'I don't want to think of it. I want to go back to Earth.'

The Third Doctor and Jo Grant, Colony in s.p.a.ce (1971)

It's the same a year later in The Curse of Peladon Jo would much rather go on a date with Captain Mike Yates than visit another world. When the Time Lords give the Doctor control over the TARDIS again in The Three Doctors (19721973), Jo agrees to go with him on a test flight and is then horrified to get caught up in three consecutive adventures in s.p.a.ce and time. She leaves him in the next story.

We can understand why the people making Doctor Who created a companion who didn't want to travel in s.p.a.ce and time: if she doesn't then neither do we, so we're less likely to resent the Doctor's exile to Earth. But the people making Doctor Who were also keen to address concerns about contemporary politics and the environment even when the Doctor was occasionally able to take the TARDIS out into s.p.a.ce.

As we saw in Chapter 2, in Colony in s.p.a.ce Jo is told that on the Earth of AD 2471 there is 'No room to move, polluted air, not a blade of gra.s.s left on the planet and a government that locks you up if you think for yourself.' When she visits Earth in the twenty-sixth century in Frontier in s.p.a.ce (1973), those who speak out against the government in favour of peace are sent to prison on the Moon where corrupt officials conspire to murder them. We see more corruption in The Mutants (1972), when Jo visits an Earth colony in the thirtieth century. We see the Earth Empire's brutal exploitation of the planet Solos with little thought for the Solonians who are kept in segregated communities and murdered by the authorities. War, pollution and corruption are all too familiar aspects of recent history, and Doctor Who seems to be warning us that just as in the past the biggest risk to human health and happiness in the future might be our own greed and violence.

Of course, human activity isn't the only danger threatening our planet's future. As we've seen, it's thought that the dinosaurs were wiped out after a large object collided with the Earth. How likely is another such collision one that could wipe out humanity? A smaller asteroid exploded above the Tunguska region of Siberia in 1908, destroying thousands of square kilometres of forest, while in 2013 a 20-metrewide asteroid blew up over the Russian city of Chelyabinsk, shattering windows and injuring more than a thousand people. Astronomers know that larger asteroids are out there and eventually one of these will inevitably strike our planet again.

Even if Earth survives the threat of impact, there is one fate our planet can't avoid. A star like our Sun has a lifespan of about ten billion years. For most of this time it will remain stable and well-behaved. At around 4.5 billion years old, our Sun is currently middle-aged, and will continue to shine reliably for billions of years to come. But as it nears the end of its life it will slowly become hotter and brighter. The 'habitable zone' will gradually migrate outwards, eventually leaving the Earth behind so that our planet begins to follow the path taken by Venus billions of years before, losing its oceans to evaporation and entering a period of runaway greenhouse heating. As if this wasn't enough, as the Sun uses up the last of its fuel it will begin to swell up, cooling and reddening as it does so, until it becomes a Red Giant a bloated, dying star around a hundred times its current size. Mercury, Venus and quite probably the Earth will all be swallowed up and incinerated and life on Earth will finally be over. In Doctor Who, the Doctor takes Rose to see this happen in The End of the World (2005).

There isn't much we can do about this destruction of the Earth except find ourselves a new place to live. But until that happens many of the dangers threatening the Earth could still be avoided if only we set our minds to it.

Perhaps the most influential of Jo's journeys in time and s.p.a.ce is her trip in Day of the Daleks (1972) to a version of the Earth in the twenty-second century. It's 'a version' of the Earth because, having seen a terrifying future in which humanity is enslaved by the Daleks, the Doctor is able to return to the present day and stop the event that creates that future. The lesson for Jo and us from exploring s.p.a.ce and time is that we can save the Earth, if only we act now.

Day of the Daleks is just one example in Doctor Who where history in this case future history can be changed. But there are consequences to making those changes, and the Doctor's own people have very strict rules about interfering in time...

The Destructions of Earth Each incarnation of the Doctor has seen life on Earth threatened in inventive ways. Here are some examples...

1. Plague and motor The Dalek Invasion of Earth (1964) The Daleks were the first of many aliens to invade Earth in Doctor Who. Their plague bombs wiped out whole continents of people and they enslaved the rest to help mine out the planet's core and replace it with a power system so that they could pilot the Earth anywhere in the universe.

2. From the depths The Underwater Menace (1967) Professor Zaroff's plan to raise the sunken island of Atlantis involved draining the sea into the centre of the Earth but the Doctor pointed out that the heat of the planet's core would turn the water to steam and crack the Earth's crust, destroying all life.

3. Roll back the clock Invasion of the Dinosaurs (1974) The appearance of dinosaurs in modern-day London was actually a diversion, not an invasion. A misguided group of scientists and politicians were so horrified by man-made pollution that they tried to use time-travel technology to rewind history returning the Earth to a 'Golden Age' before people came along.

4. Swallowed whole The Pirate Planet (1978) The hollow planet Zanak was made to materialise around other, smaller planets and then mine out their precious minerals. The Doctor stopped the pirates in charge from doing this to Earth.

5. Crash course Earthshock (1982) The Cybermen tried to wipe out humanity by crashing a s.p.a.ceship powered by antimatter into it. But the ship accidentally travelled back in time 65 million years and wiped out the dinosaurs instead.

