Two things, quickly:

1) If you’re in the NYC area, the American Museum of Natural History is hosting a Cassini exhibit, showing many of the beautiful images from the Saturn probe. You have plenty of time to see it; it runs through March 2009. Carolyn Porco will be giving a talk there in September, too.

2) Saturday is Astronomy Day! Universe Today has details on some events. Remember, if you’re in the Detroit area, I’m giving two talks there; one Friday night and another Saturday afternoon.

3) OK, a third thing: my friend Karen Stollznow is hosting the Skeptics’ Circle. I really need to start submitting to that. One more thing to do on a very long list of things to do.

Space Cynic is hosting this week’s Carnival of Space. Brought to you by… Rod Serling?

A Breaking news for sky afficianados post made me sit back and gape for a moment: "Armageddon" and "Deep Impact" came out this summer 10 years ago. Wow. I’ve been mocking the former and hyping the latter for a decade now. Cool.

I use both movies in a general public talk I give about astronomy, showing brief clips that go over mistakes made as well as some rare accuracies as well. I still think "Armageddon" is one of the worst movies ever made, ever, and "Deep Impact", though flawed, is far superior.

But wow, 10 years? BNfSA has some thoughts on this as well, seeing as how the two together have grossed nearly a billion dollars since their release.

It’s enough to make Ben Affleck cry.

On May 25th of this year, the Mars probe Phoenix will land on the red planet. This interplanetary lab may not rove about the surface like Spirit and Opportunity, but it is loaded with experiments to test the polar region of Mars to see if life ever arose there.

The landing site was chosen to be as boring as possible; they want the thing to land safely, and that means a wide, flat area. The chosen site really is dull, but happily Emily found something interesting to say about it: it sports dust devils. And she has a very cool image to back it up.

I’ll be writing more about Phoenix as landing time approaches. Stay Tuned, but check with Emily’s blog to get details more often. This is her territory more than mine.

The other day I posted about the record-breaking sighting of the youngest crescent Moon — the Moon wasn’t even new yet!

There was some confusion over the image I posted along with that article. That image was taken many hours before the actual record was made, when the Moon was still 19 degrees from the Sun.

Martin Elsässer, the man who broke the record, has posted a new image, showing what the Moon looked like just 10 minutes shy of its conjunction (closest passage) with the Sun:

That is an amazing picture. You can barely see the crescent at all; I highlighted it with red lines so you can see it. The closer the Moon gets to the Sun, the less of the lit day side we see, so this picture tells you just how close it was. At this point, it was less than 5 degrees from the Sun. Incredible.

Thanks to the folks at Fark for picking up on this story, and for the link to Herr Elsässer’s page.

Update (5/7/08): The image I had posted originally was distorted due to the wrong picture being made available to the press (like me!). I got a nice email from Joerg Dietrich, one of the astronomers who took the data, with a link to the correct image. I have updated both the image and the link. Sorry, and enjoy!

We’ve known for a long time that most of the Universe is invisible. 72.1% of it is dark energy, about which we know very little. 23.3% of it is dark matter, which was only recently tagged for real and for sure; we still don’t know what particles make it up, but we’re on the verge of finding out.

Normal matter — us — makes up just 4.6% of the Universe’s energy and mass budget. But here we are! At least, here we mostly are: actually, we only see roughly half of the normal matter in the Universe. Stars, galaxies, and warm-to-middling gas aren’t too hard to spot in general, but they only make up about half of what we expect to see of normal matter.

Where’s the other half?

Let’s turn the wayback machine to about 13.6 billion years or so ago. The Big Bang is old news at this point, but the first stars have yet to be born. Matter and energy are mixed everywhere, but some of it is different. What we now call dark matter is starting to clump together through gravity, forming long sheets and filaments far bigger than any galaxy we see today. This forms a grid, a framework, upon which normal matter starts to fall. Eventually, galaxies and clusters of galaxies and clusters of clusters of galaxies will form along these cosmic skeletons.

Fast forward to today. Bang! We see galaxies everywhere… well, not exactly everywhere. We see them lying in those long sheets and filaments, showing us where the dark matter structures are, like dew drops on a spider’s web.

But that’s just the stars and galaxies, remember? It’s only half. Where’s the other normal matter?

