0:00:02 > 0:00:04Space is a dangerous place.
0:00:06 > 0:00:09The Earth is hurtling through clouds of thousands of rocks
0:00:09 > 0:00:12as it journeys around the sun.
0:00:12 > 0:00:14Any one of them could collide with our planet
0:00:14 > 0:00:17and in the past, they have.
0:00:21 > 0:00:23So, in this programme we're talking about
0:00:23 > 0:00:26the awesome power of cosmic impacts.
0:00:26 > 0:00:27Welcome to The Sky At Night.
0:00:52 > 0:00:56We're here at Oxford University Museum of Natural History,
0:00:56 > 0:00:58home to the one of the most impressive collections
0:00:58 > 0:01:01of meteorites, lumps of rock which have collided with
0:01:01 > 0:01:02the atmosphere and fallen to Earth.
0:01:02 > 0:01:05Tonight, we're exploring the remarkable ways
0:01:05 > 0:01:08that cosmic impacts have shaped the universe around us.
0:01:08 > 0:01:10Coming up...
0:01:10 > 0:01:15Hollywood loves the idea of big impacts but the threat is real.
0:01:15 > 0:01:18We think that every year, as many as 20
0:01:18 > 0:01:21potentially destructive pieces of rock pass so close
0:01:21 > 0:01:24they travel between the Earth and the Moon.
0:01:25 > 0:01:30So we're asking you to join us on a hunt for near Earth asteroids.
0:01:32 > 0:01:35Materials specialist Mark Miodownik is here
0:01:35 > 0:01:38to explain the extraordinary physics that happens
0:01:38 > 0:01:41when something collides with our planet.
0:01:41 > 0:01:42Ready. Firing.
0:01:45 > 0:01:48We'll be finding out how the biggest impact in Earth's history
0:01:48 > 0:01:53gave birth to our celestial partner, the Moon.
0:01:53 > 0:01:55Most of the Earth probably melted
0:01:55 > 0:01:58and it would have been left in the state of being a global magma ocean.
0:01:58 > 0:02:04And impacts on a truly epic scale - what happens when galaxies collide.
0:02:09 > 0:02:12But first, impacts that are closer to home.
0:02:12 > 0:02:15These rocks that we're holding are just two of the meteorites
0:02:15 > 0:02:17that have come from space into the museum's collection
0:02:17 > 0:02:20and on this one, if you look carefully, you can see this crust.
0:02:20 > 0:02:24That was formed as this rock plunged through the Earth's atmosphere.
0:02:24 > 0:02:27It's estimated that 100,000 tonnes of material
0:02:27 > 0:02:31hits the Earth every year, most of it dust and small rocks
0:02:31 > 0:02:33that burn up in the atmosphere.
0:02:33 > 0:02:36These tiny fragments we see as shooting stars.
0:02:36 > 0:02:38But not all impacts are so benign.
0:02:38 > 0:02:41We know they have the power to reshape our planet,
0:02:41 > 0:02:45to affect the course of evolution, maybe even disturb our civilisation.
0:02:47 > 0:02:50Viewed from space, it's possible to see the scars
0:02:50 > 0:02:52left behind by collisions in Earth's past.
0:02:54 > 0:02:57The famous Barringer Crater in Arizona
0:02:57 > 0:03:00was created by a rock just 45m in diameter.
0:03:02 > 0:03:05The Manicouagan Crater in Canada, at 70km wide,
0:03:05 > 0:03:08is one of the most impressive craters
0:03:08 > 0:03:12and was formed by an impact over 200 million years ago.
0:03:14 > 0:03:19Most craters are destroyed by Earth's constantly changing surface,
0:03:19 > 0:03:22but the Vredefort Crater in South Africa
0:03:22 > 0:03:25is another rare example of a crater that has survived.
0:03:27 > 0:03:30It was carved out some two billion years ago
0:03:30 > 0:03:33as a colossal 300km wide basin.
0:03:33 > 0:03:36The energies involved in these impacts are huge
0:03:36 > 0:03:41and it makes the rocks and minerals behave in extraordinary ways.
0:03:41 > 0:03:43We asked material scientist Mark Miodownik
0:03:43 > 0:03:45to take a look at exactly what happens
0:03:45 > 0:03:48when an object collides at high speed with a planet.
0:03:52 > 0:03:54I've spent my working life studying materials -
0:03:54 > 0:03:56how they behave and interact.
0:03:56 > 0:03:58It's a science that underpins the modern world.
0:03:58 > 0:04:00I mean, everything is made from something.
0:04:00 > 0:04:05And in an everyday situation like this we understand how stuff works.
0:04:05 > 0:04:07We can predict their properties.
0:04:07 > 0:04:10One thing we've learned is that materials behave differently
0:04:10 > 0:04:12depending on the conditions.
0:04:12 > 0:04:14For instance, temperature.
0:04:14 > 0:04:16If you heat metal up it gets softer
0:04:16 > 0:04:18but if you cool it down it becomes brittle
0:04:18 > 0:04:21and sometimes, those differences are extreme.
