NARRATOR: Our planet is under attack.

Our solar system is full of deadly missiles that could strike at any time.

It's like cosmic roulette.

You can't go on playing a game of chance and expect to keep winning.

NARRATOR: Killer asteroids.

Trillions of tons of rock, hurtling through space at tens of thousands of miles per hour.

Some are on a collision course with earth.

An asteroid has more damage potential than a nuclear bomb of the same energy.

NARRATOR: 65 million years ago, a huge asteroid wipes out much of life on Earth.

But strikes aren't all ancient history.

In 2013, a much smaller asteroid terrorizes this Siberian city.

For many, this really was a wake-up event.

NARRATOR: Now the race is on to understand the danger.

We know they're out there.

When and where will the next one strike?

ED LU: Will it hit on a city?

I don't know.

The point is we have to find that out.

We really need to upgrade the search capability.

NARRATOR: We stand at a crossroads.

Only now do we have the ability to identify the threat and even the technology to neutralize it.

PETER SCHULTZ: Boom!

That is just dangerous.

Kapow!

NARRATOR: But where some see danger, others sense an opportunity.

Asteroids contains precious metals and water.

Perhaps we can learn to mine them for their wealth.

CHRIS LEWICKI: There are millions and millions of these objects in the solar system, and we've only just begun to understand their resource potential.

NARRATOR: Asteroids could be the biggest threat humanity faces... or a great opportunity.

"Asteroids: Doomsday or Payday?"

Right now on NOVA.

NARRATOR: A rock tumbles through space: material left over from when the planets first formed.

It has orbited the sun for four-and-a-half billion years.

An asteroid.

It's 30 meters, or 90 feet, across, half the size of a small office building.

Mass: 100,000 tons.

It's on a collision course with Earth.

Ground based telescopes spot it just days before impact.

In its sights: a large city.

There's panic on the streets.

Why didn't we see it sooner?

Perhaps then we could have saved ourselves.

Now it's too late.

The asteroid strikes.

It hits with the energy of over 100 atom bombs.

Everything from downtown to the outer city limits and beyond is completely obliterated.

This nightmare scenario is just fiction.

We can only hope it never happens.

But asteroids have struck Earth before.

Wind back 65 million years.

A rock approximately ten kilometers wide smashes into planet Earth.

It helps wipe out the dinosaurs and around 70% of species on the planet.

Our planet also bears the scars from much smaller strikes.

This one: 50,000 years ago.

But the danger from asteroids still hangs over us today.

They can catch us completely by surprise.

Our challenge is to understand these potentially devastating objects.

So what exactly is an asteroid?

Asteroids are rocks left over from the birth of the solar system 4.6 billion years ago.

As Mars and Jupiter form, they leave a ring of debris between them: the asteroid belt.

Asteroids are rocks that should form another planet, but Jupiter's gravity prevents them from coalescing.

The asteroid belt ranges from roughly 150 to nearly 500 million kilometers away.

Out here, asteroids pose no threat to Earth.

CATHY PLESKO: There are millions of asteroids and comets in the solar system.

Fortunately for us, most of them are on very stable orbits.

They stay where they're supposed to be and they live their lives and evolve through their part of the solar system without ever coming close to the Earth.

NARRATOR: Today, we have the technology to visit these remote objects.

2011: NASA's Dawn probe approaches the Vesta, the second-largest asteroid in the solar system.

It returns these detailed images.

It's now en route to an even larger body: Ceres.

But not all asteroids stay in the asteroid belt.

A random collision, a planet's gravity, even heat from the sun can knock them into a different orbit closer to Earth.

Many of these near-Earth asteroids cross our path time and again.

If they enter our atmosphere, they become potentially deadly meteors.

Any fragments that reach the ground, we call meteorites.

With the experience of three missions and over 200 days in space, former Astronaut Ed Lu believes it's only a matter of time before one strikes us again.

ED LU: It's like cosmic roulette.

The entire city of Las Vegas was built upon the concept that the house always wins.

And in this game, we're not the house.

NARRATOR: Our asteroid knowledge and detection technologies have improved rapidly in recent decades.

Many now believe asteroids are a problem we can solve.

CATHY PLESKO: Asteroid impacts are actually the first natural disaster that humans have a hope of really preventing.

We can maybe try and model where earthquakes happen, we can try to predict how frequently tornadoes happen.

