0:00:02 > 0:00:06This week, our skies will see a rare daytime astronomical event.
0:00:08 > 0:00:10On Monday, if the weather's better than this,
0:00:10 > 0:00:13we'll be able to watch a transit of Mercury
0:00:13 > 0:00:16when the solar system's smallest planet passes in front of the sun.
0:00:17 > 0:00:20Centuries of work allow us to predict precisely
0:00:20 > 0:00:22when this event will occur.
0:00:23 > 0:00:26But although we know exactly where Mercury is,
0:00:26 > 0:00:29it's a planet that we know surprisingly little about.
0:00:31 > 0:00:34I like to think of it as the solar system's problem child
0:00:34 > 0:00:37cos it so often confounds our expectations.
0:00:38 > 0:00:42A planet this close to the sun should be baked dry
0:00:42 > 0:00:45and yet there's ice on its surface.
0:00:47 > 0:00:51You'd such a tiny world to have solidified into
0:00:51 > 0:00:52an inactive ball of rock...
0:00:53 > 0:00:56..but Mercury is geologically alive.
0:00:58 > 0:01:00It even appears to be shrinking.
0:01:03 > 0:01:06On this month's programme,
0:01:06 > 0:01:09we'll be investigating Mercury's curious behaviour.
0:01:11 > 0:01:14And Pete Lawrence will be here to explain how you can watch
0:01:14 > 0:01:15the transit in safety.
0:01:16 > 0:01:20So, tonight, in all its glory, we give you Mercury -
0:01:20 > 0:01:23the most puzzling planet in the solar system.
0:01:23 > 0:01:25Welcome to The Sky At Night.
0:01:57 > 0:02:00At first glance, the thing that strikes you about Mercury
0:02:00 > 0:02:02is that it looks a lot like our moon -
0:02:02 > 0:02:05a bare, pale rock covered in craters.
0:02:06 > 0:02:08It's even a similar size.
0:02:10 > 0:02:13But in reality, Mercury is not at all like the moon.
0:02:13 > 0:02:17In fact, it's not like anywhere else in the solar system.
0:02:19 > 0:02:23Mercury has long been seen as an enigmatic planet,
0:02:23 > 0:02:26but it's difficult to observe because it's orbit is so close to the sun.
0:02:26 > 0:02:29So, whenever we look at it, it's lost in the sun's glare.
0:02:31 > 0:02:34Even so, with observations from Earth,
0:02:34 > 0:02:37we can map out Mercury's unusual orbit.
0:02:38 > 0:02:42Unsurprisingly, Mercury, being the closest planet to the sun,
0:02:42 > 0:02:44has the shortest orbit.
0:02:44 > 0:02:46It takes just 88 days to go all the way round.
0:02:46 > 0:02:48It also has the most elliptical orbit
0:02:48 > 0:02:51of all the planets in the solar system.
0:02:52 > 0:02:55At its closest approach, it's just 46 million kilometres
0:02:55 > 0:02:59away from the sun, and, at its furthest, 70 million kilometres.
0:02:59 > 0:03:01Now, that's unusual, but things get really weird
0:03:01 > 0:03:04when you take into account Mercury's rotation.
0:03:08 > 0:03:10Mercury rotates very slowly.
0:03:10 > 0:03:13It actually takes 59 Earth days for it to complete one revolution.
0:03:14 > 0:03:18But as it rotates, Mercury is also moving around its orbit.
0:03:19 > 0:03:22And the combination of rotation and orbit
0:03:22 > 0:03:25causes the sun to move very slowly across the sky.
0:03:26 > 0:03:30So slowly in fact that from the planet's surface, a day
0:03:30 > 0:03:34from sunrise to sunrise actually lasts two complete orbits.
0:03:34 > 0:03:38So, on Mercury, a day is twice as long as its year.
0:03:38 > 0:03:42That means that, on the surface, daylight lasts the equivalent of
0:03:42 > 0:03:47three Earth months and temperatures rise to around 450 Celsius.
0:03:47 > 0:03:49That's followed by three months of night-time,
0:03:49 > 0:03:52where the temperatures plummet to minus 180.
0:03:54 > 0:03:57This temperature difference of over 600 degrees
0:03:57 > 0:04:00is the highest experienced anywhere in the solar system.
0:04:03 > 0:04:07That made it all the more surprising when astronomers at the Arecibo
0:04:07 > 0:04:10radio telescope bounced radar pulses off the planet.
0:04:14 > 0:04:17The signals that came back showed bright spots near the poles
0:04:17 > 0:04:21that bore the unmistakable signature of water ice.
0:04:22 > 0:04:25The shape of the ice deposits clearly showed that they were
0:04:25 > 0:04:28concealed in the depths of polar craters -
0:04:28 > 0:04:32the only areas on the planet that never receive any direct sunlight.