0:04:23 > 0:04:25Conditions don't get much more extreme than when
0:04:25 > 0:04:30an asteroid travelling at over 40,000mph comes to a sudden stop
0:04:30 > 0:04:33as it crashes into the surface of a planet or a moon.
0:04:35 > 0:04:38At these speeds you enter the world of extreme physics,
0:04:38 > 0:04:41where everyday materials like metal and rock
0:04:41 > 0:04:43behave completely differently.
0:04:43 > 0:04:46Understanding what happens at the moment of impact
0:04:46 > 0:04:49helps explain the different effects meteorites have
0:04:49 > 0:04:50when they crash into a planet.
0:04:52 > 0:04:56I've come to Cranfield University, where the dynamic response group
0:04:56 > 0:04:59can recreate meteorite impacts in miniature
0:04:59 > 0:05:03and reveal the key transformations that occur.
0:05:03 > 0:05:06To see it, we're going to fire an object faster than
0:05:06 > 0:05:08the speed of sound at a solid target.
0:05:10 > 0:05:11I've got a lead ball
0:05:11 > 0:05:14and we're going to shoot it using this high-pressure gas gun.
0:05:14 > 0:05:17High-pressure helium is going to shoot it down the barrel.
0:05:17 > 0:05:19It's going to reach speeds of up to 350 metres per second.
0:05:19 > 0:05:22It's then going to hit the target.
0:05:22 > 0:05:24It's going to come to a dead stop
0:05:24 > 0:05:27and that whole process is going to take about a 100th of a second.
0:05:31 > 0:05:35Now, this is a fraction of the speed of a meteorite strike
0:05:35 > 0:05:37but even at this slower speed,
0:05:37 > 0:05:40the energy levels involved are still large enough
0:05:40 > 0:05:43to mimic one of the key processes at work in asteroid impacts.
0:05:45 > 0:05:48- OK, Dave. Tell me when. - Just about.- Just about.
0:05:48 > 0:05:50And as that energy is released,
0:05:50 > 0:05:54it's going to make a solid metal act in an extraordinary way.
0:05:54 > 0:05:57OK, ready. Firing.
0:06:01 > 0:06:03- Clear?- Clear!
0:06:06 > 0:06:07Now we can see the damage.
0:06:10 > 0:06:12Oh, yeah.
0:06:12 > 0:06:17Very nice crater produced by that impact.
0:06:17 > 0:06:19Have a look at that.
0:06:19 > 0:06:22Now, that is a shape you see all over the universe
0:06:22 > 0:06:25and what you're seeing is this material behaving
0:06:25 > 0:06:27not like you would expect a solid to behave,
0:06:27 > 0:06:29but actually, more like a fluid.
0:06:29 > 0:06:32Now, all that happened pretty quickly
0:06:32 > 0:06:36and even when we slow the footage down 800 times
0:06:36 > 0:06:39the moment of impact lasts just a second.
0:06:39 > 0:06:41The kinetic energy of the projectile
0:06:41 > 0:06:44travelling at faster than the speed of sound
0:06:44 > 0:06:47has to be absorbed into the target as it comes to a sudden stop
0:06:47 > 0:06:51and it's that shock wave of energy that causes the material to flow.
0:06:51 > 0:06:55This liquid-like behaviour is key to understanding what happens
0:06:55 > 0:07:00during meteorite impacts and it also provides another way to study them.
0:07:00 > 0:07:02So, one of the simplest ways of looking at this
0:07:02 > 0:07:05and exploring the mechanics of asteroid impact
0:07:05 > 0:07:07is to look at the impacts of fluids.
0:07:07 > 0:07:08And there's an easy way to do that,
0:07:08 > 0:07:11which is just get a pipette and a bowl of water
0:07:11 > 0:07:14and impact one fluid - i.e. a drop of water -
0:07:14 > 0:07:17into another, which is the bowl of water.
0:07:17 > 0:07:21The fact that in the moment of impact solids act like fluids
0:07:21 > 0:07:24means that you can use something as simple as a water droplet
0:07:24 > 0:07:29to reveal the processes at work when a meteorite hits the Earth.
0:07:29 > 0:07:30So, here comes the droplet.
0:07:30 > 0:07:32And as it impacts the surface,
0:07:32 > 0:07:34you can see that immediately it starts to flow.
0:07:34 > 0:07:36Now, of course it does, it's a droplet,
0:07:36 > 0:07:39but this also happens in rock.
0:07:39 > 0:07:40And in metal, as we saw earlier.
0:07:42 > 0:07:45But, still, you can just see the hemispherical droplet
0:07:45 > 0:07:47so, half of the droplet, in a way,
0:07:47 > 0:07:50doesn't know that the front end has hit.
0:07:50 > 0:07:53It's still going. The momentum's carrying it forward and off it goes,
0:07:53 > 0:07:56making the crater deeper and deeper and deeper.
