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The Sky At Night is back,

bringing you the latest in all things astronomical.

There's been a lot going on since our last episode,

and one particular object caught our eye.

The biggest story to hit the news since we've been off air

was about an asteroid called 2024 YR4.

The City Killer,

an asteroid that had the potential of hitting Earth in 2032.

The headlines were everywhere.

BBC NEWS THEME

Now, an asteroid spotted late last year is now being carefully tracked.

Scientists are trying to work out whether there's a chance of it

hitting the Earth.

Astronomers estimate that the space rock is roughly

the size of a football field.

Suddenly, the night sky wasn't a place of comfort.

With that asteroid out there

and its potential to change life as we know it,

looking up at the night sky wasn't a place of quiet contemplation,

but a place to fear.

So the level of damage that an asteroid the same size

as 2024 YR4 could cause,

it really depends on where it hits.

We're probably talking about the destruction of a city,

were it to hit the land.

Something you really don't want landing on top of you.

Not enough to wipe out dinosaurs or cause a global shift in climate,

but big enough that you want to avoid it, for sure.

But what lies behind the headlines?

And are we prepared for an asteroid strike?

Welcome to The Sky At Night.

On a beautiful day like today,

it's easy to forget that beyond that gorgeous sky,

there could be legitimate threats heading our way.

I'm not talking about aliens.

Obviously, I'm talking about asteroids.

Now we actually get hit by asteroids every single day.

In fact, in any given 24-hour period,

we might get hit by roughly 73 Earthling cows' worth of material.

So why aren't we fussed about this?

Well, the scale of this material is such

that it's like a sprinkling of dust, mostly just burning up in

the atmosphere and not even reaching the ground.

Bigger things do hit us too.

The asteroid that wiped out the dinosaurs a while back

was tens of kilometres across,

and something that size is expected to hit us

roughly every 100 million years.

But between these extremes,

mass extinction event, rather nice light show,

you have your midsize asteroids.

Now your midsize asteroid

could cause a heat blast capable of vaporising solid rock.

We're talking wind speeds

five times the strongest hurricanes.

We're talking a shock wave

that could flatten buildings for hundreds of miles.

That would mean a city the size of London and the surrounding areas,

maybe even as far as Kent, gone.

Now, at the start of this year,

we thought there was just such an asteroid coming our way

when Asteroid 2024 YR4 was spotted.

Now, at the time, it was still over 30 million miles away,

but it looked like it was on course to collide with us in 2032.

It has since been downgraded,

and we're now pretty confident it's actually going to be a near miss.

However, what it did provide is a practice run, like a fire drill,

or even an asteroid drill, if you will,

so that when the real threat does come along, we know what to do.

When YR4 was discovered,

it was no more than a dot moving amongst the background stars.

And even once it had been found,

it was just one of a million asteroids in our books.

So what was special about this one?

To explain, I've come to a football pitch, obviously,

where I'm meeting my old friend,

planetary scientist, Dr Meg Schwamb.

Hey! Hey! we've talked about football a lot,

so I thought you were the perfect person

to bring here and talk about YR4.

When we were watching the odds of it hitting the Earth go up,

I did think of it like watching

a striker's shot head towards the goal.

Yeah, definitely, exactly that playing out, right?

I think as the world,

but also astronomers sitting there watching, right,

waiting to see as they got more data,

as you get closer to that goal, right,

of whether it's going to go in or in this case,

we really want it to miss.

Yes, that's the unusual bit.

But let's go back to the beginning.

2024 YR4's an asteroid. But what are asteroids?

Asteroids are the debris left over after planet formation.

Most are between Mars and Jupiter.

Some get disturbed in their orbits and get sent inward.

That potentially can be an orbit that will cross the Earth.

On the 27th of December 2024,

YR4 was first spotted by a telescope in Chile,

as nothing more than a faint point of light.

It soon gathered a lot of attention,

as calculations showed it had around a 1% chance of hitting Earth.

So in January,

other astronomers jumped on this and started trying to observe it.

Everybody was jumping on it.

Amateur astronomers, anyone with a telescope

that had time was jumping on this

because very few objects have jumped above that 1%.

And when it hits that level for the size

that we thought this object might be,

which is about 30m to 90m,

that's when we start worrying,

because that's where you could do damage on a city scale.

What happened to the odds of YR4 hitting us?

It went up and it went up a lot.

It went from about 1.2% to 2.8

or over 3% roughly.

And so that might feel really scary, right?

So it IS interesting, I think, the fact that we went from

a low probability and then it rose before dropping off.

