On Feb. 15, a hunk of space rock 50 feet across and weighing some 7,000 tons screamed into our atmosphere at 40,000 miles per hour over Chelyabinsk, Russia, about 9:20 a.m. local Russian time.
The rocky meteorite shattered from the pressure of moving at such supersonic speeds at an altitude of 12 to 15 miles. The explosion was equivalent to a 500-kiloton nuclear weapon, several times smaller than the nuclear bombs dropped in World War II.
Ironically, the last time anything of any significant size hit Earth from space, it also exploded over Russia.
On June 30, 1908, a 150-foot-wide, rocky meteorite slammed into our atmosphere over the Tunguska Forest region of Siberia.
The explosion knocked down some 80 million trees over an area of 830 square miles.
It is estimated that the shock wave from the air blast measured 5.0 on the Richter scale. That one released an energy equivalent of 7,700 kilotons of explosive energy.
Meteorite impacts on this scale are considered a once-a-century event. Smaller events occur much more frequently. In fact, a bit of a space rock just entered our atmosphere. Whoops, there goes another. And another. They come in at a rate of about one per second. Some 50,000 tons of stuff from space comes to Earth every year. The vast majority are no bigger than a grain of sand or a dust mote. At night, we see them as shooting stars. We don't see them at all during daylight hours, unless, like that one over Russia, they happen to be big enough to glow brighter than the daylight sky.
Impact events occur with increasing rarity with larger sizes. Space rocks massive enough to destroy a city may happen only once every few thousand years and most will hit over ocean.
Asteroids with the power to destroy a state- or country-sized region — plus the economy of our planet — happen with million-year frequencies.