The hypothesis is was that it would be in the form of very hot gas strung out along those filaments as well. Hunting for it would be hard: it would be very diffuse, making it dim, and very hot, meaning it would only emit at short wavelengths, like extreme ultraviolet or X-rays.

Hey, we have telescopes that can see those!

And now we have (and more pictures can be found here). Astronomers upped the odds of finding the gas by looking around galaxy clusters, where it would be denser, and also doing something clever: looking near clusters that are near each other in the sky due to perspective. One would actually be farther away than the other, but peering very nearly along the angle separating them they would look like they’re right next to each other. Since we’d be looking along a long thin cylinder of gas, that would make it appear brighter than if we saw it through its side.

The picture above shows the galaxy clusters Abell 222 and 223, both about 2.5 billion light years away. The visible light image just shows them as clumps of points, but remember: each dot is a massive galaxy like our own! The technicolor bit is from the XMM-Newton orbiting X-ray observatory, and shows the hot gas. Since these are separate clusters, they should be detached from each other. But instead, they’re connected by a gas bridge of ten-million-degree plasma. That’s the missing stuff! That’s made up of baryons; particles like protons and neutrons, atomic nuclei and the like. Look around you: everything you see is made of baryons (and leptons, which include electrons), so this gas is your kin.

It’s a bit more rarified, though: there are only about 30 baryons per cubic meter in this bridge. Good thing it’s big (about 4 million light years wide) and we’re looking down its length! But then, that’s why so much of this stuff is missing. It’s really hard to detect.

According to the models, there is enough stuff in this bridge to extrapolate the existence of the rest of the missing normal matter. Of course, we only have a data set of one, which is a bit rocky, but I suspect more of these will be found now that we know they’re out there.

And may I add, phew! It’s always nice when half the stuff you can’t find finally turns up.

The ways the Universe can deal out death are as numerous as they are terrifying. Asteroid impacts, nearby stars exploding, wandering black holes… I spent a year or so thinking of nearly every method of cosmic catastrophe I could while writing Death from the Skies!^*.

I wrote a whole chapter on what dangers lurk in our own Milky Way galaxy, and I was surprised to find out the Sun’s orbit around the center of the galaxy is a potential problem. The galaxy is flat, like a CD (in fact, the proportion is right if you stack about 4 CDs together). The Sun does not orbit the center of the galaxy in a nice, flat plane, like planets do around the Sun. Instead, it bobs up and down like a cork in water, making about four cycles for every one time it orbits the galaxy (which takes about 200 or so million years).

In my research, I came across the idea that when the Sun is at the apex of its bobbing, towards galactic north, it’s about 100 light years above the galactic plane. That’s far enough up that the magnetic fields of the galaxy are weaker, and it’s these fields that protect the Sun (and the planets, meaning us) from intergalactic cosmic rays, subatomic particles that zip around space between galaxies. When the Sun is up high, these cosmic rays can strike us, and we have to endure this particulate rain for millions of years. The radiation can do bad things, like damage the ozone layer or induce genetic mutations.

When researchers plotted the times of the Sun’s most northerly excursions (which happen every 64 or so million years), they lined up in time with many mass extinctions on Earth. Uh oh.

The good news is that this only happens at one part of the Sun’s orbit, so while we’re deep in the plane of the galaxy we’re protected and safe.

Or, actually, things get worse.

A new result has just been announced that says that when the Sun is in the thick of the Milky Way’s plane, tides from the galaxy can induce comets from the outer solar system to plunge down toward the Sun, meaning many will hit the Earth and potentially cause mass extinctions.

Well, nuts.

According to the new study, this happens every 35 - 40 million years, which is not too far off from the calculations in the older study. Since the Sun moves up and down in the plane, it actually plunges through the mid-plane twice each cycle. If it reaches its apex every 64 million years, then it should pass through the mid-plane every 32 million years, which is reasonably close to what the second study says.

So as if it’s not bad enough that we get irradiated at the top of the orbit, we get pummeled by comets twice as often!

Bummer.

So sure, having trillion ton chunks of rock and ice rain down every few 30 million years is bad and all, but that’s not the worst part! Horrifyingly, this news came too late for me to include in the book!

We have to keep our perspective on these things, after all.