So far, we've set foot on one world beyond our own.
We discovered a desolate, barren rock.
An ancient, unchanging, cratered world.
And the footprints we left there
could last for millions of years.
Our only direct personal experience of an alien world is of our moon.
Beautiful, but a dead, inactive world, frozen in time,
whereas our planet is active and alive.
If you come to the right places on Earth,
it's literally seething with energy beneath our feet.
For a long time,
we wondered if all this activity is unique to our planet.
But now, thanks to a fleet of spacecraft,
we know our world is not alone.
We currently have over 40 probes exploring the solar system,
relaying a stream of information to Earth,
allowing us to see our sister worlds in unprecedented detail.
They're revealing planets and moons covered with volcanoes,
dwarfing anything seen on our planet.
Alien landscapes bursting with fire...
..and ice.
Eruptions so violent,
they reach into space.
So, why are some worlds vibrant and alive,
while others are cold and dead?
Now, that question is deeper than it first sounds,
because answering it will have profound implications
for our understanding of our place in the universe.
See, geological activity,
the flow of energy from the interior of a world outwards,
is necessary for the origin of life.
And that's why finding and understanding those worlds
is a necessary first step in the search for life beyond Earth.
Let's begin a journey to the volcano worlds,
by leaving Earth...
..heading away from the sun...
..and setting a course to the planet next door -
the most visited of them all.
For almost two decades,
the Mars Reconnaissance Orbiter has pointed its cameras
at the Red Planet.
And the images it has sent back
have revealed volcanoes on a staggering scale.
One so wide...
..it would span the UK.
And one so tall...
..it rises up through Mars's atmosphere,
almost to the edge of space.
Over time, these mega volcanoes have flooded
the Martian surface
with a billion billion tonnes of lava.
So much that they've tipped the entire planet over
by 20 degrees.
So, what drives a planet's volcanism?
Iceland's one of the most volcanically active
places on Earth.
This is the Icelandic Met Office,
so this is the weather forecasting site.
But it also gives you a real-time update
on earthquakes,
and earthquakes are precursors for volcanic eruptions.
These dots are all earthquakes that have happened
in the last few hours, actually.
And we, at the moment, are driving along a road...
LAUGHTER
..in there!
So, there's...
Is that OK, by the way?
Oh, yeah, that's normal.
It's normal? Yeah.
Yeah?
When you have a collection of earthquakes like this,
a lot in the same place, at the same time,
it's called the jardfraedingur.
Jard... Fraedingur
..fraedingur?
Yeah, so it's basically stirring the earth.
Stirring the earth. Yeah, yeah.
But recently, the land here did more than stir.
Just last year, over ten million cubic metres of lava
flowed out down this valley,
creating brand-new land.
This is planet-building in action.
Activity so recent,
you can still see the afterglow.
So, there's the old volcano in the distance,
which is old and cold,
and then there's all this new land.
And look, it's glowing!
To drive volcanism on this scale
takes an enormous amount of energy.
So, where does it all come from?
Think about what was happening here about 4.5 billion years ago.
So, this would've been a cloud of gas and dust and rocks,
and all those rocks falling together under the influence of gravity -
ultimately to form the primordial Earth.
During our planet's formation,
that gravitational energy was transformed into heat,
adding to the heat released by the decay of radioactive elements.
Heat is a form of energy.
Now, there's a law of physics, a law of thermodynamics -
it's called the first law of thermodynamics.
And it says that energy is neither created nor destroyed.
So, all the energy released when all those rocks
were smashing together to form the primordial Earth
is still here.
It's stored - trapped - ever since.
Just below the surface there, down that crack,
it's just glowing hot!
Mars formed at the same time, and in the same way -
the planet trapping enough heat to raise the largest volcanoes
in the solar system.
But unlike the Earth,
these giant volcanoes fell silent millions of years ago.
Something happened to Mars's inner heat.
And in the north of the planet...
..Mars Reconnaissance Orbiter
spotted a clue.
An impact crater...
..whose walls appear to be built
from an intricate array of pillars.
So perfect they look almost engineered.
They aren't, of course, the work of Martian sculptors.
They're also found here on Earth.
Just look at these beautiful geometric shapes.
They look almost carved into the rock.
They are a beautiful example of one of -
actually perhaps, in some sense, THE - most fundamental law
of nature in action, the second law of thermodynamics.
