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Space exploration is humanity's grandest adventure.

We've sent missions to visit all of the planets of the solar system

and to gaze deep into distant galaxies.

But how do we choose which missions go into space?

Only a handful are launched every decade.

And for every one that flies, dozens are left on the drawing board.

This month on The Sky At Night we're looking at the selection

of the European Space Agency's next mission.

My finalists are now waiting to hear who will get the final spot

onboard ESA's new rocket.

I do feel a bit nervous.

I slept well, but woke up ridiculously early.

Two of those teams are from the UK.

Yeah, checking my phone throughout to see if e-mails will come in,

but I've not heard any specific news yet.

And before we reveal the victor, we'll be finding out how they choose

which mission will go into space and which simply won't fly.

Welcome to The Sky At Night.

This is the headquarters of the European Space Agency in Paris.

Right now, it's hosting a meeting of the Science Programme Committee,

the representatives of the 22 member states, who have to decide

what ESA's next mission will be.

ESA is building a new rocket that can carry two scientific

probes to space.

One of these missions has already been selected.

And now teams from all over Europe are fighting for the other spot.

It will be called an F-Class mission.

And although the F stands for fast, the mission itself

won't launch until 2028.

The agency received 23 proposals covering everything from exploring

an asteroid to high-energy astrophysics.

Each one could transform our view of the universe.

They can only pick one.

And by lunchtime today, we think we'll have a winner.

This will be the first F-Class mission that ESA has announced.

They're designed to be smaller, lighter and cheaper

than their usual missions,

to test innovative ideas and different concepts.

Now, we say cheap -

this mission is due to cost around 150 million euros.

And it has to meet some pretty tight specifications, too,

including weighing less than 1,000kg.

If it meets these requirements, ESA will cover the costs of building

the spacecraft and launching it on an Ariane rocket.

And although ESA doesn't know which mission they'll launch yet,

they do know where the spacecraft will be positioned,

at a very specific point in space.

As we all know, the Earth goes around the sun in what is pretty

close to a circular orbit.

But around this orbit there are some very interesting locations.

They're called Lagrange points.

Lagrange points occur because of the way the forces

of gravity and orbiting bodies interact.

Anything that finds itself in one of these locations will stay

there in a fixed position relative to the Earth,

as they all go around the sun.

Let's start with Lagrange point one.

That sits in here,

about 1.5 million km away from the Earth.

Now this is the ideal position for a solar observatory,

because it has an uninterrupted view of the sun

as the Earth orbits the sun.

Next we have Lagrange point four

and Lagrange point five.

They sit at an angle of 60 degrees ahead of the Earth

and behind the Earth.

Despite being millions of kilometres away from the sun and Earth,

objects here will stay in a fixed position relative to the Earth,

as it orbits the sun.

Next we have L3.

That sits all the way over here, on the other side of the sun.

It is permanently out of view of the Earth

and a long, long way away from the Earth,

so we don't do much out here.

But that brings me to L2.

This point sits 1.5 million km away from the Earth

and it has unobscured views of deep, dark space.

It's unobscured by the Earth, it's unobscured by the sun

and any of the inner planets.

And it's at this location that ESA wants to put its first F-Class mission.

Lagrange point two is a perfect place to study the wonders

of deep space.

And this F-Class mission won't be alone on its journey,

it will be piggybacking on another mission, sharing the same rocket.

The Ariel Exoplanet Atmospheres mission is already being planned

at University College London.

Its principal investigator is Professor Giovanna Tinetti.

Giovanna, we're here to talk about your latest mission, Ariel.

But before that, is it weird having an interloper,

someone piggybacking on your rocket?

Well, it's not weird.

It's actually fantastic.

I'm really delighted about that idea, because you see

we were having all these, um,

big rockets just for ourselves and a lot of empty space,

and so, finally, we can fill this empty space with a great mission.

So, I'm looking forward to the selection of that mission.

So, now, let's focus a bit more on Ariel itself.

What is Ariel designed to do?

Ariel's going to look at the atmospheric composition

and the weather for about 1,000 planets in our own galaxy.

So, we are very excited about that.

But how is it going to work?

How is it going to measure these exoplanet atmospheres?

Maggie, if I can ask you to hold this up.

OK, this lamp will be the light from the star. OK.

And typically, the star has a planet that is orbiting around it.

