The threat of space rocks smashing into Earth is bigger than previously thought, according to scientists studying the terrifying meteor that exploded without warning over a Russian city last winter.
Meteors about the size of the one that streaked through the sky at 67,600 kph and burst over Chelyabinsk in February - and ones even larger and more dangerous - are probably four to five times more likely to hit the planet than scientists believed before the fireball, according to three studies published in the journals Nature and Science.
Until Chelyabinsk, the US space agency Nasa had looked only for space rocks about 30 metres wide and bigger, figuring there was little danger below that.
This meteor was only 19m across but burst with the force of about 40 Hiroshima-type atom bombs, scientists say. It released a shock wave that shattered thousands of windows and injured more than 1,600 people, and its flash was bright enough to temporarily blind 70 people and cause dozens of skin-peeling sunburns just after dawn in icy Russia.
Up until then, scientists had thought a meteor causing an airburst like the one in Russia was a once-in-150-years event, based on how many space rocks have been identified in orbit. But one of the studies now says it is likely to happen once every 30 years or so, based on how often these things are actually hitting.
Scientists said a 1908 giant blast over Siberia, a 1963 airborne explosion off the coast of South Africa, and others were of the type that is supposed to happen less than once a century, or in the case of Siberia, once every 8,000 years, yet they all occurred in a 105-year timespan.
Because more than two-thirds of Earth is covered with water and other vast expanses are uninhabited deserts and ice, other past fireballs could have gone unnoticed.
Lindley Johnson, manager of Nasa’s Near Earth Object programme, said the agency is reassessing what size space rocks to look for and how often they are likely to hit.
By readjusting for how often these rocks strike and how even small ones can be a threat, "those two things together can increase the risk by an order of magnitude," said Mark Boslough, a Sandia National Lab physicist, co-author of one of the studies.
In fact, the US government got a new sense of urgency after Chelyabinsk, quietly holding a disaster drill earlier this year in Washington that was meant to simulate what would happen if a slightly bigger space rock threatened the East Coast.
In the early part of the drill, when it looked as if the meteor would hit just outside the nation’s capital, experts predicted 78,000 people could die. But when the mock meteor ended up in the ocean, the fake damage featured a 15m tsunami and shortages of supplies along the East Coast, according to an after-action report.
The exercise and the studies show there’s a risk from smaller space rocks that strike before they are detected - not just from the long-seen-in-advance, dinosaur-killing giant ones like in the film Armageddon, said Bill Ailor, a space debris expert at the Aerospace Corporation who helped coordinate the drill.
"The biggest hazard from asteroids right now is the city-busting airbursts, not the civilisation-busting impacts from one-kilometre-diameter objects that has so far been the target of most astronomical surveys," Purdue University astronomer Jay Melosh, who wasn't part of the studies, wrote in an email. "Old-fashioned civil defence, not Bruce Willis and his atom bombs, might be the best insurance against hazards of this kind."
After the government drill, NASA and Federal Emergency Management Agency officials said they should look at the need for evacuations, figure out better ways of keeping the public informed without scaring them, and handle meteor threats in a way comparable to how they deal with hurricanes bearing down on the coast.
NASA also got a wake-up call this week on those bigger space rocks that astronomers thought they had a handle on, discovering two 20km-wide space rocks and a 1.9km-wide one that had escaped their notice until this month.
The three objects won’t hit Earth, but their discovery raises the question of why they weren’t seen until now.
Asteroids are space rocks that circle the sun as leftovers of failed attempts to form planets billions of years ago. When asteroids enter Earth’s atmosphere, they become meteors. When they hit the ground, they are called meteorites.
The studies said the Chelyabinsk meteor probably split off from a much bigger space rock.
Agencies