Astronomer: What's the REAL chance an asteroid will wipe out life on Earth?

Contributed by
Jun 25, 2015

Phil Plait is an astronomer and major sci-fi geek. He writes the Bad Astronomy Blog for Discover Magazine and is also the host of the Discovery Channel's science show "Phil Plait's Bad Universe." You can follow him on Twitter at @BadAstronomer.

It begins when an amateur astronomer spots something in the sky that shouldn't be there. A hurried email, a chat over the 'net, and then confirmation from professional observatories—but a discovery kept secret from the public.

In a few weeks' time, an asteroid hundreds of miles across will hit the Earth, wiping out all life. NASA is notified, and they scramble into action. A team of astronauts hurriedly trains, launches in the Space Shuttle, and lands on the rock hours later. They plant a nuclear bomb, and just barely escape as the explosion vaporizes the killer asteroid with the Earth hanging hugely in the background.

You've seen this movie, right? I have, in various incarnations, approximately one bazillion times. Armageddon, Asteroid, Doomsday Rock, Meteor (actually, two impact movies were made with that title), Deep Impact ... the details change—maybe it was a comet, or maybe it's a fighter jet equipped with a laser to blow up the rock—but in the end, all these movies have one thing in common: they totally screw up how this would really work. When it comes to preventing an asteroid impacting the Earth and wiping out all we know and love, Hollywood can't seem to get it right. From the discovery to the detonation to the denouement, nothing of reality survives intact.

Let's tackle each issue, point by point, and see where silver screen storytelling steps off the narrow path of science.

It begins when an amateur astronomer spots something in the sky that shouldn't be there.

This used to be true: there are thousands of amateur astronomers in the U.S. alone, and they constantly observe the heavens. But only a few actually look for asteroids, and it's an inefficient process, scanning back and forth across the sky. About 20 years ago the robot revolution hit astronomy; automated hardware and software optimized to look for moving objects. The vast majority of comets and asteroids are actually "discovered" by machines now.

I probably shouldn't mention that one of the professional robotic networks is called SkyNet. Seriously.

A hurried email, a chat over the 'net, and then confirmation from professional observatories—a discovery kept secret from the public.

The first part of this one is close to the truth. Once a potentially threatening rock is discovered, the physical parameters (coordinates, orbit shape and so on) are sent to central clearinghouse of this info: the Minor Planet Center in Cambridge, Mass. The data are checked and then disseminated to observatories all over the world. If it's a particularly interesting object—and by "interesting," I mean there's some chance of "Aiiie we're all gonna die!!!"—then other astronomers will scramble to observe it and nail down the orbit, confirming or repudiating any potential impact.

A lot of conspiracy theorists think astronomers would keep something like this a secret (I get this asked of me all the time). The thing is, astronomers are a chatty, verbose lot (perhaps you've noticed). If one discovered an asteroid that really could hit us, the problem wouldn't be getting an astronomer to talk, the problem would be shutting them up.

In a few weeks' time, an asteroid hundreds of miles across will hit the Earth, wiping out all life.

This part always makes me laugh. Objects hundreds of miles across are bright. In Armageddon, the asteroid was "bigger than Texas", which is 900 miles across. That's roughly the size of Ceres, the biggest asteroid in the solar system, and that was bright enough to be discovered in 1801!

Not only that, but space is kinda big. That's why we named it that. So it takes a long time to move from one part of the solar system to another. At regular orbital speeds, an asteroid coming in from the main belt between Mars and Jupiter would take years to get here. Decades. And that's if it's on a direct course for our blue marble; more likely we'd find it would pass several times in its orbit before it would hit us. For example, we know of no objects more than a few miles across that can hit us in the next century. Don't call Bruce Willis just yet.

Oh, another thing: when something that big and moving that fast hits the Earth, the resulting explosion is huge. It only takes an object the size of a house to hit with the energy of a megaton-sized warhead. An object a mile across would be billions of megatons, thousands of times the entire world's nuclear arsenal. In at least one movie (yes, Armageddon again), they mention the explosion will be thousands of times that of a nuclear weapon. So weirdly, in a movie as completely ridiculously over the top as that one, the one thing they actually underplay is the actual danger from the asteroid.

Brilliant.

NASA is notified, and they scramble into action. A team of astronauts hurriedly trains, launches in the Space Shuttle, and lands on the rock hours later.

Well, let's just say NASA isn't exactly nimble. A mission like this would actually take years to plan and execute ... which, in real life, they'd have.

And again, orbital mechanics will not be denied. A trip to an asteroid would take months. Especially one headed right at us; a rocket would have to head toward it, swing around and match velocities. That would take a long time, and a lot more fuel than the Shuttle could carry, unless it happened to be towing an oil tanker full of liquid hydrogen and oxygen. That turns out to have engineering difficulties.

They plant a nuclear bomb, and just barely escape as the explosion vaporizes the killer asteroid with the Earth hanging hugely in the background.

This is where the movies really part from reality. First of all, anything hundreds of miles across—or even just a few—won't get shattered to smithereens by a nuke. You might carve a chunk out of it, but it would be like tossing a firecracker at a boulder. In Armageddon, given the size of the asteroid involved, they'd need a nuke that could detonate with the same energy output as the sun.

I'm rather glad we don't have a weapon like that.

Worse, they always seem to shatter the incoming killer rock just as it's about to hit the Earth, but that won't help. It might even hurt. The explosive energy of an impact depends on the object's mass and velocity. Blowing it up doesn't change its speed, and you haven't changed the mass, either: you've just spread it out a bit. If all the material still hits, it just spreads the joy—I mean destruction, terror and death—around a little. Deep Impact, a movie that got a lot of its science correct, blew it (haha! get it?) on this one.

However, there is a way a nuke might help. Instead of blowing it up on or in the rock, you detonate it near the surface. The explosion can heat up and vaporize a layer of material, which would expand violently. That acts like a rocket, pushing on the asteroid, changing its path. If the Earth is already looming in the background it's too late for this, but if you have a few years' warning, then a nuke or two might actually push the asteroid—or, more accurately, force it to push itself—into a safe trajectory.

Even better, one group of engineers and scientists (calling themselves the B612 Foundation, after the asteroid home of the Little Prince) has proposed what's called a gravity tractor: a probe with a mass of a ton or two that can hover over the surface of the killer asteroid. The gravity of the probe, feeble as it might be, can be enough to pull the asteroid into different path. Again, given time, this can turn a potentially extinction-level impactor into just another rock that safely passes us in the sky.

I'll note I've glossed over a lot of details here, but I hope you get the picture. At the moment, no asteroid is known to have an Earth-impacting trajectory, but we're looking. It would take something about the size of a football field to do us any real damage (and even then it would be local, not global), and those are hard to see. But our technology is getting better, and the gravity tug idea—while still theoretical—is our best hope for preventing us from joining dinosaurs in some museum in the distant future when marmosets evolve intelligence.

And as long as we're making movies like Armageddon, I can't necessarily say that's entirely a bad thing.

Phil Plait has an asteroid named after him—1654347 Philplait—which is in no danger of ever hitting the Earth, thus foiling his evil plans for world domination.