0:04:34 > 0:04:36That means that, on this planet,
0:04:36 > 0:04:38three times closer to the sun than Earth,
0:04:38 > 0:04:40you can still find water ice.
0:04:40 > 0:04:42That's pretty amazing.
0:04:45 > 0:04:48But there's only so much we can tell by observing from Earth.
0:04:50 > 0:04:53To learn more about Mercury, we need spacecraft data.
0:04:55 > 0:04:58Only two missions have ever visited the planet -
0:04:58 > 0:05:01the first was Mariner 10 in 1974.
0:05:03 > 0:05:06Its camera only produced grainy pictures,
0:05:06 > 0:05:09but the other instruments it carried began to reveal major
0:05:09 > 0:05:13differences between Mercury and the other rocky planets.
0:05:14 > 0:05:18I've come to the Open University in Milton Keynes to talk to
0:05:18 > 0:05:20Mercury expert David Rothery.
0:05:21 > 0:05:23When they see images of Mercury, like this one,
0:05:23 > 0:05:25you can't fail to be intrigued by it.
0:05:25 > 0:05:27It's got a wonderful landscape.
0:05:27 > 0:05:31It's a beautiful image as well, but we've only sent two spacecraft
0:05:31 > 0:05:34two Mercury, compared to many that have been to Venus and Mars.
0:05:34 > 0:05:36Why is that? Why is Mercury being neglected?
0:05:36 > 0:05:39Well, Mars and Venus are the low-hanging fruit -
0:05:39 > 0:05:41they are closer to the Earth.
0:05:41 > 0:05:43And at first sight, certainly they are more interesting.
0:05:43 > 0:05:46Mercury, when we first sent a probe by it, it's airless,
0:05:46 > 0:05:48it's heavily cratered, it's a little bit dull.
0:05:48 > 0:05:51Now we realise it is not dull at all.
0:05:51 > 0:05:53There's all kinds of things going on there.
0:05:53 > 0:05:55Even that first mission, the Mariner mission,
0:05:55 > 0:05:59- that flew past made some remarkable discoveries.- It did.
0:05:59 > 0:06:01It was equipped with a magnetometer
0:06:01 > 0:06:03to look at the interaction between the solar wind,
0:06:03 > 0:06:05the charged particles from the Sun and the planet's surface,
0:06:05 > 0:06:09but it found that the planet's generating its own magnetic field.
0:06:09 > 0:06:12It's like a scaled-down version of the Earth's magnetic field.
0:06:12 > 0:06:14And that's surprising, for such a small planet
0:06:14 > 0:06:16to have a magnetic field.
0:06:16 > 0:06:18- It's really, really unusual. - Absolutely.
0:06:18 > 0:06:21Mars, the Moon and Venus don't generate a magnetic field,
0:06:21 > 0:06:22but Mercury does.
0:06:22 > 0:06:25And, you know, it caught everybody by surprise.
0:06:25 > 0:06:27And thank goodness Mariner 10 carried its magnetometer.
0:06:27 > 0:06:28But what does it tell us?
0:06:28 > 0:06:31What do we need to be able to generate this magnetic field?
0:06:31 > 0:06:34OK, well, some people say, "Well, iron in the core.
0:06:34 > 0:06:36"You've got a core made of iron - it will be a magnet."
0:06:36 > 0:06:38But that doesn't work. It has to be fluid.
0:06:39 > 0:06:42You have to have an electrically conducting fluid churning around,
0:06:42 > 0:06:46so we think the outer part of Mercury's interior core
0:06:46 > 0:06:50is made of molten iron, and that's the explanation that holds for the Earth as well.
0:06:50 > 0:06:54Well, that would make sense, apart from the fact that Mercury's such a small world.
0:06:54 > 0:06:57On Mars, for example, we think that there's no magnetic field
0:06:57 > 0:07:00because it's cooled down and there's no longer any fluid core.
0:07:00 > 0:07:03Yeah, well, Mercury should be cooling down as well
0:07:03 > 0:07:05cos it's got a large surface compared to its volume.
0:07:05 > 0:07:09It was probably quite hot to begin with, but there is also clearly some
0:07:09 > 0:07:12way of generating heat in the core to stop it having frozen,
0:07:12 > 0:07:15and also something to reduce the melting
0:07:15 > 0:07:17temperature of the outer part of the core -
0:07:17 > 0:07:19we think that is probably sulphur.
0:07:19 > 0:07:21- Mixed in with the iron? - Mixed in with the iron.
0:07:21 > 0:07:23There has to be enough iron to make an electrical conductor.
0:07:23 > 0:07:28So what we think is happening is iron is still today sinking
0:07:28 > 0:07:33inwards to join the frozen inner core of solid iron
0:07:33 > 0:07:35and, as the iron sinks inwards,
0:07:35 > 0:07:38it's turning gravitational energy into heat
0:07:38 > 0:07:41and that's heating the outer part of the core, which is gradually
0:07:41 > 0:07:44becoming richer in sulphur, reducing its melting temperature
0:07:44 > 0:07:47and enabling it to churn round and generate a magnetic field.