I think it's easy to get the impression

that somebody made an error, but that's not what happened.

No. It's really trying to understand where right in space

the Earth is compared to where we think

all the possible orbits that this object could have.

Well, I really think we can explain this with football,

but I decided neither you nor I should kick a ball.

Definitely not.

I've got some students here.

Gabe! Gabe! Come here.

I feel like we're bringing on a substitute.

Someone much better skilled!

So if we get Gabe to kick the ball towards the goal.

At first, we don't have much information about where it's going,

whether it's going in or not.

Tracking the asteroid after it was first spotted is like freezing

the ball just after Gabe kicks it.

It might go in the goal,

but the goal makes up just one small part of the area

the ball could end up in.

For YR4, the odds of hitting Earth were 1%.

No, we've got an initial idea of position, right?

Right when it's kicked, we have an idea of the velocity,

but it can go any direction.

But a little bit later on we've got more information

as the ball heads towards the goal. Right.

Because if you could just stop the ball for a second in midair,

you now can see more of where it's headed.

You have extra information, another data point, right?

You have another position, another possible velocity.

This additional data narrows the ellipse,

removing much of the area outside the goal.

The goal now takes up more of the remaining ellipse,

and so the odds of a hit go up.

Things are getting exciting.

For YR4, it was over 3%.

And that's what happened here.

We ruled out parts of space

where the Earth wasn't, and so the probability of impact went up.

And then if we let the shot play out...

..the third freeze frame shows the ellipses reduce down again.

We're now sure the ball will miss the goal...

Bad luck, Gabe!

..and YR4 will miss the Earth.

And it was observations from one of the most advanced telescopes

on Earth that told us that this asteroid...would not score.

And so we have the additional data

from the Very Large Telescope helped,

because that helped with all the other observations

to give us enough of the trajectory

to show it's going to come close,

but it's just going to eke away and not hit the goal.

But it's still close, it's not like it missed by miles.

No, and there's even a small chance

this object could still hit the moon.

That'd be pretty spectacular, wouldn't it?

Yes, but not threaten the Earth in any way?

No. It would just be a good show.

If we even see it, it might be the far side of the moon

and we see nothing.

I promised I'd say, by the way,

that Gabe missed that on purpose for us.

But I think the demo really works,

and it's good to know that YR4, too, is missing. Yeah.

But the threat is still there.

There's many other objects of the size

that can do damage to cities that we still don't know about.

But the nice thing is there are telescopes searching

the skies nightly to try to find these objects.

I enjoyed that. Good. It's a bit like Ted Lasso.

We brought in an American to explain football to us!

MEG LAUGHS

While multiple observations allowed us to confidently predict

a miss for YR4,

one day, there WILL be a direct hit.

So what can we do if we know an asteroid strike is imminent?

Maggie's taking a tea break to explain.

MAGGIE: Hollywood movies are full of ideas

of how we might handle an asteroid strike.

But in the world of reality,

what tools do we have in our toolkit?

Well, to understand this, I'm going to use this

as the asteroid and this as planet Earth.

Now the challenge is the asteroid

is heading straight towards the Earth.

So what can we do to deflect it?

Well, there's a number of different techniques out there.

And the first one is the nudge method.

So... Oh, sorry, they're good.

The first one is where you want to use

a projectile to hit the asteroid.

When this happens, we'll transfer some momentum to the asteroid.

And it will be deflected

and hopefully, won't hit planet Earth.

And the great thing about this method

is that it's already been tested.

In 2022, the DART mission flew into an asteroid, Dimorphos,

changing its orbit significantly.

But there are a few problems with this.

Firstly, how much of a nudge do we need?

If we don't nudge far enough, we'll still get an impact.

And if we nudge too far, for smaller asteroids,

they might crumble and break up into many pieces.

Not good.

The second technique I want to talk about involves this.

Yeah, it's a tractor, but it's a gravity tractor.

So this will actually be a spaceship which will fly

alongside the asteroid.

Its gravitational force will pull the asteroid towards it.

This will again cause a deflection.

The gravitational tractor technique is quite desirable

because we don't actually impact the asteroid at all.

It just sits alongside it

and uses that gravitational force to deflect it.

But the problem is,

we can't put anything too massive in space.

And so as it's causing the deflection,

it needs to do that for quite some time

to make sure that the asteroid will miss planet Earth.

Now our last technique is usually what Hollywood go for.

And it involves using a nuclear bomb.

Now, if the asteroid is quite close to Earth and we don't

have much time, using something like DART to move the asteroid away

might not be enough. And that's where the nuke comes in.