We can't stop them from happening.

Asteroid impacts, we might actually be able to do something about that in the very near future.

NARRATOR: So how can we overcome the asteroid threat?

Observations have revealed almost 1,000 asteroids larger than a kilometer in near-Earth orbits.

These are potentially large enough to cause a global catastrophe.

But we know where most of them are.

The chances of a strike are vanishingly small.

The immediate danger comes from smaller asteroids, less than one kilometer, or about 3,000 feet, across.

There are millions of these in orbits that pass near Earth.

DON YEOMANS: We've discovered roughly 10,000 near-Earth objects that can get fairly close to the Earth.

To put that in perspective, only about 20 years ago, we had less than 100.

NARRATOR: For every near-Earth asteroid we have detected, there are thousands that we haven't.

Ed Lu believes we need to locate and track them.

ED LU: 50 years ago, 100 years ago, if we were wiped out by an asteroid, that was just bad luck.

Today, if we are hit by a major asteroid, that is not bad luck anymore.

That is negligence.

NARRATOR: We have the technology to detect asteroids long before they hit us.

We've found many of the big ones, but smaller rocks could hit at any time.

The Minor Planet Center at the Smithsonian Astrophysical Observatory is at the center of the mission to track every asteroid in the solar system.

Gareth Williams is tasked with deciding which asteroids are a cause for concern.

GARETH WILLIAMS: The computers in this room are really the nerve center of the Minor Planet Center.

The 103 million observations and 600,000 orbits that are stored on the computers here are the primary source of information for the world on the motions of minor bodies in the solar system.

NARRATOR: Around 450 observer stations feed information to the Minor Planet Center.

But most near-Earth asteroid discoveries come mainly from professional optical telescopes.

GARETH WILLIAMS: To date, we have found most of the large main belt asteroids.

Where we are lacking in knowledge are the smaller near-Earth objects.

Anything about 50 meters diameter, we are very incomplete in our knowledge of those objects.

NARRATOR: Every single asteroid the Minor Planet Center knows about was discovered using the same basic principle.

Optical telescopes capture several images of the night sky minutes, hours or days apart.

Astronomers use computers to compare the images.

The stars stay still.

Asteroids move.

Once the computers identify a new near-Earth asteroid, further observations are requested.

The asteroid must be tracked to decide if it's a threat.

GARETH WILLIAMS: We can get an orbit from three nights of observation.

To get a good orbit, we generally require observations over an arc of a month.

The primary reason we track asteroids is to have some idea of if and when an object is going to approach the Earth very closely or hit the Earth.

NARRATOR: To complete his risk assessment, Gareth can request the help of a different kind of telescope to track the asteroid's orbit.

This is NASA's Goldstone Observatory in the Mojave Desert.

Standard optical telescopes use reflective mirrors to watch a patch of sky.

Goldstone is different.

It uses radar to zoom in on and track an asteroid.

LANCE BENNER: This is a DSS-14, a 70-meter radio telescope.

It has a high-powered radar transmitter on it that we use for tracking and imaging near-Earth asteroids when they're close to the Earth.

NARRATOR: The Goldstone telescope acts like a giant flashlight, but instead of light, it sends out a focused beam of electromagnetic radio waves toward the asteroid.

By timing how long it takes the radio waves to hit the asteroid and bounce back, astronomers can verify exactly where it is in space.

And once locked onto it, they can watch how it moves over time.

Radar is a very useful instrument in preventing newly discovered asteroids from being lost.

Usually, their orbits are not well determined, so if you get the radar measurements, you manage to extend how far in future you know where these objects are.

This automatically provides you with an early warning system.

You know if there is any chance of impact many decades, maybe even centuries in the future.

NARRATOR: The data Goldstone produces is so precise, it can predict an asteroid's path years in advance.

February 15, 2013: Asteroid DA14 comes within just 28,000 kilometers of Earth, inside the orbit of many satellites.

But NASA and other telescopes around the world have already tracked the asteroid for a year.

They know its path with great precision.

They're 100% certain that we're safe.

The asteroid passes at exactly the distance they predict.

LANCE BENNER: It was less than one-tenth of the distance between the Earth and the moon.

In advance of the fly-by, we expected that it would be roughly 50 meters in diameter, and this was the closest approach by something that size that we've ever known about in advance.

