From planetary hit-and-runs
to Galactic T-Bones,
the universe is on a collision course.
What happens when these two monsters collide?
Well, probably the most violent explosion that we know of in our universe.
When the universe unloads, our Earth could be caught in the cross-fire.
lf you're within a thousand light years, you're dead.
This thing is gonna fry you.
Our planet's past is marred by extinction.
This was an epic disaster.
A global disaster.
And our future is at risk.
There's no doubt that an impact like that will happen again.
The question is, of course, when?
lt could happen tomorrow.
Over the next hour,
we'll show you the most intense smash-ups in the universe.
Beginning in the past, with the pounding that formed the Earth and created our moon.
Then we'll see what's flying around our planet right now...
..and meet the people working to protect us from the apocalypse.
Finally, we'll look to the future,
as we travel to deep space
to witness the universe's most violent impacts...
..and see how our Milky Way galaxy
will meet its demise in one of the biggest wrecks in the cosmos.
so take cover
and get ready for fireworks,
because we're on a collision course.
There are rocks out there in space
that not even Armageddon's Bruce Willis can save us from.
lron boulders,
massive rock piles...
..and objects the size of mountains.
Asteroids may make for a summer movie blockbuster...
but is there any truth to what we see on the silver screen?
There are bigger rocks out there and they're very dangerous if they hit us,
and if you don't believe me you can ask the dinosaurs.
Oh, wait, you can't. They're dead.
We've all heard of the rock that slammed into the Earth...
..and turned the dinosaurs into museum pieces.
The most recent large impact happened a century ago
in a sparsely populated region of siberia called Tunguska.
This 90-metre-wide rock exploded with the force of a thousand Hiroshima-sized bombs.
Had it struck over a large city,
it could have killed half a million people.
The scary thing is we think that similar sized events
should happen about every hundred years,
so that doesn't guarantee that one will happen any time soon,
but it does mean that we're statistically due for another one.
Even though we haven't been hit by a big rock in the last hundred years,
we're actually being pummelled all the time.
To find proof, all you need is a hosepipe.
Believe it or not, the roof of my house is covered with meteorites.
l'm not kidding. lt's the truth.
l'm gonna use this hose and l'm gonna wash down the roof of my house
and all the water is gonna end up in that bucket over there,
and in the bottom of the bucket l placed the magnet.
All the meteorites should stick to the magnet because they're made mostly of iron.
Let's see what's in the bucket.
Oh, this looks great.
This is fantastic.
As you can see, the surface of the magnet is covered with lots of little black specs.
Now most of those are fly ash, but a few of them are actual micro-meteorites.
Here's a micro-meteorite.
And there's another one right there.
And right here in the centre is a nice, big, fat one.
And if you go down below, ah, there's a little bitty one right there.
Now most meteors burn up when they pass through the atmosphere
but a few of them made it through and landed on my roof.
l bet you've got some on your roof too.
These tiny meteorites are evidence that Earth sits in the middle of a cosmic shooting gallery.
But not all rocks out there are so small.
some of them are miles across
and to understand the threat we need to go back four and a half billion years
to study the collisions that shaped our solar system.
There were asteroids and comets by the thousands,
many more times than there are now.
They ran around the solar system, often colliding with the planets.
lt was a violent place.
''Violent'' doesn't even begin to describe some of these gargantuan impacts.
Nearly every planet in the solar system bears the scars,.
like Mercury, the closest planet to the sun.
We look at Mercury, we see a gigantic basin called the Caloris Basin,
which was created by an impact.
lt's about 1 ,700 kilometres in diameter.
That's a crater as wide as California is long.
Venus, the second planet from the sun, was hit so hard it now spins the wrong way.
We know this because the sun and all the other planets spin in the same direction.
Think about it, something hit the planet Venus so hard that it's rotating the wrong way.
And then there's Mars, home to the largest known crater in the solar system.
The reason that the Lowlands on the North Pole of Mars are low
is that the crust was removed by a gigantic impact
that left a crater almost the size of half the planet.