We can embed the nuke into the asteroid and then blow it up.

We crush up the asteroid.

SHE LAUGHS

I'm making a bit of a mess!

But we need to be careful with this technique,

because now, instead of just one lump heading towards Earth,

we have multiple lumps heading towards Earth,

which means they'll have a greater impact area,

potentially affecting many more people.

Now, to understand which of these tools we can use

for an asteroid strike, we need to get a better understanding

of the asteroid. One I prepared earlier!

And it's almost as if we need an eye in the sky.

Cue the James Webb Space Telescope.

When ground-based telescopes look up at space,

they have the atmosphere to contend with.

This means that images get blurred and measurements get distorted.

But JWST gets around this.

By sitting a million miles above the atmosphere,

its highly specialised mirrors can capture even

the faintest amounts of infrared light,

allowing for incredible accuracy.

It's further helped by an extreme cooling system.

The spectrograph on board JWST is cooled to just seven degrees

above absolute zero. All of this allows JWST

to completely zone in on an object with minimal interference,

which means astronomers can measure the sizes of incoming asteroids

to a level of precision just not feasible from the ground.

And in March, observations from JWST revealed why R4

was about 60 metres across.

A detail no ground-based telescope was able to supply.

This key information will be invaluable

for any future asteroid strikes.

So, we know something's coming and we know what size.

But another thing we might want to know is, what's it made of?

MISSION CONTROL: Three, two, one, lift-off!

Luckily, there is one mission that can provide some answers.

Atlas V takes flight.

Sending Lucy to uncover the fossils of our solar system.

Launched in 2021,

the Lucy mission is on a journey to explore

a record breaking number of asteroids in our solar system.

With 11 asteroids to explore,

the Lucy mission is on course to provide us with

a whole new understanding of the smorgasbord of asteroid types

that could be headed our way.

I'm meeting Dr Carly Howett,

who has worked on the mission from the very beginning.

Let's talk about Lucy! Let's.

Now, obviously when we think about asteroids, we think asteroid belt.

But that's not where Lucy's going to end up going?

That's right. The asteroid belt's very cool.

Lots of good science there. But we're going to a different set

of asteroids. So these are known as Jupiter's trojan asteroids,

but they're not orbiting Jupiter.

They're sort of orbiting with Jupiter.

So why these ones in particular?

What's special about them versus the asteroid belt?

Well, this region of space that they exist in are known as

Lagrangian points. And they're really stable regions of space.

If you get caught there, the gravity of the situation

all sort of cancels out, basically, there's not a way, an easy way out.

And so we think when the solar system was formed

and debris was flying everywhere, anything that got caught

in these regions of space got stuck there.

And so they're sort of giving us a little glimpse back

in what life could have been like in the very early solar system.

By observing more and more asteroids,

we can build our understanding of what could be heading our way.

So what do we know already about these objects?

Are we able to categorise them at all? A little bit, yeah.

So we can see things from the Earth.

We've got telescopes that point at the sky, and largely they fall into

a certain number of categories. So there are things like iron asteroids

that we think were probably the beginnings of a planet,

but the outside got lost, and so we're left with this iron core,

a bit like what would happen on the Earth if it lost everything

above its core. You then have sort of stony, pebbly asteroids that,

erm, that might be held together quite loosely.

And then, what we're looking at with the Lucy mission,

which are primitive asteroids. So asteroids that have stayed

a long way from the Sun, they haven't had that thermal change

that the Sun might produce, and we don't really know what's going on

in their interior. And that's something we're going to go

and try and find out. So when we understand more about

each of those classes, we'll have a better idea of, in the future,

if something is perhaps going to be a bit of a threat,

how to categorise it.

So how is Lucy going to be telling us more about

what these asteroids are made of, then?

So it's a difficult thing to see inside an asteroid,

especially if you're doing a flyby.

But we can tell a lot just by looking at the surface.

So we have some cameras, we have black and white cameras

and colour cameras, but we also have ways

of understanding what the composition is.

So we have an infrared spectrometer which measures

those compositional fingerprints.

And we can understand what the surface is made from, from that.

And then we have a thermal camera on there as well.

And so by putting all of these pieces of information together,

along with the brightness and the shape, and all

that sort of good stuff, we're going to be able to get

a really good idea of what's going on in these asteroids

in a way that we haven't been able to until now.

How does this help us in the context of maybe

a future YR4-type threat? So the more we know about asteroids,

the better. So if we understand what they're made from,

if we understand what they typically look like,

all of those things can help us develop our models of how these

asteroids evolve in their orbits over a matter of time.