NARRATOR: But our detection systems don't always work this well.

That becomes painfully clear on the exact same day that the world's telescopes track DA14 passing by.

Another asteroid approaches: one our telescopes fail to detect.

It hits us.

9:20 a.m., Siberia, Russia.

A fireball rips through the sky over the city of Chelyabinsk.

Smartphones and car dash cameras provide an unprecedented view.

A shockwave crashes down on the city.

Windows blow in.

Doors are flattened.

More than 1,000 are injured.

The Chelyabinsk asteroid impact was a game-changer in the sense that it makes it real for people.

And that's important because human psychology is such that if something hasn't happened in my lifetime, I tend to discount it.

NARRATOR: NASA asks astrophysicist Peter Brown for an early estimate of the size of the blast.

He uses a highly sensitive global system of above-ground microphones designed to detect illegal nuclear bomb blasts.

The system is part of the nuclear weapons test ban treaty.

PETER BROWN: We're listening to the very lowest, very deep bass tones reflecting the huge amount of energy in this explosion.

As the explosions get bigger, the tone of the shockwave or the tone of the sound gets lower and lower and lower until it's below the level that human hearing is able to detect.

NARRATOR: Peter is shocked by what he discovers.

PETER BROWN: In this particular case, the signal was very obvious.

It was huge, and the most startling characteristic was the fact that it was a very, very low tonal frequency, much lower than anything I'd ever seen before.

NARRATOR: His official estimate of the explosion's power is nearly 500 kilotons-- more powerful than the blast from a large nuclear bomb from the American arsenal.

The atomic bomb dropped on Hiroshima was about 15 to 20 kilotons, so 15,000 to 20,000 tons of TNT.

So this event, Chelyabinsk, is about 20 to 30 times that event, which is what makes it so unusual but also so destructive.

NARRATOR: The event is huge.

(glass breaking) So why didn't we see it coming?

Physicist Mark Boslough is one of the first scientists to visit Chelyabinsk after the meteor strike.

He's convinced the answer is tied up with the asteroid's trajectory-- the direction it came from.

He believes he can calculate this with the help of amateur video footage.

Using several key images, he returns to the precise locations where they were shot.

He has to do this at night when the stars are out.

Well, I am doing a stellar calibration.

So we got... One of our videos was from a dash cam, from a car parked in this parking lot, and the fireball streaked across the sky here.

We're looking south, it went from left to right.

And what we really want to do is determine the exact angles to the fireball as seen from this location.

NARRATOR: Mark's goal is to plot the exact trajectory of the meteor from where it appears in the sky.

Standing where the eye-witness video was originally taken, Mark records its GPS coordinates.

He then takes a picture of the night sky with his own camera.

He'll use the stars in his photo to help him pinpoint the meteor's path in the video.

So if the stars show up on the digital camera, we can get those angles and then calibrate that image that was taken from the dash cam and know exactly the angles to the trajectory of the fireball.

NARRATOR: By lining up his star field photo against the video frame, Mark can get a fix on where the fireball appeared relative to the stars.

He repeats this process at several locations Finally, Mark uses simple geometry to locate the fireball in three-dimensional space, building a model that shows exactly the angle and direction of its approach.

Mark's calculations explain why our telescopes fail to see the Chelyabinsk meteor coming: it approaches from the direction of the sun.

It's masked by the sun's glare.

And the Chelyabinsk meteor is not unusual.

Any near-Earth asteroid is completely invisible at the point when its orbit passes between the Earth and the sun.

That's a big gap in our detection capability.

ED LU: And that's the issue right now.

Our current surveys, the current things that we're doing right now to find asteroids are totally inadequate.

We need to bump this up by about a factor of 100.

NARRATOR: So we have a problem.

Ground-based telescopes can only detect asteroids by looking away from the sun into the night sky.

But half the asteroids that hit us come from the other direction.

There could now be a solution: putting a telescope into space.

DON YEOMANS: Currently, all of the near-Earth object discoveries are made with ground-based optical telescopes.

If we could get above the earth's atmosphere, we could do a far more efficient survey.

NARRATOR: NASA has already demonstrated the capabilities of space telescopes to identify near-Earth objects.

In 2010, the WISE mission detects 135 near-Earth asteroids before the telescope is retired.

The problem is there may be millions more.

Many believe we now need a longer-term mission designed for asteroid detection.

