FIRE IN THE SKY: A fireball brighter than the sun lit up the morning skies over Russia and left a long trail in the sky. Image: Courtesy Alex Alishevskikh/Cyberborean Chronicles via Creative Commons license
A surprise meteor strike over central Russia this morning lit up the skies, blew out windows on the ground and injured roughly 1,000 people in and around Chelyabinsk, a city of 1.1 million. The inbound object, thought to be a small asteroid, had not been discovered prior to impact. But already teams on the ground are reportedly collecting possible fragments of the meteorite, and researchers around the globe are scrambling to figure out what happened. Scientific American contacted Margaret Campbell-Brown, a professor in the Meteor Physics Group at the University of Western Ontario, to get the latest.
[An edited transcript of the interview follows.]
What do we know, as of now, about what caused the fireball over Russia this morning?
We’ve actually seen it from at least two infrasound stations. Infrasound is very low frequency sound waves, which are produced in, for example, loud explosions. There is a global network of infrasound sensors whose purpose is to detect nuclear explosions in the atmosphere. It’s part of the Comprehensive [Nuclear] Test Ban Treaty. Two of the nearest stations in this network, which were both in Russia, did detect this very large event.
So, from that, we know that the energy of the explosion was about 300 kilotons of TNT equivalent. So it was a very, very powerful explosion. It was the biggest explosion from a meteor that we’ve seen in the atmosphere since the Tunguska impact of 1908.
We know that the meteor lasted about 30 seconds. It came into the atmosphere at a very shallow angle, which is why it lasted so long. The object was moving at about 18 kilometers per second, which is about 65,000 kilometers per hour, which is typical of an asteroidal speed.
From the energy of the impact, we think that it was about 15 meters in size, so it would be the largest object to hit the Earth since the Tunguska impact, as far as we know—we haven’t recorded an object larger than that. It had a mass of probably about 7,000 metric tons, so it was a very large object.
You may have seen that the Russian Academy of Sciences issued a statement with a lower estimate for the size of the object—something in the few-meter range producing an explosion of a few kilotons.
Right. It’s the most uncertain part of the calculation, but I would be very surprised if it’s less than 100 kilotons. It was a very, very large event. And the fact that there was so much damage on the ground supports the conclusion that the energy was high. You need a lot of energy to shatter windows in the way that was seen.
Is there any reason to suspect that it was anything other than an asteroid?
An asteroid is certainly the most likely suspect. The size of it, the speed that it was going and so on, all point to an asteroid. The fact that it exploded in the atmosphere implies that it was probably a stony asteroid, maybe a chondritic type, for example, as opposed to something iron, because iron things are stronger and tend to make it to the ground, where they release their energy.
Where was most of the energy released as this object made its way through the atmosphere?
In this case the final destination, which seems to have been the largest deposit of energy, was somewhere around 15 to 20 kilometers altitude. The actual fireball probably started significantly higher than that, maybe 50 kilometers, but most of the energy was apparently deposited during that last explosion lower in the atmosphere.
Is it possible that if this meteor had hit over the ocean rather than over a populated area, we might not have known about it?
We certainly would have known about it. The CTBT, the Test Ban Treaty, constantly is monitoring for large explosions in the atmosphere, and this one was large enough that no matter where it occurred over the Earth it would have been detected by the CTBT array.
You mentioned that this event showed up in two nearby CTBT sensors. Is it possible that the explosion was picked up by other stations as well?
Not all the sensors are as straightforward to get the data from. We’re trying to get data from other sensors. It would surprise me if there wasn’t data on other sensors because this was a very powerful wave, and I would expect it to propagate a very long distance because infrasound can travel a very long way in the atmosphere. But we don’t have data from other stations yet.
How often should we expect to see an event like this?
In the 15-meter size range, we think it happens about every 50 years. It’s been more than 100 years since we’ve seen something of this size, but statistically it happens approximately every 50 years.
When you consider all the areas of the Earth that are uninhabited—the oceans, the ice caps, the deserts and so on—it’s very surprising that this happened over such a populated area. Very unlucky.