So when something starts threatening the Earth,

it comes close to the Earth, the details matter.

If we can know what they look like,

if we know what their interiors are like,

we can start modelling those very fine effects

in a lot better detail.

When we get all this information about these asteroids,

how does it help us reduce the unknown unknowns?

That's the $64 million question, right?

One of the things that's really important,

if you want to deflect an asteroid, is to think about how well

the grains are in touch with each other.

You can imagine if you have something that's basically

like little polystyrene balls that are just touching,

held together with gravity or maybe static,

and you could just shove your way through them, right? But if you have

something that's clumped together, if it's really sort of

like an iron core, that's going to be a lot easier to move as one,

right? If you shove it, the whole thing's going to move

rather than collapse. And so when we start to think about how to deflect

or how to interact with asteroids

that are potentially a threat to Earth,

understanding what they're made of and how they're held together

is going to be really, really important.

The Lucy spacecraft is currently passing through

the asteroid belt towards Jupiter, and taking in some sights

along the way.

In 2023, it passed asteroid Dinkinesh,

discovering it also had two moons.

Not unknown for an asteroid,

but new objects that further our knowledge.

So what is the next big thing for Lucy, then?

Well, on Easter Sunday, so easy to remember,

we're going to be flying past our next main-belt asteroid.

So this one is Donaldjohanson.

It's larger than Dinkinesh that we flew by before.

Why is Donaldjohanson exciting, then?

Well, any time you see something new in the solar system,

you learn lots of things that you didn't even know to ask.

So there's just pure exploration,

but we see that the way it reflects the light's really interesting.

So we think it's either a really interesting shape

or perhaps it has a moon, we'll find out.

Very exciting, then!

It's reassuring to know

that our understanding of these potential threats is growing,

but if an asteroid was on course to strike,

who will do something about it?

Or who are we going to call?

Maggie's in Southampton to find out.

It turns out that astronomy is pretty good

at working out the odds of an asteroid strike,

but that might be the easy bit.

After that comes a complex discussion

involving philosophy, ethics and geopolitics.

I'm meeting Professor Hugh Lewis,

who represents the UK on a global approach to asteroid threats.

Nice to meet you.

We're sitting here sort of surrounded by these flags.

I guess this is a global threat,

and so you are part of a network, a global network,

that is considering this and looking at this

and trying to assess these challenges.

Yeah, so after the incident

involving the asteroid over Russia, Chelyabinsk...

Yes. ..more than a decade ago now,

that really prompted the United Nations to take some action.

The UN were reacting to an 18-metre asteroid

that flashed across Chelyabinsk in 2013.

It released the equivalent energy of 20 atomic bombs

and created a shock wave that injured 1,500 people.

It was a stark reminder of the threat that asteroids can pose.

The United Nations recommended the creation of two groups.

One was the International Asteroid Warning Network,

and that's the group of scientists and facilities

that would observe the solar system

and identify the potentially hazardous objects

and then issue warnings.

And then the second group

is the Space Mission Planning Advisory Group,

commonly referred to as SMPAG, erm...

and that's the group that I was a member of.

I see, yes. And this was the group of engineers, scientists and experts

who would consider how we would go about deflecting

or mitigating the threat from an asteroid using a space mission.

I think one of the challenges is that often many countries

are exposed to a risk for an asteroid impact.

Yes. And the problem is is that those countries

don't necessarily have the capability to take action.

Yes. So they have to rely upon other countries who do,

and that means that we need to have that international response

and community that works together. Yes.

An asteroid is heading towards a country

that doesn't have a space capability,

so it's reliant on those that do,

and any space capability will be quite expensive.

So weighing those two up seems a challenge. Yeah.

It is, and I think that's one of the key aspects

of the involvement through the United Nations

and the international community

to understand that particular issue and to try and address it.

Right. Gosh, it's a quagmire.

Are there conferences on things like this?

Yeah, so every two years, scientists and engineers

get together for the Planetary Defence Conference.

That sounds like...the Planetary Defence League, superheroes!

Yeah, I guess you can see it in that way.

These are people who are trying to

understand the threat from asteroids

and what to do about it.

So, at these conferences, what sort of things do they do?

This particular conference is really quite unusual,

because they run a kind of a simulated exercise,

where they create an asteroid,

an asteroid that has a potential impact with Earth,

and they ask the participants at the conference to play particular roles.

It could be journalist, it could be scientist, erm...

it could be, erm... you know, part of the government.