But the funds haven't been allocated.

Though NASA is rebooting WISE as an asteroid-hunter, a tailor-made asteroid detection system from NASA looks a long way off.

So the private sector is stepping in.

B612, a non-profit foundation, is financing a new infrared space telescope based on existing designs like this one.

Its sole aim: finding more near-Earth asteroids.

Former astronauts Ed Lu and Rusty Schweikart are leading the $400 million fundraising drive.

RUSTY SCHWEIKART: This is a public service.

We're a non-profit corporation.

We're not doing this to make money.

We're doing this, you know, to help extend the domain of life on Earth here.

NARRATOR: Codenamed Sentinel, the telescope will orbit the sun just like Earth and the other planets But with a similar orbit to Venus, it can look out towards Earth's path with the sun behind it.

Looking away from the sun will enable it to detect asteroids we cannot see from Earth.

Sentinel is an infrared space telescope that the B612 Foundation is building, and we're going to launch it in 2018.

As it moves around the solar system, it will scan Earth's orbit, and we'll discover just in its first year 200,000 near-Earth objects.

Again, compare that to our current discovery rate of about a thousand.

NARRATOR: Sentinel will help detect asteroids between Earth and the sun.

It will make it easier to find that potentially dangerous group of near-Earth asteroids: not just those over 140 meters wide, but also millions of smaller objects.

Historically, these have been hard to detect.

Sentinel's technology could change that.

Sentinel will be built here by Ball Aerospace, who also helped design and produce the Spitzer and Kepler Space Telescopes.

Program Manager John Troeltzsch is planning to use their proven technology in the new telescope.

So Sentinel has come in at the right point in time.

We've developed a lot of technologies over the last 20 years in detectors, in telecommunications systems, in spacecraft, in solar power.

All these things are coming together today to help produce the Sentinel spacecraft.

NARRATOR: Sentinel's most important technology will be its infrared detector.

This will find the smaller asteroids that most worry scientists by searching for the heat they emit.

Asteroids are very hard to see against the dark background in deep space.

Here's a piece of a meteorite.

Same composition as an asteroid, really hard to see in visible light.

However, when I take my infrared camera, wow, it shows right up.

And I can see it clearly against the background.

This is the advantage that Sentinel has for infrared detection of asteroids.

NARRATOR: Sentinel's infrared capability is essential for detecting small asteroids.

Asteroids around a kilometer across are easier to spot.

Even though they're dark, their size means they reflect enough sunlight for us to see them.

Smaller dark asteroids reflect less light, but infrared detectors will pick up the heat they emit.

That gives Sentinel a huge advantage.

Our design for Sentinel is designed to find 90% of 140 meter or larger asteroids that might threaten the Earth, the things that can really destroy large areas of a continent, really do major damage.

NARRATOR: The goal is to launch Sentinel in 2018.

Meanwhile, NASA is helping fund a ground-based system to complement existing telescopes in Hawaii.

Codenamed ATLAS, it will provide a few weeks' warning, enough for at least some people to evacuate.

Even this won't find every asteroid that threatens us.

Earth is still flying blind through a solar system full of undetected asteroids.

The nightmare scenario is still entirely possible.

A small asteroid undetected by our current technologies could strike at any time.

And we now know that the threat is not just from rocks 140 meters across.

Even far smaller asteroid strikes can be devastating.

When scientists use data from the Chelyabinsk blast to calculate the asteroid's size, they discover it's tiny: 20 meters across, an asteroid so small, no one thought it could cause a problem.

In fact, rocks this small can do far more damage than we saw at Chelyabinsk.

It all depends on four factors: the asteroid's composition, its speed, angle of attack, and where it strikes.

In the immediate aftermath of the Chelyabinsk strike in Siberia, physicist Mark Boslough continues his research.

He hunts through the snow to find meteorites: fragments of the asteroid.

What's amazing to me, though, when you think about it, I mean, this is part of an asteroid that had been, you know, floating through space, orbiting the sun for billions of years.

And two weeks ago, it exploded in the atmosphere, dropped to the ground, and here I am, holding it in my hand.

That's amazing.

NARRATOR: Mark takes the fragment to a lab in Yekaterinburg and studies it under an electron microscope.

There's a crack here.

A crack?

Yes.

What's the scale on that?

About three microns.