0:07:56 > 0:07:59The result is that the back of the droplet
0:07:59 > 0:08:03drives on through the middle, pushing the surface out and up.
0:08:03 > 0:08:07Meanwhile, the shock wave is making it wider and wider as it comes out
0:08:07 > 0:08:13and on the process goes until you get this classic crater shape.
0:08:13 > 0:08:16It's only because both the surface and the projectile flow
0:08:16 > 0:08:19that you get this unique shape.
0:08:19 > 0:08:21Now, this looks like it's all finished but actually,
0:08:21 > 0:08:22the impact is still going on.
0:08:22 > 0:08:25The momentum of the droplet has gone all the way down to the bottom
0:08:25 > 0:08:28of the crater and in a minute will come back up again as it rebounds.
0:08:28 > 0:08:30There it is!
0:08:30 > 0:08:31So, it's rebounding back up.
0:08:33 > 0:08:35And the extraordinary thing is that you see this effect
0:08:35 > 0:08:40also in real impacts in asteroids and meteors.
0:08:40 > 0:08:46We can see it on the Moon, on Mars and even here on Earth.
0:08:48 > 0:08:49Now, these experiments give us
0:08:49 > 0:08:54an idea of certain types of impacts and the craters they form,
0:08:54 > 0:08:56but there are still many types of craters
0:08:56 > 0:08:58that we don't fully comprehend.
0:09:00 > 0:09:03Dr Ken Amor is an impact crater specialist.
0:09:03 > 0:09:05What are the things we see on the craters of other
0:09:05 > 0:09:08planets that we don't understand or don't quite make sense?
0:09:08 > 0:09:11So, the experiments that we do with a gas gun produce
0:09:11 > 0:09:14a very sort of simple bowl-shaped type of crater.
0:09:14 > 0:09:16And we can see from this picture, here,
0:09:16 > 0:09:18here we have two impact craters.
0:09:18 > 0:09:21The much smaller one is a younger impact crater
0:09:21 > 0:09:23and the larger one is a much older one.
0:09:23 > 0:09:26And this sort of smaller impact crater has the classic bowl shape,
0:09:26 > 0:09:28quite a sharp rim.
0:09:28 > 0:09:33But this is in marked contrast to the much larger impact crater.
0:09:33 > 0:09:36So, you lose this well-defined rim.
0:09:36 > 0:09:38You've got much more material slumping inwards
0:09:38 > 0:09:40and forming these terraces.
0:09:40 > 0:09:43You have a sort of flat-bottomed floor to the crater.
0:09:43 > 0:09:46And then the thing that we really don't understand very well at all
0:09:46 > 0:09:49is these central peak-type structures.
0:09:49 > 0:09:51There are a number of theories as to how they might form.
0:09:51 > 0:09:57So, rocks under normal pressures do behave in an elastic way.
0:09:57 > 0:10:03So, it's the bonds between the atoms, they're being compressed,
0:10:03 > 0:10:06and then once that pressure is released they're decompressing,
0:10:06 > 0:10:08just like a spring, and they're sort of bouncing back.
0:10:08 > 0:10:11So, there's a certain amount of elastic rebound going on
0:10:11 > 0:10:13which is pushing up the central peak.
0:10:13 > 0:10:16But because we can't really generate them in the gas gun experiments
0:10:16 > 0:10:19we don't fully understand what's going on.
0:10:19 > 0:10:21So you're saying that two different scales of impact,
0:10:21 > 0:10:25different physics operating in different crater characteristics.
0:10:25 > 0:10:26Is there a bigger scale still
0:10:26 > 0:10:29where you might even get a different morphology of crater?
0:10:29 > 0:10:30There is.
0:10:30 > 0:10:34So, the really large impacts which form the basins,
0:10:34 > 0:10:36particularly on the moon -
0:10:36 > 0:10:39and a really good example is called Mare Orientale -
0:10:39 > 0:10:42and this has this sort of concentric ring type structure,
0:10:42 > 0:10:45very much like a bull's-eye, over an enormous distance.
0:10:45 > 0:10:49And so, these are the really huge, enormous, perhaps anywhere
0:10:49 > 0:10:52in the region of sort of 50 to 100km diameter impactors.
0:10:52 > 0:10:57Do we know how they form? Is the physics becoming clear?
0:10:57 > 0:11:00That's really where our physics lets us down.
0:11:00 > 0:11:04It would be difficult to reproduce that type of scenario
0:11:04 > 0:11:07within a gas gun experiment.
0:11:07 > 0:11:10Very interesting. Thank you.
0:11:15 > 0:11:17And next, we're sticking with the Moon.
0:11:17 > 0:11:20We used to think about impacts as destructive processes,
0:11:20 > 0:11:24things that vaporise rocks and wipe out dinosaurs.
0:11:24 > 0:11:26But impacts can be creative, too.
0:11:26 > 0:11:30And that's especially true for the biggest impact in Earth's history,
0:11:30 > 0:11:33something so large that it reshaped our planet
0:11:33 > 0:11:36and probably caused the formation of the Moon.
