After spending not a few words talking about Armageddon and fake space rocks last Wednesday I was slightly surprised when an actual real-life asteroid tore through the skies above Russia and disintegrated/detonated in midair somewhere above Chelyabinsk. Thanks to the asteroid’s passage over populated areas and the modern ubiquity of smartphones with some kind of video capture capability – not to mention the uniquely Russian preponderance of car dashboard cameras to provide some protection against the now-famous driving standards in the country, as well as the notoriously corrupt traffic police – this has been by far the most well-documented asteroid “strike” in history, so I thought I’d take a little while to talk about it, and the reaction to it.
First, there’s the question of nomenclature. I’m going to be calling the Russian object an asteroid in this piece, but you should know that the term “asteroid” is rather ill-defined (in fact all the terminology used to refer to things smaller than dwarf planets is rather ill-defined), and the best I can come up given currently-existing IAU guidelines is “a lump of rock that is smaller than a dwarf planet but larger than a meteoroid”. Meteoroids are currently defined as “a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom”, but that definition dates from 1968 and we’ve since determined that meteoroids and asteroids are basically the same population of bodies, with the term “meteoroid” being used to refer to any small-scale asteroid (10m or below) that manages to enter Earth’s atmosphere. Most meteoroids burn up in the atmosphere; the ones that make it through to hit the ground are called meteorites, which is what the majority of the press is calling the Russian asteroid.
Are they correct? Well, yes and no. There are two problems with calling the Russian asteroid a meteorite:
1) It was actually quite large – NASA says around 17 metres – which puts it out of the generally accepted size range for a meteorite.
2) It didn’t actually hit the ground. While it didn’t burn up completely, the Russian asteroid broke up just above ground level thanks to the immense heat and pressure of entering the atmosphere.This will have scattered chunks of asteroid everywhere and there’s already been several fragments and craters reported found. Astronomers have a specific term for asteroids that create fireballs and break up in the atmosphere like this: we call them bolides.
However, if the press went around referring to the “Russian bolide” in their news reports nobody would have any idea what the hell they were talking about, so in the absence of any better term calling it a meteorite is fine. This is the IAU’s fault for not ever bothering to classify small Solar System objects properly and relying on fuzzy definitions that are decades-old, not bad reporting.
Now, let’s take a moment to watch a video of the asteroid’s passage.
(I have desperately tried to avoid lifting the video links straight from Bad Astronomy guy’s excellent rundown of what was going on at the time, but the originals I saw have since been buried by recycled clips from news agencies and I didn’t save them for future reference.)
That gets across just how bright the asteroid’s entry into the atmosphere was; it’s a large object travelling at around 18 kilometres per second (i.e. far, far faster than a supersonic jet aircraft, which would be lucky to manage a kilometre and a half per second) so it’s generating an awful lot of ram pressure, which heats up both the asteroid and the air flowing around it. If you direct that amount of force and energy against a lump of rock and metal it is rather understandably going to start falling to pieces, which is exactly what the Russian asteroid did; it lasted barely thirty seconds in the Earth’s atmosphere and broke into bits over the Urals near Chelyabinsk. But did it explode?
This video shows that there was a very large bang about thirty seconds after the asteroid’s passage through the atmosphere. This is caused by a sonic shockwave, which is what broke all the windows, smashed up that zinc factory and injured a thousand people, but the shockwave is not the product of the asteroid exploding or hitting the ground. Instead it’s been caused by the simple passage of the asteroid through the air at such a ridiculous speed. You’ve heard aircraft make sonic booms as they break the sound barrier? This is that, except magnified about a hundred times. The shockwave of a sonic boom is created by air piling up in front of the aircraft, and as the aircraft goes faster and faster the air simply cannot get out of the way fast enough and becomes compressed into a single shock front moving at the speed of sound. Since the aircraft is moving faster than the speed of sound you’ll see a supersonic jet a few seconds before you hear its sonic boom. Scale up from an aircraft to a large chunk of rock 17 metres on a side, and make the time lag thirty seconds (because it was around 30-50 kilometres up, an altitude much higher than aircraft fly at) and you get the shockwave caused by the Russian asteroid.
So the bang wasn’t caused by any explosion in the conventional sense; it’s not like the asteroid reached the end of its trajectory and abruptly exploded like a nuclear weapon. Instead, as bits and pieces sheared off of the asteroid it would have released energy with the same effect as a trail of smaller explosions. NASA puts the total energy released during the asteroid’s passage through the atmosphere at around 500 kilotons, which is roughly equivalent to a mid-sized nuclear bomb, but this energy was released gradually instead of all in one go which is why the property damage was mostly limited to a lot of smashed glass.
That’s what we think happened, but sadly there’s a couple of myths that have already grown up around the Russian asteroid:
- That it was something to do with the asteroid 2012 DA14, which was scheduled to make a very close approach to the Earth on the same day. This is stupid because the two objects were on completely different trajectories as demonstrated by this video; the Chelyabinsk asteroid grazed the Earth’s atmosphere on an east-west trajectory, while DA14 passed it by north-south. They are completely different bodies that just happened to get noticed by the human race on the same day; thousands of meteoroids/meteorites enter the Earth’s atmosphere every year and the vast majority of them are completely unknown to us because they burn up too high, or they come in during the day, or they descend over the ocean. The only things that make the Russian one special are its size and that it happened to come down over a populated area.
- That it was intercepted by Russian air defence forces and destroyed. This is insane for a number of reasons but I’m going to pick the most salient one: the best air defence technology on earth currently cannot make a reliably successful interception on an ICBM moving 7-8 kilometres per second. It would have no chance of hitting an asteroid moving at 18 kilometres per second.
Finally there’s the media reaction to it, which if you look past the usual idiotic misreporting is basically one of total ignorance. Despite reporting on 2012 DA14 for a week or two the fact that an asteroid could actually make it through (most of) the atmosphere and cause some damage to a populated area seemed to take them by surprise. I lost count of the number of hastily-commissioned opinion pieces I saw which basically said “Whoa, maybe we should be a bit worried about this!” like they previously thought it was something that only happened in movies. The good news is that even if 2012 DA14 had hit it would have caused only localised damage (roughly equivalent to one city, and that’s only if it had managed to score a direct hit), and that really threatening impacts only occur over geologic timescales. The bad news is that there is literally nothing we could do to stop either kind of impact without a decade of lead time to prepare – and as the Russian asteroid showed, often we don’t even see them coming at all.
(Just as an aside, every news article under the sun mentioned the Tunguska event as another meteorite event which happened in Russia, but nobody brought up the Sikhote-Alin meteorite which came down over what was then the Soviet Union in 1947. Unlike Tunguska this was witnessed by people who understood what they were looking at, including an artist who immediately drew this. It’s strikingly similar.)