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Lift-off of Messenger
on NASA's mission to Mercury.

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00:01:09,880 --> 00:01:11,234
Far, far away...

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00:01:16,640 --> 00:01:18,757
...beyond Mars...

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00:01:21,400 --> 00:01:25,599
...past the storms of Jupiter
and the rings of Saturn...

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...conditions become very different.

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Temperatures plummet...

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...and distances between worlds

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are measured not in millions
but in billions of kilometres.

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Here lie the most mysterious planets
of them all.

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Uranus.

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A pale blue marble hanging
in the dark, frozen depths of space.

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00:02:03,560 --> 00:02:05,074
And further out...

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...the solar system's final true planet.

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Neptune.

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Beyond, we thought we'd only ever find
tiny, lifeless worlds...

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...frozen to the core.

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How wrong we were.

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We live at a time

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when our civilisation
has unprecedented reach.

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And we have a near permanent presence
at Mars, both in orbit and on the surface,

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and we've had spacecraft orbiting
Mercury, Venus, Jupiter

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and Saturn for extended periods.

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But once we get beyond Saturn,
things become much more difficult.

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The distances are vast.

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Travel times are measured
not in months but in years, decades even.

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And that means that we've only visited
the outer planets once.

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Each visit is a voyage into the unknown

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as we strive to reach
the most distant planetary systems

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ever visited.

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And when we do get there,
our spacecraft are travelling so fast

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that they fly straight through,
only spending a few hours in the system.

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But those few hours of precious data

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have allowed us to begin to understand
the mysteries that lie there,

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on the outer edges of the solar system.

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We have ignition
and we have lift-off.

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Reports coming back indicate
those twin large solid motors

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are functioning perfectly.

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Our first mission into the dark
was only possible

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thanks to some help
from the planets themselves.

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Each planet orbits at a different speed.

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And for most of the time,

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they're scattered around the Sun.

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But once every 175 years...

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...something rather wonderful happens.

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The outer planets align...

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...creating a path...

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...that leads from Earth,
past Jupiter and Saturn...

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...all the way to Uranus and Neptune.

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Voyager 2 had been launched
at precisely the right moment

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to take advantage
of this rare planetary alignment.

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It arrived at Jupiter
in a little under two years.

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Two and a half times more massive
than all the other planets combined...

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...Jupiter gave Voyager
a gravitational kick.

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Another two years on,

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and Voyager witnessed
one of the most beautiful sights

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in the solar system.

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The icy rings of another gas giant.

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Saturn.

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And then, on and on...

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00:07:07,680 --> 00:07:09,592
...further into the darkness.

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Almost nine years after leaving Earth,

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Voyager approached
an entirely new class of planet.

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Just like Jupiter and Saturn,

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the planet's upper atmosphere

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is composed mostly
of swirling hydrogen and helium gas.

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00:08:01,720 --> 00:08:02,995
And hidden beneath

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lies an exotic icy mix
of methane, ammonia and water.

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00:08:18,080 --> 00:08:23,075
But unlike the other gas giants,
Uranus is almost featureless.

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00:08:26,960 --> 00:08:29,714
For all the time
that Voyager stared at the planet,

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it saw just ten cloud formations.

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And we soon discovered why.

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Uranus, at minus 224 degrees Celsius,

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is the coldest planet in the solar system.

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The first of the ice giants,

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in a permanent state of deep freeze.

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Voyager spent just six hours with Uranus.

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And as its gaze widened,
it took in the entire system.

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Just like Saturn, Uranus has rings.

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The rings are so dark, so faint

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that they're very difficult to see
from Earth.

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In fact,
we didn't discover them until 1977.

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They must be made
of some kind of material

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that doesn't reflect
a lot of sunlight back.

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And we don't know what that is.

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It can't be water ice,
like the rings of Saturn,

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because they would be much brighter.

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They're also extremely delicate and thin.

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And again, that's a great mystery
cos you'd expect the particles to collide

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and the rings to spread out over time -
that doesn't happen.

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So something must be holding those rings
in place.

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And the answer can be seen
in this photograph taken by Voyager 2.

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You can see a bright thin ring,
which is known as the Epsilon Ring,

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and above it and below it,
you can see two moons.

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The inner one is called Cordelia,
and the outer one is called Ophelia.

95
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Now, the thing you need to know
about orbits -

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a moon or a ring particle
that's orbiting closer to a planet

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is moving more slowly

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than a moon or a ring particle
that's orbiting further away.

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00:11:09,120 --> 00:11:11,715
And so, if there's a particle
in the Epsilon Ring

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that maybe has a collision,
loses a bit of energy

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and starts to drop down,

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then Cordelia tends to accelerate that
particle back up into the ring again,

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to speed it up
by its gravitational influence.

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Whereas, if there's a particle
on the outer edge of the Epsilon Ring

105
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and it gets a bit more energy
and raises up in altitude,

106
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then Ophelia tends to slow it down,

107
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and so it falls back into the ring again.

108
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And by that mechanism,
these moons keep the ring

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nice and neat around Uranus.

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And for that reason,

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these moons have become known
as shepherd moons.

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Now, we haven't discovered
any more shepherd moons.

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I mean, Voyager was only in the system
for a few hours.

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But we assume that they're there,

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orbiting around Uranus,

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keeping the other rings
precisely in place.

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The missing moons
are not the only mysterious thing

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about the planet.

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Uranus orbits the Sun in a unique way.

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A consequence of events
that happened long ago.

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Uranus was born
from the vast cloud of material

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that surrounded the young Sun.