0:11:36 > 0:11:38I've been speaking to lunar expert Sarah Russell
0:11:38 > 0:11:40about this unique event.
0:11:40 > 0:11:44Hi, Sarah. I'm a lunatic and I love everything about the Moon.
0:11:44 > 0:11:47So, can you tell me how we think the Moon was formed?
0:11:47 > 0:11:49Well, before the Apollo missions actually brought stuff
0:11:49 > 0:11:51back from the Moon,
0:11:51 > 0:11:53there were three main theories that were around.
0:11:53 > 0:11:57One was that the Moon was a captured asteroid,
0:11:57 > 0:11:59so an asteroid just got too close to the Earth
0:11:59 > 0:12:02and it got captured by the Earth's gravitational field to form a moon.
0:12:02 > 0:12:05And that's how we think many moons form in the solar system?
0:12:05 > 0:12:08Yes, exactly. So, the moons of Mars, for example, Phobos and Deimos,
0:12:08 > 0:12:10are probably captured asteroids.
0:12:10 > 0:12:12But the moons of Mars are much smaller compared to the size
0:12:12 > 0:12:15of the planet than our moon is compared to the size of the Earth.
0:12:15 > 0:12:19Another theory is that the Earth and Moon are created together,
0:12:19 > 0:12:22that they just formed, they were always twin planets.
0:12:22 > 0:12:24- Like a binary system? - Like a binary system, exactly.
0:12:24 > 0:12:27But that doesn't quite work either
0:12:27 > 0:12:29because the lunar core is very, very small
0:12:29 > 0:12:33and you would expect it to be the same proportion of the Moon
0:12:33 > 0:12:35as our core is to the Earth.
0:12:35 > 0:12:37And the other theory is that when the Earth formed,
0:12:37 > 0:12:42it was spinning so fast that a blob just sort of got flung off the Earth
0:12:42 > 0:12:44and that went on to form the Moon.
0:12:44 > 0:12:46So, it's coming off fast enough to be thrown off
0:12:46 > 0:12:49- but yet it's still near enough to be captured.- Exactly.
0:12:49 > 0:12:52- The dynamics of that... - ..are tricky, yes!
0:12:52 > 0:12:54But then, when the Apollo rocks were brought back
0:12:54 > 0:12:57and the lunar rocks were brought back by the Russians,
0:12:57 > 0:13:01we had a chance to look in detail at the chemistry of the Moon rocks.
0:13:01 > 0:13:04And a consensus started to merge in the 1980s about
0:13:04 > 0:13:06how we really think the moon formed
0:13:06 > 0:13:10and we think it formed by this massive collision of a planet
0:13:10 > 0:13:14about the size of Mars, which is sometimes called Theia,
0:13:14 > 0:13:15smashing into the early Earth.
0:13:15 > 0:13:18We've got an artist's impression here.
0:13:18 > 0:13:21So, it was a pretty catastrophic event both for Theia
0:13:21 > 0:13:22and for the Earth.
0:13:22 > 0:13:24What happened to the Earth after that?
0:13:24 > 0:13:26Most of the Earth probably melted
0:13:26 > 0:13:29and it would have been left in the state of being a global magma ocean
0:13:29 > 0:13:31with bubbling silicate rock covering its whole surface
0:13:31 > 0:13:33and then most of the stuff that formed the Moon
0:13:33 > 0:13:35would have originated from Theia,
0:13:35 > 0:13:37and that's the stuff that got thrown out from the Earth.
0:13:37 > 0:13:39So, we have these rocks, you've analysed them.
0:13:39 > 0:13:41What in the composition is telling us this?
0:13:41 > 0:13:44Well, there are a few lines of evidence but the main one
0:13:44 > 0:13:49is that the composition of the Earth and the Moon is surprisingly similar,
0:13:49 > 0:13:52and that suggests that they must share some genetic link.
0:13:52 > 0:13:54So, you've analysed these samples -
0:13:54 > 0:13:56just samples returned from the Moon or do
0:13:56 > 0:13:58we have any other Moon samples?
0:13:58 > 0:14:00Well, since the Apollo missions happened,
0:14:00 > 0:14:05we actually found out that some samples of the Moon come to Earth
0:14:05 > 0:14:08naturally in the form of meteorites, and I've got one here to show you.
0:14:08 > 0:14:11This one was found in the Sahara desert
0:14:11 > 0:14:14and it's a sample of what's been called the lunar highlands,
0:14:14 > 0:14:15which is the pale part of the Moon.
0:14:15 > 0:14:18One of the great things about lunar meteorites is that
0:14:18 > 0:14:21they randomly sample the whole surface of the Moon,
0:14:21 > 0:14:24so they give us a better idea of the global composition of the Moon.
0:14:24 > 0:14:26Whereas the Apollo astronauts just visited
0:14:26 > 0:14:28a very tiny proportion of the Moon.