NARRATOR: Mark's analysis reveals that the fragment is mostly rock, and it's shot through with cracks.

These are scars from previous collisions in space.

When it smashed into our atmosphere at 40,000 miles per hour, and reaching more than 3,600 degrees Fahrenheit, the cracks help break the meteor apart.

BOSLOUGH: The way we understand it is it hits the atmosphere going so fast that there's so much stress on it that it actually breaks the asteroid.

It exceeds the strength of the asteroid, and that can happen very, very fast.

NARRATOR: The Chelyabinsk meteor disintegrates 15 miles above the ground.

It releases a shockwave with the energy of almost 30 atom bombs It's this shockwave that does the damage.

The Chelyabinsk meteor explodes because it's made of fractured rock.

But not all asteroids are made this way.

Some are made of metal.

The Natural History Museum in London holds one of the world's largest collections of meteorites.

Caroline Smith studies them, and it's clear to her that not all meteorites are alike.

Meteorites come in three different flavors: stones, stony irons, or irons.

These two that I have here, these are called stony iron meteorites, and these are a mixture of rock and metal.

Because they've got so much metal in them, they are much heavier than a normal rock from Earth.

I mean, that's a good sort of 12, 15 kilograms.

And this is an iron nickel meteorite, and it's heavy.

This is about three times as heavy as the one that I've just shown you.

It does put into context the type of damage that you can get from something of that size.

NARRATOR: Metallic asteroids are denser and tougher than rocky ones, so a metallic asteroid the size of the Chelyabinsk rock could punch through the atmosphere instead of breaking up, hitting Earth's surface more or less intact.

And here's the proof: Meteor Crater, Arizona.

Over one kilometer across and more than 150 meters deep, yet the meteor that creates it 50,000 years ago is only one-and-a-half times the size of the Chelyabinsk meteor.

The difference is that it's metallic.

It smashes into the ground with the power of 500 atom bombs.

An iron meteorite slammed into the rock here with most of its cosmic speed still intact at about 40,000 miles per hour.

The energy released in that impact was something on the order of ten megatons of TNT equivalent, and it's that energy that was responsible for excavating this enormous hole in the ground and spewing that rock into the desert around us.

NARRATOR: Composition makes a big difference to the destructive power of an asteroid.

Metallic asteroids stand the best chance of surviving the brutal atmosphere.

But even stony asteroids can still be devastating.

Russia, 1908: a meteor explodes in the sky above Tunguska, Siberia.

Calculations show that the asteroid is just twice the width of the Chelyabinsk rock.

But where the Chelyabinsk blast knocks out windows, the Tunguska strike flattens hundreds of square miles of forest.

Its effects are similar to this nuclear test blast captured on film.

Even 20 years later, when these images were taken, the effects are still evident.

So why the difference between the two meteors?

Video images show the Chelyabinsk asteroid approaches at a particularly shallow angle.

Mark Boslough feeds that information into a computer used to model the behavior of nuclear weapons.

This simulation shows the effects of the meteor's trajectory on its destructive impact.

And you get this enormous fireball, and that fireball continues to move downward and it pushes a shockwave ahead of it.

So these are like mushroom clouds with two big, giant nuclear explosions at the bottom.

The shock from the explosion continues to push forward and it starts to move downwards.

You can see that it's descending.

It's down to about ten kilometers above the surface here, at ground zero.

NARRATOR: The simulation shows that because the meteor enters at a shallow angle, much of the energy is lost horizontally.

The shockwave is destructive, but it would have been far worse if had been focused more downward and the explosion had been closer to the ground.

This is what Mark believes happens in Tunguska in 1908.

The Tunguska explosion was much closer to the ground and it was much more intense in terms of energy release, and therefore the blast wave was much stronger, and that's why it blew down trees over this wide area almost a thousand square miles.

NARRATOR: No one dies in the Tunguska blast, but only because no one lives here.

Someday, a similar strike could happen over a city.

It could kill millions.

I've seen maps of the Tunguska blast area overlaid with a map of Washington, D.C., and it extends beyond the Beltway in every direction.

An asteroid has more damage potential on the ground than a nuclear bomb of the same energy.

NARRATOR: Fortunately, most asteroids are relatively small and strikes are rare.

Better detection will help, but detection alone is not enough.

If we find an asteroid heading toward Earth, what can we do about it?