0:07:47 > 0:07:50So we know about one process that is happening inside Mercury -
0:07:50 > 0:07:52what do we know about the rest of its structure?
0:07:52 > 0:07:55We do know that the core must be very,
0:07:55 > 0:07:59- very large compared to the size of the planet.- Wow!
0:07:59 > 0:08:03Beyond the iron-rich inner core and outer core, you've got the rock.
0:08:03 > 0:08:05Most of the rock is what we call the mantle,
0:08:05 > 0:08:08there's a crust on the outside that is slightly chemically different.
0:08:08 > 0:08:10This rocky outer part is much,
0:08:10 > 0:08:13much thinner on Mercury than it is on any of the other rocky planets.
0:08:13 > 0:08:16It's the opposite of the earth, where we have quite a thick mantle and a thin core.
0:08:16 > 0:08:19So, to me, this is one of the great mysteries of Mercury.
0:08:19 > 0:08:22Why do we have this large core surrounded by a thin mantle?
0:08:22 > 0:08:23Absolutely.
0:08:24 > 0:08:28Mariner had started to reveal Mercury's inner secrets,
0:08:28 > 0:08:31but we had to wait another 35 years for a really good
0:08:31 > 0:08:33look at Mercury's surface.
0:08:35 > 0:08:40That finally came in 2011, when NASA's MESSENGER probe
0:08:40 > 0:08:43started returning high-definition images like these.
0:08:45 > 0:08:49The team here in Milton Keynes are using these images to create
0:08:49 > 0:08:52detailed geological maps of Mercury,
0:08:52 > 0:08:56but they're also finding plenty more evidence of Mercury's strangeness.
0:08:58 > 0:09:01So, Jack, if we can interrupt, what are you working on?
0:09:01 > 0:09:04I'm making a geological map of a region on Mercury, this area
0:09:04 > 0:09:08you see here. I'm using data from NASA's MESSENGER satellite,
0:09:08 > 0:09:10planetary images mosaiced together,
0:09:10 > 0:09:13and I'm interpreting the geological units I see at the surface.
0:09:13 > 0:09:16And so what I see when I look at this globe of Mercury is
0:09:16 > 0:09:18craters and I think that's what people think of.
0:09:18 > 0:09:21Like the moon, it's a grey body with a cratered surface.
0:09:21 > 0:09:22You're absolutely right.
0:09:22 > 0:09:25It is a heavily cratered surface in places.
0:09:25 > 0:09:26However, there is more to Mercury.
0:09:26 > 0:09:30For example, these lobate scarps that you see here,
0:09:30 > 0:09:31this one is called Carnegie Rupes.
0:09:31 > 0:09:34So that's this line running from top left to bottom,
0:09:34 > 0:09:37- right across the image.- Absolutely.
0:09:37 > 0:09:40This is an escarpment in the landscape caused by faulting
0:09:40 > 0:09:41within the crust of Mercury.
0:09:41 > 0:09:44So we see this kind of thing on Earth,
0:09:44 > 0:09:46these places where you have this sort of raised up area.
0:09:46 > 0:09:48Yes, you see these things on Earth.
0:09:48 > 0:09:50On Earth we have plate tectonics
0:09:50 > 0:09:53and, because of the collisions of plates, we find them building
0:09:53 > 0:09:56mountains and making escarpments in the landscape such as this one.
0:09:56 > 0:10:00However, there's no evidence to suggest Mercury has multiple
0:10:00 > 0:10:03tectonics plates that move around and collide with each other,
0:10:03 > 0:10:06so instead this is internal deformation within one plate
0:10:06 > 0:10:08that's being drawn in from within.
0:10:08 > 0:10:11So it's almost as if the whole planet is shrinking.
0:10:11 > 0:10:13Yes, the planet is in a state of global contraction.
0:10:13 > 0:10:16And people have looked at the distribution of these lobate
0:10:16 > 0:10:18scarps over the planet and added up all their effects,
0:10:18 > 0:10:20and they calculate that perhaps the planet has
0:10:20 > 0:10:24- lost as much as 7km of its planetary radius.- That's enormous.
0:10:24 > 0:10:29- That's Everest-ish...- It's hard to imagine a planet shrinking,
0:10:29 > 0:10:31but Mercury demonstrates that it has.
0:10:31 > 0:10:35So what's causing this? Why is there this shrinking?
0:10:35 > 0:10:38As the planet loses heat, this causes a volume reduction,
0:10:38 > 0:10:41particularly because as the liquid part of Mercury's iron core freezes,
0:10:41 > 0:10:44that causes a volume reduction, which pulls the crust in everywhere.
