“Don't wake me for the end of the world unless it has very good special effects.” -Roger Zelazny

It's always the ones you least expect that get you the worst, it seems. I went to bed last night excited that Asteroid 2012 DA 14 , a 200,000 ton asteroid, was going to pass within just 28,000 km (or 17,000 miles) of Earth's surface, which would make it the closest pass of an asteroid that large that we've ever observed.

Image credit: NASA / JPL Near-Earth Object Program Office.

I thought that would be the best way to celebrate today, which would be Galileo's 449th birthday. After all, it was Galileo who first discovered that there was no way that Earth could be at the center of all the heavens. By looking at Jupiter through a telescope, he discovered that it had its own set of large moons that very clearly orbited their own planet, indifferent to the workings of Earth.

We now know that these moons orbit Jupiter because of Jupiter's gravity; this is the same reason all planets, moons and solid objects move the way they do in space. Jupiter is the second largest gravitational source in our Solar System, behind only the Sun. And since it's been orbiting the Sun for over four billion years, it's had literally hundreds of millions of passes by each object in the asteroid belt.

Image credit: NASA / ESA / STScI.

Every solar system has an asteroid belt, and here's why: as you move away from the Sun (or any star), the temperature of the interplanetary space around you drops. Near the planet Mercury, interstellar space is somewhere around 800 °F, out by Pluto, it's nearly -400 °F. But there's a critical place -- out beyond Mars but before Jupiter in our Solar System -- where the temperature is too cold for water to exist in any state other than frozen ice.

And once you reach that point, you're going to get little frozen chunks of ice and rock mixed together. So every solar system has an asteroid belt. But ours also has a large planet nearby, and over billions of years and millions of passes outside this asteroid belt, Jupiter changes the orbits of these rocky objects.

Image credit: Jen Christiansen. Via NASA Jet Propulsion Laboratory.

And these repeated gravitational interactions of asteroids with the other planets -- primarily Jupiter -- changes their orbits over time. This is important to us here on Earth for a few reasons, but nothing makes it more apparent than seeing the havoc a collision with one of these asteroids can wreak here on our world.

Image credit: D. Roddy, U.S. Geological Survey.

This is meteor crater, from an asteroid strike about 50,000 years ago, of an asteroid that was comparable in size (maybe 50 meters in diameter) to 2012 DA 14 , the one that just missed us today. Asteroids of this size -- 40 meters or larger in diameter -- strike Earth a couple of times every 100,000 years, and could wipe out an entire London-sized city if they struck there.

One of the big problems is that we only know of about 1% of the asteroids that are that size, so we can't even tell when most of them are coming. It's only the ones that we get a good view of for a long time that we can track well enough to predict when they're going to strike us. The hardest ones to predict are the ones that come towards us from the direction of the Sun; we literally never see those coming.

Image credit: Mark Oliver from the UK's Telegraph.

Which is why it was such a shock when this meteor appeared in the skies over Russia early this morning!

What exactly happened here? A much smaller asteroid than the ones we aspire to track, one only maybe 2-3 meters* in diameter, entered the Earth's atmosphere. Asteroids this small rate a zero on the Torino scale , as they pose no serious danger to Earth, but that doesn't mean they can't cause destruction.

Take a look from a different view; this is what happens when a roughly 50-tonne asteroid -- a mix of ice, rock and other chemicals -- enters the Earth's atmosphere.

Wow!

The physics of what's going on here is amazing. Let's do some Q&A about this:

Q: Why does it make a fireball in the sky?

A: The Solar System is a fast-moving place. Most objects move in excess of 25,000 miles per hour relative to Earth, and you probably think the wind was problematic when you put your arm out of the window while driving down the highway! At the astronomical speeds achieved by meteors, the outside of the meteor heats up tremendously, by many hundreds of degrees, and the fire you see is from a heat so hot that the meteor is disintegrating before your eyes.

Q: Why does it appear to explode in mid-path?

