When Will the Earth Get Hit by Another Asteroid?

Your odds of dying in an asteroid impact are about one in 700,000.

Surprising, isn’t it? That’s about the same chance you have of dying in a flood or a fireworks accident over your lifetime. It may be even more surprising when you consider that there has never even been a confirmed human death resulting from an impact. But this number involves something of a trick: A big enough impact will kill everyone on Earth. A smaller impact might devastate a local region on Earth, but a big one can wipe out entire species. Just ask the dinosaurs …

For a global event, you get these odds roughly by dividing the time between impacts by the average human lifespan. But it’s still a little misleading because it’s similar to the lottery: The chance is 100 percent that someone will win the lottery, but the chances are extremely low that you specifically will. Your odds of dying in an impact event are pretty low, but the odds of some random person somewhere getting killed are higher.

Of course, asteroid impacts are a lottery you get to play whether you want to or not.

Today is a good day to think about all this: It’s the first anniversary of the Chelyabinsk impact over Russia. On Feb. 15, 2013 (it was still Feb. 14 in U.S. time), a rock the size of a house came screaming in from space. In a single moment, its huge energy of motion was converted into light and heat. The resulting explosion was the equivalent of a half-million tons of TNT detonating all at once. Even though it exploded dozens of kilometers above the Earth’s surface, the shock wave shook the ground, set off car alarms, and shattered windows. More than 1,000 people were injured, some seriously, by flying glass.

Amazingly, no one was killed, but it shows quite vividly that the threat of asteroid impacts is quite real.

So when will the Earth get hit again? And what can we do about it?

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The Earth gets hit by about 100 tons of material every day, but that’s in the form of tiny pebbles that burn up high in the atmosphere and produce shooting stars.

Big impacts are rare. The Chelyabinsk asteroid was 19 meters (62 feet) in diameter, and, on average, we should expect an impact from an object that size somewhere on Earth about once every 25 years. (Because most of the planet is covered in water, many of these go unnoticed.)

Bigger impacts are more rare. In 1908 an object 30 meters or so in diameter came across the Earth, exploding high over a swampy region of the Russian countryside near the Tunguska River. The yield was equivalent to a 15 megaton nuclear bomb! Something like this Tunguska event (as it’s now called) happens every few centuries on average.

You probably know that the dinosaurs were taken out by an asteroid or comet about 10 kilometers wide. Happily, those events are extremely rare, occurring on a timescale of tens of millions of years. As it happens, we’re pretty sure there’s no dinosaur-killer on its way to Earth for the next few centuries. But the lesson of Chelyabinsk and Tunguska is that it doesn’t take a flying mountain to ruin your whole day. A hill will do nicely.

If we want to prevent asteroid impacts from happening, the first thing we need to do is spot these threats. And we’re working pretty hard on that.

Astronomers have built quite a few observatories dedicated to patiently scanning the heavens looking for blips of light. Thousands of near-Earth asteroids have been found this way, their orbits meticulously calculated, projected into the future, and determined to be potentially threatening or not.

As things stand now, we don’t have the capability to find them all. But we will, soon. The huge Pan-STARRS telescope is looking deep for threats and is already producing data. LSST is a planned monster 8-meter telescope specifically designed to look for near-Earth objects and is expected to catalog hundreds of thousands of them.

To extend our vision, two spacecraft are currently in the works, too. They’ll have a better view above our turbulent atmosphere, and by using infrared detectors they can more easily spot the warm glow of rocks orbiting the Sun.

Even with all these eyes on the sky, there are roughly a million such rocks that could potentially, someday, impact the Earth. Eventually, it’s bound to happen: We’ll spot one with our name on it. What then?

* * *

You won’t like this answer much, but: nothing. As things currently stand, even if we had a few years’ warning, there’s little we could do to stop an impact. It takes time to plan a mission, build it, launch it, travel to the rock, and then do whatever it is you plan to do.

But let’s say we did have a rocket waiting to launch. What do we do with it?

If you watch movies, the answer is easy: Nuke it! But that might not be the best option. For one thing, it’s a bit ham-fisted and hard to control. You might hit the asteroid and shatter it, turning one big rock into a bunch of slightly smaller, still-dangerous (and radioactive!) ones. Also, timing is a problem; remember, these asteroids are moving at high speed, so detonating the bomb at just the right exact moment might prove tricky.

It helps to remember the goal: You want to make the asteroid not hit the Earth. When you think of it that way, a better solution is to change its velocity—speed it up or slow it down—so it’ll whiz past us without an impact.

For this, you don’t need a bomb, just the rocket itself. Literally aim the rocket directly at the asteroid and hit it as hard as you can! The purpose would be to use the momentum of the rocket to change the momentum of the rock enough to do the job.

The key to this trick is time. The Earth is 8,000 miles across, so at most you have to move the asteroid 4,000 miles to make it miss. If you have, say, two years of advance warning you need only change the velocity of the rock by one-fifth of a mile per hour to make it miss. That’s far less than walking speed! And the longer lead time you have, the less the change needs to be. For big rocks that’s still a problem, but again, finding them farther in advance is our best bet.

The B612 Foundation has made it its mission to do just this. The group includes scientists and engineers who investigate how best to stop an incoming asteroid or comet, and they have found this impactor method to be feasible. But they found a subtle problem: You might push the rock into an orbit that’ll just make it hit us at some other time. You’ve prevented an impact in two years, but now you’ve forced one to happen in, say, five years!

You could hit it again, but then the same problem might occur, and rockets are expensive. This is where the B612 folks came up with a fantastic idea: a gravity tractor.

Instead of a single payload to whack the asteroid, you also send along a probe that has a lot of a mass, maybe a ton or so. It has some small amount of gravity that can be used to literally tow the asteroid into a safe orbit once the impactor has done its job. That way, you can ensure no impact from that rock will occur, ever.

I love this idea. It’s right out of Star Trek! And it has the added bonus of being actually doable.

Mind you, we could build such a mission right now, since much of the tech is off-the-shelf. But it’s never been tested together in this way, and I think this should be a priority. Hitting a rock only a few hundred meters across at multiple times the speed of a rifle bullet is something I think we should test before it counts for the final grade. A mission like that might cost a billion or so dollars, but it’s worth it.

Think of it this way: It costs a fraction of a percent of the total price of a car to add seat belts. The cost of a mission to prevent an asteroid impact would be far less than that percentage of our national annual budget; it would be less than one percent of just NASA’s budget over the 10 years it would take to build and test it. You can drive your whole life and never need a seat belt, but given enough time there will be an asteroid impact big enough to cause major damage.

That sounds like a good investment to me.

The science-fiction author Larry Niven once said that the difference between us and the dinosaurs is that we have a space program.

I would add that while we do have a space program, we still haven’t made the choice to use it this way. The cost to do so is small, but the price to not do so is unthinkable.

So, we can now answer our original question: When will the Earth suffer its next big impact?

If we make the right choice, then the answer to our question is: never.