Clara took one last look around. 'So no one will ever come here again?'

'I've sealed the fences and put in a skynet,' said the Doctor. 'These hunting grounds are closed.'

'I could never come back here anyway,' said Clara with a shudder, looking at the thick dust covered in their footprints. Motes floated in the air, lit by the hole in the ceiling that let in the sunlight, the whirling ghosts of dancers forever departed.

'Things decay,' said the Doctor. 'But remember, Clara, this isn't a fixed point. It doesn't have to be like this.'

Clara nodded glumly as they crossed the once pristine floor.

The Doctor looked at her stricken face. 'So maybe I do dance,' he said quietly. 'A bit.'

She looked at him.

'Obviously,' said the Doctor. 'You don't infiltrate the deadly French court without mastery of the gavotte.'

'The gavotte?' said Clara. 'Is that the only dance you can do?'

'No,' said the Doctor. 'Also, the quadrille. Take it or leave it.'

He knocked his blackened screwdriver several times hard against his boot, until it emitted a tiny peep and beam of light, and directed it towards what was left of the organ, which started up, creakily, painfully, its old programme, a very slow, mournful version of 'We Do Like to Be Beside the Seaside' in a minor key.

Clara put on her bravest smile as he reached for her hand.

The lion had long gone by the time they got down to street level and marched the chastened men to the perimeter fence. The Doctor saw Clara desperate to ask questions, and halted her with a look.

'Don't,' he warned gently. 'And you,' he said to Triss. 'If I hear of you treating your wife with a fraction of the contempt with which you treat the rest of creation and I hear everything I will happily bounce you off that tower myself, do I make myself understood?'

'Yes, sir.'

'Good boy. You can go.'

They stood on the beach, throwing pebbles in the water. The Doctor looked at Clara. 'You need to work harder in your own time, don't you think? Educate all those millions of children of yours? It's not fixed... yet.'

Clara nodded. Then she looked up at him. 'You do know they're not actually my children? I just teach them?'

The Doctor ignored her, and spoke on, gazing out at the sea.

Suddenly he was almost knocked into the surf by a donkey launching itself on him, licking his face like an overgrown dog.

'Hey!' he said. 'Hey, Meghan! How are you? How are you, girl? There you are.' He scratched behind her ears and she rubbed her head against him adoringly. Then she got down on all fours.

'Oh no,' said the Doctor. 'No. No. Definitely not.'

Clara smiled. 'I think she wants to. You should roll up the bottom of your trousers so you don't get wet.'

The Doctor gave her a look.

'Ooh!' She felt into her pocket and pulled out the pink plastic hat she'd grabbed from the console. 'And here. Stick this on!'

'No hats!'

'Stick it on!' She reached up and put it on for him.

'I don't want...' Meghan had already nudged herself underneath him, and got to her feet, lifting him up. 'I don't...'

But it was too late. Donkey and Time Lord were already proceeding at a stately pace through the shallow waters against the twisted wreckage of the pier. Clara, giggling, watched them splashing away in the light of the huge pink setting sun, as the Doctor gently rubbed the donkey's ears and, when he thought he was un.o.bserved, planted a very quick kiss on the animal's head.

'Why are Earth people so parochial?'

The Fifth Doctor, The Visitation (1982)

Learning more about outer s.p.a.ce has taught us a lot about the Earth and our existence here. But some of what we've learnt is pretty scary...

What do we gain from exploring outer s.p.a.ce? As we saw in Chapter 2, it's very expensive to send s.p.a.cecraft and people into orbit round the Earth, let alone to other worlds. Government-funded s.p.a.ce programmes have to justify that expense to the taxpayer, while privately funded programmes are paid for by people who want to see a return on their investment.

On 20 July 1969, a few hours after the first two people to walk on the Moon began their journey back to Earth, the BBC's Panorama listed the benefits of the s.p.a.ce programme as they were seen then. Reporter Julian Pettifer explained that 'the a.n.a.lysis of pictures taken hundreds of miles out in s.p.a.ce can reveal new mineral deposits. They've already led to the discovery of new oil fields. In agriculture, satellite pictures can provide data on erosion and irrigation, and their value to the weather forecaster is already established. Soon, urban planners may be using astronauts' photographs to help them plan road systems.'

In going to s.p.a.ce, we learnt more about the Earth, but pictures from s.p.a.ce did more than tell us about new ways to exploit our planet's resources. One photograph in particular helped transform our ideas about our relationship with Earth.

Before Apollo 11 could put the first people on the Moon, scientists needed to test their equipment and calculations, and also know more about the lunar surface. In December 1968, Apollo 8 became the first manned mission to be sent there. It didn't land on the Moon but orbited ten times, taking pictures of potential landing sites.

Then, on Christmas Eve, Apollo 8 astronauts William Anders, Frank Borman and James Lovell saw the most incredible sight over the lunar horizon. Their voices were recorded, so we know their reactions as they struggled with a camera:

'Look at that picture over there! There's the Earth comin' up. Wow, is that pretty!'

'Hey, don't take that it's not scheduled.'