When the planet is passing in front of the star it is basically masking

some of the light.

And when that happens, then we can work out how big is the planet

compared to the star, which is great. Yes.

But we want to do even more.

And that's where Ariel comes in.

To measure the different gases in the atmosphere at these

distant worlds, Ariel will use an instrument called a spectrograph.

What we'll do with a spectrograph is to look at the light from the star

and split the light into colours or wavelengths.

Which is what we see here? Exactly.

So, this is the spectrum of the star. Yeah.

And then we wait for the planet to pass in front of the star.

And when that happens, the planet has an atmosphere with some molecules

and each molecule will absorb a different colour in a different way.

And, you know, what is quite incredible is that every molecule,

every atom has its own unique signature.

And that's why we can find out, then, what is the composition,

what is the chemistry of our planet.

So if you see the same fingerprint of a chemical that we've measured

here on Earth, in that spectrum, you'll know that that chemical

exists in that atmosphere? Absolutely.

So, how long until we get those first results?

So, both Ariel and the other mission will be launched in 2028,

and then we need to wait about six months before they really arrive

where they're supposed to be. At least in the case of Ariel,

we will start to get spectra, and so we will start

about six months after launch.

So, we're talking a ballpark figure of ten years

before we start getting results?

It's a long time. I know, I know.

But actually, if you talk to the engineers who are really building

and doing the hard job of building the spacecraft and the payload,

then they will tell you the ten years is not really such a long time.

Well, I think we should put a date in the calendar for ten years' time,

so we can come back and speak to you and find out about

the amazing results Ariel's discovered.

I look forward to it.

It was only last year that ESA invited scientists to pitch

their ideas for a space mission to launch

in the rocket alongside Ariel.

It was a galvanising call -

23 proposals from all over Europe were submitted,

each proposing different areas of outer space to explore.

Following an intensive selection process,

these have been whittled down to five finalists,

and two of them originate from this unlikely looking

British Space Centre, deep in the Surrey countryside.

We asked Lucie Green to meet the teams.

This is the Mallard Space Science Laboratory or MSSL to its friends.

It's a Department of UCL and for over 50 years

it has been at the cutting edge of space science.

I've been working at MSSL for over 15 years,

and now two of my colleagues are in with a chance

to launch their ideas into space.

The first team is led by Professor Geraint Jones.

His mission is called the Comet Interceptor.

Hi, Lucie.

Congratulations on being shortlisted. Thank you, yeah.

Tell me about Comet Interceptor and what the aims are for the mission.

OK. As its title suggests, we want to go to a comet,

but unlike previous missions, we want to go to a comet

that we don't know about yet.

So, one that's coming in towards the sun for the very first time.

We've visited comets before,

so why is it so important that we visit another one?

So, from previous comet missions we've learnt a huge amount,

so Giotto to Halley, Rosetta to Churymov-Gerasimenko.

So, there have been big advances in our understanding,

but the comets that have been visited by spacecraft up to now

have all been past the sun many times.

Each time they pass close to the sun, the ices on the surface

get heated, they change and they also get covered in this layer of dust

that falls back down onto the nucleus.

So, we'd like to see a pristine one coming in from the outer solar

system for the very first time.

Why is it so important for you to get to a pristine comet

that hasn't had any of this processing? What does it tell us?

So the expectation is that if we can visit one of these objects

that hasn't been processed, so one that was around when the proto

planetary disc was here, before the planets were formed,

but was ejected out of the solar system into what's known

as the Oort cloud, this huge cloud of icy objects stretching

all the way out to roughly halfway to the nearest star,

if we can visit one of those objects, we will truly see

what the original material was like when the planets were being formed.

But getting a good view of a comet is going to take some clever

manoeuvring and camerawork.

PhD Student George Brydon is working on this.

Now you've been involved in the mission since the start,

how is the spacecraft going to work?

So the spacecraft sits at L2,

while telescopes search for a suitable comet,

and then, once one's found, it will head off and intercept it.

When it reaches the comet, a few days before the flyby,

it splits into separate spacecraft.

They pass by the comet, but they take slightly different paths,

so they get a different view.

The advantage of this is that, because you've got spacecraft

from different points, you're able to learn more

about the structure of the comet.

And what data will be collected on these flybys?

Particular interest, I think, is the large number of cameras

onboard this spacecraft.

