WASHINGTON - Scientists studying the meteor that exploded without warning over a Russian city last winter say the threat of space rocks smashing into Earth is bigger than they thought.

They say meteors about that size -- 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. That's according to three studies published in the journals Nature and Science.

Scientists say the meteor that streaked through the sky at 42,000 miles an hour and burst over the city of Chelyabinsk was only 62 feet across -- but it burst with the force of about 40 Hiroshima-type atom bombs. It released a shock wave that shattered thousands of windows and injured more than 1,600 people.

Its flash was bright enough to temporarily blind 70 people and cause dozens of skin-peeling sunburns.

Until then, scientists had figured that 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 says it is likely to happen once every 30 years or so, based on how often these things are actually hitting.