe.

But it’s a lie, an illusion.

The Universe is a dangerous place. We sit here in the Milky Way, isolated and out of danger… for now. But other galaxies are out there. They can get close, and their immense gravity can distort our Galaxy, fling stars out like a dog shaking off water, rip out long streams of gas and stars, and then eventually merge with us.

What evidence do I have for this chilling scenario? For one thing, we see it happening in the Milky Way. Our Galaxy is actively eating at least one other one, for example. We’re also pretty sure that in 3 billion years, we’ll merge with the Andromeda galaxy too.

And, for the clincher, we just plain old see other galaxies colliding. The image above, just released by the Spitzer Space Telescope, shows two spiral galaxies named NGC 2201 (left) and IC 2163 (right). Each is similar to ours, except that they are in the process of colliding. The red color in the image is actually from infrared light, showing dust warmly glowing (the blue and green parts are from a Hubble image, and added together make the false-color image). See how the dust is all in the spiral arms? When the two galaxies passed each other, the gravity of each disturbed the other, causing clouds of gas and dust to collide, collapse, and form stars. This happens predominantly in the spiral arms, which is why they are so obvious in the image.

The two galaxies will pass each other, swing around due to gravity, and pass again. This will happen multiple times, until, about a half billion years from now, they will merge into one colossal galaxy. It’ll probably be elliptical in shape, like a football, the spiral arms distorted beyond recognition. Probably, the same fate awaits us when we merge with Andromeda.

A thought: In the image above, the two galaxies are superposed on the other. Can you tell which galaxy is in front of the other? No? Maybe this image will help:

That’s the Hubble image of the same galaxies that was used in the Spitzer image above (click on it for a higher-res version). See on the right, how the spiral arm from NGC 2207 cuts across the nucleus of IC 2163? In optical light, dust is dark, absorbing light behind it. Since we see the dark arm in front of IC 2163, NGC 2207 must be in front. If it were behind the smaller galaxy, we wouldn’t see that arm at all, because it would be blocked by dust in the other galaxy.

As usual, it’s the combined might of telescopes that see in different wavelengths of light that reveals clues about the goings-on of our Universe. And if you don’t think knowing this stuff is important, then just wait three billion years. When Andromeda is filling up the whole sky, and the Sun gets tossed out of the Galaxy like you’d flick some noisome goo off your finger, well, you’ll be sorry then! Or maybe you’ll have other things on your mind. Or maybe, realistically, it won’t happen until three billion years from now, so maybe it’s not so pressing.

But it’s cool. Don’t ever underestimate that.

I just remembered to check how well the poster from The Amaz!ng Meeting 4 did on Ebay.

It sold for over $900! Not bad. It’s nice to know that money goes toward an organization dedicated– and I mean dedicated — to teaching people to think critically.

Now J.J. Abrams is saying the Trek movie rumors are not entirely accurate:

“The whole thing was reported entirely without our cooperation,” says the director with a hint of regret. “People learned that I was producing a Star Trek film, that I had an option to direct it, they hear rumours of what the thing was going to be and ran with a story that is not entirely accurate.”

To be clear, it does look like he will direct Trek XI, but he is being coy about the movie revolving around Kirk and Spock.

This is pretty cool. BA Reader Karen Davis alerted me to this page by a young Norwegian man. He has links to a ton of skeptical podcasts and radio interviews. If you read BA, you’ll recognize a lot of the names: interviews with Michael Shermer, James Randi… and hey, who’s that guy with the NASA cap on?

Yeah, it’s me. Actually, there’s some stuff there I forgot about: like talking about the Venus Transit in 2004 with a reporter from MSNBC. I remember doing it, but I did a lot of short, four minute interviews that day at Goddard Space Flight Center– like literally 30 of them. The one I remember best was with a local reporter from San Francisco. I thought it was funny to fly all the way across the country, then wind up on the air with my own local news! I’ll have to write up that little adventure some day; it was fun, and the transit was truly excellent.

I watched that MSNBC video again (I had to use — shudder — IE), and besides looking a little stiff I think it was OK. But then the reporter mispronounced my last name. Sigh. It’d better than another reporter who introduced me as being from "the Goddard Space Station". Yikes.

Anyway, go to that skeptical site, and check out those links. Randi’s page has some great stuff. The debunking of Uri Geller is particularly fun.