The crater is so big, you can only see it in its entirety from space.
lt's the blue and green from this image.
so how big was this impact?
lmagine a rock the size of Australia coming screaming across the sky
at 20 times the speed of a modern jetliner.
lt smacks into Mars and leaves behind this enormous hole in the ground.
To give you an idea of how much energy was in these mega impacts,
it's estimated that the one on Mars was in the order of one hundred billion gigatons of TNT.
That's 200 trillion times more powerful than the largest nuclear weapon ever detonated.
All of our planetary neighbours were beaten up,
but what about Earth?
Did it suffer the same kind of punishment?
Amazingly, we think the Earth was involved in a collision bigger than the one on Mars.
A blast of such gigantic proportions that it created our moon.
The moon itself was born in a gigantic collision
between the Earth four and a half billion years ago and a proto-planet the size of Mars.
Our moon formed after something roughly 4,000 miles across slammed into Earth.
so if you had a time machine and you could go back in time to right before the moon formed,
you would see the primitive Earth that's just cooling off and everything is going pretty well
and then, oh, what's that over there?
You'd look up and see this Mars-sized thing coming and you would say,
''Oh, this is gonna be good.''
As the two objects close in on one another,
their gravity starts stretching them and ripping them apart.
lt would hit and the impact would blind you.
This would be like exploding every nuclear device on the Earth
probably a billion times over.
As the object ploughed into Earth,
it blasted off hundreds of billions of tons of rock that fell into orbit around the planet.
The Earth would be hot and molten.
This ring of material would already be in place
and it would only then take a relatively short amount of time,
years or hundreds of years for this moon to re-coalesce and then start forming in place.
so how lucky was Earth to be hit just hard enough to create the moon
but not hard enough to have the entire planet destroyed?
Oh, Jeez!
OK, stop.
To find out how Earth survived,
Brown University astronomer Peter schultz
is using heavy artillery to recreate that collision in the lab.
What we want to do is to look inside an impact when it happens,
so we're using this clear, acrylic ball.
lt allows us to look inside the planet.
To recreate the impact, shultz is using NAsA's vertical gun.
lt will shoot this ceramic sphere 1 0 times faster than a bullet into the acrylic ball
inside this reinforced blast chamber.
lt's coming in so fast,
it's gonna be the equivalent of a much larger body at slower speeds.
shultz carefully positions the gun hoping to recreate the same angle here
as the rock that slammed into Earth billions of years ago.
Ready!
Oh, Jeez!
Ha! OK, stop.
Well, say goodbye to my little friend.
l wanna see what l got.
Huh, good catch.
That's amazing.
so we slammed it right here.
The experiment shows that whatever struck the Earth all those billions of years ago
came in at a very shallow angle.
This is the type of impact that would have produced the moon.
Oh Jeez!
Now, that got hammered.
lf it were too direct it would have just shattered the Earth.
lf it were too glancing, it would have continued on its way with a lot of the pieces following it.
Luckily for us, Earth didn't meet its demise and we got our moon,
which goes to show that cataclysmic collisions and impacts
are the universe's way of putting things together.
The Earth and solar system were born from collisions
and it's a process that continues to this day.
But what's good for creating planets isn't always good for the things that call them home.
The massive, high-energy impacts that created our solar system
may be the reason we're here today.
But ironically, those collisions also left a lot of stuff flying around
that presents a constant danger for everything living here on Earth.
There's chunks of stuff out there that never made it into a planet,
rocks from the size of little pebbles to the size of a major state.
We call these left-over rocks asteroids.
They're shooting through space at tens of thousands of kilometres an hour.
But luckily most are out between Mars and Jupiter
in a region of space made famous in movies and video games.
You know it as the Asteroid Belt.
Most people have their impression formed by star Wars,
where you're taking the Millennium Falcon
driving around the Asteroid Belt and you go, ''Whoa! Watch out!''
And you can suddenly go in there. This is totally untrue.
lt's untrue because for every asteroid in the Belt,
there's, on average, 2.5 million square kilometres of empty space around it.
The Asteroid Belt forms a ring all the way around the sun,
but if you collected all the rock out there,
you'd only have enough to build something about half the size of the moon.