Sci-fi movies always have the answers.

The hero flies up and blows the asteroid apart.

So this is the real Hollywood way of looking at an asteroid's impact hazard mitigation.

So they detect, with just enough time, that there's this giant asteroid or comet or something coming in towards the Earth, and everyone decides, "A-ha, well, we're gonna blow it to kingdom come.

We'll just nuke it."

NARRATOR: Blowing an asteroid apart sounds extreme, but right now, scientists are exploring ways to harness technology to save us from the next big asteroid strike.

We need to think about, "What is the response of the object?"

"Are we going to fragment it?

"Is that okay?

"Are the fragments going to recombine "or fly off out of the way, "or are fragments going to hit the Earth that we need to worry about?"

NARRATOR: To learn how these fragments might behave, we can simulate a similar impact here on Earth.

And Peter Schultz is the man who pulls the trigger.

He operates one of the few guns in the world fast enough to model an asteroid strike.

This hydrogen-powered gun is targeted into a reinforced metal safety chamber.

Inside this chamber is a small resin replica of an asteroid.

It's there to illustrate two different approaches to keeping Earth safe: destruction and deflection.

So today, we're going to do two experiments.

The first one will actually destroy the asteroid by using this projectile, about a quarter inch across.

And the second one, we'll use this projectile: it's about an eighth inch across.

This will not destroy the asteroid but instead will form a crater, and that's really intended just to deflect it without destroying it.

NARRATOR: The aluminum pellets will hit the asteroid replica at almost 12,000 miles per hour.

That's over three miles per second.

High-speed cameras are needed to record every millisecond of the impact.

Even this small simulation will be violent.

The team keeps a safe distance in the control room.

MAN: High voltage is good.

We have ready lights.

(alarm blaring) Oh, jeez!

We really busted the devil out of this.

NARRATOR: The high-speed video allows Peter to analyze the impact of the larger projectile in minute detail.

Here's the projectile coming in, you can actually see it coming in.

It's going to hit right there.

Boom!

That is just dangerous.

Kapow!

(laughing) There's nothing left except for something right in the center.

The last piece, that core still could be a threat, but it's a lot smaller.

We're way down in terms of a threat.

So this is one option we got: we blow the whole thing to smithereens.

But we always have to wonder, "Is there something left over?"

NARRATOR: Scale this up to blowing apart a real asteroid and the problem is clear: deadly fragments of rock could still rain down on Earth.

So the possible problems with destroying an object, or disrupting it, we say, is that you need to have enough time for the fragments that are then flying off in all kinds of directions to get out of the way before the Earth passes through there.

NARRATOR: Blowing an asteroid apart could make the problem worse instead of solving it.

Luckily, there is another option.

Rather than blast the asteroid apart, we could use an impact to change its course.

PLESKO: For a lot of objects, you don't actually need to blow them up to prevent them from hitting Earth and making a crater.

For me, the most realistic are what we call kinetic impactors for small asteroids, and that's basically a cannonball shot from a spacecraft to knock it off course.

NARRATOR: That might reduce the fragment problem, but it still needs to be tested.

Peter sets up the gun with a smaller projectile.

This shot should create a crater, but not an explosion.

MAN: Here we go.

Okay, so let's see what we have.

(loud bang) Whoa!

Oh, look at that!

NARRATOR: This time, the asteroid stays mostly intact.

PETER SCHULTZ: And this is the crater forming.

So what's going to be interesting is to see, "Does it make it move?"

Boom!

Then we see that jet come out-- that's really hot gas, it's a plasma-- and we can see the ejecta forming, and now it begins to move.

NARRATOR: The collision creates a blast of super-heated impact material spraying out from the simulated asteroid.

It acts like a rocket firing out into space.

The asteroid is nudged off course.

So the impact hit it, its momentum is now shoving this, and with enough time, maybe this would get it out of our way.

So I think we're illustrating that we don't have to destroy the whole thing to make it move; we just have to have enough energy and momentum to give it a nudge.

NARRATOR: Could our planet and our lives be saved by a mere nudge?

The reason it's so easy is because the Earth's a moving target.

The Earth's about 8,000 miles across, and it's moving 65,000 miles an hour.

So every eight minutes, the Earth moves a distance equal to its size.

So all I'm really going to do is upset the timing a little bit.

