"What time of the day was it?"

"It was 12:45."

"12:45."

"And this comes through the roof and hits you."

She was napping,

when the rock crashed through her ceiling

and bounced off the radio into her stomach.

Because it was small, her injuries were minor.

"The first person ever to be hit by a meteorite.

Imagine that."

But much bigger objects have collided with Earth.

65 million years ago,

a rock ten kilometers wide

slammed into the Gulf of Mexico.

An event that likely caused dinosaurs

to go extinct.

Elsewhere, mammoth meteorites have been discovered

everywhere from The US

to Russia,

and blasted craters in North America,

Australia,

and Africa.

Falling objects from the sky

have always fascinated humans,

but they have also made us fearful,

making us wonder whether someday

a giant asteroid

could come for us.

"Run!"

In movies like Deep Impact and Armageddon

those fears are played out.

[clock ticking]

"One minute!"

And heroes save the day.

"We all gotta die right?

I'm the guy who gets to do it saving the world."

"YEAHHHH!"

You might think this only happens in Hollywood,

but that’s not exactly true.

There is a small chance, a

of a large asteroid hitting Earth.

So scientists are getting ready,

just in case.

"Asteroids" are hunks of rock

that weren't big enough to become planets

when the solar system formed.

Smaller asteroids are called "meteoroids"

and when they fall through Earth’s atmosphere

they become "meteors".

If they make it to the surface,

they are called "meteorites".

Between 1988 and 2017,

NASA counted over 700 fireballs

created by objects entering our atmosphere.

In order to detect asteroids,

NASA takes multiple pictures of the night sky

and uses computers to scan for moving objects.

As Earth orbits,

scientists make several observations

from different locations

to detect how close the asteroids are to Earth.

Here, the nearest objects are labelled in green.

At least 16,000 of them

have been classified as “near-Earth”,

meaning they orbit within roughly a third

of the distance from the Sun.

And based on the object’s speed and brightness,

scientists can map a trajectory

to predict whether it will collide with Earth.

The impact energy of a 10-meter object,

just a little smaller than a school bus,

would be 100 kilotons.

But the larger the object,

the nastier the impact.

A meteorite just a little larger

than The Great Pyramid at Giza,

at about 150 meters,

would generate 288 megatons of energy.

For reference,

the number of megatons in the payload of the

Castle Bravo hydrogen bomb

was only 15.

But asteroids don’t need to hit Earth to cause damage.

In 2013, a 17-meter-wide meteor

exploded over Chelyabinsk, Russia

before it ever reached the ground

and the resulting shockwave

released about 440 kilotons of energy

that damaged structures and injured over 1,500 people.

Following that event in Russia,

US politicians called on NASA

to ask about the threat of future collisions.

"I said, 'If we saw one coming toward Omaha,

what could we they about it?'

[Scientists] said they could use a laser."

"First of all, it would not be practical

to have a laser powerful enough to split it in half."

Their questions sounded like

plot lines for a new blockbuster,

but their ideas weren’t actually that far off.

"How far inland could a

reasonably sized asteroid

make water come in?"

Scientists have analyzed

how asteroid impacts would kill people

and getting hit by the "ejecta",

meaning space rock and other debris it kicks up,

is one of the least likely ways to die.

The most lethal cause is

violent wind generated by the impact blast,

followed by scorching heat,

and massive tsunamis.

"The odds of a near-Earth object strike

causing massive casualties

and destruction of infrastructure

are very small,

but the potential consequences are so large

that it makes sense to take the risk seriously."

"All agents are go."

In Armageddon, annihilation is avoided

by nuking the killer asteroid.

But in real life, our ability to prevent impact

depends on how big it is and when we detect it.

A 140-meter-wide space rock

is large enough to destroy a city

and if it were on a path to collide with Earth

in less than a year,

the only thing to do would be

evacuate the impact zone.

But if we are lucky enough

to spot an asteroid seven to ten years out,

NASA would have enough time

to try deflecting the object,

which they could do a number of ways.

First, NASA could launch a spacecraft to

act as a battering ram

and if the asteroid is far enough out,

it would only need to be pushed a few centimeters

off course to avoid hitting Earth.

Right now, NASA is in the early stages of "DART":

The Double Asteroid Redirection Test,

which will try out this approach

by trying it on a non-threatening asteroid,

called “Didymoon”,

which will pass near Earth in 2022.

Another method would be using something

called a "gravity tractor",

which pulls the asteroid in a new direction

and points it away from Earth.

Theoretically, a spacecraft could do this

by hovering near an object to create

a slight gravitational attraction

that could be used to redirect the asteroid.

The third option would be to go full Hollywood

and use a nuclear device to vaporize part of the surface.

"What you can do, in principle,

if you have a very powerful laser,

is to cause jets of material heated by the laser

to fly off of the asteroid

and that is essentially the equivalent of

a jet engine pushing the asteroid off course."

So far, NASA hasn’t found any asteroids

big enough to threaten Earth,

but we haven’t been searching that long.

There is still lots of space to survey

and the big one might be on its way,

we just haven't found it yet.