You could be standing on the surface of an asteroid and not be able to see another one
because they'd all be so far away they'd be too faint to see.
But that doesn't mean there aren't that many there.
There are millions and billions of these things out there.
The problem is they don't stay there.
Every so often, asteroids bump into each other,
knocking them out of the Belt and towards the planets of the inner solar system.
some of these rocks can be hundreds of yards across
and if these came in and hit the Earth, they would put severe hurt on us.
The last time the Earth really got pummelled was 65 million years ago
when an asteroid nine kilometres wide slammed into the planet at 25,000 kilometres an hour.
lt's hard to imagine what that must have looked like but think about that: six miles across.
That is comfortably larger than Mount Everest.
We've all heard about that asteroid - it's the one that's said to have wiped out the dinosaurs -
and it exemplifies everything that makes them dangerous.
lt actually hit, we think, just off the Yucatan peninsula in the Gulf of Mexico.
When it was all said and done, it made a crater hundreds of miles across,
but while it was happening this thing punched into the Earth,
probably punched a hole right through the crust.
The impact set off a chain of events
that would have been terrifying to watch
and nearly impossible to survive.
lt sent a furnace-like blast wave shooting a thousand kilometres in every direction...
..and it blasted millions of tons of pulverised rock into space,
which then came raining back down on the planet.
lt's not even enough that you had a giant asteroid smacking into the Earth.
All over the Earth you had huge chunks of rock
coming back down and burning up over the Earth's atmosphere.
ln a matter of minutes,
this rain of red-hot material cranked up the temperature and baked the Earth's surface.
l had calculated the amount of heat that you expect
is equal to 1 0 kilowatts per square metre.
Now, 1 0 kilowatts per square metre, what is that?
ls like the heat from a pizza oven.
lt fried the Earth and everything on it.
Anything that couldn't get into shelter would feel this tremendous heat baking on their skin.
lt would be like being under 1 0 noonday suns and there's no place to go.
This was an epic disaster, a global disaster, and it wiped out 95% of all species on Earth.
Everything bigger than, you know, this big wouldn't have been able to find food.
Yet, as apocalyptic as this scenario sounds,
we probably wouldn't be around to talk about it if it hadn't happened.
Mammals really got their opening
when the dinosaurs were cleared away and they could start out afresh.
lf dinosaurs had survived, the small mammals that evolved into hippos and humans
would never have had the opportunity to be anything other than lunch.
so what are the odds another rock will some day come in and wipe us out?
There's no doubt that an impact like that will happen again.
The question is, of course, when?
Asteroid impacts happen all the time.
Every day, up to 40 tons of dust and sand-sized particles rain down
and burn up in the atmosphere.
A few times a year, something the size of a car comes crashing in.
And about every century, an asteroid a few hundred feet across slams into the planet,
like the one that nailed Tunguska in 1 908.
And every hundred million years or so, you get something like the dinosaur killer.
The question is should you lie awake at night in a cold sweat worrying about them
and the answer is really no.
The bigger the object is, the much rarer it is.
But they don't need to be that big.
Even asteroidsjust a few hundred feet across present a threat.
Though they're not big enough to snuff out life on the planet,
they do have the potential to kill hundreds of millions of people.
The problem is these objects are travelling so fast.
some of them are travelling 20, 30 times faster than a rifle bullet.
That's almost 65,000 kilometres an hour.
And they're the size of a city block.
And so they carry a lot of energy.
When they hit the atmosphere, the friction is so intense, it generates incredible heat.
ln miniature, it's a lot like peppering a plate of steel with machine-gun fire.
When an asteroid impacts the atmosphere,
it's travelling so fast the air up there acts like concrete.
All that heat and pressure does something to that rock
and a good way to demonstrate that is with a steel plate
and my friend Bob and his machine gun.
Take it away, Bob.
When a speeding asteroid hits the atmosphere, the air can't get out of the way fast enough.
so the bullet is like an asteroid and the steel plate is like our atmosphere.
Whoa! That was loud.
Let's go see what happened.
Ah, there's one. l'll pick it up.