And so the blue spacecraft that gets close

actually has a camera onboard that's able to scan the whole sky.

Similar in a way to this commercial 360-degree camera we have here.

So, we have several snapshots passing through the comet.

Using those and the data from the other cameras as well,

we'll be able to reconstruct in 3-D

the nucleus itself, and also the gas and dust jets coming off it.

So, if we get selected and we gather the data,

then we can imagine putting on a VR helmet and actually looking it

down towards the nucleus and see the gas and the dust coming off

this object, which is boiling away for the first time

after being formed billions of years ago.

And how are you feeling? The decision is imminent.

Nervous, yeah.

If we're not selected, um,

it will of course be disappointing.

But we have a mission concept,

which I'm sure we'll be proposing again in the future.

Well, fingers crossed.

Thank you.

The atmosphere here is tense,

as situated down the hall from the Comet Interceptor team sits

the competing British mission

that's amongst the final five on the short list.

A mission called Debye, led by Professor Rob Wicks.

Debye plans to study how energy is transferred

out in the vacuum of space.

The big question we're trying to answer is how does heat

travel around in outer space and in the universe?

So, here in the atmosphere, if I lit a candle over here

and it got warmer where you are,

that heat has got to you through conduction and convection,

and that is molecules bumping into each other in the air.

In space, that doesn't happen. It's a vacuum.

Collisions between electrons and protons, or other bits of the gas,

don't happen very often.

So something else has to pass that energy from electron to electron

to transfer the heat from here to there.

How does the Debye mission help us with that question?

So this is an electron detector that you would typically use

on a space mission.

And here is a tiny little gap in this detector

and an electron can enter there.

And then, I can open this, in here...

..we can supply an electric field in this instrument, to the sphere,

and that will then deflect an electron when it comes in

and the electron will be bent down,

until it hits a detector down here on this plate.

And that tells you then about the properties of the electrons.

It's very challenging to do this measurement,

because the electrons are so light.

So you've got your electron measurements,

and then you need to, what, join the dots with the other data

that you're collecting from the spacecraft to understand

what's heating the plasma?

So what we're going to measure is, simultaneously, the electrons

themselves, the magnetic fields and the electric fields

in the plasma.

So, if I deposited a lot of energy in one place, the electrons

over here get hot, what they also do is they make electric fields

and they make magnetic fields.

The mission is to look for a small population of electrons

that are going a bit faster or a bit slower

than they should be.

And at the same time,

we'll look for a little wave in the magnetic field,

and a little wave in the electric fields.

If we see those things at the same time,

we can say that wave has scattered

or collided with those electrons

and caused them to be moved around.

The decision's not far off now.

Are you feeling nervous? A little bit, yeah.

It's actually a lot like waiting for your A level results.

Daniel and I worked together very closely, with more than 120 people

across Europe to put this proposal together,

and so, no matter what happens tomorrow,

I'm happy with what we've done.

I think we've done ourselves proud.

Here in Paris, whilst the committee is working hard to make

a final decision, there's nothing anyone can do but wait.

ESA's science programme costs each of us a euro a year.

It's not much, but we can't fund everything,

so hard decisions have to be made.

And two British suggestions illustrate the diversity of ideas

that's out there.

You don't get much more different, after all, between a fundamental

physics mission, and a trip to a comet.

So how do you choose?

How do you pick the one mission with the star quality to succeed?

To find out, I sat down with Dr Fabio Favata,

Head of ESA's Strategy Planning and Coordination Office.

One founding principle of the ESA Science Programme

is that all of our science choices, science priorities,

science selections are done by a peer review committee,

by a committee of scientists... By other scientists?

By other scientists that don't work for ESA.

They're selected from the community for their, of course,

seniority, for their competence,

and they make the final scientific selection.

Yeah, but how do you pick from such a diverse set of options?

You could go to Mars, or look at an asteroid,

or study the High Energy Universe.

Those don't feel like the same kind of thing.

How do you compare such different missions?

The proposals that make it to the final shortlist, usually,

are all so good, they all deserve implementation.

So it's a very frustrating activity for the members

of the peer review committee.

Yet it's a choice that we have to make, because of the resource

limitations, and we can only afford to fly one mission

at the current time.

Well, let's talk about this Fast mission.

This didn't exist a couple of years ago,

the idea that you'd fly this mission.

Can you tell us how that idea came about?