0:10:44 > 0:10:46So it's quite a simple process but, Dave,
0:10:46 > 0:10:49this isn't the only thing that's happening on the surface.
0:10:49 > 0:10:52If we look elsewhere, we can find evidence of other processes.
0:10:52 > 0:10:56Far from it. There's all kinds of things that have gone on on Mercury.
0:10:56 > 0:10:59One thing that comes to mind is something that we didn't know about
0:10:59 > 0:11:02until MESSENGER got there, which is these areas called hollows.
0:11:02 > 0:11:05There's a view here that's about 20-30km across
0:11:05 > 0:11:09and it's showing an area of surface where the top 20m of material
0:11:09 > 0:11:14is just gone, it's been stripped away to leave that irregular area.
0:11:14 > 0:11:16There's some smaller hollows nearby.
0:11:16 > 0:11:20It looks a bit like mould or Swiss cheese, or something like that.
0:11:20 > 0:11:22Absolutely. How is it being removed?
0:11:22 > 0:11:24It's not falling into caverns,
0:11:24 > 0:11:27it's not being blowing away in the wind, there's no wind.
0:11:27 > 0:11:30It's turning to vapour somehow and just being lost to space,
0:11:30 > 0:11:33so something in the surface is volatile.
0:11:33 > 0:11:36Volatile enough to turn to vapour and just go.
0:11:36 > 0:11:40- So what could that be?- It's a big problem and we can't tell from this.
0:11:40 > 0:11:42It could be sulphur, could be chlorine.
0:11:42 > 0:11:45Is it driven by heat or charged particles breaking chemical bonds?
0:11:45 > 0:11:48But there is evidence of volatile richness in the planet,
0:11:48 > 0:11:50which was completely unexpected.
0:11:50 > 0:11:54Something this close to the sun ought not to be rich in volatiles.
0:11:54 > 0:11:55Why not?
0:11:55 > 0:11:57Close to the sun you should be losing volatiles as you're
0:11:57 > 0:12:01trying to grow a planet because it's hot and, because Mercury has
0:12:01 > 0:12:05a large core, people thought it's had a violent birth.
0:12:05 > 0:12:07How do you get such a large core and a thin, rocky area?
0:12:07 > 0:12:09You blast the rock away,
0:12:09 > 0:12:12but that should be stripping away the volatiles as well,
0:12:12 > 0:12:14and yet Mercury has retained it's volatiles
0:12:14 > 0:12:16and still got a large core.
0:12:16 > 0:12:20- It doesn't fit.- And it's a world that's changing now.
0:12:20 > 0:12:22Some of these processes are still happening.
0:12:22 > 0:12:25The hollow-forming processes are still going on today.
0:12:25 > 0:12:28When you look at fields of hollows, you don't find impact craters
0:12:28 > 0:12:32superimposed. We think the hollows are still growing in some areas.
0:12:32 > 0:12:35Who would have thought Mercury would be an active planet?
0:12:35 > 0:12:38That adds up to Mercury being a very exciting place. Thank you very much.
0:12:38 > 0:12:40Pleasure.
0:12:42 > 0:12:45One of the reasons that we knew so little about Mercury for
0:12:45 > 0:12:49so long was that it's difficult to observe from the Earth.
0:12:52 > 0:12:56But this week's transit will give everyone the chance to see
0:12:56 > 0:12:57this mysterious planet...
0:12:59 > 0:13:03..and Peter's here to explain the best way to view the transit safely.
0:13:07 > 0:13:09As planets go, Mercury isn't far away.
0:13:10 > 0:13:13Only 48 million miles at closest approach.
0:13:16 > 0:13:19But it's surprisingly hard to observe...
0:13:19 > 0:13:21because being so close to the sun,
0:13:21 > 0:13:23it is only ever visible for a short period,
0:13:23 > 0:13:26just before sunrise or after sunset.
0:13:28 > 0:13:31But this week's transit is a great opportunity to see
0:13:31 > 0:13:33the planet during broad daylight.
0:13:34 > 0:13:38We won't get another chance as good as this until 2049.
0:13:40 > 0:13:44Eclipse glasses are a great way to look at the sun safely,
0:13:44 > 0:13:47but unfortunately Mercury is going to be too small to be seen
0:13:47 > 0:13:48with the naked eye.
0:13:48 > 0:13:52It's about 1/155th the apparent size of the sun,
0:13:52 > 0:13:55so to see it at all you're going to need something
0:13:55 > 0:13:56with a bit more power -
0:13:56 > 0:14:00say a telescope or a powerful pair of binoculars.
0:14:00 > 0:14:02But to be safe, these must be fitted
0:14:02 > 0:14:04with a certified solar safety filter.