Wow! That's hot. That's hot.
Wow.
Take a close look at our bullet.
lt flattened out and that can happen to an asteroid when it impacts our atmosphere.
lt can flatten out or pancake
and with all that heat and pressure, it can explode,
and if that happens,
you should bend over, grab your ankles and kiss your ass goodbye.
Now scale that bullet up a few hundred thousand times
and increase its speed by a factor of 20
and you begin to get the picture.
All that friction and the heat and stress of entry can cause these rocks to explode.
When you're talking about chunks of rock that are 1 0 metres across, 50 metres across,
a 1 00 metres across, the size of a football field,
these guys can blow up in the Earth's atmosphere.
Then you're talking about megatons of energy.
Nuclear weapon-sized explosions.
A hundred metres doesn't sound very big,
but that would hit the Earth with the force of hundreds of megaton nuclear warheads.
so it would wipe out a large region of a country.
so considering what one of these things can do,
surprisingly we know very little about asteroids
and even less about where they're located.
The good news is we're looking for them and it's something that people take seriously.
But are we taking the threat seriously enough?
ls there an asteroid out there with our name on it?
ls there anything big enough out there targeted on the Earth right now?
When it comes to asteroids, we have near misses all the time.
Almost every year we hear about one of these huge rocks just missing Earth.
The scary thing about a lot of these is we don't see them until after they've already passed us.
so that's when you say, ''Oh, yesterday a 1 00-yard wide asteroid
missed us by 50,000 miles.''
Yeah, you don't want to hear that.
You want to hear ''We have two years' warning.''
Ed Beshore is among the few on the front lines of asteroid defence.
Working at an observatory outside Tucson, Arizona,
Beshore scans the skies looking for anything that moves.
What we do with our telescope is we select an area on the sky
for observation over the course of about 45 minutes
and we take repeated images of that same area of the sky
spaced by about 1 0 minutes apart.
By comparing the images, Beshore is able to pinpoint Near Earth Objects or NEOs.
They appear as faint, moving blips.
On this image, you see that all the stars are not moving, but this object, this NEO as it turns out,
is readily made obvious.
Beshore usually spots dozens of NEOs a night.
They're space rocks that have fallen out of the Asteroid Belt
and into the neighbourhood of Earth.
And while none of these giant NEOs are on a collision course with Earth,
there are a lot more smaller rocks out there that threaten our planet.
The asteroids that we think are particularly threatening are something that we call PHAs,
or Potentially Hazardous Asteroids.
Their orbits bring them close enough
for the Earth to actually bring them onto a collision course.
While none of these PHAs are aimed at Earth right now,
we need to be constantly on the lookout.
When l started doing this job, there were only about 1 70,000 asteroids known.
Now there's over 400,000.
We don't walk away from this frightened
because we know we're actually doing something about it.
some day we will probably have to act against an asteroid,
and that day may come sooner than most of us realise.
There is one asteroid that astronomers are a little bit concerned about, just a little bit.
Let me say that.
lt's called Apophis.
On Friday 1 3th April 2029, this iron rock, bigger than a football stadium,
will pass within 35,000 kilometres of Earth.
lt sounds like a lot, but it's really a very near miss.
lt's going to pass so close to the Earth
that it's going to be between us and our geo-synchronous satellites.
That's a close call and we're sure it's gonna miss the Earth in 2029,
but there's a problem.
That problem is what's called a keyhole.
Keyholes are little pockets of space
where Earth's gravity is strong enough that it will pull an asteroid slightly off its current course
and send it on a new one.
ln the case of Apophis, the keyhole is about half a kilometre wide
and if this 250-metre rock passes through it,
Earth's gravity will nudge it just enough
so that it will return seven years later
and hit the planet somewhere between California and the Hawaiian lslands.
so what will we do?
Blast it with a nuclear missile?
That may not be the best idea.
lnstead of solving the problem, you could make it worse.
lf we're ever really threatened by an asteroid, nuking it might not be the answer.
Now, why is that?
Let's see if we can find out.
Let's just say that this boulder is an asteroid
and my friend scott is loading an explosive charge on its surface.
