It's a scientific fact that the planet Earth will be hit by cataclysmic asteroids in the future.

Assuming we can see them coming, what can we do to stop them?

[MUSIC PLAYING] Space is swarming with lots of really fast moving rocks, like the planet Earth, but also like that 19-meter-wide wide chunk, that blazed across the sky above the Russian town Chelyabinsk, in 2013, exploding in the air and across Russian dash cams, with the power of up to half a megaton of TNT.

(RUSSIAN ACCENT) In post-Soviet Russia, space explore you.

Now, Russia does seem to get hit a lot.

The Tunguska event of 1908 was even bigger, when a 100-meter space rock exploded to flatten 2,000 square kilometers of forest in Siberia.

Now, there were no recorded fatalities in either case.

Tunguskia was very isolated.

And the Chelyabinsk asteroid came in at a shallow angle, so it had lots of atmosphere to burn up in.

Had it been a direct hit, then a city of over one million people would have been devastated.

Now that's pretty lucky.

But so far, we've all been lucky.

Impacts have tended to happen in remote regions.

But now that more and more of the world is covered in urban areas, the chance of a catastrophic impact is increasing rapidly.

It's really not a matter of if, just of when.

So when can we expect the next killer asteroid?

And can we stop it?

That depends on how bad the situation is.

Let's run through the levels of catastrophe.

OK. A rock less than 10 meters in diameter vaporizes harmlessly in the atmosphere.

This stuff isn't a problem.

At least 40,000 tons of it rains down on the Earth every year, mostly in the form of pebbles and smaller.

These are the shooting stars that you see on a dark night.

Meteors 20-100 meters in size have the kinetic energy equivalent of a thermonuclear explosion.

And this is a city killer on a direct hit.

We expect something like this once every century or so.

Now, we just had one.

But that does not mean we won't get one tomorrow.

For a rock around 500 meters in diameter, the strike will have the energy equivalent of all currently operational nuclear weapons in the world.

This devastates a country or makes a tsunami.

Expect one of these at least every 50,000 years or so.

Of course, it gets worse as you get bigger.

At two to three kilometers, the sky goes dark from the ejector, and nuclear winter effects global climates.

These happen every couple of million years.

Much larger than this and the planet is showered in molten rock.

Nuclear winter lasts decades.

Oceans are acidified.

And most life on Earth dies.

An asteroid of this magnitude probably killed the dinosaurs 65 million years ago, hitting the Yucatan Peninsula with the power of a billion megaton hydrogen bombs.

Earth gets smacked this hard every 100 million years or so.

Now the chance of any one of these killers on a given day is tiny.

But small probabilities have a way of adding up to a certainty, over time.

That means a big one will hit again.

And we can only stop it if we know it's coming.

There's been some good work.

Astronomers around the world have worked together as part of the Spaceguard Program to find and track most of the biggest, one kilometer plus, comets or asteroids that cross Earth's orbit.

Those are called Near Earth Objects, or NEOs.

Good news-- none are coming close for centuries.

There are a couple of 500 meter bad boys that have a very small chance of hitting in the late 2100s, and the 300 meter Apophis, who will buzz us inside the orbit of the geosynchronous satellites in 2029 and 2036, but will almost certainly not hit.

So we've got some time.

But what about the NEOs the size of the Chelyabinsk or Tunguskia rocks, Potential city killers.

We have no idea where most of these are.

These things are very faint specks in the deep dark of space.

And current monitoring programs can only find a fraction.

This is probably something we want to deal with.

There are efforts.

The nonprofit B612 organization is fund raising to launch the Sentinel infrared telescope, that will orbit the sun and look outwards, tracking hundreds of thousands of NEOs.

They need $450 million.

But that's less than the box office take of the movie Armageddon.

So you know, we either pay to save the world, or pay to watch Bruce Willis pretend to.

And speaking of Armageddon, what do we do if we spot an incoming city killer, or for that matter, a planet killer that we somehow missed?

Is there even anything we can do?

Absolutely.

But the approach is going to depend on how fast we can act.

Caught early, we have options.

See, the Earth is a very small target on the scale of the solar system.

And Earth itself is moving pretty fast, traveling its own diameter once every seven minutes.

Let's say we catch this killer rock at 10 years out.

We need to slow it down or speed it up so that it is behind or in front of the Earth.

That means making it arrive just seven minutes later or seven minutes earlier.

So divide seven minutes by 10 years.

We need to change its speed by roughly one part in a million, and we save the world, and get to have a parade.

We can also nudge it by a similar amount to the side.

The further out we catch it, the less we need to nudge it.

There are a few different ways to do this.

Number one-- brute force.

Just slam a heavy spaceship into the NEO to knock it off course.

This is kinetic impact.

A one ton impact gives the necessary one part to a million momentum change needed to divert city killers.

Now unfortunately, this doesn't work for planet killers, which are at least a million times more massive.

Number two, nuke it, but miss.

Yup, explode a nuke right next to the rock and vaporize part of the surface.

The pressure of the nuke, and of the ejected asteroid bits, will push the NEO off target.

A regular nuke would be fine for smaller asteroids.

But for a planet killer, you're going to need a few 50 megaton Tsar bombs.

Thanks again, Cold War.

Number three, graffiti.

Seriously, arm a spacecraft with a cosmic-sized can of spray paint, and color one side of the asteroid white.

The sun's light will push harder on the more reflective side, slowly pushing it just far enough off course to miss us.

A similar proposal fixes a giant light sale to the side of the rock.

Now for this option you're going to have to catch it early.

Number four, the weirdest deflection option-- a gravitational tractor.

Fly a 20 ton spacecraft alongside the object for 10 years, and their mutual gravity will pull the impactor off course.

Options are great.

In fact, I don't want to attempt more than one, because once the first one fails, it may be too late to start again.

The real trick is getting politicians to agree to something that involves scientific uncertainty and long term thinking.

Hm, maybe we should do a Kickstarter to save the world.

OK, let's address the worst case scenario.

What if we catch this thing at the last minute, or fail to act for other reasons?

The asteroid or comet is heading straight for us.

And we have a month or two, at most.

At this point, we can't deflect a large impactor.

We have to blow it up.

Now people often say that you shouldn't explode NEOs.

You'll just get lots of hard to track fragments.

But at this point, tracking isn't even that helpful.

And lots of small bits are much better than one big bit.

My favorite idea so far is the Hypervelocity Asteroid Intercept Vehicle.

The idea is that you launch a thermonuclear device at the rock, but you lead with a very high speed kinetic impactor, that creates a nice, deep crater for the nuke to explode in.

This uses the nuke's energy much more efficiently, sending shockwaves through the asteroid, that will break it into much smaller pieces.

That is, if you believe the supercomputer simulations of HAIV team, a one megaton H-bomb should break up a 500 meter asteroid nicely, although it depends on the structure of the rock.

A loose, gravitationally-bound rubble pile is obliterated, while a single rock will be fragmented.

For a giant asteroid, you're going to need a gigaton blast.

And such devices don't exist.

Should we build one just for this?

I don't know.

But without question, a better option would be to build the infrastructure needed to detect and deflect all dangerous objects much, much earlier.

To me, this is amazing.

We are a species capable of defending our world from a planet-killing meteor strike.

Or we could be, soon.

Science has given us such incredible power-- among other things, to serve as custodians of the planet Earth.

But what does it take?

Just a little support from enough of us.