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!

In July of last year, I wrote about a comet that passed extremely close to the Sun. Astronomers have now had a chance to pore over that data, and were able to determine some very cool stuff.

First, here’s the video of the comet’s fiery demise (watch it in HD to make it easier to spot the comet):

See it? It’s faint, but there. Actually, there are a lot of observations from multiple observatories and detectors, which allowed astronomers to find out quite a bit about this doomed chunk of ice and rock.

For one thing, it was screaming along at about 650 kilometers per second (400 miles/second) as it flamed out. To give you an idea of how flippin’ fast that is, it would’ve crossed the entire United States in about eight seconds.

Yeah, I know.

It also passed an incredible 100,000 km (62,000 miles) above the Sun’s surface. Have you ever stood outside on a hot day, and thought the Sun would cook you? Now imagine the Sun filling half the sky. That’s what that comet saw. No wonder it disintegrated.

As it approached the Sun, it was watched by NASA’s Solar Dynamics Observatory. In its final 20 minutes or so, the comet broke up into a dozen pieces ranging from 10 – 50 meters in size (and no doubt countless smaller ones too small to detect), with a tail of vaporized material streaming behind it that went for thousands of kilometers. For that size, it would’ve had a mass of hundreds of thousands of tons — about what a loaded oil tanker weighs on Earth!

We’ve learned a lot about how comets break up and disintegrate by observing this event, but it’s raised further questions: like, why did we see this at all? Comets are faint, and to be able to see it this way against the bright Sun is odd. It was definitely one of the brightest comets seen, but it’s interesting to me that it appears to glow in the ultraviolet, as it did in the above video. That means, at that wavelength, it was brighter than the Sun! It wasn’t like a meteor, burning up as it slammed through material, so some other process must have affected it. I suspect that the Sun’s strong magnetic field may have had something to do with it; in the far ultraviolet magnetism is a strong player. Gas under the influence of intense magnetic fields can store a lot of energy, which is why sunspots — themselves the product of magnetic squeezing — look bright in UV.

Perhaps as the comet broke up, the particles inside got excited by the magnetic fields of the Sun and glowed. I’m no expert, and I’m spitballing here. The thing is, no one is exactly sure. But that doesn’t mean we won’t find out. Nothing makes a scientist’s noggin itch as much as a mystery like this, something apparently misbehaving.

One of the single most important words in science is "yet". We don’t know yet. But we will. Someone’ll figure this out, and we’ll have one more victory in our quest to better understand the Universe.

Science! I love this stuff.

Credits: Credit: NASA/SDO; SOHO (ESA & NASA)

… and I mean that literally. Here he is, supercooling a beer.

Supercooling is when a liquid is chilled to a temperature below its freezing point, but it remains a liquid. Water, for example, will crystallize when it freezes, but it needs a starting point for that to happen, like a particle of some impurity (a mineral, for example), or the rough wall of its container. If you freeze a container of (distilled) water without jostling it, it’s possible to supercool it. If you then carefully remove it from the freezer and shake it or pour it over ice, it’ll freeze instantly^*.

This is similar to superheating, where a liquid can be heated beyond its boiling point but remain a liquid. This happens all the time for me when I boil water in my microwave using one particular Pyrex measuring cup. I have to be careful — I might say super careful — when removing it, because if jostled the water will erupt with steam and explode outwards. To call that dangerous is a massive understatement; water can carry a lot of heat and the resulting burns are no fun at all.

… which is how I discovered that particular measuring cup superheats the water. Ow.

Anyway, Brian Brushwood makes the great video series Scam School, and this video is for a book version he’s doing. I can’t wait to see that!

^* I don’t suggest trying this without knowing what you’re doing; if the water does freeze inside the container it can rupture: ice has a larger volume than water.

Yesterday, I was in a live video chat session with several other scientists and science journalists. I wrote up the details of it yesterday, and it went pretty well! We had a lot of fun talking about the new GRAIL Moon mission, the fiery future return of Phobos-Grunt, 2012, and of course President Obama’s purported teleportation trip to Mars many years ago.

Wait, what?

Well, if you wanna know more, now you can: the video’s online.

The plan is to do these every week on Thursdays, and have a rotating cast of characters over time. I hope you like it. And I strongly suggest people join up over at Google+. I really like it there, and post quite a few things you won’t see here or on Twitter.

Fraser Cain (from Universe Today) and I are trying something new… and by new, I mean new. We’re going to be holding a live video weekly astronomy and space roundup on Google+! We’ll have a roundtable group of scientists and science journalists discussing the latest cosmic news, explaining it, and letting you know what it all means. We have a pretty good group of folks lined up for this, and the first one will be held today, Thursday, January 5 at 18:00 UTC (1:00 p.m. Eastern US time).

[UPDATE: We're live now!]