Let's see what will happen.
- Ready to do this, scott? - Ready.
OK. Here we go.
Three, two, one.
That was incredible.
Let's check out our asteroid and see what happened.
Doggonit, not much.
You know, we put the equivalent of two or three sticks of dynamite on top of this thing
and it's still intact.
Hmm.
l wonder what would happen if we drilled a hole down through the centre of our asteroid
and put the charge there.
Maybe that would eliminate our threat.
Hey, scott, you ready to go all Bruce Willis on this one?
- Ready. - OK.
Oh, whoa! Man, that was great!
l think we just made a big mess.
Man, this is a mess.
We started out with one big problem, a big asteroid,
and now we've got lots of little problems.
You see, if the original asteroid had been, say, two miles across,
well, each one of these would then be 1 00 or 200 yards across and that's still a threat.
so maybe that tells us we shouldn't nuke a space rock.
To make matters worse, not all asteroids are solid rock or metal.
Consider something called ltokawa.
This 600m Near Earth Object is what's called a rubble pile.
lt's not solid.
lt's really just a pile of dust and boulders held together by gravity.
Even though they're fragile,
rubble piles are some of the most dangerous asteroids out there.
That's because they're impervious to just about anything we could throw at them.
A rubble pile asteroid is a little bit like a pile of sand,
where all the little particles are held together by their mutual gravitational attraction.
Now what happens if we hit a pile of sand like this with a missile or a rocket?
Let's find out.
so l've got my missile.
lt's a heavy lead ball.
Let's see what happens when it hits the rubble pile.
Bombs away.
Bingo.
so let's see what happened to our rubble pile here.
Not much.
That's what makes these so dangerous,
because anything we throw at them, they absorb the energy of the impact.
lf we're going to have to save the Earth from this kind of asteroid,
we're going to have to come up with something else.
lf our biggest weapons are powerless against these cosmic invaders,
then what hope do we have of saving the Earth from the inevitable?
This former Apollo astronaut has a plan.
You just pull up with a small spacecraft in front of this big asteroid
that may be tumbling in space or whatever
and you just hover directly in front of it or directly behind it.
40 years ago, Apollo 9 astronaut Rusty schweickart impacted the Earth
and today he's using that unique insight to save our planet from apocalypse.
l've been a Near Earth Object, right.
But l have also rendezvoused in orbit and that's exactly what we're talking about here.
We're talking about essentially an asteroid rendezvousing with the Earth.
schweickart's plan involves something called a gravity tractor.
lt's a spaceship that would use its own gravity to nudge a killer asteroid off course.
You're towing it with a very weak gravitational tow rope
and gradually the gravitational attraction between the two of you
will very gradually accelerate that big rock, that asteroid.
That change of speed might only be a few millimetres an hour,
but give it enough time and it's enough to deflect one of these killers.
And it would work on any kind of asteroid.
You don't care if it's nickel iron or a puff ball or stones or a rubble pile,
l mean, you don't touch it.
so as long as it has mass, that's all you need to tow it.
The only drawback to the gravity tractor is that you need a lot of lead time.
Because the forces exerted on the asteroid are so small,
we need to act years, even decades, in advance to avoid a devastating impact.
lmpacts are the most terrifying natural disasters we can imagine,
yet we have the power to predict and maybe even prevent them.
However, asteroid impacts aren't the only kind of collisions in the cosmos
that can wreak havoc here on Earth.
lt's time to witness some of the biggest bangs the universe can muster.
They're collisions so powerful, they can deal a lethal blow to Earth from trillions of miles away.
Unless we can come up with some way to deflect it,
an asteroid will some day slam into Earth.
But space rocks aren't the only things that threaten our planet.
ln fact, compared to some of the other things out there in the universe,
that type of collision
is insignificant.
One frightening example..
neutron stars.
Neutron stars are one of my favourite objects in space
because they are one of the weirdest things in the universe.
A neutron star is the core of a supernova explosion,
a titanic explosion where a giant star rips itself apart
but there's a super-dense little core left over.