And then they present you with the information about the asteroid

in kind of pieces, they give you a little bit of information at a time,

and you have to make decisions.

Yes, so drip-feed the information, as it would be in a real situation.

Exactly right. Right.

And then in some cases, the asteroid ultimately misses,

yet you still have to run through the scenarios

and make sensible decisions.

And in some cases, the asteroid does hit.

And I think it really does showcase the fact

that whenever we're dealing with this particular threat,

it's never certain.

There's always a chance that it misses us.

But if we don't know that for absolute certain,

an absolute certainty is a hard thing to get.

And it's that kind of scenario that they use

at the Planetary Defence Conference.

It's never, "It's a huge asteroid,

"it's definitely going to hit, what do we do?" Yes!

You have to kind of really consider what the...

..not just what the numbers are telling you,

but what the moral imperative is in terms of protecting the planet.

That's why it's called "planetary defence", not UK defence.

Yes! And so the fact that we are working together,

I find quite comforting.

CHRIS: While space can seem like a violent, scary place...

..it's good to stop, look up

and enjoy our beautiful universe,

asteroids and all.

You can even see a few yourselves.

Our resident stargazer, Pete, is on hand to guide us

through some of the upcoming highlights in the night sky.

For amateur astronomers, the big news while we were off air

was the display of planets that were out in force.

Now most of them are disappearing from view,

but Jupiter and Mars are still there to spot,

visible in the early evening sky.

Jupiter is especially obvious,

low above the west-northwest horizon as darkness falls.

It can be seen in conjunction with a beautifully thin,

13%-lit waxing crescent moon on the 30th of April.

A wonderful sight if the weather is clear.

Another easy spot this month is the Lyrid meteor shower,

and this even has a tenuous link to asteroids,

because the bodies which create the debris stream

which passes through the Earth's atmosphere

to create a meteor shower,

are comets or sometimes asteroids.

A meteor shower's radiant is the small area of sky

its meteors appear to emanate from,

which in this case is near the bright star Vega.

The shower reaches its peak around midday on the 22nd of April,

meaning the best nights for a watch will be

the nights of the 21st of April into the morning of the 22nd,

and the 22nd of April into the morning of the 23rd,

with the most activity seen just before dawn.

Watching Lyrid meteors is a fairly relaxed affair.

All you have to do is make yourself comfortable,

find a good dark location

and keep watching the sky.

Look at a height about two-thirds of the way up the sky in any direction.

If you want to try your luck photographing Lyrid meteors,

it's fairly easy to do.

You just need a camera with a wide-angle fast lens.

You need to set the camera's ISO fairly high

and fully open the lens,

and then just take continuous images,

hoping to grab a meteor trail.

If your sky is light-polluted and bright,

your image may look like this.

If so, reduce the exposure length until they aren't overexposed

and you can still see stars.

Then just let it run for as long as you are out,

or your batteries allow, and see what happens.

You never know, you might get lucky.

But it's common to take thousands of images

without a single meteor on any one of them.

But if you have more advanced kit,

there are two actual asteroids we'd like to point out.

Neither of them are going to be hitting the Earth,

but they are fairly easy to see.

The first is 4 Vesta,

which, with a diameter of 330 miles,

is the largest asteroid

in the main asteroid belt which lies between Mars and Jupiter.

To locate the asteroid,

use the two brightest stars in Libra,

Alpha Librae, or Zubenelgenubi,

and Beta Librae, or Zubeneschamali.

Vesta will be located in northern Libra,

heading towards the border with Virgo as we head into next month.

Record the suspected star field over several days

where you think Vesta is located.

If you've got it right, comparing the records

should reveal Vesta as a moving star-like dot.

Owners of smart telescopes have got it easy

as all they have to do is search for Vesta and start imaging.

For those that like a bit of a challenge,

then there's 9 Metis,

which comes to opposition on the 9th of May.

"Opposition "is a term which describes

when an object is in the opposite position in the sky to the Sun,

so that means it's the best time to see it.

It's currently also within Libra,

and we can use the same two stars we used to locate Vesta

to find 9 Metis.

Just as with Vesta, record the star field over several nights.

Align the stars and look for the moving object.

Whether you're snapping Jupiter with your phone

or you manage to track Metis across the sky,

we'd love to see any photographs you've got,

so send them in to our Flickr account.

You can find details of this at

www.bbc.co.uk/skyatnight

and we'll show some of our favourites.

Meanwhile, here are some of the pictures you've sent in

while we were off air.

Happy stargazing! Goodnight.