One was the fact that, you know,

the European rocket launcher programme is evolving.

Ariane 6 is more powerful than the previous option,

and so we've been designing missions for a certain size,

and now we've discovered that we can carry more.

So we said, "Can we implement faster a smaller mission

"that allows us to take benefit of this?"

So there's a meeting downstairs to make this decision.

What do you think will come out of it? I don't know.

All I can tell you is that the leading contenders are so good

that there certainly will be a mission that I'm sure

will be exciting, and we'll be happy to discuss whatever choice will be

in the future. We'll come back and talk to you when we know

what the mission's going to be. It would be a great pleasure.

Luckily, you don't need to be selected for a space mission

to gaze out into the night sky.

This month, we're going to observe one of its most

spectacular sights - Jupiter.

Jupiter is currently very low in the UK sky.

In fact, it's almost as low as it can possibly get

in the sky itself.

Now, that means the observational window for us is pretty short,

but if you've got a telescope and use a bit of magnification,

you can see plenty of detail on Jupiter's disc.

Now, the skies are a bit cloudy at the moment, but hopefully

we will get some clear gaps where I can see Jupiter,

and if it does appear, I'm going to grab it

with my 14-inch telescope.

You don't really need a massive telescope to do this,

but I want to get in nice and close, so we get a good picture of it.

And I've got a high frame rate camera.

I'm also using an infrared filter, that helps to steady the image.

What remains is for us to wait and hope that the cloud gaps

come over in time.

Jupiter's atmosphere appears as segregated bands of gas,

so we'll be looking out for belts on its disc.

But viewing conditions are far from ideal.

I can see Jupiter,

so we have got a gap in the clouds.

First thing you notice with Jupiter is you get a disc

which looks squashed, because it's a fast-rotating gas planet.

So it expands at the equator, and it looks squashed at the poles,

but you can see that Jupiter's definitely banded.

As the planet rotates, you get these vicious jet streams,

basically dragging material round, creating this banding effect.

But we have got something, which is really good.

Another sight to behold is the four moons that orbit the planet -

Io, Europa, Ganymede and Callisto.

Tonight, we hope to observe the transit

of Jupiter's largest moon.

I can see Ganymede very close to the edge of Jupiter.

It's about to just head off as Jupiter ends the transit.

Now, a bit later on, the shadow of Ganymede will start to cross

the disc as well, so that will give us a Ganymede shadow transit.

And that's actually a much easier thing to see, because it's

a dark shadow, it's very large, and it stands out really well

against Jupiter's bright disc.

Unfortunately, clouds stopped me from observing the shadow transit...

It's gone again.

..but what is it that transits can tell us?

Ganymede or its shadow passes across Jupiter's discs,

so a tiny amount of light is blocked from the planet,

and it's possible to measure that minute dimming of Jupiter,

and work out that there's something in orbit around it.

This is similar to a method used to find exoplanets -

by observing a distant star and looking out

for a reduction in light.

And if that dimming happens for a fixed length of time,

at regular intervals, then it's probably due to a planet.

As we know, the aerial mission will be studying exoplanets,

and to do this accurately,

the team at UCL will need to know the precise transit time

of the exoplanets across their stars.

And this is where amateurs can get involved,

because Ariel is planning to bring together individuals

with all different sizes of telescopes to create a network

dedicated to the observing of exoplanet transits.

Now, these observations will help refine the data about the planets

and their orbits, paving the way for Ariel

to characterise their atmospheres.

More details of how to get involved can be found

on the Ariel Mission website.

It's decision day.

Which one of the missions will be chosen to be launched

on ESA's latest rocket?

The leaders of both UK teams are not in Paris.

They're actually in the same presentation,

in the same room at UCL,

anxiously waiting for news.

A little nervous. I did get some sleep last night,

but I woke up incredibly early.

Rob and Daniel are in the room here with me as well,

so we've got a day of listening to project talks today.

Hopefully, at some point, we'll hear the news.

It's a nail-biting morning, but at two o'clock,

the moment they've been waiting for finally arrives.

Er... Yeah, just received an e-mail from ESA,

confirming the Comet Interceptor has been recommended

for further study.

So, um...

Yeah, quite a relief.

If all goes well from here on, then it will keep us busy

for between 12 and 15 years into the future.

So should keep me busy all the way to retirement, I think.

