What would really happen if a huge asteroid hit Earth?

Credit: Johns Hopkins University

Last month, thousands of people in southwestern Russia learned very suddenly what happens when a bus-sized meteor arrives unannounced. But a city full of shattered windows and 1,200 injuries from shattered glass is pretty minor compared to what could have happened — in a lot of ways, the residents of Chelyabinsk were lucky.

For one thing, the 11-ton meteorite that exploded above their city was crumbly, and shed lots of its energy as it broke apart in the upper atmosphere. It also came in at a shallow angle, so frictional forces slowed it down significantly. But what if it wasn’t crumbly, but made of iron, as other meteorites are? What if it was a little bigger? Or what if it wasn’t an asteroid, but a much speedier comet?

Change any one of those variables, and the result would not have been pretty, said Margaret Campbell-Brown, an associate professor at the University of Western Ontario and a member of the Western Meteor Physics Group. A stony asteroid, fast-moving comet, or much larger impactor would have wrought havoc. It’s a good thing none of these are very common — so far as anyone can tell.

Thousands of tons of solar system material falls to Earth every year, but most of it is space dust formed from micrometeorite collisions in the high atmosphere. Big rocks are pretty rare.

Generally, the larger the asteroid size, the less often they hit Earth. Something the size of a boulder [a couple feet in diameter] hits about once a week," Campbell-Brown told DVICE. These mostly fall in the ocean or unpopulated places. “But that’s not to say that every week there’s something the size of a boulder," she added. "Something the size of Chelyabinsk hits us every 100 years-ish, but that’s not to say that another one won’t hit us tomorrow."

If it did, here’s what could happen.

Chelyabinsk Asteroid, 18 meter diameter (iron)

An iron asteroid would be less likely to crumble in the upper atmosphere, producing larger meteorite pieces. It would still shrink as it streaked through the sky, releasing energy and slowing down to a speed of a couple miles per second.

"An iron one would be likely to give you a real impact crater. If I had to guess, it would be in the 10-meter (32 feet) to a few tens of meters range," Campbell-Brown said. "It’s hard to know, exactly, but in general, impact craters don’t come small."

A comet the size of the Chelyabinsk meteorite could do some damage just by virtue of its speed, said Mark Bailey, director of the Armagh Observatory in Northern Ireland and a comet expert. Asteroids enter the atmosphere at speeds around 26,000 to 56,000 MPH (astronomers discuss them in kilometers per second), but comets can travel at 156,000 MPH. Energy is a function of speed squared, so if you double an object’s speed, you have four times as much energy to dissipate.

"You wouldn’t want that to happen," Bailey told DVICE. "Comets, because of their longer-period orbits, have the potential to collide with greater speed, and therefore greater kinetic energy." And that means more damage on Earth. That's the case for even larger asteroids, too.

Asteroid 2012 DA14, 30 meter diameter

The other space rock that visited Feb. 15, passing within 17,000 miles of Earth, was almost double the size of the Chelyabinsk meteorite. If it had collided with Earth, it would have produced an air burst equivalent to 2.4 megatons of TNT — by comparison, the atomic bomb dropped on Hiroshima, Japan, in WWII was equivalent to 17 kilotons. And in that explosion, some 70,000 people died in an instant.

Along with a gigantic shockwave, an impactor this size could excavate a crater comparable to the 4,000-foot diameter Meteor Crater near Flagstaff, Arizona. People several miles away from the blast center would feel like their clothes were on fire. 

Tunguska Comet, 60-100 meter diameter

The June 1908 Tunguska event unleashed about 10 times the energy as Chelyabinsk, wiping out 80 million trees across more than 800 square miles. Its blast wave, estimated at 10 to 15 megatons, was roughly 1,000 times more powerful than the Hiroshima bomb. Scientists are still debating its original size, which is hard to pin down because it didn’t form a crater. That’s led to ongoing debate that it could have been a disintegrating comet and not an asteroid. One thing it did for certain is spark a huge but short-lived forest fire, Campbell-Brown said.

"It was so bright, the light was so intense, that it heated everything to the point where trees burst into flames," she said. "But the blast wave was so powerful that it blew the flames out, so the fire didn’t have long to burn."

The Earth has much more water than inhabited land areas, so an impactor this size is more likely to fall in the ocean, where it could trigger a devastating tsunami. If a similarly-sized chunk did enter the atmosphere over a city, anything in its path would be obliterated.

"It would be very unpleasant if something came overhead that could set your clothes on fire," Campbell-Brown said. "If it was over a city, it would cause lots of fatalities and massive damage. It might not completely total a city, but it would kind of be like New Orleans after Hurricane Katrina. You’d have a lot of rebuilding to do."

Around this size, asteroids or comets also start to have global effects, although they wouldn’t cause long-term changes. In the same hemisphere as the explosion, the sky would seem to glow for a few days, a result of ice particles formed high in the atmosphere.

Asteroid Apophis, 350 meter diameter

When it was discovered, early estimates suggested asteroid 99942 Apophis had a roughly 2.7 percent chance of hitting Earth in 2036. New estimates from its recent flyby have pretty much ruled that out, however. If it did collide with Earth, the greatest risk along its predicted path would be in northern South America, where casualties could top 10 million, according to at least one research paper. Apophis-sized rocks could produce a devastating tsunami, too.

"When you get above about half a kilometer in size, it doesn’t really matter where on Earth it hits," Bailey said. "It would produce such a dramatic explosion, it would affect the global climate in the form of water vapor ejected into the atmosphere. It would make a crater, and uplift a mass of dust . At that point, you have a global catastrophe."

Asteroid 1950 DA, 1.1 to 1.4 kilometer diameter

At this scale, asteroids have the power to destroy nations; kilometer-sized rocks would create tsunamis capable of destroying entire coastlines. Dust would fill the atmosphere, causing widespread crop failures and changing climate patterns. Asteroid 1950 DA, at 1.1 kilometers in diameter, is one of a handful of near-Earth objects of this size.

Around 10 kilometers, we're talking about extinction events. The asteroid (or comet) that did in the dinosaurs is thought to have been around 10 to 16 kilometers in diameter. At those scales, Earth would be on fire, long-term changes to the climate would ensue, and and mass extinctions would be inevitable.

Luckily, scientists know about roughly 94 percent of those giant asteroids, and they know we don’t have anything to worry about for at least 100 years. It’s trickier to nail down the threat from smaller space rocks or dirty snowballs, partly because astronomers don’t know exactly how many there are, where they are or whether they’re close enough to Earth’s orbit to pose a threat. But new measurements, and new telescopes, could provide a clearer picture of those dangers, too.

2012 DA14 was discovered in an Earth-based sky survey, but the Chelyabinsk meteorite wouldn’t have shown up even if astronomers had known where to look, because it was coming from the same direction as the sun. Space-based surveyors could have seen it, however. The nonprofit B612 Foundation’s Sentinel mission, for instance, aims to launch an infrared space telescope in orbit around the sun, where it would discover and track asteroids whose orbital paths come close to Earth’s.

Campbell-Brown said at the very least, the recent rash of comets and asteroids is a reminder that we are inhabiting a fragile planet. "We are part of an active solar system," she said.

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