These will be held on Google+ using Hangouts on Air – a live video stream that can be watched by an unlimited number of people. You have to be on Google+, and then circle Fraser Cain — that’s G+’s version of adding friends. He’ll have the link to the video feed in his stream once it’s set up (and I’ll update this very blog post as well). And once you’re in, you can ask questions for us in the comments section on the post! You can read more about this on Universe Today.

I’m very excited about these live video news session. For one thing, we’ve done this a few times already in a rather impromptu way, and it’s worked out really well. We can talk about news, switch from one person to another, and take questions from people watching. It’s all live and real-time — yesterday, we even had a live feed from a telescope in Bucharest where we observed the Moon! That was amazing.

Also, Google+ is turning out to be a really cool place to be, with a lot of very intelligent and thoughtful participants. It is not Facebook, with endless announcements of games, ads, and such. It’s far more of a discussion and an exchange of ideas. The addition of live video conferencing is a huge benefit too. Fraser and I think that this will change a big chunk of the internet… and maybe more. If you’re on G+ please circle me, and if you’re not, you’re missing out.

We have big, big plans. Just you wait.

But until then, I hope to see you on G+ for our roundup!

Mynd you, Møøn bites Kan be pretti nasti…

Today, NASA successfully put a new mission into lunar orbit: GRAIL, for Gravity Recovery and Interior Laboratory. Great acronym, weird name, right? What this mission will do is map the gravity field of the Moon, and use that to probe the interior composition. The basic idea isn’t all that complicated: fly a probe around the Moon. If it goes above a region where the density is higher, there will be a slightly stronger gravitational pull, and the spacecraft will accelerate a bit. By carefully measuring the spacecraft position and velocity, you can make the lunar gravity map.

In detail, that’s a bit tougher! What NASA has done is launch two probes, GRAIL-A and GRAIL-B, that will fly in the same orbit, one behind the other^*. They’ll stay in constant communication, sending radio pulses to each other. The timing of these pulses allows an extremely accurate determination of their separation: their distance will be known to an accuracy of about a micron: that’s a hundredth the width of a human hair, or the size of a red blood cell!

So how does that help? If one of the two probes speeds up or slows down, the radio signal timing will change, taking more or less time to get from one probe to the other. The amount of change is related to the force of gravity felt by the probe, and that in turn is related to the density of the material below. In practice, making a gravity map this way is extremely complex, but it’s been done before here at Earth using probes like GRACE and GOCE. It’s tried and true.

(more…)

Professor Brian Cox is a physicist in England, very well-known there as a popularizer of science. The reasons for this are many-fold, including his ubiquity across media (including podcasts, Twitter, and of course TV)… but also because he has an infectious enthusiasm for science coupled with a boyish charm.

This was all on display recently when he hosted a great segment on the BBC’s show A Night With The Stars, where he simply and effectively demonstrates why atoms are mostly empty space:

Pretty cool, isn’t it? It helps if you can enlist Simon Pegg to help, too!

I like this demo a lot. On a very tiny scale, objects act like both particles and waves. On a big scale, like our solar system, we can think of planets as discrete particles, interacting through gravity only, and it works pretty well. Our semi-evolved brains want to think of electrons that way as well: little spheres whizzing around atomic nuclei. But that’s not the way the Universe works on the quantum scale; electrons act like waves, and that means they can interfere with each other. When a crest meets a trough they cancel, when a crest meets a crest they add together. If you have a wave bouncing around inside a box the result can be chaos.

I like to use the example of sitting in a tub, and rhythmically pushing your body along its length with your toes. It’s hard to do unless the rhythm is just right; otherwise the waves smack into each other chaotically and it’s a mess. But get the pattern timed just right and you’re in sync. That timing is just a simple multiple (like 1 or 1/2) of the time it takes a wave to move from one end of the tub to the other. You can actually feel it as you push; the correct timing just feels natural.

Electrons around an atomic nucleus work the same way. It’s more complicated than your bathtub, but the principle is the same. The electrons can only exist where the wave crests and troughs add up correctly. They literally cannot exist anywhere else. They’re like standing waves, as Brian shows.

We teach kids that atoms are like little solar systems, but that model is really bad! In principle, planets can orbit the Sun at any distance — give a planet more orbital energy and it’ll move away from the Sun and continue orbiting, happy as you please. But electrons can’t do that. They can only be at energy levels where they don’t interfere with themselves (and each other). It’s more like a staircase; they can only move up or down by discrete amounts. Once you figure this out, a ton of stuff becomes possible: lasers, semiconductors, fluorescent bulbs, atomic bombs… it’s quantum mechanics, and it’s a huge, huge field of science.

And it’s all because, as Brian demonstrates, a rope held at both ends won’t vibrate at any old frequency. Amazing, isn’t it?

Post script: can you imagine a show like this running on American TV? No, I can’t either, unless they had a toll number you could call to vote for atoms being a hoax perpetrated by Big Little Science.