The second law of thermodynamics, put really simply,
is that if you get a hot thing - high temperature -
and bring it into contact with a cold thing -
low temperature -
then it is inevitable that energy will be transferred
from the hot thing to the cold thing
until they reach the same temperature.
That's absolutely fundamental.
That's what's happened here.
The hot lava has come out from underneath the ground,
that inner heat,
it's met the cold atmosphere,
and it's cooled down, it's lost energy.
And what's true here on Earth is also true on Mars.
On both planets, the pillars started life as hot molten rock.
As the lava cooled, it contracted,
causing cracks to form on the surface
that then grew downwards...
..creating the symmetrical columns.
They are a direct consequence of the second law in action,
as the lava released enormous amounts of heat -
ultimately out into space.
But the pillars on Mars are likely millions of years older.
The flows that built them died,
just as Mars's volcanoes did.
Mars lost its inner heat far faster than Earth.
The question is, why?
God, it's...
BRIAN LAUGHS
..the single simplest invention in human history.
If I'd have been the cavemen,
we wouldn't have even domesticated animals.
Ah!
Success!
Take one Earth-sized rock,
add a smaller Mars-sized one,
and roast for 30 minutes.
So, these two rocks have been in the fire,
they've been heating up,
and I've just got them out of the fire.
At the moment, they're at...
..the same temperature.
You can see there,
they're both about 200 degrees.
But now remove them from the fire.
In accord with the second law of thermodynamics, well,
they're going to start losing energy.
So, if we wait,
then the rocks will cool down.
Well, now these two rocks have been out of the fire
for about 20 minutes or so,
and not surprisingly they've cooled down -
cos they're in contact with a colder environment.
The big one has cooled down to about...
..150, 155 degrees or so.
But the little rock has cooled down way more.
It's now only at a temperature of about 50 degrees or so.
I can pretty much touch it with my finger.
And that's because the small one is small.
To be more specific, these rocks are losing heat
to the environment through their surface area -
and the small one has got much more surface area
in relation to its volume than the large one.
That means that it loses heat more quickly, cools down.
And this is exactly what's happened to Earth and Mars.
Earth is large enough to have held on to much of its internal heat.
But Mars's radius is about half that of Earth's.
So, since the glory days,
when its volcanoes were raised on a scale seen nowhere else...
..Mars's inner heat has escaped,
lost to the cold of space,
bringing the grandest volcanism
the solar system has ever seen...
..to an end.
Size, then, sets a powerful limit on volcanic activity.
Yet the next volcano world
seems to break this rule.
Out beyond the asteroid belt
lies the first of the gas giants.
Jupiter commands its own system of moons -
over 90 at the last count.
Including one that is truly unique.
Io.
Nasa's Juno probe has been circling Jupiter since 2016.
Its orbit taking it ever closer to Io.
Its infrared camera saw a world consumed by fire,
each bright patch a volcanic eruption.
Right now, rivers of lava are pouring
across its tortured surface.
In places, the volcanic eruptions are so violent...
..they throw columns of gas and dust far out into space.
Io is the most volcanically active world in the solar system...
..yet its radius is just over half that of Mars.
You remember that scene in Alien,
where John Hurt and all the astronauts
descend into the cave?
And remember what happens to 'em?
This has a bit of that feel.
The scale of volcanism on Io is hard to comprehend,
to visualise,
until you come to a place like this.
Here's a photograph of the surface of Io.
Can you see all those colours?
All those beautiful yellows and oranges?
Now look at the walls of this cave.
Same colours.
And that's because these are the same chemical elements -
it's elements like sulphur.
Now, in this case, they were deposited on the walls
of the cave when the magma seeped away
around 5,000 years ago.
But here, on the surface of Io,
they've been constantly replenished.
Just look at the scale of it!
Imagine that, all on a small world, no bigger than our moon.
Its small size means that Io's heat of formation has long gone.
Something else is fuelling these fires.
The giant planet that looms so large in its skies.
Io orbits around Jupiter -
and Jupiter, being a very massive planet,
raises tides on Io.
And that's pretty much the same mechanism
by which the moon raises the tides on Earth.
But Jupiter is extremely massive,
and so the tides on Io are extremely violent.
It actually raises the tides in the rock
of something like 100 metres.