HE LAUGHS

In Paris I caught up with Chris Lee,

the UK's man on the committee.

So, you're just out from the meeting.

We hear a decision has been made. What happened?

Right, so, Comet Interceptor has been selected.

Oh, that's good news.

And that's a British-led mission as well.

What we really wanted with the F mission was something

that was innovative, something that put technology together

in really interesting ways.

And the thing I think we all agreed in the meeting was that

Comet Interceptor came up with a really imaginative approach

of using existing solutions for instrumentation and spacecraft,

but put together in a package we'd never seen before,

and which will be very fast.

We have to move quickly, because we already have a mission

that's going to go. That's Ariel? That's the Ariel programme.

And so it's the ride that joins the Ariel initiative,

and the clock is ticking already.

What are the next stages that this mission will go through?

It's got approval today. What happens next?

OK, so, first of all, it was a genuine competition,

so it's won because of science merit,

but what we now need is a detailed study to work out exactly

what the technology needs to be, what the instrument needs to offer,

what the spacecraft needs to address,

and then we will get a costed proposal, and it's at that point,

the mission is what we call "formally adopted".

And that will be around November 2020.

What does that mean for the UK Space Agency in the UK?

Well, first of all, it means we have to pay for it.

So, the thing that we have to try and emphasise to people

is that our investment in ESA pays for the spacecraft,

pays for the launch, pays for the operations,

but we actually pay for the instruments ourselves

through the national programme, so it's a partnership.

Like the cameras, and the things that will do the science?

Absolutely. So yes, it's led by the UK, but we have to recognise

it's a strong European project.

So these are European missions,

but both Ariel and Comet Interceptor, now,

are UK-led missions.

Why do these missions get people excited?

Well, I think it's important to realise that we are

a UK Space Agency for science, as well as for industry and economics.

So from our point of view, it's great to have a motivational

programme from a space science perspective, because we are living

in a golden age of European space science, and we can show that

the UK's not only being involved in the building of the spacecraft,

but actually contributing to the reason you're flying it

in the first place.

Well, it's a good story, and I hope we'll come back to it

again and again in the years leading up to and after launch.

Chris, thank you very much.

Thanks, Chris.

Geraint can't wait to share the exciting news with his mission

co-leader, Dr Colin Snodgrass, from the University of Edinburgh.

Hi, Colin. Hi.

I guess you got the same e-mail as I just did.

Yes, very good e-mail to get. Yeah. Exciting news.

Huge relief, so, yeah, sorry we're not in the same place

to share the news, but somewhere out there,

beyond the orbit of Pluto,

there's something approaching the sun,

and we don't know about it yet, but hopefully we'll get to see it.

OK, I'll speak to you again later.

Thanks for all your help with this.

Yeah, thanks a lot. See you.

All right. Bye. Bye.

Congratulations. Thanks very much. Very well deserved. Well...

The other team has taken the news graciously,

but ESA's announcement wasn't all bad news for the Debye mission.

So I've just received an e-mail saying that the first-choice mission

is Comet Interceptor, and the second choice, and a backup

if Comet Interceptor runs into trouble, is Debye - my mission.

So, a fantastic result.

Obviously we'd rather be number one, but if you can't be

number one, you better be number two, and that's what we've done.

So, a great result for UCL,

great result for Mars Space Science Laboratory,

and I couldn't be... Well, I could be slightly happier,

but I'm not unhappy.

Considering that we started from scratch last year,

with a completely new mission concept, that's very exciting,

very good news for us.

Of course, if ESA needs us at any point, we will be ready.

We won't throw away the mission concept, and maybe we'll find

another home for this proposal, but at this point,

it's really just a great result.

We're very happy.

It's been an exciting day for UK space science.

In 2028, ESA will propel two British scientific probes

out beyond Earth's orbit, to make discoveries about exoplanets

and the origins of the solar system.

Congratulations to the Comet Interceptor team,

and we'll look forward to following their progress

on The Sky at Night, as they head towards launch.

That's all we've got time for this month, but do join us again

for the next programme, where we'll be finding out about

the latest research into exoplanets.

Also, in the autumn, we'll be holding The Sky At Night

Question Time, where Pete, Chris, myself and some selected

special guests will be answering your questions

in front of a studio audience.

So if you've got a question you'd like to put to the team,

send it in to...

Until then, goodnight.