0:14:04 > 0:14:08There are various filters available, but one of the easiest ways
0:14:08 > 0:14:13to achieve this is to get hold of an A4 sheet of solar safety film
0:14:13 > 0:14:15and then make your own filter,
0:14:15 > 0:14:19which slips on the front of the telescope - just like that.
0:14:19 > 0:14:21You can point the telescope at the sun...
0:14:23 > 0:14:25..and you're good to go.
0:14:28 > 0:14:29With the filter in place,
0:14:29 > 0:14:32you should get a view of the whole of the sun's disk,
0:14:32 > 0:14:35on which it's possible to make out small sun spots.
0:14:37 > 0:14:39This one is about the size that Mercury will
0:14:39 > 0:14:41appear during the transit.
0:14:44 > 0:14:48The transit will begin just after noon, with the sun high in the sky.
0:14:51 > 0:14:54You will see Mercury make first contact with the eastern
0:14:54 > 0:14:57edge of the sun and will then track southwest
0:14:57 > 0:15:01until the transit finishes at 7:42 in the evening.
0:15:04 > 0:15:07If you don't have a filter then there is another way to get
0:15:07 > 0:15:11a decent view of the sun and that's to project it.
0:15:11 > 0:15:14Now, projection is only really suitable for small refracting
0:15:14 > 0:15:17or lens-based telescopes. But using a small refractor,
0:15:17 > 0:15:20if you point this directly at the sun with
0:15:20 > 0:15:23an eyepiece in the eyepiece holder, it's then possible to project
0:15:23 > 0:15:26an image of the sun onto a piece of white card
0:15:26 > 0:15:29and it actually gives you a really good view.
0:15:29 > 0:15:32One final warning, if you're using this method,
0:15:32 > 0:15:36is to not keep the telescope pointed at the sun for too long a period.
0:15:36 > 0:15:39If you do, you run a risk of damaging the internal
0:15:39 > 0:15:41components of the telescope.
0:15:41 > 0:15:44If it's got plastic bits inside, for example, they may melt
0:15:44 > 0:15:47and plastic eyepieces may melt, as well.
0:15:47 > 0:15:48Also, be careful
0:15:48 > 0:15:52because the temperature just behind the eyepiece is really hot.
0:15:52 > 0:15:55I can demonstrate that with this little piece of black card.
0:15:55 > 0:15:58Look at that. That didn't take very long at all.
0:15:58 > 0:16:01Makes Bear Grylls look pretty pathetic, doesn't it?
0:16:01 > 0:16:02HE LAUGHS
0:16:08 > 0:16:10Hopefully, on the day of the transit,
0:16:10 > 0:16:13the skies will be lovely and clear like they are today.
0:16:13 > 0:16:15But if the clouds do come,
0:16:15 > 0:16:20in the event is so long at 7.5 hours from beginning to end that we do
0:16:20 > 0:16:23stand at least a decent possibility of some clear breaks
0:16:23 > 0:16:25where we'll see something of it.
0:16:25 > 0:16:27The end of the transit occurs with the sun just
0:16:27 > 0:16:30nine degrees above the northwest horizon,
0:16:30 > 0:16:34so if you do intend to watch the entire event then make sure
0:16:34 > 0:16:37you've got a clear view in that particular direction.
0:16:37 > 0:16:39So clear skies and good luck.
0:16:42 > 0:16:44If you don't have the right equipment,
0:16:44 > 0:16:46there will be lots of events happening all over the country,
0:16:46 > 0:16:49like here at the Open University, where you can watch the transit
0:16:49 > 0:16:51in the company of your local astronomers.
0:16:51 > 0:16:54And if it does happen to be cloudy, like today,
0:16:54 > 0:16:57then Isa are streaming the transit live from space using
0:16:57 > 0:17:00satellites that will get a great view whatever the weather,
0:17:00 > 0:17:02and we'll have details of that stream
0:17:02 > 0:17:04and the list of events on our website.
0:17:04 > 0:17:07But transits are more than just rare and remarkable events -
0:17:07 > 0:17:09they also have significant scientific value.
0:17:09 > 0:17:11We asked public astronomer Marek Kukula
0:17:11 > 0:17:14from the Royal Observatory Greenwich to investigate.
0:17:18 > 0:17:21To understand the importance of transits,
0:17:21 > 0:17:23we need to geo back to the 17th century,
0:17:23 > 0:17:26to the time just after the founding of the Royal Observatory.
0:17:29 > 0:17:32This is what was known as the solar system in the second
0:17:32 > 0:17:36half of the 1600s. The six inner planets all orbiting around the sun.
0:17:36 > 0:17:40The outer planets, of course, hadn't been discovered yet.
0:17:40 > 0:17:43It was a model we'd had since the time of Copernicus and Kepler.
0:17:43 > 0:17:47We knew the order of the planets and we knew the shape of their orbits,
0:17:47 > 0:17:50but there was one thing about the solar system we didn't know -
0:17:50 > 0:17:52we didn't know how big it was.