These little cores are small, hot and incredibly dense.
The best way l can say that is if you think about a cubic centimetre,
which is about the size of a keyboard key on a keyboard.
A cubic centimetre of neutron star material would have the mass of Mount Everest.
Neutron stars often come in pairs
and with all that mass they swirl around each other each,
each pulling on the other in a fiery death dance.
You have two neutron stars whipping around each other.
What happens when these two monsters collide?
Well, probably the most violent explosion that we know of in our universe.
This is called a gamma ray burst.
Doesn't sound that scary.
ln fact, these things are one of the few objects in the universe
that really give me the heebie-jeebies.
Gamma ray bursts release more energy in a microsecond
than our sun will produce in its entire 1 0-billion-year lifetime.
lf you're within a thousand light years of a gamma ray burst
and the beaming energy happens to be pointed right at you,
you're gone, you're dead. This thing is gonna fry you.
A micro-second after the burst, it's over.
The combined mass of the neutron stars collapse into a black hole
with so much force, scientists suspect it actually distorts space.
Einstein predicted that if you have two objects like this and they collide,
they can actually send out ripples in the fabric of space itself.
These are called gravitational waves
and it's a little bit like the ripples on the surface of a pond when you throw a rock in.
so what exactly does a gravitational wave do to space?
Well, let's pretend this balloon is space itself.
As the gravitational wave passes,
the balloon is first compressed in one direction and stretched in the opposite
and then a fraction of a second later it reverses, like this.
so a gravitational wave compresses and expands space-time alternately
as it moves through.
We haven't seen these waves yet because when they reach Earth they're very weak.
scientists think they compress space by as little as a millionth of a millionth of a metre.
Though we haven't detected a gravitational wave,
scientists believe they exist
because they were predicted by Einstein in his Theory of General Relativity.
One of the weirder aspects of Einstein's Theory of Relativity is that space is a thing.
lt's something that we're embedded in.
lt's not just empty space.
When objects move through it, they can create these ripples.
lt's called gravitational radiation.
These ripples rush out through the cosmos at the speed of light.
They stretch and squeeze space and everything in it.
Gravitational waves are really distortions in the very fabric of space.
They're caused by large masses, like stars, or pairs of stars, moving very rapidly.
This kind of motion really distorts space itself.
Because they happen so far away,
by the time these waves reach Earth, they're compressing space by the tiniest amount.
That's not enough to cause a problem for us here on Earth,
but if some titanic collision happened within a few hundred kilometres of our planet,
the waves wouldn't gently lap at our cosmic shores.
They'd crash upon our beach with all the force of a tsunami.
As the gravitation wave moved through the planet,
everything on it would be stretched and squeezed.
Mountains would be flattened, then spike up.
Trees would be squashed
and people would be compressed, then lengthened,
over and over again
until the wave train passed.
Depending on the intensity,
it could kill billions of people.
And that'sjust what happens if Earth is very close to a neutron star collision.
lf we're too close to this, it could actually rip the planet apart,
which is hard to imagine, you know, something disastrous on that scale.
The danger doesn't end with a gravitational wave.
The black hole that's left over from the collision then moves through the galaxy,
consuming everything that crosses its path.
A black hole is simply an object
that is so small and has such strong gravity
that even light cannot escape.
Black holes are created by dying stars that explode
and trigger a gravitational collapse that goes berserk.
The star gets smaller and smaller
until you're left with something that's four times the size of the sun
packed into a space that's too small to measure.
They keep collapsing until they're infinitesimally small, so they have zero volume.
They have no size.
Yet they have such intense gravity, they can rip entire stars apart.
so how many of these tiny phantoms are there in the galaxy?
We think there are millions of black holes in the Milky Way galaxy.
And you might think, ''God, millions of black holes roaming the Milky Way galaxy,''
but space is really, really big.
The odds of even a normal star getting close enough to the Earth
to do any kind of damage is incredibly low,
even over the lifetime of the universe.
But what would happen if a black hole was on a collision course with us?
As the black hole nears Earth, things start to get very strange.
lt's going to draw the Earth's atmosphere off.