0:14:28 > 0:14:31So, these are fantastic at giving us
0:14:31 > 0:14:33extra clues about the rest of the Moon.
0:14:33 > 0:14:35And by analysing these,
0:14:35 > 0:14:39we're still finding a high percentage of Earth-like substances?
0:14:39 > 0:14:42Yeah, so one of the main arguments in favour of Theia was
0:14:42 > 0:14:47the similarity between the Apollo rocks and the terrestrial rocks.
0:14:47 > 0:14:49But, actually, geochemists are never happy
0:14:49 > 0:14:51and doing more and more analyses,
0:14:51 > 0:14:53they found out actually they're too similar.
0:14:53 > 0:14:54Too Earth-like?
0:14:54 > 0:14:57They're far too Earth-like, which is actually a problem
0:14:57 > 0:15:00because the modelling suggests that most of the Moon
0:15:00 > 0:15:03should actually be made of Theia and by all probability,
0:15:03 > 0:15:05Theia would have a different composition to the Earth.
0:15:05 > 0:15:07So, it could have been a collision of two bodies
0:15:07 > 0:15:09that were a similar size to each other,
0:15:09 > 0:15:12and then they would have mixed up more thoroughly.
0:15:12 > 0:15:15Or it could be that there was a period after the collision
0:15:15 > 0:15:18where there was a hot vapour that allowed
0:15:18 > 0:15:20everything to equilibrate and everything to mix up.
0:15:20 > 0:15:23But this is all research that is ongoing at the moment.
0:15:23 > 0:15:25What we really need, Maggie, is to go back to the Moon.
0:15:25 > 0:15:27I'm volunteering! I want to go!
0:15:27 > 0:15:30So, if we can get samples - the problem with the Apollo missions
0:15:30 > 0:15:33is that they only sampled a very small part of the Moon and the problem with meteorites
0:15:33 > 0:15:37is that they sample probably most of the Moon, but we don't know exactly where they're from.
0:15:37 > 0:15:40So, we need to go back to targeted areas to get new samples
0:15:40 > 0:15:43and then we can solve this problem.
0:15:43 > 0:15:47- Well, thank you very much, and sign me up!- Thank you, Maggie.
0:15:53 > 0:15:54Coming up...
0:15:54 > 0:15:58How you can help protect the Earth from deadly meteorite strikes
0:15:58 > 0:15:59by hunting for asteroids
0:15:59 > 0:16:02that could be on a collision course with our planet.
0:16:04 > 0:16:07But first, Pete has this month's Star Guide,
0:16:07 > 0:16:11including a tour of the spectacular craters on the Moon.
0:16:13 > 0:16:16When it comes to impact, there's no better place
0:16:16 > 0:16:19for amateur astronomers to see their lasting effects than on the Moon.
0:16:20 > 0:16:25There are over 300,000 impact craters at least 1km wide on its surface
0:16:25 > 0:16:29and many more smaller ones that we're still counting.
0:16:31 > 0:16:33And on the Moon, they're perfectly preserved,
0:16:33 > 0:16:36frozen in time for us to admire and study.
0:16:38 > 0:16:39The wonderful thing about the Moon
0:16:39 > 0:16:42is that so long as it's above the horizon
0:16:42 > 0:16:45it can be seen at any time and from anywhere.
0:16:45 > 0:16:46Even here, in the centre of London,
0:16:46 > 0:16:49where it's impossible to escape the city lights,
0:16:49 > 0:16:51we can still enjoy what the moon has to offer.
0:16:52 > 0:16:55After new moon, the first phase which allows you
0:16:55 > 0:16:59to see some surface detail is the waxing crescent.
0:16:59 > 0:17:03This is when just a thin sliver of moon is visible in the sky.
0:17:03 > 0:17:04Close to the terminator,
0:17:04 > 0:17:07that's the line which divides the lunar night and day,
0:17:07 > 0:17:10that's when you'll see the fantastic shadows
0:17:10 > 0:17:13which really define some of the craters there.
0:17:13 > 0:17:14Five days after new moon,
0:17:14 > 0:17:18look out for a trio of craters in the south-east quadrant.
0:17:18 > 0:17:22Called Theophilus, Cyrillus and Catharina,
0:17:22 > 0:17:24they're easy to see with binoculars.
0:17:24 > 0:17:28As the nights pass, the terminator sweeps across the lunar landscape
0:17:28 > 0:17:32revealing more of the Moon's surface to us.
0:17:32 > 0:17:35At first quarter, the Moon appears in the sky as a half circle
0:17:35 > 0:17:39and in the following days, two really impressive examples
0:17:39 > 0:17:42of a distinctive type of crater become visible.
0:17:42 > 0:17:45These are Tycho and Copernicus and they're ray craters
0:17:45 > 0:17:47and they're really quite spectacular.
0:17:47 > 0:17:50These spiderlike patterns of bright rays
0:17:50 > 0:17:53can extend for hundreds of kilometres.