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Over time, this material drew together

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under the influence of gravity...

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...forming each of the planets...

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...all orbiting
in the same anticlockwise direction

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as the primordial cloud of gas and dust.

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This rotation remains to this day.

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Almost all the planets spin on their axes

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in the same anticlockwise direction.

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But for reasons
we don't yet fully understand,

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Venus and Uranus spin on their axes
in the opposite direction.

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And Uranus is even more curious.

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The entire planet is on its side.

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00:14:28,560 --> 00:14:33,237
It's not known for certain
why Uranus spins in this way,

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but it seems likely that, at some point,
probably in the distant past,

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it was hit by another planet,

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possibly the size of Earth,
or even larger,

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which knocked the planet over.

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And indeed,
modern computer simulations suggest that,

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when you do that to a planet,

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all the moons follow,

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and you end up with a planet
and its system of moons,

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and now its rings,

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sort of corkscrewing around the Sun
on its side.

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Voyager had taken us

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to the most distant planet
we had ever visited.

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And now, an almost unthinkable distance

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of 1.6 billion kilometres
of icy cold space

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lay between the spacecraft
and its next encounter.

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00:16:36,440 --> 00:16:39,797
The size and scale of the solar system
is impossible to imagine.

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But if we scale everything down,
then it becomes easier to visualise.

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The Sun is 1.4 million kilometres
in diameter.

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00:16:51,160 --> 00:16:55,154
But scale that down
by a factor of around 600 million,

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and it becomes the size of that light.

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And the Earth, Mars and the rocky planets

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are about that big on this scale.

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00:17:06,400 --> 00:17:10,519
And the distances are, I think,
even more remarkable.

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So Mercury,
the closest planet to the Sun,

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would be all the way over there
by the sea.

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00:17:25,360 --> 00:17:29,274
So this would be where Mercury orbits.

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00:17:29,400 --> 00:17:32,996
And Mercury is 96 metres away.

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It's about just under a centimetre
in diameter.

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And the next planet, Venus,
is somewhere over there.

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We'd fly past Venus
about 180 metres away from the Sun...

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...and then head onwards and outwards
towards the Earth.

167
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Here, at the Earth's orbit,

168
00:18:08,040 --> 00:18:12,432
the true scale of our fragility
and isolation becomes apparent.

169
00:18:15,520 --> 00:18:18,479
The Sun, there, 250 metres away,

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00:18:18,600 --> 00:18:22,594
with the twinkling stars
of the galaxy beyond.

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00:18:22,720 --> 00:18:26,680
And here's the Earth -
two centimetres across.

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On that pebble,
all of human history played out.

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00:18:39,720 --> 00:18:42,872
At around 380 metres, we pass by Mars,

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the last of the rocky planets,

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and then onwards, into the darkness.

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Crossing the harbour takes us
to the fringes of the outer solar system.

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00:19:18,040 --> 00:19:23,069
And now the distances and the sizes
are beginning to get really big.

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Jupiter, the largest planet
in the solar system,

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is over ten times the diameter
of our Earth.

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00:19:33,720 --> 00:19:39,432
And, I mean, there's the Sun,
1.3 kilometres across the harbour.

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At almost twice the distance
from the Sun to Jupiter,

182
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we reach Saturn...

183
00:20:04,880 --> 00:20:08,635
...2.4 kilometres on our scale.

184
00:20:09,720 --> 00:20:13,555
And we really are now entering
the twilight of the solar system.

185
00:20:13,680 --> 00:20:18,357
Light levels are like early morning
or late evening here on Earth.

186
00:20:18,480 --> 00:20:21,279
But we can still see Saturn
in the night sky

187
00:20:21,400 --> 00:20:25,155
because the planet, and its rings,
are highly reflective.

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As far again as all the rocky planets,

189
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and Jupiter and Saturn,

190
00:20:36,440 --> 00:20:38,113
we reach Uranus.

191
00:20:44,320 --> 00:20:46,755
Three billion kilometres from the Sun,

192
00:20:46,880 --> 00:20:51,352
and we begin to fully appreciate
the distances Voyager had travelled...

193
00:20:56,560 --> 00:20:59,473
...on its journey to the farthest reaches
of the solar system.

194
00:21:14,840 --> 00:21:17,480
Journey's end - Neptune,

195
00:21:17,600 --> 00:21:21,435
a ball of gas
about seven and a half centimetres across,

196
00:21:21,560 --> 00:21:25,315
7.4 kilometres away from the Sun,

197
00:21:25,440 --> 00:21:26,669
on this scale.

198
00:21:33,040 --> 00:21:38,354
And <i>we</i> are still down there,
on the island, in Reykjavik Harbour.

199
00:21:38,480 --> 00:21:44,272
Little microscopic ants crawling
around on the surface of a pebble.

200
00:22:09,080 --> 00:22:10,912
It had taken 12 years,

201
00:22:11,040 --> 00:22:15,398
but Voyager had finally crossed
the great expanse of space

202
00:22:15,520 --> 00:22:18,911
between Earth
and the most distant planetary system.

203
00:22:33,920 --> 00:22:34,920
Neptune.

204
00:22:38,720 --> 00:22:41,440
17 times the mass of the Earth

205
00:22:41,560 --> 00:22:44,871
and even more massive than Uranus.

206
00:23:02,600 --> 00:23:05,559
But in contrast to its sister planet,

207
00:23:05,680 --> 00:23:08,878
Neptune's atmosphere
is bursting with activity.