0:17:52 > 0:17:55Although the relative sizes of the orbits were understood,
0:17:55 > 0:17:57for instance, we knew that the Earth was three times
0:17:57 > 0:18:00further from the sun than Mercury,
0:18:00 > 0:18:02the actual distances weren't known.
0:18:03 > 0:18:06It was a solar system without scale.
0:18:08 > 0:18:11And that's where this guy comes in. He's Edmund Halley.
0:18:11 > 0:18:15He'd later become Astronomer Royal himself, but in 1677 he was
0:18:15 > 0:18:19just an assistant astronomer, here at the observatory in Greenwich.
0:18:19 > 0:18:22And he'd been sent to St Helena in the South Atlantic to observe
0:18:22 > 0:18:23the southern skies.
0:18:23 > 0:18:26While he was there, he watched a transitive Mercury,
0:18:26 > 0:18:29just like the one that's due this week.
0:18:29 > 0:18:32Watching Mercury crawl across the face of the sun,
0:18:32 > 0:18:35Halley realised how a transit could be used to measure
0:18:35 > 0:18:37the size of the solar system.
0:18:39 > 0:18:42Halley's breakthrough was to understand that
0:18:42 > 0:18:46if you viewed the transit from two widely spaced locations,
0:18:46 > 0:18:50thousands of miles apart, then you'll see the transit differently.
0:18:51 > 0:18:53Viewed from here, south of the equator,
0:18:53 > 0:18:56the planet will appear here against the disk of the sun.
0:18:56 > 0:18:58But viewed from north of the equator,
0:18:58 > 0:19:01the planet will appear further down on the sun's disk.
0:19:02 > 0:19:04What Halley realised was that
0:19:04 > 0:19:07if you could measure the apparent separation between the points,
0:19:07 > 0:19:09the parallax, then you could work out
0:19:09 > 0:19:11the distance between the earth and the sun.
0:19:14 > 0:19:16It was a calculation that required
0:19:16 > 0:19:18some fiendishly complicated geometry.
0:19:20 > 0:19:23But to produce an accurate figure, it also needed a number
0:19:23 > 0:19:27of extremely precise measurements to be made during the transit...
0:19:28 > 0:19:29..including, most crucially,
0:19:29 > 0:19:32the exact time it takes for the planet to cross the sun.
0:19:35 > 0:19:37But here Halley faced a problem.
0:19:38 > 0:19:42Mercury was so small and travelled so fast that it would be almost
0:19:42 > 0:19:45impossible to make the measurements required.
0:19:47 > 0:19:49A more suitable target was Venus,
0:19:49 > 0:19:53which appears both bigger and slower as it transits the sun.
0:19:55 > 0:19:59But the next transit of Venus wasn't due for another 84 years.
0:20:00 > 0:20:03Halley knew he'd be long dead by then,
0:20:03 > 0:20:06so he laid down the gauntlet for future generations.
0:20:08 > 0:20:12Almost a century later, the world's astronomers rose to that challenge.
0:20:14 > 0:20:17There were transits of Venus in 1761 and 1769,
0:20:17 > 0:20:21and expeditions were sent out all around the world to observe them.
0:20:21 > 0:20:24These expeditions were among the first great international
0:20:24 > 0:20:26scientific collaborations
0:20:26 > 0:20:29and, in many ways, they were like the Large Hadron Collider
0:20:29 > 0:20:31or International Space Station of their day.
0:20:42 > 0:20:45The French, British and Austrians went to Siberia
0:20:45 > 0:20:46and Northern Canada,
0:20:46 > 0:20:49where they had to brave polar bears and hostile locals.
0:20:49 > 0:20:51They went to the Indian and Pacific Oceans.
0:20:51 > 0:20:55Captain James Cook was sent to Tahiti in 1769.
0:20:55 > 0:20:57And these were major expeditions for the time,
0:20:57 > 0:21:01involving perilous sea voyages sometimes lasting several years.
0:21:01 > 0:21:04There was one French expedition to Mexico from which only one
0:21:04 > 0:21:05person returned alive.
0:21:05 > 0:21:08And to make matters worse, during some of this time,
0:21:08 > 0:21:09France and Britain were at war
0:21:09 > 0:21:12and special arrangements had to be made to give safe passage
0:21:12 > 0:21:14to scientists from each side.
0:21:18 > 0:21:20Once they'd arrived, each team had to spend weeks
0:21:20 > 0:21:23calculating their latitude and longitude.
0:21:23 > 0:21:26And this is dated from Cook's voyage in 1769.
0:21:26 > 0:21:28You can see how detailed it is.
0:21:28 > 0:21:32They're even using the moons of Jupiter to calculate their longitude.
0:21:32 > 0:21:34And once they'd done that, they had to hope for clear
0:21:34 > 0:21:37skies for the transit itself.