'You got a color film, Jim? Hand me that roll of color quick.'

William Anders and Frank Borman, Apollo 8, 24 December 1968

Borman was joking, but the astronauts hadn't been scheduled to take pictures of the Earth from s.p.a.ce only of the Moon, and those in black and white. The focus of the mission was getting out into s.p.a.ce, not to look back at where the astronauts had come from. However, Anders quickly put colour film in his camera (a reminder of how long ago this was and how basic the technology was back then!) and took pictures of that extraordinary view...

The photographs show the blue and white Earth hanging brightly in the black of s.p.a.ce above the cratered, grey lunar surface. One of the photographs, known as 'Earthrise', became one of the most recognised and reprinted images of the twentieth century: it was soon used on the cover of the influential magazine Time and issued as a stamp.

Why was 'Earthrise' so effective? Satellites had been taking increasingly detailed pictures of Earth from s.p.a.ce since 1959. What made 'Earthrise' different was that it also shows the lunar landscape in the foreground. We're used to seeing the same view the other way round: the Moon in the sky over Earth's horizon. More than that, including the lunar landscape also provides a disturbing sense of scale. The Earth looks tiny and vulnerable and at the time, to many, it was a surprise to learn that our planet is brightly, vividly blue.

Something similar is going on at the start of Spearhead from s.p.a.ce (1970), the first adventure of the Third Doctor. The opening shot is of stars in s.p.a.ce. The camera pans left and finds the shining, blue-white Earth, while eerie sound effects suggest an alien threat.

It's a quick and simple way of establishing the new format of the series. With this Doctor stranded on Earth without the use of his TARDIS, we would no longer travel in s.p.a.ce and time to find monsters, so they would have to come to us. But Earth was at threat from more than just monsters. In Spearhead from s.p.a.ce, our dependency on plastic is made to work against us. In the next story, Doctor Who and the Silurians, a research project that aims to develop cheap atomic energy unleashes a deadly force. In the same year, Inferno sees a mining project unleash its own terrible threat to humanity. The characters running these projects are keen to press on, despite the warnings from the Doctor and other scientists. One politician ignores the Doctor and unwittingly spreads a killer disease across London. One scientist ignores the evidence in front of him and ends up destroying the world. (Luckily, it's a parallel world, as we discussed in Chapter 3.) These stories seem to reflect wider concerns from the time in which they were made about the inherent dangers in the way we exploited our planet's resources. As Inferno was being recorded in April 1970, the first 'Earth Day' saw 20 million Americans take to the streets to call for a healthy, sustainable environment. Earth Day was directly inspired by a large oil spill in California, but it was also the result of an emerging public awareness about pollution of the water and air.

That awareness had been helped by a 1962 bestseller, Silent Spring, in which marine biologist Rachel Carson spelt out the damage being done to birds and the countryside by the uncontrolled use of pesticides. She also said that the makers of the pesticides lied about the effects of their chemicals, and that the government was too willing to believe whatever they said.

The evidence presented in Silent Spring led to the banning of the pesticide DDT in the United States of America. It also clearly influenced Doctor Who stories such as Planet of Giants (1964) which we discussed in Chapter 3 and The Green Death (1973), which we'll come on to shortly. However, it's been argued that environmental concerns remained abstract ideas to the general public until the 'Earthrise' photograph suddenly made the issues very clear. Whatever the case, going to the Moon and what we learned there taught us a lot about how our own environment can be threatened.

For example, evidence collected by going to the Moon proved what caused the lunar craters. Before that, theories had included volcanic eruptions or the movements of glaciers, but now it is known they are the result of meteorites and asteroids colliding with the Moon. Scientists wondered if Earth might also have suffered such impacts and found evidence of that, too. Then, in 1980, physicists Luis and Walter Alvarez suggested that an asteroid hitting the Earth 65 million years ago wiped out the dinosaurs. That theory was used as the basis for the Doctor Who story Earthshock (1982) although it wasn't until 2010 that an international panel of scientists finally favoured the Alvarez hypothesis over other theories.

We also now know that the presence of the Moon stabilises the tilt of the Earth's axis, helping to regulate the seasons, and affects the tides and movement of tectonic plates all things that it has been suggested helped life to develop and thrive here. As Kill the Moon (2014) showed, changes to the Moon's...o...b..t and gravitational influence would be disastrous for life on our planet.

It's not just the Moon we have learnt from. Venus is the nearest planet to Earth. As we saw in Chapter 1, Venus, like Earth, is just about at the right distance from the Sun for water to be in liquid form in a region known as the 'habitable zone'. However, the atmosphere of Venus is many times thicker than Earth's and is mostly composed of carbon dioxide, with a dense layer of sulphuric acid cloud. In 1962, the Mariner 2 s.p.a.cecraft orbited Venus and confirmed a theory that the carbon dioxide acts like a blanket, trapping the Sun's heat and raising the temperature of the planet. We now know that, despite being further from the Sun than Mercury, Venus actually has the highest average surface temperature of any planet in the Solar System an incredible 462C! Water would evaporate in that incredible heat, and tin and lead would melt!