It's not in water, it's in rock!
And it's about the height of this cavern!
But Io's orbit is not circular.
It's elliptical,
so that means that the moon comes close to Jupiter,
and far away.
Close, and far away.
Once every 42 hours.
So, that hundred-metre rock tide is going up and down
and up and down every 42 hours,
as Io goes around Jupiter.
So, imagine the friction as that rock tide rises and falls,
and rises and falls.
That introduces immense amounts of heat into the moon.
It's actually about half the energy
that we know is needed to power the volcanoes.
But it's only about half.
So, where does the other half come from?
Well, that's where it gets really cool.
So, let's say Jupiter...
..is there,
and let's say that Io...
..is orbiting around Jupiter...
I'm going to exaggerate it a lot.
..orbiting around Jupiter in an elliptical orbit.
So Io is moving around like this.
In an elliptical orbit,
there are two foci.
The cross, out here in empty space, which we call the empty focus,
and the other centred on the planet.
And it turns out that Io...
"It can be shown that..." That's what you say.
I'm not going to show it, cos it's a load of mathematics.
But it can be shown that Io is locked to the empty
focus of the ellipse -
the other focus, not the planet.
But the tide is raised by Jupiter's gravity.
So, that big, sort of huge, towering tide
in the rock always points towards the planet.
As Io goes around,
that tide is dragged backwards and forwards
across the face of the moon.
So not only have you got this big hundred-metre tide in the rock
going up and down as it goes around,
it's going side to side,
being dragged backwards and forwards across the face of the moon.
That also injects a tremendous amount of energy
into the moon,
and that's the other half of the energy
that's required to power Io's volcanoes.
These colossal tides are what enabled Io, despite its size,
to become so violently volcanic.
The friction may even have melted so much of the moon
that there's a global ocean of magma
just below the surface.
But there's another twist to Io's tale.
This is a series of photographs of Io taken -
it's only a few months ago now -
over a period of several weeks.
And you see the volcanoes, you see all that activity,
the hot spots switching on and switching off.
This is an infrared photograph.
So what you're seeing here is heat,
which is useless energy being radiated off into space.
Energy is being removed from Io's orbit.
Now, if you remove energy from an elliptical orbit,
it gets more and more circular.
And if the orbit was circular,
then the tidal heating would die away
and the volcanoes would fall silent.
So, if all there was
was Jupiter and Io,
then Io would not look like that.
Io's extreme activity should've killed off the tides
that create its internal heat.
So there must be something else
beyond the squeezing of the moon
keeping its fires alive.
Io is not alone in orbit around Jupiter.
It's one of the four big moons known as the Galilean satellites.
And Io orbits in what's called an orbital resonance
with two of them - Europa and Ganymede.
So here's Jupiter,
and for every four orbits of Io,
Europa goes around twice
and Ganymede goes around exactly once.
That means, on every fourth orbit, the moons line-up,
and they give a gravitational kick to Io.
They put energy into the orbit,
which keeps the orbit elliptical.
And so, whereas here on Earth the volcanoes are driven
by the primordial heat down at the Earth's core,
Io's volcanoes, ultimately, are driven by gravity.
This bizarre volcanic moon...
..locked in a seemingly endless cycle
of eruptions by its sister moons...
..is the furthest world from the sun
where we've seen molten rock erupting onto the surface.
But, beyond Jupiter,
another mission has encountered an entirely different
type of volcano.
Crossing the great gulf of space,
we encounter the next planet.
Saturn's rings loop for hundreds of thousands of kilometres
through space.
And just beyond them lies a glittering gem.
A frozen moon -
perhaps the last place you'd expect to find a volcano.
Enceladus's surface is a hard mantle of frozen water
that's a deathly -200 degrees Celsius.
On such a cold world,
everything should be frigid, unchanging.
Yet, in 2005,
the Cassini probe witnessed an extraordinary sight.
Explosive jets roar from the surface...
..reaching hundreds of kilometres into space.
The largest volcanic plumes in the solar system.
How are such epic eruptions possible
on a tiny frozen moon?
Even on Earth, eruptions don't have to be molten rock.
The geothermal activity so close to the surface here in Iceland
is kind of a double-edged sword.
I mean, on the one hand it can be dangerous.
But here, that geothermal activity is also used
for the benefit of the population of Iceland.