0:17:53 > 0:17:55They form because at the moment of impact,
0:17:55 > 0:17:59light-coloured material from underneath the surface is pulled up,
0:17:59 > 0:18:01creating the streaks.
0:18:03 > 0:18:07Finally, two weeks after the new moon we arrive at the full moon.
0:18:07 > 0:18:10This is when the full face of the Moon is illuminated,
0:18:10 > 0:18:12which is what we've got tonight.
0:18:12 > 0:18:13Now, when that occurs,
0:18:13 > 0:18:16the Sun's light is falling straight down onto the lunar surface
0:18:16 > 0:18:18and there are no shadows cast,
0:18:18 > 0:18:21so it's really difficult to make out any crater detail.
0:18:21 > 0:18:23But there is something else which is on view.
0:18:25 > 0:18:30A huge circle of dark lava covers the northern hemisphere.
0:18:30 > 0:18:34It's over 1,000 kilometres wide and at least three billion years old.
0:18:35 > 0:18:37It's called Mare Imbrium
0:18:37 > 0:18:41and though it looks very different to the classic craters we've seen,
0:18:41 > 0:18:43it is in fact a giant impact basin,
0:18:43 > 0:18:47one of about 40 dark basins that litter the Moon's surface.
0:18:50 > 0:18:53And there are lots of other great things to see this month
0:18:53 > 0:18:55besides the Moon, so here's this month's Star Guide.
0:18:57 > 0:19:02It's a great time of year to see the Milky Way.
0:19:02 > 0:19:05The three bright stars, Deneb, Vega and Altair,
0:19:05 > 0:19:09form a large pattern known as the Summer Triangle.
0:19:09 > 0:19:11This is visible towards the South,
0:19:11 > 0:19:14roughly two thirds of the way up the sky during June.
0:19:16 > 0:19:18Deneb is the brightest star in Cygnus,
0:19:18 > 0:19:22a constellation with another distinctive pattern in its centre
0:19:22 > 0:19:23known as the Northern Cross.
0:19:26 > 0:19:27If you have dark skies,
0:19:27 > 0:19:31look out for the Milky Way passing down the length of the cross.
0:19:32 > 0:19:33The Milky Way is the merged light
0:19:33 > 0:19:36of billions of distant stars in our own galaxy,
0:19:36 > 0:19:39too distant to be seen individually with the naked eye.
0:19:42 > 0:19:45Dark dust blocks this delicate light in Cygnus
0:19:45 > 0:19:48and the Milky Way appears to split in two.
0:19:48 > 0:19:50The split is known as the Cygnus Rift
0:19:50 > 0:19:53and it's a magnificent sight in a dark sky.
0:19:57 > 0:20:01When we think of impacts, we tend to think of our own solar system
0:20:01 > 0:20:03but, of course, collisions are happening
0:20:03 > 0:20:06throughout the universe on many different scales.
0:20:06 > 0:20:09Chris is speaking to cosmologist Karen Masters
0:20:09 > 0:20:13about perhaps the biggest collisions of all - when galaxies collide.
0:20:15 > 0:20:19When we look beyond the Milky Way at other galaxies in the universe,
0:20:19 > 0:20:23we see that often these enormous structures are moving together.
0:20:24 > 0:20:28And in some cases, we can catch them in the moment of collision.
0:20:30 > 0:20:33We think that this is an important part of how the universe evolves
0:20:33 > 0:20:36and a trigger to make stars form.
0:20:39 > 0:20:40We're here to talk about collisions,
0:20:40 > 0:20:44and perhaps the most spectacular of all is going to happen
0:20:44 > 0:20:46to the Milky Way in a few billion years' time.
0:20:46 > 0:20:47So, what's going on?
0:20:47 > 0:20:51The universe is expanding but gravity is always acting.
0:20:51 > 0:20:53So, for example, the Milky Way and the Andromeda Galaxy,
0:20:53 > 0:20:55our nearest large spiral neighbour,
0:20:55 > 0:20:57the gravity between those two galaxies
0:20:57 > 0:21:00are pulling them together and we believe they're going to collide
0:21:00 > 0:21:04and merge sometime in about five or six billion years from now.
0:21:04 > 0:21:05So, that sounds spectacular,
0:21:05 > 0:21:07but what does that actually mean for our galaxy?
0:21:07 > 0:21:10What happens to the Milky Way when it undergoes such a merger?
0:21:10 > 0:21:13So, the distances between the stars and galaxies are vast
0:21:13 > 0:21:15compared to the size of stars.
0:21:15 > 0:21:17So, as the galaxies merge and collide,
0:21:17 > 0:21:20no two stars are ever going to hit each other
0:21:20 > 0:21:23but the combined gravitational action of all those stars moving together,
0:21:23 > 0:21:26they distort the galaxies, they pull out these amazing tidal arms
0:21:26 > 0:21:30and tidal tails, there are bridges built between the galaxies.
0:21:30 > 0:21:33You've brought along a simulation of the future of the Milky Way.