0:21:37 > 0:21:40The crucial measurement was to time how long it took Venus to
0:21:40 > 0:21:44cross the disk of the sun to within a couple of seconds.
0:21:44 > 0:21:48And these are drawings by Captain Cook himself and they perfectly
0:21:48 > 0:21:52illustrate a really crucial problem that they discovered.
0:21:52 > 0:21:56It's an optical illusion - the so-called "black drop effect".
0:21:56 > 0:21:59And as Venus starts to cross the disk of the sun,
0:21:59 > 0:22:02the disk of Venus appears to stretch out and blur,
0:22:02 > 0:22:04and that makes it very difficult to measure
0:22:04 > 0:22:07the precise time at which the transit begins.
0:22:08 > 0:22:11The black drop effect made it impossible to record
0:22:11 > 0:22:14the length of the transit with the desired accuracy...
0:22:17 > 0:22:20..but the data they did collect was enough for astronomers
0:22:20 > 0:22:21to start their calculations.
0:22:22 > 0:22:26In 1771, the French astronomer Jerome Lalande calculated
0:22:26 > 0:22:29a value for the astronomical unit,
0:22:29 > 0:22:33the distance between the earth and the sun, of 153 million km,
0:22:33 > 0:22:37which is impressively within 2.5% of the modern value.
0:22:37 > 0:22:39Suddenly, the solar system had a scale.
0:22:42 > 0:22:44That's why transits were important historically.
0:22:46 > 0:22:48But today, transits are still important in helping us
0:22:48 > 0:22:51understand our position in the universe
0:22:51 > 0:22:54because of the role they play in showing how many other planets
0:22:54 > 0:22:56there are outside the solar system.
0:22:59 > 0:23:02Whenever a planet passes in front of the sun,
0:23:02 > 0:23:05it blocks out a small but measurable amount of its light.
0:23:07 > 0:23:10The same principle applies when we look at other stars -
0:23:10 > 0:23:14it's very difficult to observe their planets directly -
0:23:14 > 0:23:16but we can see the tiny drop in brightness
0:23:16 > 0:23:19as the planet passes in front of the star.
0:23:19 > 0:23:23It's exactly this technique that the Kepler space telescope uses.
0:23:24 > 0:23:29It monitors 100,000 stars, looking for the telltale dip in luminance
0:23:29 > 0:23:31that indicates a transiting planet.
0:23:34 > 0:23:37By using this method, it has in the last seven years detected
0:23:37 > 0:23:40nearly 6,000 possible exoplanets.
0:23:43 > 0:23:45When you're watching a transit, like the one this week,
0:23:45 > 0:23:47you should bear in mind a couple of things.
0:23:47 > 0:23:50One is that what you are watching is a clear example
0:23:50 > 0:23:52of the solar system in action -
0:23:52 > 0:23:55planets actually moving along their orbits in real time.
0:23:55 > 0:23:58But also what you're seeing isn't just a pleasing spectacle
0:23:58 > 0:24:01because transits, perhaps more than any other phenomenon,
0:24:01 > 0:24:05have helped us to understand the scale and scope of our universe.
0:24:09 > 0:24:12Mercury is undoubtedly a strange world.
0:24:12 > 0:24:15With its large iron core and its thin mantle,
0:24:15 > 0:24:19it's not like any of the rest of the family of rocky planets.
0:24:19 > 0:24:22So what happened in Mercury's formation to make it this way?
0:24:23 > 0:24:27Maggie has been talking to planetary scientist Craig Agnor to
0:24:27 > 0:24:29discuss the latest ideas.
0:24:31 > 0:24:32Craig, can you describe to me
0:24:32 > 0:24:35the old theory of the formation of Mercury?
0:24:35 > 0:24:38One of the initial ideas was that Mercury's mantle was removed
0:24:38 > 0:24:39through a giant impact.
0:24:39 > 0:24:43And the initial modelling of this suggested a smaller object,
0:24:43 > 0:24:48maybe a third the mass of Mercury, smashed in at very high velocity,
0:24:48 > 0:24:50vaporised the mantle, blasted off into space
0:24:50 > 0:24:55and this would have predicted a very hot but iron-rich planet.
0:24:55 > 0:24:58OK, so an iron-rich planet, so a large core and a thin mantle,
0:24:58 > 0:25:00so that does tie in with what we see of Mercury.
0:25:00 > 0:25:02That's exactly right.
0:25:02 > 0:25:04The problem is that recent spacecraft data has shown
0:25:04 > 0:25:06that Mercury's mantle retains a significant
0:25:06 > 0:25:09inventory of volatiles that wouldn't have survived the extreme
0:25:09 > 0:25:11heating of this initial scenario.
0:25:11 > 0:25:14They would have been blown off into space with the temperature.
0:25:14 > 0:25:16- That's right.- OK, so there's a challenge there.- That's right.