I mean, here you see thermodynamics in action.
This is a power station.
These two power stations in this region
provide over 400 megawatts of power.
It's enough to power Reykjavik, and also half its hot water.
And so, you can feel the energy,
that primordial energy of the Earth, rising to the surface,
heading off into the cold of the atmosphere.
This is precisely what's happening out there on Enceladus.
It's just...
BRIAN LAUGHS
You get... You do get a sense of the raw power
just sitting just a few...
Not far, in this case,
below our feet, actually.
But this is nothing compared to Enceladus...
..where over 300kg of water vapour and ice
erupts every second
from giant cryovolcanoes.
It was Cassini that first spotted something odd
about the motion of Enceladus.
As it orbits Saturn, it wobbles on its axis...
..by a very small but, it turns out, very significant 0.12 degrees.
Consider an egg.
EGG CRACKS CREW LAUGH
Now, when you spin an object,
so when an object spins on its axis,
it rotates around what's called its centre of mass.
And for a solid object -
like this hardboiled egg -
if I spin it, it spins nice and evenly.
Uniformly.
But now...
..look what happens if I take an egg that hasn't been hardboiled -
so it's filled with fluid.
If I spin this...
..it wobbles all over the place
because the fluid inside is sloshing around.
Because this egg is raw,
the shell and liquid inside move independently
of each other when spun, making the egg wobble.
So the reason that Enceladus wobbles
is because it's not completely solid.
And we now think,
by high-precision measurements and simulations
of exactly how Enceladus wobbles,
that there is a global liquid ocean
beneath the frozen, icy surface of Enceladus.
We can infer that
because the laws of physics
that apply to eggs here on Earth
also apply to moons.
I mean, Enceladus isn't going to do that.
So Enceladus has an outer shell of ice
sitting on top a global ocean of water.
But how is that water managing to force its way
through 5km of solid ice?
To find out, Cassini took a much closer look
at the moon's south pole.
Oh, wow. That's changed, hasn't it?
It's changed - I don't know whether it's changed for good or bad.
Crikey!
HE CHUCKLES
We've found a really nice,
relaxing place to explain some complicated physics!
Now, here, about 2km down below my feet,
there's a hot reservoir of water that's under pressure.
Now, under normal circumstances, that couldn't escape, but we've
drilled a borehole and, the moment that that borehole is present,
then those pressure and temperature differences will equalise.
And, in this case,
the water comes out of the borehole as superheated steam.
Now, here is a photograph of Enceladus's south pole
from Cassini, and you can immediately see there's
something interesting here - interesting geology.
Cracks in the thin ice of the south pole -
these things became known as the tiger stripes.
They are revealed most clearly by Cassini's infrared instruments.
The red shows freshly deposited ice crystals...
..hints of activity along the entire length of the cracks.
But the real insight comes when you measure their temperature.
Because those tiger stripes are hot, really hot, compared to the surface.
The surface of Enceladus is -200,
maybe -220 degrees Celsius.
These tiger stripes are at -80 degrees Celsius.
You might say, "Well, it's still cold."
It is cold, but it's a lot hotter than the surface surrounding
those structures.
And so, what you can see here is high-temperature,
high-pressure ocean beneath the surface,
and there's a cold, low-pressure environment of space above,
and there's a weakness here in the surface.
That allows that gradient to equalise.
It's exactly what you see there,
other than there, someone has drilled a hole down into the deep,
underneath the Earth, whereas, here, the ice happened to be thinner.
We're not really sure why, actually.
Could've been that there was some kind of impact here.
But the upshot is the same.
You get plumes of water,
ice in this case,
erupting out into space.
The tiger stripes also create a window into Enceladus's interior.
As Cassini flew through the plumes, it detected
traces of molecular hydrogen and silicon dioxide, chemistry that
most likely comes from ocean water interacting with hot volcanic rock.
This suggests that the ocean beneath Enceladus's icy shell
has something that on Earth we call hydrothermal vents.
The discovery of active geology on Enceladus took
everybody by surprise.
Nobody expected to see it on such a small world.
But there might be more to Enceladus than just geology.
See, hydrothermal vents of the kind we think might be
present on Enceladus are one of the prime
candidates for the cradle of life on Earth.
The reason is that, if you think about what the origin of life
has to be, it has to be, in a sense, a transition from geochemistry
to biochemistry, from active geology to active biology.