0:21:33 > 0:21:35So, here we are. This is the Milky Way.
0:21:35 > 0:21:37Yes, this is a visualisation
0:21:37 > 0:21:40where they've simulated two large spiral galaxies.
0:21:40 > 0:21:42That one's the Milky Way and here we have the Andromeda galaxy.
0:21:42 > 0:21:46The gravity between these two galaxies is pulling them together
0:21:46 > 0:21:48and it takes a few seconds in the movie but in reality,
0:21:48 > 0:21:50it could take about five billion years.
0:21:50 > 0:21:53You're going to see all sorts of structures shooting out of them.
0:21:53 > 0:21:56- You see these tidal tails, ridges... - Beautiful long arms.
0:21:56 > 0:21:58Yeah, beautiful long arms.
0:21:58 > 0:22:00These are all the stars from these galaxies
0:22:00 > 0:22:03being thrown out of the galaxy by the gravitational interaction.
0:22:03 > 0:22:05But you end up with something rather boring?
0:22:05 > 0:22:08Yeah, that's right. So the product of all this merging
0:22:08 > 0:22:11tends to be rather spherical in shape, rather boring - the elliptical galaxy.
0:22:11 > 0:22:13And what about the Sun? What will happen?
0:22:13 > 0:22:16I mean, imagine we're still here in six billion years,
0:22:16 > 0:22:18I'm sure The Sky At Night will still be going
0:22:18 > 0:22:21so, what should we be expecting to see and what happens to us?
0:22:21 > 0:22:24The sun will either remain in this remnant
0:22:24 > 0:22:25or become part of an elliptical galaxy.
0:22:25 > 0:22:28Or there's a small chance, something like 10%,
0:22:28 > 0:22:30that it'll get kicked out into intergalactic space.
0:22:30 > 0:22:34A rather sober thought. Has it happened to the Milky Way before?
0:22:34 > 0:22:37We know the Milky Way cannot have had a major merger
0:22:37 > 0:22:40where the thing that merged with it was comparable in size
0:22:40 > 0:22:43to the Milky Way itself in the last ten billion years
0:22:43 > 0:22:45because it has an incredibly thin disk.
0:22:45 > 0:22:49So, you can't sustain that kind of thin disk in a major merger.
0:22:49 > 0:22:51Because a merger would inevitably kick stuff up out of the disc?
0:22:51 > 0:22:54That's right, yeah. It kicks stuff up out of the disc.
0:22:54 > 0:22:57It would drive stars and gas into the centre of the galaxy
0:22:57 > 0:23:00and build the bulge, the spherical egg yolk component, of the galaxy.
0:23:00 > 0:23:03But there are other important effects as well.
0:23:03 > 0:23:05So, we've seen the change in the shape,
0:23:05 > 0:23:08but we also get things like star formation within the galaxies?
0:23:08 > 0:23:11Yeah, so galaxies aren't just made of stars.
0:23:11 > 0:23:14They also have quite a lot of neutral hydrogen gas,
0:23:14 > 0:23:15the fuel for star formation.
0:23:15 > 0:23:19And when the galaxies collide, those gas clouds can be put under pressure
0:23:19 > 0:23:21and that pressure can induce star formation.
0:23:21 > 0:23:23And so, we see bursts of star formation
0:23:23 > 0:23:25in galaxies that are colliding.
0:23:25 > 0:23:29And this is something that astronomers have changed their minds about just over the last few years.
0:23:29 > 0:23:33It's tempting to say, look, stars form in major mergers
0:23:33 > 0:23:35and therefore, most stars must form in mergers,
0:23:35 > 0:23:37- but that doesn't seem to be the case.- No.
0:23:37 > 0:23:40It makes a very nice story, I think, a very simple story.
0:23:40 > 0:23:43But these processes that compress the gas clouds that happen in mergers
0:23:43 > 0:23:46can happen due to other effects going on in the galaxy -
0:23:46 > 0:23:50the effects of the spiral arms, the gas clouds ride on them
0:23:50 > 0:23:53like surfers riding on waves in the ocean,
0:23:53 > 0:23:55and compress things and cost star formations.
0:23:55 > 0:23:57You don't need a major merger to form stars.
0:23:57 > 0:23:59- Karen, thanks a lot. - Thank you.
0:24:06 > 0:24:08Now, closer to home.
0:24:08 > 0:24:12We want you to join us on an asteroid hunt.
0:24:12 > 0:24:15The aim is to discover near Earth asteroids
0:24:15 > 0:24:17that have never been observed before.
0:24:17 > 0:24:19Objects that are close to our planet
0:24:19 > 0:24:22or cross its orbit and could potentially hit us.
0:24:24 > 0:24:26History has shown that in the past,
0:24:26 > 0:24:29meteor strikes can have a devastating effect on our planet
0:24:29 > 0:24:32like the ones that struck at the time of the dinosaurs.
0:24:35 > 0:24:4165 million years ago, a meteorite 10km wide crashed into the Earth.