It's thought that, until four billion years ago, conditions on Venus were more like those on Earth, with lots of liquid water. What changed is much debated among scientists, but it seems a moderate increase in temperature meant that the surface water evaporated into the atmosphere where it began to trap the Sun's heat, causing the temperature to rise still further. Without any water on the surface, carbon dioxide from volcanoes also began to build up in Venus's atmosphere leading to a runaway warming effect and creating the searing temperatures which we find there today. An Earth-like world was transformed into a planet so hot that it soon destroys any s.p.a.cecraft we send there.

The climate of Mars has also been transformed. It's a cold, dry world now, but the robots currently exploring its surface Opportunity and Curiosity have found minerals that are formed only in water. This and studies of the networks of valleys on the Martian surface suggest that Mars was once warmer, wetter and much more like Earth. It seems that conditions changed about four billion years ago roughly the same time that they changed on Venus, too. Here the problem was reversed: Mars's thin atmosphere is also mostly composed of carbon dioxide but there is too little of it to hold on to the Sun's warmth. As its temperatures plunged, Mars's water was frozen into the icecaps and soil, and the planet became a chilly desert with an average surface temperature of -55C. (In Doctor Who, this change in climate is the reason why the Ice Warriors were forced to abandon their Martian home.) Planetary scientists are still trying to understand the details of how Mars and Venus could start out in such a similar state to the Earth and yet end up looking so different. But the fact that conditions on the two planets closest to us transformed so dramatically is a worrying prospect especially when we look at how conditions on Earth are currently changing.

Here on Earth, carbon dioxide and other gases in the atmosphere also act to raise the planet's average temperature. Without this 'greenhouse effect', Earth would have an average surface temperature of just -18C rather than its current balmy value of +15C. Our planet would be too cold for liquid water, and life could probably not exist here.

Up until very recently, Earth's natural carbon dioxide levels had a beneficial effect, helping to keep the planet warm and habitable. However, evidence is mounting from satellites and ground-based measurements that this natural thermostat is increasingly out of balance. For the past few decades, the average temperature of the world has been increasing. As the planet warms, ice sheets, snow cover, glaciers and Arctic sea ice are all rapidly declining. The melting of large areas of ice on land produces huge volumes of water that flow into the sea. This means that sea levels have risen more in the past decade than they did in the last century. The oceans are warming, too, as is the planet as a whole: ten of the warmest summers since 1880 have occurred in the past twelve years.

This rise in global temperatures affects our weather: a warmer climate can carry more water vapour in it, so we experience heavier rainfall and there's a higher likelihood of extreme weather conditions, such as major storms and tornadoes. It's highly unlikely that the Earth will ever suffer such dramatic transformations as Mars and Venus but any change in sea level or seasonal patterns of weather and temperature could make life very uncomfortable for our planet's natural ecosystems. As Clara and the Doctor found when they visited Blackpool in 2089, climate change will have serious consequences for all of us.

What could be causing this rapid rise in temperature? Unfortunately, the most likely culprit is us. Since the Industrial Revolution began in the eighteenth century, humans have been burning fossil fuels such as coal, oil and gas at ever-increasing rates. The waste product of all this energy-guzzling activity is carbon dioxide. At the same time, we've been clearing large tracts of the Earth's surface for agriculture and urban development, removing the forests and marshlands which once helped to suck additional carbon dioxide from the air. Our atmosphere's carbon dioxide level is now almost fifty per cent higher than it was before the Industrial Revolution began, just 250 years ago and it's still going up.

If learning more about s.p.a.ce helped reveal what we're doing to Earth, perhaps it can also help us find solutions to slow or even reverse the damage. Perhaps the Doctor Who story The Green Death might show us how.

At the beginning of The Green Death, the Doctor and his companion Jo Grant talk at cross purposes. He wants to show her the famous blue sapphires of the planet Metebelis 3, while Jo wants to rush to Wales to take part in a protest against pollution caused by a chemicals firm. The Doctor makes the same offer he makes all his companions the chance to see the universe. Yet Jo turns him down.

'Jo, you've got all the time in the world, and all the s.p.a.ce. I'm offering them to you.'

'But, Doctor, don't you understand? I've got to go! This Professor Jones, he's fighting for everything that's important, everything that you've fought for. In a funny way, he reminds me of a sort of younger you.'

The Third Doctor and Jo Grant, The Green Death (1973)

Note that Jo is keen to join the protest because of what she's learnt from the Doctor travelling with him in s.p.a.ce and time has made her better appreciate what needs to be done to save Earth in her own time.

In fact, Jo's reaction to travelling in the TARDIS is unlike that of any other companion. When she first meets the Doctor in Terror of the Autons (1971), he can't operate the TARDIS and is stuck on Earth. The Doctor and Jo both work for UNIT, their job to help keep Earth safe from alien menaces. Jo doesn't visit another world until her fifteenth episode and she isn't impressed:

'That's an alien world out there, Jo. Think of it.'