So, all the conditions seem to be present on Enceladus
for the origin of life, and we don't even need to land or find
some way of getting into that ocean to test that hypothesis because
Enceladus is throwing the evidence, potentially, out into space.
All we need to do is fly a spacecraft through those plumes.
So Enceladus has to be one of the prime candidates for exploration
in the solar system to search for the origin of life beyond Earth.
Enceladus is not the only world with cryovolcanoes.
Even at the furthest planet from the sun...
..we've found evidence of them.
Only one ship has ever made the journey.
It was on one of Neptune's frozen moons that Voyager 2 caught
a glimpse of recent activity.
Its camera sent back images of dark smudges on Triton's face.
Trails left by plumes erupting from its surface.
Making Triton the most distant of the active volcanic worlds
that we've witnessed.
It seemed that the inventory of the solar system's
active volcano worlds was complete.
But, recently, we found something we'd missed far closer to home.
Venus is shrouded in thick clouds of sulphur dioxide...
..making it very difficult to see the surface.
So the spacecraft deployed here use radar to peer through
the dense atmosphere.
Magellan's radar imagery revealed Venus to be a hellish world...
..its landscapes dominated by volcanoes...
..over 85,000 at the last count.
Including truly bizarre examples, with deeply rutted sides...
..and lines of flattened volcanic domes like chains of pancakes.
But with only snapshots from orbit to go on, no-one knew
if any of these volcanoes were active...
..until, in 2023, a new analysis
of the Magellan data revealed,
on a volcano the size of Mount Everest,
an eruption along its northern flank.
Proof after all that there's activity on the most
volcano-ridden planet in the solar system.
So why does Venus have such strange
and diverse volcanoes littered across its surface?
A clue can be found in Iceland's remote volcanic interior.
In 1783, for a period of eight months, one of the most
catastrophic volcanic eruptions in human history happened here.
15 cubic kilometres of lava emerged from these eruptions.
You see this, it's a remarkable landscape, a line of volcanoes.
And they're really classic volcanoes,
like a child has drawn a volcano.
And then, everywhere else that you look across this valley,
it's just lava.
The fact that such a violent eruption happened here
is not down to chance.
If I take a map of the Earth and draw all the volcanoes,
then they form a very distinct pattern.
So, there's a line all the way down North and South America,
on the Pacific Coast, and then, the other side of the Pacific,
there's another line of volcanoes through places like Indonesia.
Down here in the Rift Valley, Tanzania and Ethiopia.
And then there's a line of volcanoes through Iceland and, actually,
under the ocean, down the middle of the North and South Atlantic.
So there's a very distinct pattern here.
And that's because the surface of the Earth is not just one big
slab - it's carved up into plates.
The Earth has what's known as plate tectonics.
So, here, for example, down the Pacific Coast of North
and South America, the Pacific Ocean crust, the floor of the Pacific,
is moving down, this way,
underneath the continent.
And you get eruptions, you get volcanoes.
In the Atlantic, here, through
Iceland, the opposite is happening.
The Earth's crust is spreading.
You can see it, actually. I'm sat on it.
So, over there, in the west, is North America, the North
American Plate, and, over there, in the east, is the Eurasian Plate.
They're spreading apart here, literally here,
and that's why there's a line of volcanoes moving down through
here and straight onwards down into the South Atlantic.
So, Earth's pattern of volcanoes is telling us
that there's what's called plate tectonics on the Earth.
Now look at a map of the volcanoes on Venus.
Look at that.
It's absolutely covered, completely randomly,
in pretty much every kind of volcano you can imagine,
scattered across the entire face of the planet.
And the reason for that is that there are no
plate tectonics on Venus.
We don't fully understand why Venus and Earth are so different.
Why Earth developed plate tectonics and Venus didn't.
But we do know that Venus's outer crust is much thinner.
The planets Venus and Earth are roughly the same size,
they probably started life with about the same amount of internal
heat, but it's how the heat escapes that makes all the difference.
So, here on Earth,
it escapes mainly at those boundaries between the plates.
But Venus has a much softer and thinner crust, a lithosphere,
than Earth, and so the heat can escape anywhere.
And that's why you see this surface covered in a plethora of volcanoes.
With less of a barrier, Venus's inner heat has built
vast lava flows that run for thousands of kilometres.