0:24:47 > 0:24:49It unleashed an Armageddon on our planet,
0:24:49 > 0:24:54resulting in a nuclear winter that blocked out the Sun for decades.
0:24:54 > 0:24:56The impact led to the loss
0:24:56 > 0:24:59of three quarters of all species alive at the time.
0:25:01 > 0:25:04What I have in my hand is evidence for that impact,
0:25:04 > 0:25:07and it's one of the most remarkable things I've ever seen.
0:25:07 > 0:25:10This line, here, marks the point, the exact moment at which
0:25:10 > 0:25:12the asteroid that did for the dinosaurs hit the Earth.
0:25:12 > 0:25:16And up here above it is clay infused with iridium.
0:25:16 > 0:25:20That iridium is the fallout from the meteorite. It came from space.
0:25:20 > 0:25:22What's really remarkable is that
0:25:22 > 0:25:25this rock doesn't come from Mexico, where the impact occurred,
0:25:25 > 0:25:28but from Copenhagen, more than 5,000 miles away,
0:25:28 > 0:25:33showing that the asteroid changed the future of the entire world.
0:25:33 > 0:25:35But such big impacts are rare,
0:25:35 > 0:25:38maybe they happen once in a billion years or so,
0:25:38 > 0:25:41so what should concern us now are the smaller impacts.
0:25:42 > 0:25:45Because even the small ones can be dangerous.
0:25:45 > 0:25:47We are constantly being bombarded
0:25:47 > 0:25:50by rocks small by astronomical standards
0:25:50 > 0:25:53but big enough to create explosions with the kind of force
0:25:53 > 0:25:56associated with nuclear weapons.
0:25:56 > 0:25:58Since the year 2000, a network of sensors
0:25:58 > 0:26:00have been monitoring the planet
0:26:00 > 0:26:03with the aim of detecting nuclear explosions.
0:26:03 > 0:26:06In that time, it's detected 26.
0:26:06 > 0:26:12These apparent explosions range in force from 1 to 600 kilotons.
0:26:12 > 0:26:15That's 40 times the power of the Hiroshima bomb.
0:26:16 > 0:26:18But none were nuclear weapons.
0:26:18 > 0:26:22They were all asteroid impacts blowing up in the upper atmosphere.
0:26:22 > 0:26:26They reveal we are being bombarded at an alarming rate,
0:26:26 > 0:26:31at least three times more frequently than previously thought.
0:26:31 > 0:26:33All this demonstrates why predicting
0:26:33 > 0:26:37when and where a meteor might hit the planet is so important.
0:26:37 > 0:26:39We think we found most of the large ones
0:26:39 > 0:26:42but even the smaller asteroids could take out a city.
0:26:42 > 0:26:46The first step to protecting ourselves is to find them
0:26:46 > 0:26:47and track them and so far,
0:26:47 > 0:26:51we've managed to trace the orbits of 10,000 near Earth asteroids.
0:26:53 > 0:26:57But that's less than 1% of the city killers that might be out there,
0:26:57 > 0:26:59asteroids bigger than 30m which would hit the Earth
0:26:59 > 0:27:02with all the power of an atomic bomb.
0:27:02 > 0:27:05So, we need to find more and that's where you come in.
0:27:05 > 0:27:08We want to harness an ability that you have
0:27:08 > 0:27:11that beats most computers - the ability to detect movement.
0:27:11 > 0:27:13A special website has been designed
0:27:13 > 0:27:19based on images compiled over 20 years by the Catalina Sky Survey.
0:27:22 > 0:27:24Each patch of sky is imaged multiple times,
0:27:24 > 0:27:26normally ten minutes apart.
0:27:28 > 0:27:31And most things in the sky don't move on that timescale
0:27:31 > 0:27:33but near Earth asteroids will.
0:27:33 > 0:27:38And so, if you see something moving, you might just have caught one.
0:27:38 > 0:27:39So, go to asteroidzoo.org.uk
0:27:39 > 0:27:43or to the Sky At Night website, to find out how you can participate.
0:27:43 > 0:27:47Every image you see will also be checked by several other volunteers
0:27:47 > 0:27:51and we hope to follow up on the best targets with our telescopes.
0:27:51 > 0:27:53It's really easy to take part
0:27:53 > 0:27:57and we'll report back on what you've found in the next few months.
0:28:02 > 0:28:04So, that's it for this programme.
0:28:04 > 0:28:06Next month we'll be looking at what seasons are like
0:28:06 > 0:28:08on other planets in our solar system
0:28:08 > 0:28:11and how it's possible to do stargazing in the middle of the day.
0:28:11 > 0:28:14Our website at bbc.co.uk/skyatnight
0:28:14 > 0:28:17contains details of everything we've talked about on today's programme
0:28:17 > 0:28:20as well as details of more than 600 astronomical events
0:28:20 > 0:28:22happening all over the country.
0:28:22 > 0:28:26- In the meantime, get outside and get looking up.- Good night.