A: Because it really does explode! Think about it: you're heating this mostly frozen ice-and-rock-ball by hundreds and hundreds of degrees. Inside the meteor, you've got frozen water, frozen methane, and other weird, carbon-rich molecules. What happens when you heat these ices up? They melt, and eventually boil. As this boiling causes fissures in the meteor, oxygen -- common in our atmosphere but rare everywhere else -- can combine with these combustible gases under very high heats.

And that combination of things very quickly goes boom.

Image taken long enough ago that copyright has expired. :-)

Q: Why -- like this one and the Tunguska event -- do so many of these occur over Russia?

A: They don't preferentially occur over Russia, if that's what you're asking. Events like these -- I call them super-bolides -- occur on average about once every few years. There was a comparable one, another 50-100 ton asteroid, that encountered our atmosphere and burned up over Indonesia in 2009; in reality, most of them occur over the ocean and so go unobserved and unrecorded.

The Tunguska event was special: it's the largest one in recorded history, and was probably just a little smaller than the asteroid that made meteor crater. The reason these feel like they occur over Russia is simply because Russia has a huge amount of land area.



Image credit: NASA / JPL-Caltech.

Q: Why didn't we see this one coming?

A: First off, it's small. It's very difficult to see something that's just two-three meters across until you get very close to it, even with the most powerful telescopes in the world. Even the most ambitious survey proposals of asteroids that could be potentially hazardous to Earth don't go smaller than about ten times the volume of this one. And second off, it came from the direction of the Sun, the hardest direction to monitor. (Because if you're going to build an expensive telescope, the first rule is do not fry your optics, which you'll do if you point it too close to the Sun!)

Image credit: Meteosat-10.

Q: THIS IS A HOAX! IF IT WAS REAL WHY DIDN'T NASA PHOTOGRAPH IT FROM SPACE?

A: NASA doesn't monitor all places on Earth all the time from space. But this part of the world was actually monitored at the time, by the EU's Meteosat program. Above is the image of this meteor event from the Meteosat-10 satellite.

Image credit: Oleg Kargopolov / AFP / Getty Images.

Q: How much damage did it cause, and how did it happen?

A: There was some localized property damage, and probably around 1,200 injured people. When the big "flash" (or explosion) occurs, both a sonic boom and an intense pressure wave emanate from the source. This can do things like blow out windows (imagine the damage if one hit New York City!), damage eardrums, and -- in the case of Tunguska -- knock even large objects completely over. The building above had its roof collapse from the blast.

But this doesn't destroy the meteor, it just breaks it up into smaller chunks. Many of these fragments reach the surface, still traveling at speeds that are often in excess of terminal velocity and capable of causing some pretty intense damage, similar to a cannonball strike. The ice, below, had this giant hole created in it from a meteorite fragment.

Image credit: Chebarkul town head Andrey Orlov.

So what you hear reports of come from a combination of the initial blast wave and the secondary falling debris.

And finally...

Image credit: Don Davis.

Q: How long before the big one strikes, and all of humanity dies?

Believe it or not, an event like the one that caused the mass extinction of the dinosaurs is thought to occur only every few hundred million years. These events occur at random, which means -- like getting struck by lightning -- there's no way to predict it, not with our current state of knowledge.

But we could know this, what we'd have to do is find and start tracking each one of these potential Earth-killers, or any asteroid larger than a few kilometers in size. And we could do it with our current technology, too; all we'd have to do is invest in it.

Want to hear more about this meteor and have even more of your questions answered? I'll be on my local TV station tonight at 7 PM Pacific Time, and you can watch it live from anywhere in the world! See you then! The video permalink is up here, and you can watch the segment embedded below!

* -- Note that these and other numbers may change as reports continue to come in, and as we continue to learn more about this asteroid that struck us!

UPDATE: Just got my hands on this information, courtesy of Peter Brown, the director at the Centre for Planetary Science and Exploration in Canada and one of the world's experts in meteor fireballs. Be aware that these numbers are still preliminary, and many of them -- especially the size, mass and yield -- may change.