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The latest analysis of the bollide that burst over Chelyabinsk, Russia in February suggests that the risk from such airbursts — which occur when friction in our atmosphere heats up a meteor — may be greater than previously thought. Meteorite collisions are often compared in size to nuclear explosions, but because they are speeding toward Earth they have momentum that makes them far more destructive. And to make matters worse, they may occur more often than currently estimated. On the morning of Feb. 15, a fireball lit up the skies above the town of Chelyabinsk. A 12,000-ton bollide estimated to be roughly 20 meters in diameter came screaming into the atmosphere at more than 42,000 mph. Locals could feel the heat from the blast while dozens of dashboard cameras made recordings of the event, which were disseminated widely on social media. The best estimates of how much energy was released by the Chelyabinsk explosion come from infrasound measurements taken by an array of sensors all over the world. These instruments detect low-frequency sound waves traveling through the atmosphere. The longer the waves’ period is, the larger the explosion. Infrasound measurements are calibrated from atmospheric nuclear testing done in the 1950s, which is why asteroid explosions are often described in megaton units. The bomb that exploded at Hiroshima had a yield of 16 kilotons while the most powerful nuclear weapon active in U.S. service, the B83 bomb, has a yield of up to 1.2 megatons. The Chelyabinsk blast is estimated to have been between 200 and 800 kilotons, on par with a huge atomic weapon. But meteors explode in a very different way than a typical nuclear bomb, says physicist Mark Boslough of the Sandia National Laboratories, who studies asteroid impacts and is presenting a talk today about the Chelyabinsk event at the American Astronomical Society’s 2013 Division for Planetary Science meeting in Denver. Boslough said. For that reason, the people who live in Chelyabinsk explosion are very lucky to be alive, he added. If the bollide had come into the atmosphere at a less steep angle, its blast would have been aimed right at the ground, likely doing much more damage. That an airburst continues traveling in the same direction as a meteorite was only appreciated starting in the 1990s, particularly after the impact of Shoemaker-Levy 9 on Jupiter. This understanding has led to revisions in estimates of the size of the asteroid that exploded over the Siberian tundra in 1908. This blast, known as the Tunguska event, flattened trees over a 2,000-square-kilometer area. Scientists in the mid-20th century used nuclear blast comparisons to estimate Tunguska’s power. To make trees fall down over that large an area, a nuclear weapon would have to be 10 to 20 megatons. Now knowing how asteroid impact bursts can deliver more energy to the ground, the Tunguska bollide estimate has gotten smaller, suggesting that an object of roughly 100,000 tons entered the atmosphere and delivered a blast of between 3 and 5 megatons. Tunguska and Chelyabinsk are thought to be among the most powerful asteroid impacts in recent history. That both would come within about 100 years of one another is slightly worrying to scientists like Boslough. That’s because current estimates are that an impact the size of Chelyabinsk should happen roughly once a century while a Tunguska-level event should happen once every millennium. To see two such once-in-a-long-while events within close succession makes “you wonder if you’ve got your probabilities right,” said Boslough. He gave a rough back-of-the-envelope calculation suggesting that the chances of these two occurrences — plus a third airburst near South Africa in 1963 (.pdf) that was somewhere in size between Chelyabinsk and Tunguska and was was only observed by infrasound sensors — is somewhere on the order of 0.2 percent. Our current probability estimates of asteroid impacts are most calculated using astronomical data. Telescopes search for space rocks and note the number that cross Earth’s orbit. But models based on these asteroid surveys have a lot of assumptions built into them, mostly because detecting asteroids is a big challenge, particularly smaller ones that would cause airbursts, and we don’t know exactly how many more of them we have yet to find. It’s possible that we’ve missed many and that airbursts like Chelyabinsk and Tunguska happen more than once a century or millennium. Agreeing with this assessment is geoscientist Peter Schultz of Brown University, who said that Chelyabinsk “should be kind of an eye-opener.” After all, he added, Earth experiences an airburst explosion similar in energy to Hiroshima almost every year, but they are more likely to happen over the ocean or uninhabited areas and go unnoticed by people other than the scientists who track them. Geological evidence also suggests that larger asteroids that hit the Earth’s surface strike more frequently than we think. In Argentina alone, scientists have found glass that was formed in impacts from about eight or nine large events that occurred in the last 10 million years. said Schultz. On the other hand, that Chelyabinsk and Tunguska happened in close succession might just be a fluke. Boslough said that two data points in of themselves shouldn’t make us believe that asteroid impacts happen more frequently than we think. he said. As with anything relying on probability, we will simply have to wait and see. The longer we observe asteroid impacts on Earth, the better we will be able to estimate their frequency. It’s pretty much a guarantee that “eventually we’ll have a close encounter of a bad kind,” said Schultz.