208
00:23:23,160 --> 00:23:27,074
What we found was a planet
of extreme weather,

209
00:23:27,200 --> 00:23:31,911
where high-altitude winds
whip white methane clouds around

210
00:23:32,040 --> 00:23:35,158
at speeds
of over 2,000 kilometres per hour.

211
00:23:35,280 --> 00:23:38,398
Those are the highest wind speeds
anywhere in the solar system.

212
00:23:52,200 --> 00:23:57,320
And Voyager saw a great dark spot,
not unlike Jupiter's Great Red Spot.

213
00:23:59,280 --> 00:24:01,954
A storm system the size of Earth that,

214
00:24:02,080 --> 00:24:05,152
when we looked
only four or five years later,

215
00:24:05,280 --> 00:24:07,078
we found had vanished.

216
00:24:11,080 --> 00:24:12,230
But over the years,

217
00:24:12,360 --> 00:24:15,398
we've observed more storms
raging over the planet.

218
00:24:22,240 --> 00:24:25,358
It's one of the great mysteries
of planetary exploration -

219
00:24:25,480 --> 00:24:28,154
why a planet so far from the Sun,

220
00:24:28,280 --> 00:24:31,990
with so little energy falling
into its atmosphere from sunlight,

221
00:24:32,120 --> 00:24:35,079
can have the most extreme winds
in the solar system.

222
00:24:36,960 --> 00:24:39,919
And Voyager made
yet another puzzling discovery.

223
00:24:43,960 --> 00:24:48,193
Although further from the Sun,
the planet is warmer than Uranus.

224
00:24:52,920 --> 00:24:56,118
The source of this extra heat
remains a mystery.

225
00:25:00,040 --> 00:25:01,269
But for some reason,

226
00:25:01,400 --> 00:25:06,191
Neptune emits over two and a half times
the heat it receives from the Sun.

227
00:25:10,600 --> 00:25:15,072
And this internal heat
helps to explain the ferocious storms.

228
00:25:19,160 --> 00:25:21,755
As the heat makes its way
from the core of the planet

229
00:25:21,880 --> 00:25:23,758
and out into space...

230
00:25:27,840 --> 00:25:30,594
...it churns up the entire atmosphere...

231
00:25:32,400 --> 00:25:37,759
...creating winds unlike anything
seen elsewhere in the solar system.

232
00:25:42,320 --> 00:25:46,075
The probable reason
that wind speeds can be so high

233
00:25:46,200 --> 00:25:49,876
is because there's no solid surface
on a planet like Neptune.

234
00:25:50,000 --> 00:25:53,630
It's exotic liquids further down
and gases further up,

235
00:25:53,760 --> 00:25:55,797
but there's no rocky surface.

236
00:25:55,920 --> 00:26:02,554
There are no mountains and continents
to break up the flow of atmospheric gases.

237
00:26:02,680 --> 00:26:08,278
And so, the winds can just whip
around the planet at supersonic speeds.

238
00:26:24,920 --> 00:26:29,597
Voyager 2 had almost completed
its grand tour of the solar system.

239
00:26:46,120 --> 00:26:48,555
But before it began
its lonely journey out,

240
00:26:48,680 --> 00:26:50,353
into interstellar space...

241
00:26:54,760 --> 00:26:57,195
...it had one last world to visit.

242
00:27:07,160 --> 00:27:08,913
Triton.

243
00:27:09,040 --> 00:27:12,716
A vast moon covered
in a sheen of frozen nitrogen.

244
00:27:15,320 --> 00:27:19,109
We expected it
to be a silent, still world,

245
00:27:19,240 --> 00:27:23,439
but Voyager was in for one last surprise.

246
00:27:57,080 --> 00:27:59,595
But when Voyager arrived,

247
00:27:59,720 --> 00:28:04,158
we saw geological activity
on that frozen world.

248
00:28:04,280 --> 00:28:07,352
We saw geysers erupting up into space,

249
00:28:07,480 --> 00:28:09,756
eight kilometres high,

250
00:28:09,880 --> 00:28:14,830
and then carrying dark material
a hundred kilometres downwind.

251
00:28:23,120 --> 00:28:27,478
Now, the key to understanding
what process caused those eruptions

252
00:28:27,600 --> 00:28:30,877
was that we noticed
that the geysers tended to erupt

253
00:28:31,000 --> 00:28:34,596
on a place on Triton's surface
below the faint Sun,

254
00:28:34,720 --> 00:28:37,758
even though it's so many billions
of miles away.

255
00:28:37,880 --> 00:28:39,075
What's happening is

256
00:28:39,200 --> 00:28:43,797
the sunlight is falling
on the thin layer of nitrogen ice,

257
00:28:43,920 --> 00:28:45,274
going through,

258
00:28:45,400 --> 00:28:50,350
then heating up a layer of darker
particles a metre below the surface.

259
00:28:53,600 --> 00:28:56,240
A difference of just four degrees Celsius

260
00:28:56,360 --> 00:28:58,750
is enough for the heat to radiate up...

261
00:29:00,240 --> 00:29:04,439
...and vaporise the frozen nitrogen,
creating gas.

262
00:29:08,360 --> 00:29:09,999
That gas is under pressure,

263
00:29:10,120 --> 00:29:12,840
and ultimately
it punched through the nitrogen ice,

264
00:29:12,960 --> 00:29:18,513
carrying those dark particles up
and making the geysers of Triton.

265
00:29:23,200 --> 00:29:25,271
In the furthest reaches
of the solar system,

266
00:29:25,400 --> 00:29:28,359
the faint light of the Sun is still enough

267
00:29:28,480 --> 00:29:32,156
to power the most distant
of geological wonders.

268
00:29:36,280 --> 00:29:41,355
But this can't explain
the rest of Triton's mysterious features.

269
00:29:50,920 --> 00:29:55,039
Terrain on this scale
could only be created

270
00:29:55,160 --> 00:29:56,958
by a much more powerful force.

271
00:30:01,640 --> 00:30:06,237
And a clue seems to be lurking
in Triton's odd orbit.

272
00:30:13,320 --> 00:30:15,915
Unlike every other large moon
in the solar system,

273
00:30:16,040 --> 00:30:21,115
Triton orbits in the opposite direction
to the spin of the planet.

274
00:30:21,240 --> 00:30:23,197
That means that it's highly unlikely

275
00:30:23,320 --> 00:30:26,154
that Triton and Neptune
formed at the same time.

276
00:30:26,280 --> 00:30:28,033
If you formed a planet and a moon

277
00:30:28,160 --> 00:30:31,392
out of the same collapsing cloud
of gas and dust,

278
00:30:31,520 --> 00:30:35,150
then they tend to spin and orbit
in the same direction

279
00:30:35,280 --> 00:30:37,556
as the spin of the initial dust cloud.

280
00:30:37,680 --> 00:30:40,070
So the most likely explanation to that

281
00:30:40,200 --> 00:30:43,989
is that Triton is a visitor
to the Neptunian system

282
00:30:44,120 --> 00:30:46,919
that, unlike Voyager, never left.

283
00:30:57,080 --> 00:31:03,190
One theory is that billions of years ago
Triton was not a moon at all.

284
00:31:08,200 --> 00:31:11,955
It grew up in a region of space
way beyond Neptune.

285
00:31:15,760 --> 00:31:17,160
The Kuiper Belt.

286
00:31:23,600 --> 00:31:27,594
Here, trillions of frozen lumps of water,

287
00:31:27,720 --> 00:31:30,872
ammonia and methane circle the Sun.

288
00:31:34,760 --> 00:31:38,071
The frozen leftovers from the formation
of the solar system.

289
00:31:47,520 --> 00:31:51,275
Perhaps Triton ended up
on the inner edge of this region...

290
00:31:57,760 --> 00:32:02,437
...and ventured close enough to Neptune
to be drawn in by its gravity...

291
00:32:06,680 --> 00:32:11,232
...until, eventually,
it was plucked from the Kuiper Belt

292
00:32:11,360 --> 00:32:15,559
and forever trapped in orbit
around the distant blue planet.

293
00:32:25,400 --> 00:32:28,791
Today, Triton orbits
in a nice, regular circle around Neptune.

294
00:32:28,920 --> 00:32:31,799
But it wouldn't have started out that way.

295
00:32:31,920 --> 00:32:34,879
When it was first captured,
it would have glanced past the planet

296
00:32:35,000 --> 00:32:37,720
and ended up in a wide elliptical orbit,

297
00:32:37,840 --> 00:32:41,754
sometimes being far away from the planet
and sometimes close.

298
00:32:41,880 --> 00:32:45,032
That means that,
as Triton orbited around Neptune,

299
00:32:45,160 --> 00:32:48,119
the gravitational pull
was constantly changing.

300
00:32:48,240 --> 00:32:50,914
And that stretches
and squashes the moon,

301
00:32:51,040 --> 00:32:53,600
and heats up the interior, by friction.

302
00:32:53,720 --> 00:32:55,916
It's a process called tidal heating.

303
00:32:59,160 --> 00:33:03,074
The molten interior would have exploded up
through cracks and faults

304
00:33:03,200 --> 00:33:04,475
in Triton's crust...

305
00:33:06,320 --> 00:33:09,119
...creating the ragged surface
we see today.

306
00:33:11,400 --> 00:33:15,519
And what you're left with
is a cooling moon orbiting in a circle,

307
00:33:15,640 --> 00:33:18,200
the wrong way around the planet,

308
00:33:18,320 --> 00:33:21,950
the dramas of its past hidden from view.

309
00:33:22,080 --> 00:33:24,151
All that remains are the geysers,

310
00:33:24,280 --> 00:33:26,715
powered by the faint light of the Sun,

311
00:33:26,840 --> 00:33:30,754
painting dark streaks
across the now quiescent surface.

312
00:33:42,800 --> 00:33:45,838
Voyager had fulfilled its mission

313
00:33:45,960 --> 00:33:49,158
and now headed onwards,
out of the solar system...

314
00:33:52,080 --> 00:33:54,834
...where it would never encounter
another planet.

315
00:34:05,640 --> 00:34:09,520
But elsewhere,
another world awaited our arrival...

316
00:34:12,280 --> 00:34:14,351
...far out in the darkness.

317
00:34:18,760 --> 00:34:22,640
We have ignition and lift-off
of NASA's New Horizons spacecraft

318
00:34:22,760 --> 00:34:28,040
on a decade-long voyage to visit
the planet Pluto, and then beyond.

319
00:34:45,360 --> 00:34:48,034
The New Horizons spacecraft,

320
00:34:48,160 --> 00:34:50,994
on its way to the very edge
of our solar system.

321
00:35:02,120 --> 00:35:07,070
Our ambition to learn more
about a small, intriguing world

322
00:35:07,200 --> 00:35:10,637
depended on the success of New Horizons.

323
00:35:17,600 --> 00:35:20,399
As the tiny probe,
less than three metres in length,

324
00:35:20,520 --> 00:35:22,751
was sent out into the abyss...

325
00:35:28,920 --> 00:35:32,709
...most of its electronics
and all non-essential systems

326
00:35:32,840 --> 00:35:34,991
were shut down to save power.

327
00:35:37,800 --> 00:35:43,876
It would hibernate for almost
its entire 4.8 billion kilometre trek.

328
00:35:53,560 --> 00:35:57,110
Whilst New Horizons continued
its voyage to Pluto,

329
00:35:57,240 --> 00:36:02,872
a controversy arose here on Earth
about the definition of Pluto itself.

330
00:36:03,000 --> 00:36:08,314
See, we always knew that Pluto is small,
smaller than Neptune's moon Triton,

331
00:36:08,440 --> 00:36:12,036
but it was discoveries made
in the vicinity of Pluto

332
00:36:12,160 --> 00:36:13,992
by the Hubble Space Telescope

333
00:36:14,120 --> 00:36:18,160
that really challenged
its status as a planet.

334
00:36:22,000 --> 00:36:26,870
From Earth, Pluto, far away
on the inner edge of the Kuiper Belt,

335
00:36:27,000 --> 00:36:29,754
is so distant that, even with Hubble,

336
00:36:29,880 --> 00:36:33,112
it appears as nothing more
than a fuzzy image.

337
00:36:36,280 --> 00:36:38,511
But Pluto was not alone.

338
00:36:40,960 --> 00:36:43,759
We began to detect other distant objects.

339
00:36:56,160 --> 00:36:59,631
Then, Hubble discovered that another world

340
00:36:59,760 --> 00:37:02,036
is almost the same size as Pluto.

341
00:37:10,600 --> 00:37:14,560
The discovery of these new worlds
has forced us to reconsider

342
00:37:14,680 --> 00:37:17,320
what we mean by the word planet.

343
00:37:20,840 --> 00:37:26,359
The International Astronomical Union
has a three-part definition of a planet.

344
00:37:26,480 --> 00:37:28,437
Firstly, it has to orbit the Sun,

345
00:37:28,560 --> 00:37:30,040
and Pluto certainly does that.

346
00:37:30,160 --> 00:37:32,880
It goes round once every 248 years.

347
00:37:33,000 --> 00:37:35,117
Secondly, it has to be massive enough

348
00:37:35,240 --> 00:37:39,200
for gravity to sculpt it
almost into a sphere,

349
00:37:39,320 --> 00:37:42,518
and Pluto indeed is almost spherical.

350
00:37:42,640 --> 00:37:47,271
But thirdly, it has to clear its own orbit
around the Sun.

351
00:37:47,400 --> 00:37:50,791
And that is essentially
a mass constraint as well.

352
00:37:50,920 --> 00:37:56,314
It has to have enough gravity
to throw things out from its orbital path.

353
00:37:56,440 --> 00:37:59,080
And that's where Pluto falls down.

354
00:37:59,200 --> 00:38:01,715
Now, why that extra idea?

355
00:38:01,840 --> 00:38:06,517
Well, ultimately, it's because
if Pluto is big enough to be a planet,

356
00:38:06,640 --> 00:38:08,950
then Eris is big enough to be a planet

357
00:38:09,080 --> 00:38:11,720
and Makemake is big enough
to be a planet.

358
00:38:11,840 --> 00:38:15,959
And tens, or even potentially hundreds,
of objects out there

359
00:38:16,080 --> 00:38:18,276
are big enough to be planets.

360
00:38:18,400 --> 00:38:21,040
Now, does it really matter?

361
00:38:21,160 --> 00:38:22,958
Well, I don't think so.

362
00:38:23,080 --> 00:38:25,754
Ultimately, Pluto is a world,

363
00:38:25,880 --> 00:38:28,839
and therefore we should explore it.

364
00:38:42,440 --> 00:38:46,957
And in July 2015,
that's exactly what we did.

365
00:38:53,120 --> 00:38:57,319
After nine years of flight,
New Horizons awoke.

366
00:38:59,200 --> 00:39:03,797
Copy that. We are in lock
with telemetry with the spacecraft.

367
00:39:36,520 --> 00:39:42,756
We had our first glimpse
of the most distant world ever visited.

368
00:40:29,960 --> 00:40:32,429
Pluto is beautiful.

369
00:40:32,560 --> 00:40:36,270
Far from being
a frigid, featureless world,

370
00:40:36,400 --> 00:40:38,835
it turns out
that it has all the characteristics

371
00:40:38,960 --> 00:40:41,156
of a dynamic, living planet.

372
00:40:41,280 --> 00:40:43,636
And, yes, it is cold on the surface,

373
00:40:43,760 --> 00:40:46,229
about minus 230 degrees Celsius -

374
00:40:46,360 --> 00:40:49,751
I'd be walking over solid nitrogen -

375
00:40:49,880 --> 00:40:52,520
but the blotches that could be made out

376
00:40:52,640 --> 00:40:55,553
in the grainy Hubble Space Telescope
images

377
00:40:55,680 --> 00:41:00,755
turned out to be a tremendous variety
of geological form and structure.

378
00:41:06,560 --> 00:41:09,712
But by far the most recognisable feature

379
00:41:09,840 --> 00:41:12,275
is the region known as Tombaugh Regio,

380
00:41:12,400 --> 00:41:16,474
or to give it its more popular name,
Pluto's Heart.

381
00:41:21,560 --> 00:41:25,520
The western lobe of the Heart
is called Sputnik Planitia.

382
00:41:30,720 --> 00:41:35,715
A giant plain of frozen nitrogen,
methane and carbon monoxide...

383
00:41:38,880 --> 00:41:41,679
...that stretches
for a million square kilometres.

384
00:41:46,760 --> 00:41:52,757
And at its edge lies a range of mountains
made of pure frozen water ice...

385
00:41:54,440 --> 00:41:57,877
...that rise up to six kilometres
above the plain.

386
00:42:03,160 --> 00:42:06,676
But there's something very strange
about the region,

387
00:42:06,800 --> 00:42:11,113
something that sets it apart
from the rest of this dwarf planet.

388
00:42:15,560 --> 00:42:18,439
The surface of Pluto is covered
in craters -

389
00:42:18,560 --> 00:42:22,395
the scars of impacts that have taken place
over many billions of years,

390
00:42:22,520 --> 00:42:24,398
just like the surface of our moon -

391
00:42:24,520 --> 00:42:27,797
except if you look on Sputnik Planitia.

392
00:42:27,920 --> 00:42:30,071
It is absolutely smooth.

393
00:42:30,200 --> 00:42:34,638
There are no craters there at all,
not a single one.

394
00:42:36,440 --> 00:42:39,433
From space,
the lack of craters is striking.

395
00:42:41,320 --> 00:42:45,075
And the closer you look,
the stranger the mystery gets.

396
00:42:54,240 --> 00:42:57,119
Detailed imagery beamed back
by New Horizons

397
00:42:57,240 --> 00:43:00,711
revealed a network of hexagon
and pentagon shapes...

398
00:43:06,680 --> 00:43:09,400
...that crisscross
the frozen nitrogen surface.

399
00:43:20,720 --> 00:43:25,158
A clue to what might be happening
can be seen in these images.

400
00:43:25,280 --> 00:43:29,559
And those kind of patterns
are found elsewhere in nature.

401
00:43:29,680 --> 00:43:32,559
Here, for example,
on the surface of the Sun.

402
00:43:34,520 --> 00:43:38,594
Or here, in a liquid
that's been heated from below.

403
00:43:38,720 --> 00:43:42,600
These patterns
are characteristic of convection.

404
00:43:42,720 --> 00:43:46,396
There's a heat source,
which is causing material to rise up,

405
00:43:46,520 --> 00:43:48,989
and then it cools and falls back again.

406
00:43:49,120 --> 00:43:52,192
And in those circulating
convection currents,

407
00:43:52,320 --> 00:43:54,118
you tend to get patterns like this.

408
00:43:57,000 --> 00:44:00,550
So what could be happening
below the surface of Sputnik Planitia

409
00:44:00,680 --> 00:44:02,797
is that there is a heat source deep down

410
00:44:02,920 --> 00:44:05,276
which is melting the nitrogen ice

411
00:44:05,400 --> 00:44:08,791
and causing it to rise, cool
and then fall again.

412
00:44:08,920 --> 00:44:13,039
And those convection currents
are constantly resurfacing the area.

413
00:44:18,000 --> 00:44:22,756
That a small world like Pluto
is still active was a huge surprise.

414
00:44:27,080 --> 00:44:29,595
Our best theory of how this could be

415
00:44:29,720 --> 00:44:32,030
is that somewhere deep in its interior...

416
00:44:34,360 --> 00:44:38,912
...there are radioactive elements
that generate heat as they decay.

417
00:44:43,080 --> 00:44:47,074
This heat warms
a sunless, half-frozen ocean of water

418
00:44:47,200 --> 00:44:49,840
that has existed for billions of years

419
00:44:49,960 --> 00:44:51,679
beneath the surface of Pluto.

420
00:45:02,320 --> 00:45:06,997
Now, the evidence from Sputnik Planitia
that there's still an internal heat source

421
00:45:07,120 --> 00:45:10,750
and studies of other geological features
on Pluto

422
00:45:10,880 --> 00:45:13,031
have led some scientists to suggest

423
00:45:13,160 --> 00:45:16,073
that that ocean of water
might still be there.

424
00:45:20,760 --> 00:45:23,320
And the relatively warm ocean

425
00:45:23,440 --> 00:45:26,751
could explain the lack of craters
on Sputnik Planitia.

426
00:45:30,920 --> 00:45:34,357
The entire area
is constantly being repaved,

427
00:45:34,480 --> 00:45:38,474
as the nitrogen surface
is slowly turned over.

428
00:45:49,000 --> 00:45:53,153
But why all this activity here
and nowhere else?

429
00:45:57,120 --> 00:46:00,511
Perhaps Pluto's Heart
is the site of a huge impact...

430
00:46:04,040 --> 00:46:07,112
...that long ago punched a large hole
in the surface,

431
00:46:07,240 --> 00:46:10,597
almost down to the vast ocean beneath.

432
00:46:13,200 --> 00:46:17,831
A hole that slowly filled
with soft nitrogen ice...

433
00:46:20,160 --> 00:46:24,279
...that now gently churns
just above a warmer ocean.

434
00:46:26,840 --> 00:46:31,915
Imagine that, an ocean,
billions of miles away from the Sun.

435
00:46:32,040 --> 00:46:34,919
On the frozen frontier
of the solar system,

436
00:46:35,040 --> 00:46:37,430
on this most surprising of worlds,

437
00:46:37,560 --> 00:46:40,951
there may still be an ocean of water.

438
00:46:51,040 --> 00:46:55,557
New Horizons' closest encounter
with Pluto lasted just hours...

439
00:47:03,920 --> 00:47:06,754
...during which only one hemisphere
was visible...

440
00:47:08,960 --> 00:47:11,395
...leaving the other side a mystery.

441
00:47:30,440 --> 00:47:33,239
Pluto held a final surprise.

442
00:47:33,360 --> 00:47:37,877
As New Horizons turned its camera back
for one last look...

443
00:47:41,600 --> 00:47:46,629
...it captured an image
of Pluto's atmosphere glowing in the dark.

444
00:47:53,120 --> 00:47:57,353
A thin blue sky
over a hidden ocean of water...

445
00:47:58,800 --> 00:48:02,396
...4.8 billion kilometres from Earth.

446
00:48:29,480 --> 00:48:31,199
We've come a long way.

447
00:48:33,080 --> 00:48:36,152
After just half a century
of space exploration...

448
00:48:37,440 --> 00:48:41,992
...we've reached every one of the planets
in the solar system...

449
00:48:45,080 --> 00:48:48,073
...and begun to tell their story.

450
00:48:57,280 --> 00:49:01,194
What's emerging is a tale
of never-ending change...

451
00:49:03,480 --> 00:49:05,517
...of dramatic origins...

452
00:49:09,080 --> 00:49:11,959
...moments of hope...

453
00:49:16,800 --> 00:49:18,120
...and loss...

454
00:49:31,840 --> 00:49:35,550
...where just eight motes
of rock, ice and gas...

455
00:49:36,800 --> 00:49:40,077
...set against the dark backdrop of space

456
00:49:40,200 --> 00:49:46,197
have conspired together to produce life
on at least one world.

457
00:49:59,400 --> 00:50:02,279
"Why do we explore?" some people ask.

458
00:50:02,400 --> 00:50:05,598
"Shouldn't we deal with the problems
here on Earth before committing

459
00:50:05,720 --> 00:50:10,556
"time and energy and resources
to exploring the stars?"

460
00:50:10,680 --> 00:50:15,072
Well, I think focusing entirely
on our mote of dust

461
00:50:15,200 --> 00:50:17,999
would be a profound mistake.

462
00:50:18,120 --> 00:50:20,430
It would mean that we'd taken the decision

463
00:50:20,560 --> 00:50:24,110
to sit huddled in a tiny corner
of the solar system

464
00:50:24,240 --> 00:50:26,038
wondering what we're doing here.

465
00:50:28,360 --> 00:50:30,920
It would mean that we'd taken the decision

466
00:50:31,040 --> 00:50:35,034
to fight amongst ourselves
for ever more precious resources

467
00:50:35,160 --> 00:50:39,359
confined below a thin shell of air
on a small rock

468
00:50:39,480 --> 00:50:41,836
rather than following
the three-dimensional path

469
00:50:41,960 --> 00:50:44,759
marked out by the lights in the night.

470
00:50:54,400 --> 00:50:57,677
We live in a solar system of wonders,

471
00:50:57,800 --> 00:50:59,837
of planets of storms and moons of ice,

472
00:50:59,960 --> 00:51:03,317
and landscapes and vistas
that stir the imagination

473
00:51:03,440 --> 00:51:05,432
and enrich the soul -

474
00:51:05,560 --> 00:51:08,029
a system of limitless resources,

475
00:51:08,160 --> 00:51:11,551
limitless beauty and limitless potential.

476
00:51:13,400 --> 00:51:17,110
A system
that we've only just begun to explore...

477
00:51:19,800 --> 00:51:24,397
...in a journey
that has already rewarded us with so much.

478
00:51:49,920 --> 00:51:54,312
Pluto was the most distant world
that NASA had ever planned to visit.

479
00:51:55,360 --> 00:51:59,320
Ten, nine, eight, seven...

480
00:51:59,440 --> 00:52:03,195
The launch
of New Horizons was incredibly dramatic.

481
00:52:05,200 --> 00:52:07,237
Ignition and lift-off of NASA's...

482
00:52:07,360 --> 00:52:10,159
And it just shoots up so fast.

483
00:52:10,280 --> 00:52:12,272
It's the fastest launch ever from Earth.

484
00:52:12,400 --> 00:52:14,119
About ten miles per second.

485
00:52:19,120 --> 00:52:22,352
In order to ensure
that it was going to last for the decade

486
00:52:22,480 --> 00:52:25,176
across space and actually work there,
they built in a lot of redundancy.

487
00:52:25,200 --> 00:52:26,680
There were sort of two of everything,

488
00:52:26,760 --> 00:52:30,117
computers and the guidance systems,

489
00:52:30,240 --> 00:52:32,232
and they figured out how to hibernate.

490
00:52:32,360 --> 00:52:34,591
The first spacecraft that hibernated.

491
00:52:36,480 --> 00:52:38,472
To put the spacecraft largely asleep,

492
00:52:38,600 --> 00:52:41,035
let the spacecraft just silently coast...

493
00:52:42,240 --> 00:52:44,960
...then we could extend
the life of the electronics.

494
00:52:47,200 --> 00:52:49,715
No spacecraft mission
had ever really used hibernation

495
00:52:49,840 --> 00:52:53,117
as a day-to-day way
to cross the solar system.

496
00:52:55,840 --> 00:52:57,354
As the spacecraft slept,

497
00:52:57,480 --> 00:53:00,473
it sped away from Earth
towards its destination...

498
00:53:01,640 --> 00:53:04,951
...to a world that had lain hidden
amongst the stars

499
00:53:05,080 --> 00:53:07,231
for almost all of human history.

500
00:53:08,960 --> 00:53:12,032
The discovery of Pluto was in 1930.

501
00:53:12,160 --> 00:53:13,640
Clyde Tombaugh,

502
00:53:13,760 --> 00:53:16,992
a Kansas farm boy
who was using the telescope

503
00:53:17,120 --> 00:53:20,272
at the Lowell Observatory in Arizona.

504
00:53:20,400 --> 00:53:22,915
Every clear night, Clyde would go out

505
00:53:23,040 --> 00:53:26,556
and take images of the sky
on photographic plates.

506
00:53:26,680 --> 00:53:28,478
And when it was cloudy or during the day,

507
00:53:28,600 --> 00:53:30,034
he would compare those plates.

508
00:53:30,160 --> 00:53:34,518
And Clyde Tombaugh
noticed that there was this little speck

509
00:53:34,640 --> 00:53:36,677
moving across the stars.

510
00:53:36,800 --> 00:53:40,919
And then, in 1930,
Clyde Tombaugh discovered Pluto.

511
00:53:42,240 --> 00:53:45,039
But Tombaugh
would never see Pluto up close.

512
00:53:47,680 --> 00:53:52,038
He died nine years
before the launch of New Horizons,

513
00:53:52,160 --> 00:53:53,992
at the age of 90.

514
00:53:55,160 --> 00:53:58,119
He had asked
if a mission ever did get launched,

515
00:53:58,240 --> 00:54:01,836
some of his ashes could be sent
on the journey.

516
00:54:01,960 --> 00:54:04,634
And to think that Clyde's ashes,
you know,

517
00:54:04,760 --> 00:54:07,400
flew by Pluto and, you know,

518
00:54:07,520 --> 00:54:12,311
he was finally visiting the place that
he discovered, that was pretty cool.

519
00:54:23,040 --> 00:54:26,875
No, I mean, Pluto was a shock
and a revelation in so many ways.

520
00:54:27,000 --> 00:54:28,957
Our jaws just dropped to the floor.

521
00:54:30,520 --> 00:54:32,432
This amazing world

522
00:54:32,560 --> 00:54:34,756
that we would never have known about

523
00:54:34,880 --> 00:54:36,553
unless we sent a spacecraft out there.

524
00:54:48,840 --> 00:54:52,436
But New Horizons didn't stop
after its flyby of Pluto.

525
00:54:54,520 --> 00:54:57,991
It continued on for three more years,

526
00:54:58,120 --> 00:54:59,998
ever deeper into the Kuiper Belt.

527
00:55:01,880 --> 00:55:04,161
The temperatures out there
are almost absolute zero,

528
00:55:04,200 --> 00:55:06,954
so everything is so well preserved.

529
00:55:08,480 --> 00:55:11,120
The Kuiper Belt
completely changed everything.

530
00:55:11,240 --> 00:55:12,913
It's an archaeological dig, if you will,

531
00:55:13,040 --> 00:55:15,874
into the early history
of our solar system,

532
00:55:16,000 --> 00:55:17,753
so it's a scientific wonderland.

533
00:55:20,120 --> 00:55:22,476
On New Year's Day, 2019,

534
00:55:22,600 --> 00:55:25,638
New Horizons had reached
the centre of the region...

535
00:55:28,480 --> 00:55:33,032
...and was about to encounter
the most distant object ever visited.

536
00:55:49,120 --> 00:55:51,680
This object may be
the most primitive object

537
00:55:51,800 --> 00:55:53,917
ever encountered by a spacecraft.

538
00:55:55,600 --> 00:55:57,557
By looking at Ultima Thule today,

539
00:55:57,680 --> 00:56:02,232
we think we're looking back in time
to the origin of the solar system.

540
00:56:14,120 --> 00:56:15,713
Ultima Thule's formation

541
00:56:15,840 --> 00:56:19,311
was perhaps just one
of the countless collisions

542
00:56:19,440 --> 00:56:21,511
that formed the planets themselves.

543
00:56:28,200 --> 00:56:29,953
But somehow it escaped

544
00:56:30,080 --> 00:56:33,517
and was flung far out
into the Kuiper Belt.

545
00:56:35,080 --> 00:56:39,359
The worlds out there,
which are billions of miles from the Sun,

546
00:56:39,480 --> 00:56:42,598
there are so many mysteries kept
in these objects,

547
00:56:42,720 --> 00:56:45,554
and we can really go out and explore them

548
00:56:45,680 --> 00:56:48,275
and start to understand these mysteries.

549
00:56:50,320 --> 00:56:52,437
Like every mission before it,

550
00:56:52,560 --> 00:56:56,395
New Horizons has helped write the story
of the planets.

551
00:57:00,440 --> 00:57:04,434
A story
that we're only just beginning to tell.

552
00:57:07,320 --> 00:57:10,358
Planets of our solar system,
they're in our back yard.

553
00:57:10,480 --> 00:57:12,437
We have the technology to be able to go

554
00:57:12,560 --> 00:57:15,598
and stand on these surfaces.

555
00:57:17,120 --> 00:57:20,716
To be able to go out there
and be able to continue doing something

556
00:57:20,840 --> 00:57:23,230
that's very natural and intrinsic
to all of us,

557
00:57:23,360 --> 00:57:25,431
and that is answering the big questions

558
00:57:25,560 --> 00:57:27,517
and exploring the unknown.

559
00:57:32,200 --> 00:57:34,590
I feel as though
we're absolutely just at the beginning

560
00:57:34,720 --> 00:57:35,756
of planetary exploration.

561
00:57:35,880 --> 00:57:39,999
There's a lot of exploration left for us.