If I make the asteroid show up at the collision point three minutes early or three minutes late, the Earth's gone, and that's all it takes, that's why it's so easy at large distances.

NARRATOR: But this would be a gradual process.

The projectile will have to be launched many years before the asteroid comes close to Earth.

This is an urgent problem because if there is an asteroid out there that is already on a trajectory to hit the Earth and we're not finding out about it today, then we're losing valuable time it takes to deflect that asteroid, because what you really need is time.

You need a decade of warning, and then it's actually easy to do.

But you need time.

NARRATOR: So there are two clear options to prevent asteroid strikes: Blast the asteroid apart or deflect it.

And there is another way.

You could do something called a gravity tractor, which is something invented by myself and an astronaut named Stan Love.

And you just hover a small spacecraft near it and you tow it.

NARRATOR: Even an object as small as a space probe has its own gravity.

It's incredibly weak, but over time, it acts like a tow rope, altering the asteroid's course.

ED LU: Now, it's not as effective as a kinetic impactor, but it's controllable.

The purpose of the gravity tractor is really to fine tune the deflection.

NARRATOR: So it appears that we have the technical capability to deflect an asteroid.

But it will take a major investment to prevent the nightmare scenario: a small asteroid we cannot detect obliterating a major city.

And now there's a new question: could that same technology be applied to turn asteroids from threat to opportunity?

To answer that question, consider the birth of our planet.

I think it's a fairly widely-held consensus view that asteroids' initial impacts with the Earth four-and-a-half billion years ago seeded the Earth with a veneer of water and carbon-based materials that allowed life to form.

These are the building blocks of life.

NARRATOR: Asteroids probably dumped organic compounds onto the early Earth, and alongside those may have come other beneficial materials: platinum, gold and water, all valuable resources on Earth.

Could asteroids be a new source of these materials in space?

Chris Lewicki is president and chief engineer of Planetary Resources, a company that hopes to mine asteroids for profit.

We are mining asteroids today.

Some of the most productive metal mines on Earth are the site of asteroid impacts hundreds of millions of years ago.

So to go to space, in some cases, to the source, we can get these materials in much more abundance than they exist naturally in the Earth's crust.

NARRATOR: A single asteroid of between 200 and 500 meters could contain as much platinum as we have mined in the whole of human history.

But Planetary Resources and other companies are most interested in another resource: water locked up in the asteroid's rock.

I think water is the most important resource in developing space because it enables everything that follows, and by taking the water and breaking it into hydrogen and oxygen, we can create rocket fuel, the same rocket fuel that we use to launch space shuttles and to explore space.

NARRATOR: According to Dale Boucher, CEO of another space mining company, producing rocket fuel from water is simple.

So now we're making rocket fuel.

This is just water in here, and we've got a very simple process run by a couple of little batteries passing electric current through here.

We're going to separate this water into hydrogen, hydrogen bubbling off this side.

And on this side is oxygen.

As you can see, there's twice as much hydrogen as oxygen because the formula is H20.

NARRATOR: Many rockets use liquid hydrogen and oxygen as fuel.

Mix them together and add a spark, and you get explosive thrust.

BOUCHER: Okay, so we're going to capture the hydrogen in this tube, we're going to bleed this valve off.

All of the hydrogen that was bubbled off is coming up in here.

Because it's lighter than air, it will float up.

And what we're gonna do is tilt it like this and strike a match.

(loud popping noise) NARRATOR: We have lift off!

The weight of a rocket at lift-off is almost all fuel.

It burns almost all that fuel just getting into orbit.

If fuel could be produced from asteroid water, rockets could maneuver more easily and even travel faster.

LEWICKI: This is something that is abundant in space.

On a single 75-meter asteroid, we have enough water to have fueled the entire US space shuttle program.

NARRATOR: So how would the ability to refuel in space affect the future exploration of our solar system?

BOUCHER: In my mind, it makes sense to go to an asteroid, mine water, produce rocket fuel and hold it there in an orbital station, let's say, so that ships or equipment or robotic missions can stop in, have a sandwich, have a coffee, refuel the rocket ship and go on to Mars.

Imagine driving your car somewhere without the opportunity to stop by and gas it up.

This is what we've been doing in exploring space to date.

We now have the potential of being able to refuel a spaceship on its way not only from Earth into outer space but to refuel for farther destinations in the solar system.

NARRATOR: The rocket fuel is out there.