There will be like a long tendril of air that will go swinging in and spiralling into the black hole
and as it gets closer, objects closer and closer to the Earth will feel that even more strongly.
And at some point, even you sitting here on the surface of the Earth
will feel more gravity from the black hole than you will from the Earth
and it will lift you right off the surface and... (Makes sucking sound) Gone.
Then in very short order, the black hole's gravity starts tearing the planet apart.
The tidal forces would rip the Earth apart,
pulverising it completely,
and its matter would fall in lots of little pieces,
not a single Earth going over the edge of the event horizon.
And then eventually all of that matter will fall into the point of no return and that will be it.
We'll be gone, dead.
Neutron stars and black holes...
As epic as these smash-ups are,
they're nothing compared to two galaxies colliding
and our Milky Way is heading straight for the Andromeda galaxy
at one and a half million kilometres an hour.
Look long enough,
and you'll see gravity is pulling everything in the universe onto a collision course.
lt might be an asteroid smashing into a planet at tens of thousands of kilometres an hour,
or a star that careers into another.
lmpacts are the way of the universe.
And nothing in it is big enough to escape their destructive power.
And the biggest collisions in the universe involve the biggest things in the universe, galaxies.
lf you want to talk cosmic smash-ups, it doesn't get any bigger than galaxy collisions.
Galaxies are the largest objects in the universe.
They're collections of hundreds of billions of stars
spread out over tens and even hundreds of thousands of light years.
You imagine two galaxies as these enormous things with huge amounts of mass,
a hundred billion times the mass of the sun,
and you might think the collision would be truly spectacular.
You might think that until you realise that despite all the stars,
galaxies are still mostly empty space.
The average distance between any two stars in our galaxy is around 50 trillion kilometres,
so stars rarely, if ever, bang into one another during these cosmic mergers.
A star might be a million miles across,
but a galaxy is a hundred thousand light years across and a light year is six trillion miles,
so 600,000 trillion miles of galaxy and a million miles of star,
even though there are lots of stars in the galaxy,
chances are two of them can pass right through each other
and there won't be a single star collision.
so how does a collision where nothing actually smashes into one another wreak such havoc?
ln these gargantuan smash-ups, it's gravity that does the destruction.
The combined mass of the stars in each galaxy
generates immense tides that toss stars thousands of light years in every direction.
As they fall together, they completely change shape.
Gravity creates this beautiful new elegant dance.
some stars actually get thrown off the discs of the galaxies - the beautiful tails.
Others get dropped in towards the core.
Galactic collisions take hundreds of millions of years to unfold...
..and at the end what's left is a bigger galaxy,
loaded with the materials to build new stars and even planets.
Our galaxy, the Milky Way,
has been through a few wrecks
and, believe it or not, it's on course for another epic collision.
l hope you're prepared for an epic train wreck
because the Andromeda galaxy is on a collision course with us...
..and we are in fact going to collide with it.
The Andromeda galaxy and our Milky Way
are speeding towards one another at one and a half million kilometres an hour,
and in less than four billion years they're going to plough into one another.
When that happens, both galaxies will be ripped apart.
What happens to the Earth?
One possibility is that we get thrown off into space,
so over the course of hundreds of thousands of years,
we would peel away off the Milky Way galaxy,
which might give us a spectacular view
as we actually move farther and farther away up off the disc of our own galaxy.
Chances are the sun will just keep orbiting this galaxy,
whatever it's going to be called,
and we'll have a tremendous view of this entire event.
lt'll be really something to see.
The problem is, it's not going to be for a long time
and will take a hundred million years or billion years to unfold.
And over the hundreds of millions of years it will take for this crash to unfold,
Earth will be taken on the ride of its life
as it whips this way and that, light years through the cosmos.
The universe is not a place of serene, unchanging beauty.
Everything out there is on a collision course,
be they asteroids smashing into Earth,
planets crashing into one another,
stars impacting,
or entire galaxies careering through one another.
Collisions play an integral role in nature's plan.
With the universe on a collision course,
we on Earth are most certainly
at the mercy of powers we barely comprehend.