The fireball that exploded over Russia last week left a wake of destruction but thankfully no lost lives. Here's what S&T's staff has managed to piece together about what happened.On February 15th a meteor blast over the Russian Chelyabinsk region wowed the world.
The bolide streaked through Russian skies at 9:20 a.m. local time and outshone the Sun. One minute and 28 seconds later, shock waves from its deceleration and explosion collapsed walls and blew out roughly 100,000 square meters (1 million square feet) of windows. (The delay is due to the speed of sound being much less than the speed of light.)
The meteoroid exploded almost directly over Chelyabinsk, an industrial city of about 1 million people. Damage varied widely, even within the same building: shock waves behave like bouncing balls, bludgeoning some buildings and sparing others depending on how they’re directed and how well they resonate with certain materials (glass is particularly vulnerable). More than 1,000 people were injured, mostly by flying glass. (One amazing photo of this damage is from a theater in Chelyabinsk.)
Collected fragments are ordinary chondrites, meteorites that make up more than 95% of all stony meteorites recovered. Despite early reports that the event blasted a hole in the frozen Lake Chebarkul, about 80 km (50 miles) west of Chelyabinsk, it’s now unclear whether the fireball and the hole are actually related.
By an amazing coincidence, the impact occurred just 16 hours before the predicted flyby of asteroid 2012 DA14, whose estimated diameter of 50 m (160 feet) is more than twice as big as the Russian meteoroid’s estimated size. But the two objects were moving in completely different directions: the asteroid moved south-to-north, the bolide east-to-west. Initial calculations put the outer end of the Russian object’s former orbit in the asteroid belt, unlike 2012 DA14’s, which has an orbit similar to Earth’s.
“This is not related to 2012 DA14,” says Peter Brown (University of Western Ontario, Canada). “This is perhaps the most certain aspect of the entire event.”
While 16 hours might seem a short period of time, Earth moves about 30 km/s (67,000 mph) through its orbit, meaning that in those few hours the planet traveled more than 1 million miles.
“This innocent asteroid was run over by Earth,” says Lindley Johnson, program executive of NASA’s near-Earth object (NEO) program. (Note: innocent, not harmless.)Brown’s analysis suggests the meteoroid entered Earth’s atmosphere at about 20 km/s, a speed typical of NEOs. It entered at a grazing angle of less than 20° and burst at an altitude of 20 to 30 km. Based on ground-based infrasound measurements, between 20 and 30 seconds passed between atmospheric entry and the meteor’s airborne disintegration. The estimated explosive energy, equivalent to 470,000 tons of TNT, corresponds to about 30 Hiroshima bombs. Given a chrondritic composition, that energy release implies that the original object had a diameter of about 17 m (54 feet) and a mass of about 10,000 metric tons.
An event of this magnitude happens about once per century. It’s the biggest known impact since the Tunguska explosion in 1908, which exploded with 185 Hiroshimas a mere 8½ km above the ground and flattened trees covering 2,000 square kilometers of rural Siberia. The most recent event was a fireball over Indonesia in 2009, with an estimated energy of 50,000 tons of TNT.
Thankfully no one was killed by the Chelyabinsk event, but that’s no guarantee for future safety. The one-two combo with 2012 DA14 should serve as a wake-up call, experts say.
“It’s one thing to talk in the abstract about blown-out windows, et cetera,” says Dan Durda (Southwest Research Institute). “It’s quite another to be reminded that there are very often people behind and under that shattered glass. That’s why we care about these events and that’s why it’s worth the investment to survey for smaller NEOs and to plan for preventing larger impacts.”
Brown adds that the value isn’t just in startling us from our cosmic doze. “The major significance is that this is the first well-documented event which has clear ground-level effects,” he says. “This will permit calibration of entry models for tens-of-meter-sized objects for the first time.”
Such small objects are difficult to track, and of the 9,500 near-Earth asteroids discovered so far most are less than 300 m (1,000 feet) across. Former astronaut Ed Lu, who now head the B612 Foundation, estimates that we’ve only found 1% of objects the size of 2012 DA14.
Robert Naeye and Kelly Beatty contributed to reporting for this blog.