The challenge is to find the right asteroids, get to them, and then extract the water.

All of this demands new technology, and several companies are working on that right now.

Planetary Resources plan to use a newly designed telescope called Arkyd to find suitable asteroids.

LEWICKI: Mining in space starts much the same as mining does here on Earth.

We start with prospecting and identifying the resource.

And we will survey maybe dozens of asteroids until we find the one that has the most interesting and most economically viable combination of features.

NARRATOR: Assuming they find an asteroid to mine, the next challenge is to reach it.

Here, the technology for this is still on the drawing board.

NASA is studying ways to grab an asteroid and tow it nearer to Earth.

This is a very ambitious engineering project.

So the idea is to send a spacecraft out to a near-Earth asteroid-- not to the asteroid belt outside of Mars, but to one that actually on its own comes near Earth and is small enough-- and then take the asteroid with a Kevlar bag.

And the asteroid, of course, is spinning relative to the spacecraft, and so it would tangle itself up in the Kevlar, which would slow it down and give the spacecraft something to hold on to.

They would then haul it back and put it into orbit around the moon and have someplace for scientists to go to look at this large piece of space rock still in space.

NARRATOR: For now, it's unclear whether this program will go forward.

But even if the right asteroid can be found and captured, the next challenge will be extracting the resources.

Mining expert Dale Boucher believes there are lessons from mining on Earth that will help with the much tougher task of mining in space.

BOUCHER: Terrestrial mining is a brute force type of activity.

You know, when we're drilling, we've got huge 100-pound drills driving into the rock with massive air pressure behind it, trying to excavate.

The excavator itself is massive, weighs many tons.

It's using that dead weight to its advantage.

That's what it needs to get the traction to push its bucket into that rock and pull that rock out of there.

NARRATOR: None of this will work in space.

There's very little gravity, and the rocks can be cold: hundreds of degrees below zero.

Mining in space demands whole new technologies.

The big difference is gravity, which means that we have more problems with trying to push enough force on a drill bit, let's say, or on an excavator bucket to try and make that do our work for us.

So we have to start thinking about how we anchor ourselves to these very virtually weightless bodies.

NARRATOR: Dale has been involved in developing specialized mining and drilling technology to meet these extraterrestrial challenges.

This drill bit has been designed for use in the lower temperatures of the moon.

The same technology might possibly be adapted for mining asteroids.

BOUCHER: So this drill is trying to drill through this simulated lunar material that's got water in it, and we've cooled it down to liquid nitrogen temperatures to simulate the temperature on the surface of the moon, near the south pole, where this drill is supposed to end up.

NARRATOR: This is different from the mining drills used here on Earth.

To save energy and weight, it uses less power and is much slower.

BOUCHER: So the drill runs on about less than 100 watts of power.

It takes a very long time to drill: it's like watching paint dry.

To do a full meter sample would take about an hour to an hour-and-a-half, depending on how hard the material actually is.

NARRATOR: Even if the technology is within reach, it would be extremely costly to find suitable asteroids, relocate them and extract their resources.

Is there a big enough payoff?

If we're going to exploit asteroids, I think we need to really do a cost benefit.

It's expensive to get up there.

It's plausible.

We can do this-- after all, we've been to the moon-- but I think this requires some very serious study.

NARRATOR: We know it's possible to reach an asteroid and bring samples to Earth because the Japanese Hayabusa probe has already done it.

But the samples it brought back in 2010 were microscopic.

To mine significant quantities is an entirely different challenge.

So is the idea of mining asteroids premature?

Some believe there's no point in trying to exploit asteroids until we are safe from them.

Well, I can tell you that you can't mine an asteroid if you don't know where it is in the same way you can't deflect an asteroid or protect the Earth from an asteroid if you don't know where it is.

So the first step to all of this is building Sentinel, because Sentinel will find the asteroids in our solar system.

We can deal with the threat of asteroids and the opportunity that they present simultaneously.

As we develop the technology to detect, characterize and ultimately mine asteroids, it's that very same technology that will allow us to identify potentially hazardous asteroids and take action.

This will drive expansion and exploration of the solar system, enabling humanity to eventually live and work and play in space permanently.

NARRATOR: A paradox.

Are asteroids the biggest threat humanity faces or a new opportunity?

As our knowledge and technology improve, could it be that they are both?

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