And we now think that at least one of its volcanoes,
and we suspect many more, remain active to this day.
But only further missions will reveal just how alive
volcanoes on our sister planet really are.
Our exploration of the solar system has shown us that there's
active geology in the strangest and most unexpected of places.
The ice fountains of Enceladus, the Galilean moons of Jupiter,
even the frozen outer moon of the solar system, Triton.
But amongst all those geologically active worlds
scattered across the solar system, it still remains the case
that there's only one place where we know for certain that the
active geology became biology, and that's here, on Earth.
And if that really is the case, if we're alone here on Earth,
then I think that raises a deep and very profound question.
It's why? What is so special, possibly, about this place?
Wonderfully, at least part of the answer appears to be
a consequence of plate tectonics.
Volcanoes, when they erupt,
emit huge amounts of greenhouse gases, like carbon dioxide.
And as we all know, greenhouse gases heat a planet up.
Now, Earth has a natural regulatory system.
When it rains, the carbon dioxide is dissolved in the water
and falls on the ground and the carbon dioxide reacts with
the rock of the mountains to form minerals.
Then, plate tectonics can take those rocks and send them
back down into the Earth.
So there's a cycle from volcano to atmosphere to land
and back into the interior of the planet.
Over geological time, this wonderful relationship between volcanoes,
plate tectonics and our atmosphere has kept Earth's climate in check.
And that stability has helped sustain an unbroken
chain of life that stretches back almost four billion years.
It's only here on Earth that a range of geological processes,
from volcanoes, to plate tectonics, and hydrothermal vents,
have conspired together to produce an environment that not only
allowed life to begin, but also was stable enough to allow life to
flourish, from the simplest living organisms to the endless forms
most beautiful that we see covering the surface of the Earth today.
The question is, how special is Earth?
Well, I think the answer might be found in this giant laboratory,
the solar system, in exploring the eclectic
and diverse collection of worlds that we find orbiting the sun.
Current velocity is 145 metres per second, at an altitude
of about 9.5km above the surface.
In February 2021,
an astonishing new piece of hardware arrived on the surface of Mars.
Perseverance is looking for evidence of ancient life, which may have
started on the planet, thanks in part to its giant volcanoes.
Volcanism played such an important role in the history of our planet,
but also in the origin of life and evolution of life.
Mars is like Earth's cousin.
Very early in their history, they had these volcanic activities.
We found evidence that Mars had liquid water on its surface,
it had a thicker atmosphere.
So, at that time, when life was emerging on Earth, Mars also
was creating similar environments.
So it's possible that there was
the potential for life on Mars.
Mars's volcanism faded away, and so did the water on its surface
and the chance for life to flourish on the Red Planet.
But if life did at least get started, crucial evidence
could be locked in the Martian rocks, waiting to be discovered.
Perseverance, or, as team members called it, Percy, went to Mars,
to a crater known as Jezero, which used to be an ancient lake.
And so Percy is looking for evidence
about the habitability of this environment.
We're looking for signatures that there was life on the planet,
but it would be absolutely amazing if we actually found cells,
or something similar, in these rocks
that indicated that there is life on Mars today.
As it makes its way across the dry lakebed, Perseverance
leaves behind a series of small, carefully sealed rock samples.
The plan is to analyse these in a lab here on Earth.
But right now, they're stuck on the surface of Mars.
Retrieving our samples from Mars is not going to be an easy task.
First we have to land on the surface,
then we have to pick the samples up,
make sure they're packed into the spacecraft...
..and make sure that the spacecraft gets back to Earth.
So there's quite a bit of coordination that has to be done.
The schedule is still uncertain.
But Nasa's hope is to return the cannisters back to Earth...
..in the mid-2030s.
It's exciting to me, because I study these rocks,
and so this would be a unique opportunity to have samples
directly collected from the surface that I could analyse.
Being able to have samples from a planet is
so much better than just having to look at a planet through
a telescope or through data sent back by a spacecraft.
So, regardless of all the effort it's going to take to get
the samples back from Mars, it's definitely going to be worth it.
At that point in time, we'll have a piece of Mars in our hands.
Next time, we venture to the hidden realms of our solar system.
The dark worlds...
..where mysteries lurk in the shadows...
..and a distant hinterland sends unexpected visitors
hurtling towards Earth.