Most talk about asteroid impacts is about what could happen. Two events show what already has. In 1908 something exploded over Siberia and flattened a forest. In 2013 another object lit up the sky over a Russian city and broke thousands of windows. Neither left a big crater, and both teach the same lesson about how Earth really gets hit.
Early on the morning of 30 June 1908, a fireball crossed the sky over central Siberia and exploded near the Podkamennaya Tunguska River. Witnesses in the sparsely populated region described a bright light, a heavy blast and a shaking ground. Records from distant weather and seismic stations also picked up the disturbance.
The damage was found later, because the area was remote. Scientists estimate that trees were knocked down across more than 2,000 square kilometers, far larger than most large cities. Popular sources often say tens of millions of trees fell, but the exact count is an estimate, not a measurement.
The first scientific expedition led by Leonid Kulik reached the area in 1927, almost two decades later. He expected a huge crater and a pile of meteorite fragments. He found neither. Instead, trees near the center were stripped and left standing, while those farther out lay fallen in a radial pattern pointing away from the blast.
That pattern is the signature of an airburst: an object that heats and breaks apart in the atmosphere several kilometers above the ground. Most researchers estimate the explosion happened at roughly 5 to 10 km altitude. The size of the object is debated; many studies suggest a stony body of about 50 to 60 meters, and energy estimates range from around 10 to several tens of megatons of TNT. Some researchers have suggested that a small nearby lake, Lake Cheko, is a crater left by a fragment, but that idea remains disputed.
No one is known to have died in the blast itself, though the thinly populated area makes it hard to be certain. A similar explosion over a modern city would be a very different story.
On the morning of 15 February 2013, a bright fireball streaked over the southern Urals in Russia. Thousands of people filmed it, thanks to car dashboard cameras, which is why Chelyabinsk is among the best-documented impacts in history.
The object was roughly 17 to 20 meters across and entered the atmosphere at around 19 km/s, many times faster than a rifle bullet. Analyses of infrasound recordings from a global monitoring network put the released energy at about 500 kilotons of TNT, with significant uncertainty. For comparison, the Hiroshima bomb is usually quoted at around 15 kilotons. Most of that energy was deposited in the air tens of kilometers up, which is the only reason the event was not a catastrophe.
The fireball was brighter than the Sun for a moment. The real harm followed a minute or two later, when the shock wave reached the ground. It shattered windows over a wide area and damaged thousands of buildings. More than 1,000 people sought medical help, and the majority of injuries were cuts from flying glass, often to people who had walked to their windows to look at the light.
Small fragments fell to the ground as meteorites. A large piece, over 500 kg according to reports, was later recovered from the bed of Lake Chebarkul. These pieces gave scientists a chance to study the rock directly, which is rare for an airburst.
Chelyabinsk was not detected before it hit. Objects of that size are faint, and this one approached from the direction of the Sun, where telescopes cannot look. By coincidence, a different asteroid, 2012 DA14, passed Earth the same day at about 27,700 km, which is why the world's attention was on the sky for a harmless reason.
Detection has improved since. In 2008, asteroid 2008 TC3, only a few meters wide, was found hours before it burned up over Sudan, the first time an impact was predicted in advance. To see how surveys catalog such objects, read what a near-Earth object is and how NASA tracks asteroids.
Estimates vary, but the rough picture is consistent. Rocks a few meters across hit the atmosphere every year, mostly over oceans and empty land. Chelyabinsk-size events are thought to happen on the scale of decades. Tunguska-size events are much rarer, perhaps once in several centuries to a thousand years. Impacts like the one that ended the dinosaurs happen over tens of millions of years, as covered in Apophis vs. the dinosaur asteroid.
Apophis, at about 340 meters, is much larger than either object. But it is not a threat, as explained in Is Apophis dangerous?. The more realistic danger is a small, unseen object, exactly the kind surveys are now built to find.
Facts in this article are drawn from public scientific sources. For primary information, see: