Outer Limits The Sky at Night

Good evening.

For this programme we're going to go right out

to the edge of our known solar system.

Beyond Jupiter and Saturn there are two more giant planets -

Uranus

and Neptune.

We call them the ice giants because they're

very different from the two gas giants, Jupiter and Saturn.

With me, two experts, both from Oxford.

Dr Leigh Fletcher and Chris Lintott.

Can I ask you, Leigh, so what are the main

differences between the ice giants and the gas giants?

In fact, we knew very little about the ice giants before Voyager 2

flew past in the late 1980s.

Voyager flew past Uranus and saw a greenish disc with very little

cloud activity taking place over the disc itself.

It was a really boring planet, let's be honest about this!

Lots of scientists looked at it and were disappointed -

we were used to Jupiter and Saturn,

-and seeing incredible storms and banded systems.

-You won't see storms and rings there.

They happen, you won't see them.

And in fact that's been revealed in the 25 years since Voyager 2

that we've got a large data set of Hubble space telescope

and ground-based observations of Uranus, which show that we simply

flew past at a rather boring time

and in fact it's become a lot more active now, with white spots,

incredible colours around the north pole and the south pole.

Uranus and Neptune are twins, they are dissimilar twins. Why?

Voyager 2, when it flew past Uranus, gave us an idea of what we thought and ice giant should be like.

When we got out to Neptune we realised those ideas were again wrong

because Neptune has its own source of internal heat.

It basically emits out more energy that it's receiving from the sun

and that emission of heat is driving a really complicated weather pattern.

We see incredible storms,

such as the great dark spot which was present in Neptune

when Voyager 2 flew past in 1989.

But we haven't got that with Uranus.

Apparently there there's little or no source of internal heat.

Why not?

Well, this has got something to do with the dim, distant past

of our solar system and the way the ice giants formed in the first place.

Something, back in Uranus's distant history,

must have changed the planet into its present inclination.

-Because it's really weird.

-It's extremely strange.

What we should say is that Uranus has been tilted completely on its side.

Now, the Earth, and Neptune in fact, have a tilt of about 23 degrees.

-26 for Neptune.

-So if this is the sun and this is the Earth, you're there.

There you go.

It means that the planet is rotating like this

so it suffers from seasons in the same way the Earth does.

Uranus, on the other hand,

has got such an extreme tilt that it spends many, many years

with just one pole of the planet

facing towards the sun and, in fact, when we flew past

in 1986, the south pole of Uranus

was pointing straight back towards the sun.

What do you mean by the south pole?

The south pole has been defined by the International Astronomical Union

-as being the pole which we flew past.

-Yeah, so stop being awkward. Another point...

-Point taken!

There's another critical point here, as well. If this is our...

our Uranus, and the planet's spinning like this,

all the moons are in line with that axis as well.

So why do people think this is tilted?

It can't be, surely?

Well, one of the most popular theories out there today

is that something in the past hit Uranus...

-A large impact.

-Some kind of large impact.

-Where is it now?

The impact would have melded the two planets together, such that

what we see as Uranus today is the agglomeration

of multiple impacting bodies.

-You don't sound convinced.

-I'm not.

So how would you tilt your axis?

Years of planetary migration. Interaction with Uranus

and the other giants gradually tilted it over.

Well, during the migrational period, there is a very good chance

that collisions of this sort were happening.

In fact, the motions in these planets is causing gravitational instabilities

and it's flinging material around all over the place.

I have a silly question, which... You were talking earlier about ice giants, Patrick.

-But we're not looking at ice, we're looking at gas.

-Of course.

So why do we call them ice giants - why aren't they just gas giants?

-Cos they're icy!

-Well, yes, but where's the ice?

-It's an important point and it does confuse a lot of people.

-Confuses me!

The reason we call these things ice giants

is because the material they're actually made of, planetary scientists call ices.

So things like methane, ammonia, water

that have condensed to form solids in the outer solar system.

Now, these have all been incorporated into the interiors of these

two ice giants, but even then they're not ices

as you would imagine water ice being.

If I'd come through the atmosphere and stayed alive,

would I hit a solid icy surface?

You won't hit solid surfaces, you'll see these ices are slowly being pressure cooked.

The pressure is so immense at the centre of these giant planets

that the ices take on a very alien and exotic form.

Now, in Jupiter and Saturn it's hydrogen and helium

which consists of most of the bulk of these planets

and hydrogen takes on something called a metallic form and this is conducting

and that's where we believe we get the magnetic field of the giant planets from.

Now, on Uranus and Neptune,

there isn't as much hydrogen helium

cos there simply wasn't as much available in the outer disc when it formed.

So the thing that gets pressure cooked are these ices

that you're talking about and those ices that are in a sort of molten,

fluid state within the interior,

they are still conducting,

so Uranus and Neptune do have magnetic fields,

but even those are extremely odd - tilted and offset.

-There's a lot that we simply don't know about them.

-It occurs to me.

I mean, we talked about the great dark spot a bit

and you now tell me that Uranus is interesting again.

I know I've been abusing Uranus so maybe I should stop being cruel to it.

So over the last quarter century, Uranus, which was dull and bland

at the beginning, seems to have taken on a more Neptune-like appearance.

By that I mean it's become a lot more active.

So it must now be, what, autumn or spring on different bits of Uranus?

So the north pole of Uranus is slowly coming into sunlight

for the first time in 40 years. It takes eight years to go round

so it's springtime in the northern hemisphere and that means

suddenly you've got energy being deposited from the sun into Uranus's atmosphere,

which is triggering all these fantastic weather-like systems.

And that's why I believe we must ultimately send another space craft back to Uranus

other than Voyager 2. Voyager 2 flew past so quickly, it gave us just a snapshot

-of the dynamics...

-We want a Uranus orbiter.

We would love to see something going into orbit around Uranus

to explore the moons and to explore the rings of Uranus

and the atmosphere and interior,

but I think the big step change in our understanding of ice giants in general

would be if we were to send a probe into Uranus.

Now, we did this once with Jupiter, back in 1995.

The Galileo probe went in and sampled the gases and figured out what the chemical make-up of Jupiter was.

But if we did that for an ice giant as well,

it would be a wonderful advance for our science.

Both Uranus and Neptune have rings and satellites, so what about them?

I think the most interesting out of all of the Uranus and Neptunian moons

is a moon called Triton. Now, Triton is a big moon in orbit around Neptune.

-Not a genuine moon, though.

-But it's not a genuine moon.

By that we mean that instead of forming from the same disc that Neptune formed from,

it looks like it's been captured by Neptune's gravitational pull.

-We know that cos it's going backwards.

-It's going in a different direction to all of the other moons.

Very like Pluto.

That's right, in fact, the surfaces of both Pluto and Triton -

if they did form in the same place, this shouldn't be a surprise -

they're covered in surfaces of nitrogen and possibly methane

and carbon monoxide, condensed onto the surface.

Now, the interesting thing about that

is that when Pluto and Triton get close to the sun,

the sun's energy heats them up

and it causes them to sublimate off - basically they form a gas

which forms a very, very thin tenuous atmosphere around these moons.

The last thing to say about Triton is that with Voyager's fly-by,

we actually saw active geysers on the moon itself,

and that makes Triton one of the most interesting places

in our solar system in having some sort of volcanic activity.

It's a bit like the moon is being flexed by the gravity

of Neptune, I would assume.

What they saw with the geysers themselves is that most of them

were clustered around where the sun's energy was the greatest,

so it looks like the influence of solar radiation has some effect

on destabilising the surface,

and that's why you see these amazing plumes in the old Voyager movies.

Well, we've come a long way, and thank you both very much.

Uranus was discovered in 1781 by William Herschel.

There's a Herschel museum in Bath and Paul Abel went there.

The Herschel Museum is in the heart of Bath

and this beautiful Georgian house was once home

to the great William Herschel and his sister Caroline.

German-born Herschel arrived in England fleeing the Prussian War.

He was penniless and made the spa town of Bath his home,

earning his living teaching young ladies music.

He brought over his sister Caroline to run his household,

and together they became two of our greatest astronomers.

Well, this is Herschel's piano because he was first a musician,

then he became a rather prominent astronomer, and look at this -

this is brilliant - a picture of

the "distinguished men of science of Great Britain living in 1807"

and there's Herschel, right on the end.

'Wandering thought the museum,

'you really feel Herschel's passion for astronomy.

'These are his original telescope eyepieces.

'Just think, Herschel would have looked through these

'to map the night sky.'

'Herschel also made his own mirrors,

'striving to improve the magnification

'so he could see further into the heavens.'

Well, here we are in Herschel's workshop

and it's here that he made the telescope mirrors,

not just for himself, but for other astronomers as well.

Back then, telescope mirrors were made of metal

and he'd take the blank, put it in this device and spend

many, many hours polishing the mirror until it gained the desired shape.

It's here that he made the telescope mirror for his famous

seven-foot telescope, which is upstairs.

The one he used to discover Uranus. So let's go upstairs and take a look.

Herschel was eventually to become famous for his enormous 40-foot telescope in Slough.

But in Bath, he had a more modest reflector.

Here it is, a replica of the seven-foot telescope

used by Herschel to discover the planet Uranus.

In doing so, he became the first person in human history

to discover the existence of another large planet

far beyond the orbit of Saturn,

thus opening up the icy depths of the solar system to us.

With the discovery of Uranus,

the Herschels became overnight 18th-century celebrities.

The King and many others came calling.

Jonathan Hall is one of the resident astronomers helping to promote and preserve

the work of William and his sister Caroline.

Back in the 18th century, Herschel's night-time activities

were viewed by the good people of Bath as unusual.

His local reputation as being a little bit eccentric

and very keen on the sciences and mathematics and optics.

He would set his early telescopes up in the street

and the horse and carriages were obliged to go round him.

He wasn't what today we might think of as a professional scientist.

He was purely amateur

and he was building these telescopes

with the aid of his brother and local craftsmen

to arguably the best standard of the time.

And he made mirrors for other people as well,

-many other prominent astronomers.

-He did indeed. Yes.

Even in his youth,

he started to read books on astronomy,

read books on optics.

He'd been teaching himself a little bit of Greek.

He taught himself Italian and Latin,

partly because of his music interests as well.

-So much self-taught.

-Incredible, yes. Very tenacious.

But it was really when he came to Bath

that it really started to take off.

This is how the night sky would have looked on March 13th 1781.

In the constellation of Gemini, there was an object, which other

eminent astronomers such as Flamsteed and even Galileo had overlooked.

Herschel immediately knew that this was something special.

He wasn't actually looking for a planet.

He'd only just moved back into the house.

They'd been in a previous property.

In fact, rather unusually, he was on his own.

-Because Caroline normally observed with him.

-Absolutely.

He set the telescope up here in the garden

doing the systematic review of the heavens and there trained it on

the constellation of Gemini and that's where he made the discovery.

-And that brand-new world needed to be named.

-It did indeed.

-For a long time, people wanted to call it Herschel.

-Yes.

He wanted to name it after his patron, King George III.

In a lot of the old texts,

you can find it listed as Georgium sidus for George's star.

That's right.

Herschel, of course, was no fool.

Patronage in those days was immensely important.

The other link, of course, is that King George III,

part of the Hanoverian Empire.

Herschel was from Hanover.

So, as part of the patronage,

the King created a brand-new post for him.

He had to leave his life here in Bath

so he could be at the beck and call of the King and the Royal Family

to be able to go and show them the wonders of the heavens.

It's remarkable, over the two or three years

immediately following the discovery, all the eminent people

from around the world and Europe, who would come and pay homage

and their respects and want to look through a Herschelian telescope.

The planet was eventually named Uranus,

which in Greek mythology is the father of Saturn.

It's in our night sky at the moment

in the constellation of Pisces, the fishes.

It took some time to find it,

but I made this drawing of it the other night.

Herschel discovered this strange world over 200 years ago.

It changed his life for ever and guaranteed him

a place in astronomical history.

The Herschel Museum, well worth a visit if you are in Bath.

Herschel always makes me think of you,

because he was a musician and an astronomer as well.

You've played some of Herschel's music?

I'm a very amateur musician. I've never had a lesson in my life.

But I borrowed some of Herschel's music once

and played it live on the piano.

Was it any good?

-His music is!

-I wasn't asking about your piano playing!

I got through it.

GENTLE PIANO MUSIC

We've come a long way now,

so let's go even further out of the solar system

and come to the Kuiper Belt,

from which the best-known member is of course Pluto.

Pluto is a very similar object to Triton,

in that, it has, over the surface,

ices of things like nitrogen and carbon monoxide,

that over the course of Pluto's extremely long orbit around the sun,

will sublimate off to form a kind of atmosphere.

When it was discovered, by my old friend Clyde Tombaugh,

it was thought to be at least the size of Mars, probably larger,

and therefore of planetary status.

So we regarded it as a planet

and it has been demoted.

It's been demoted, that's true,

but I'm not sure it's particularly because

Pluto's smaller than we thought -

though it is, it's smaller than Mercury.

-But it's more that we've found lots of other things this size.

-It's not the only one.

-No.

So we have the choice of having eight planets,

as we do, from Mercury out to Neptune,

or many, many, many planets.

And we call those the Kuiper Belt -

Pluto and Pluto-like objects on one part of it.

Of course, the most exciting thing about Pluto

is that's there's a spacecraft on the way, New Horizons,

which was planned - not that it matters -

but it was planned while Pluto was still a planet,

and a big part of their pitch

-was this was the last planet that we haven't seen.

-Quite.

But where's New Horizons got to now? It's a few years away yet.

So it's still on its way to Pluto,

it's going to get there in July of 2015

and it's just got past the orbit of Uranus.

That doesn't mean it visited Uranus,

but it's on its way to do a slingshot by Pluto.

And after Pluto,

it's going to go on to look at another Kuiper Belt object.

-Which hasn't been discovered yet.

-No.

It's something that we're still looking for at the moment.

Pluto is the brightest member of the Kuiper Belt,

it's certainly nothing more than that.

Recently there was a big conference in France about these things

and Dr Chris North went there

and brings us back news from the Kuiper Belt.

This year's annual gathering of planetary scientists in Nantes

was the largest for many years.

For space scientists,

it's a great place to talk about missions to the planets,

and for the astronomers,

a platform to launch their new ideas.

This year, the icy, dark depths

of our solar system have caused a bit of a stir.

The Kuiper Belt stretches beyond Neptune

and has previously been dismissed as the dustbin of the solar system,

the leftovers of its formation.

Meg Schwamb has been raking through the rubbish,

to find clues about the Kuiper Belt and its formation.

We now know that there are lots of exotic objects out there.

They come in a range of sizes

and some, such as Pluto, even have moons.

While many formed in these icy outer depths,

others formed closer in

and were flung out there by the movement of the giant planets.

The Kuiper Belt really is

the icy remnants left over from planet formation.

You can think of it as the embryos and the failed planets

that didn't form into the terrestrial planets

or the cores of the giant planets

and have been scattered out, or may have formed out,

in orbits beyond Neptune.

And so Pluto is one of the well-known and largest of these objects

and there are now many Pluto-sized bodies known in this region,

as well as many hundreds of thousands of other smaller bodies

that orbit well beyond Neptune.

In 2007, Meg discovered an object nicknamed Snow White,

one of the largest Kuiper Belt objects

at around half the size of Pluto.

Despite its name, Snow White is actually red.

The colour is caused by a tarry gloop called tholins,

a term coined by Carl Sagan.

We think tholins are methane and other chemicals

broken down by weak sunlight over billions of years.

I don't think we've quite touched tholins in the outer solar system,

but we think that if you take these methane ices and nitrogen ices

and you keep radiating the methane,

you get a higher and higher order of hydrocarbons,

and so you get more gunk and tar.

And so, that's what we think it is

and we think it's reddened their surface.

And so it's sort of this red crust, or radiation crust,

sitting on the surface.

-It's goo.

-It's goo, it's sort of, yeah...

-It comes from the Greek word for muddy, I believe.

-Yes.

Kuiper Belt objects are the preserved remnants of our early solar system

and they tell us what is was like billions of years ago

when the planets were forming.

But studying it can be very frustrating,

as the objects are so small and so very far away.

Occasionally, however,

we get visitors from the outer limits - comets.

'Mike Ahern has led one of the most successful mission to not one,

'but two comets.

'In July 2005,'

part of the Deep Impact probe was smashed into Comet Tempel 1,

giving our first view of material from inside a comet.

Following this successful mission,

the Deep Impact spacecraft was re-routed

and in November 2010,

flew past Comet Hartley 2

where it saw a very different world.

Every comet we've visited

has been very different

from what we would have predicted from the previous one we visited.

So that's the reason for going to multiple ones -

to try to find a pattern.

One difference between Tempel 1 and Hartley 2,

the two that Deep Impact spacecraft visited,

is they're very different sizes

and very different activity for their size.

On Tempel 1 we saw differences in the outgassing

from different parts of the comet.

Now, does this mean different parts of the comet

came from different parts of the proto-planetary disc?

It was hard to tell...

-Because they're made of different stuff.

-Yeah.

But there could be still strong seasonal effects on Tempel 1.

Now, at Hartley 2, it's in this excited state rotation

where in addition to going around like this, it rolls.

So everything gets illuminated all the time

and there are no strong seasonal effects.

And we still see big differences

between the outgassing at the two ends,

which says that they really must have come,

in the early solar system, from different distances from the sun.

-So they are different?

-They really are different.

Comets, the archaeological remains of our early solar system,

will help us understand

how the planet Earth became such a unique world.

It's important, because, for one thing,

we think the comets bring all the water to Earth,

or at least some of us have thought that all along.

Is that just because Earth's the only planet we know with standing water...

-That's correct.

-..on its surface.

-Yes.

-And the early Earth,

-that should have all boiled off.

-Exactly.

-So where did it come from?

-So it has to have been brought in at a somewhat later stage -

-only half a billion years after the formation, but...

-A long time ago.

The Kuiper Belt is clearly not the dustbin of the solar system

and the exotic objects that live there are far more than just rubbish

thrown out during its formation.

Instead, each one is a fascinating world

and the more we find out about them,

the more we find out about our own origins.

Plenty of news this month

so we'll go now to Chris North and Chris Lintott.

A huge storm going on on Saturn.

A huge storm that's been going on for most of the last year.

It was discovered last December as this white spot,

which is the form storms on Saturn normally take,

but this one spread out until it covered quite a lot of the disc.

-Huge.

-You can see it in these new images that we just got from Cassini

that were taken over the last year.

Of course, the amateurs saw this as well.

Then they saw the storm fade over time

and it seemed like all was quiet on Saturn.

But if you talk to the professionals who can use

some really big telescopes and they can look in the infrared,

they see that actually what's happened is up in the top

of Saturn's atmosphere, in what would be the stratosphere,

there's still a lot going on.

The storm now covers about a quarter of the disc,

so all of the northern bit you can see on one side.

But as it emerges in January, we're going to see

some interesting things happening

where this storm was last year.

-I wonder what we'll see.

-We'll find out.

Coming into Jupiter and the second moon there, Europa,

interesting news?

Europa's this moon that's got the very weird terrain.

We call it chaotic terrain. It was terrain that the Voyager probes and Galileo saw

covered in cracks and fissures and all sorts.

-Map-maker's nightmare.

-Indeed. Map-maker's complete nightmare.

What we think is happening still is that that's cracked

and broken up by the motion of a sub-surface ocean.

We can compare that to stuff we see on the Earth.

Some of the features look like things we've seen in Antarctica,

with a sub-surface sea or lake that's not quite that deep.

I'm quite excited about this because

it's been my favourite idea for a mission anywhere in the solar system,

which is you land on Europa, melt your way through the surface

and you see something with teeth coming towards you.

The reason people care about this ocean is

it's an interesting place to think about life.

It could well be.

Life on Earth, we think, started in deep oceans

without much sunlight, so Europa's a fascinating world.

They all are. Of course, you can't get away from Mars.

Always something new there and,

of course, this new probe Phobos-Grunt.

Yes, this was a Russian probe that was meant to go to Mars's moon, Phobos,

collect a sample, bring it back to Earth.

Unfortunately, it made it into orbit, it's been stuck there.

It's not sure what will happen, which is very sad.

The Russians have never had any success there.

It was incredibly ambitious. That would have been the first sample-return mission,

BUT we shouldn't forget there's good news for Mars this month as well,

which is that the American Curiosity rover is successfully on its way.

Lift off of the Atlas V with Curiosity.

We're going to find out a huge amount about Mars's past

and about whether there are the conditions for life there.

-So I'm really excited about Curiosity.

-So far, all goes well.

We have spacecraft separation.

APPLAUSE

One more news item.

We found a long-lost Sky At Night programme

on a recording machine and here it is.

I'm talking about moon bases tonight for two reasons.

First, because they are very topical

and we've just shown you one new design study.

But secondly because I am delighted to have with me

my old friend Arthur Clarke.

Arthur, you, of course,

were forecasting developments of this kind more than 20 years ago.

Thank you, Pat.

I'd like to begin with a flashback to the communication satellite

which you mentioned,

by showing this illustration which has some sentimental interest to me

because I displayed this on BBC TV about ten years ago.

This was undoubtedly the first time that anything

about communication satellites appeared on TV anywhere in the world.

So, this is a programme from 1963.

To put us in context, that was before Apollo.

Apollo was still in the planning.

We'd had people into space, but only a few probes whizzing past the moon.

Arthur C Clarke was predicting not only these communication satellites,

which were starting to become a reality back then,

but also was talking about bases on the moon, bases on Mars,

all incredibly exciting for what we might do

from then, in the next 20, 30 years.

I think a great deal has depended there upon

the question of the lunar atmosphere

because if the moon had proved to have anything of an atmosphere,

then the surface dome would have been the answer.

Do you think that this means that the lunar base

has got to go underground?

If it turns out that a meteor bombardment is a serious danger,

then, undoubtedly, we will have to put our bases underground.

But I rather hope that the necessity of a lunar underground movement

does not arise.

-I hope we can stay on the surface and look at the stars.

-So do I.

He and I were great friends.

This is one of the few recordings we made together

that's actually been preserved.

Yes, it's wonderful.

If you could have picked one Sky At Night out of those that were lost,

-this one would have been high on the list.

-I would agree.

You can see more clips

on the website, which is...

I'm going to watch it again and dream of Mars bases.

Thank you both very much indeed.

When I come back next month, we're going even further

and look at planets on other stars.

Until then, good night.

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