0:25:16 > 0:25:19So you have to look at the different types of giant impacts that
0:25:19 > 0:25:21occur during planet formation.
0:25:21 > 0:25:23One of the new ideas about this origin of Mercury is that
0:25:23 > 0:25:27maybe Mercury hit a larger object at slower velocity
0:25:27 > 0:25:31and this collision may be able to remove the mantle without
0:25:31 > 0:25:33the extreme heating of the earlier scenario.
0:25:33 > 0:25:35OK, so we keep the volatiles.
0:25:35 > 0:25:36Wonderful.
0:25:36 > 0:25:40So what we see in this animation here is kind of a proto-Mercury
0:25:40 > 0:25:42and a proto-Venus on crossing orbits that will eventually
0:25:42 > 0:25:44result in a giant impact.
0:25:44 > 0:25:47- So the impact was actually between Venus and Mercury.- Right.
0:25:47 > 0:25:50So what actually happens during impact?
0:25:50 > 0:25:52The way we study this is through computer simulations.
0:25:52 > 0:25:54You can model a planet in this
0:25:54 > 0:25:58simulation from Arizona State by Erik Asphaug and Andreas Reufer.
0:25:58 > 0:26:02An iron core shown in blue, rocky mantles are shown in red or orange.
0:26:02 > 0:26:05This type of collision is called a hit and run collision,
0:26:05 > 0:26:08where the two objects slam into each other,
0:26:08 > 0:26:10they sheer off a portion of their mantles
0:26:10 > 0:26:12and they leave the scene of the crime.
0:26:12 > 0:26:15The impact happens at a modest velocity,
0:26:15 > 0:26:18so this is quite a bit more gentle than the smaller,
0:26:18 > 0:26:21high velocity impact collisions.
0:26:21 > 0:26:24OK. So this could explain the core, the mantle,
0:26:24 > 0:26:26but the volatiles as well.
0:26:26 > 0:26:28- That's right.- So, in this scenario, what happens to Venus?
0:26:28 > 0:26:33Part of the proto-Mercury's mantle may have been deposited onto Venus
0:26:33 > 0:26:36and that may help to explain why Venus has a little more
0:26:36 > 0:26:40mantle material relative to the size of its core than the Earth.
0:26:40 > 0:26:42So this theory is looking pretty good now
0:26:42 > 0:26:43because we've got this collision,
0:26:43 > 0:26:46we've got Mercury left with a large iron core and a thin mantle,
0:26:46 > 0:26:48we've got Venus with an extra mantle,
0:26:48 > 0:26:50which is what we actually see in reality,
0:26:50 > 0:26:52so it does seem to stand up. So what happens next?
0:26:52 > 0:26:54It's not the end of the story
0:26:54 > 0:26:56because its orbit continues to evolve
0:26:56 > 0:26:58and, over the next five billion years,
0:26:58 > 0:27:02there's about a 1% chance that its orbit can become so eccentric
0:27:02 > 0:27:05that it again crosses the orbit of Venus.
0:27:05 > 0:27:09It can suffer giant impacts with Venus or Earth,
0:27:09 > 0:27:11or it may collide with the sun.
0:27:11 > 0:27:14Gosh. 1% probability is quite high, really,
0:27:14 > 0:27:16- that our solar system could change radically.- That's right.
0:27:16 > 0:27:19So it's not as static as I take for granted.
0:27:19 > 0:27:22- No, the solar system is an amazingly dynamic place.- Thanks very much.
0:27:22 > 0:27:23Thank you.
0:27:27 > 0:27:31We still don't understand everything about Mercury
0:27:31 > 0:27:33or how it became the planet it is today.
0:27:36 > 0:27:38It remains the problem child of the solar system.
0:27:40 > 0:27:42But by studying Mercury's peculiarities
0:27:42 > 0:27:46and troubled beginnings, we are producing new insights into
0:27:46 > 0:27:50the processes that formed and shaped the whole of the inner solar system.
0:27:57 > 0:28:00When we started working on this, I thought Mercury was the least
0:28:00 > 0:28:02interesting of planets,
0:28:02 > 0:28:04but the more you find out about it the more fascinating it is.
0:28:04 > 0:28:07I was particularly interested in the fact that the formation
0:28:07 > 0:28:10of Mercury tells us more about the dynamics of the early solar system.
0:28:10 > 0:28:12Well, that's all we've got time for this month.
0:28:12 > 0:28:15But if you're going to watch the transit tomorrow, good luck.
0:28:15 > 0:28:17If you're watching us on repeat, I hope it was clear.
0:28:17 > 0:28:19That's it for this month,
0:28:19 > 0:28:22but do check out the website, where we've got Pete's star guide.
0:28:22 > 0:28:25And in the meantime, get outside and get looking up...
0:28:25 > 0:28:28- but never directly at the sun. - Goodnight.