6. Moving places The Trial of a Time Lord (1986) At some point in the future, the Time Lords moved Earth and its 'entire constellation' by a couple of light years. The effect was like that of a solar firestorm. Some humans survived in the London Underground system.

7. Industrial progress The Curse of Fenric (1989) The Doctor describes a dying Earth some thousands of years in the future, the surface just a chemical slime as a result of pollution. However, his actions seem to stop this future happening.

8. Inside out Doctor Who (1996) When the Master opens the Eye of Harmony that powers the Doctor's TARDIS, the effect is to warp reality and threaten to suck the Earth inside out.

9. Here comes the sun The End of the World (2005) The Doctor takes Rose some five billion years into the future to watch the Earth destroyed by the expanding Sun.

10. Reality Bomb The Stolen Earth / Journey's End (2008) Davros and the Daleks move the Earth across s.p.a.ce to join other planets and a moon as the power source of a Reality Bomb that can destroy all life in the universe.

11. Crash course 2 Dinosaurs on a s.p.a.ceship (2012) A large s.p.a.cecraft threatens to crash into the Earth in 2367 just like the one that wiped out the dinosaurs in Earthshock. Except, the Doctor discovers, this s.p.a.cecraft is itself full of dinosaurs.

12. Wood you believe it? In the Forest of the Night (2014) The Earth is engulfed by forest overnight, but the Doctor discovers that the trees are not invading: instead, they act as a shield from solar flares that would otherwise destroy life on Earth.

'This is a very important thing,' said the Doctor, thrusting the small plastic container into Martha Jones's hands. A skinny fellow in hi-tops, a pinstripe suit and a long brown coat, he gave her an intense stare that made it hard to look away. 'I need you to keep this safe,' he went on, 'for about the next five minutes or so.' A thunderous crash of noise sounded off the black stone walls and the network of waterlogged caverns around them. The Doctor shot a worried glance over his shoulder. 'Maybe ten minutes,' he corrected.

The brackish, alien seawater of the planet Karadax was sloshing over Martha's shoes, getting her feet soaked and making her jeans damp. And somewhere nearby, something very large and very angry was moving through the caves on hissing mechanical piston-legs, getting closer. It wasn't an optimal situation.

'OK.' She held the box gingerly.

'Good!' The Doctor fished in his pocket for his sonic screwdriver. 'What's in there is vital! The fate of hundreds of worlds turns on you making sure nothing happens to it.'

'You're overselling it,' she told him.

'Little bit,' he admitted. The sonic's tip flashed as he activated it. 'Here we go, then! Stay out of sight till I come back.' And with that, he dashed away through a gap in the rock, his coat flapping open behind him.

The unseen big and noisy thing reacted a second later, with a hooting, robotic snarl that made Martha cringe. She retreated into a corner, and decided not to look out into the tunnels beyond. Martha climbed up a hump of damp volcanic sand where the seawater didn't reach and clutched the box to her.

Distantly, she heard the Doctor yell out 'Ha-hah!', followed by another echoing crash. And before she was even aware of it, Martha was flipping the latches on the plastic box it was like the ones you'd use to keep leftovers in and lifting the lid. What exactly was so vital that it couldn't...

Martha's shoulders sank. Inside the box was a limp object, curled up in a rough knot. She poked it with a finger and realised her first impression was correct: that the Doctor's important item was, in fact, a sock.

'Are you making fun of me?' she snapped, but the Time Lord couldn't hear her, not with the crashing and banging going on outside the cave. 'This is just a-'

'Yeah,' said a voice behind her. 'It is.' Martha felt the hairs on the back of her neck stand up, and she spun around as a shadowy figure stepped through a shimmering tear in the rock wall. But not a tear, not really. More like some kind of portal, that snapped shut the moment she was through.

Because the person the voice belonged to was a she; and while this person didn't have Martha's red leather jacket and Martha's spiked hair, what she did have was Martha's face and voice.

'h.e.l.lo, you,' said the new arrival. 'This might take some explaining...'

Martha looked at the other woman, at a face that was a bit more drawn than hers, a hairdo a bit more serious and mumsy. 'You're me,' she said, 'but... older?'

'Oh,' said the second Martha. 'Maybe it won't.'

The clawed tentacles of the Karadax drone slammed down into the ankle-deep water with a crash, narrowly missing the Doctor as he twisted away. A second slower, and the sleek, deadly war machine would have turned him into Time Lord confetti. He ducked low, calling on moves he'd learned in dance lessons with a Spanish Contessa, and deftly sidestepped to avoid another pair of weapon-limbs. Each was festooned with clacking, buzzing, stabbing implements that he really did not want to go anywhere near.

'Could we just chat about this?' he called, as the machine gave a juddering growl. 'I know you think you'd like to destroy me, but you're making a big mistake. Stop for a second, take a moment. It's not the best idea.'

As he spoke, the drone pulled away. The Doctor guessed the artificial intelligence core inside its body was processing new methods to separate bits of him from other bits of him in the most unpleasant manner possible. The machine the Karadax had created to take over the universe vaguely resembled an Earth octopus, with a roughly ovoid body and eight long, flexible tentacles. But there the comparison ended, as it was made of polymorphic phase-metal, capable of fabricating dozens of different weapon templates on the fly and was the size of a battle tank.

That the Karadax a race of bad-tempered shark-people had built such an ingenious weapon did not surprise the Doctor. He'd met plenty of people who thought the way to get ahead was to make everyone else's lives miserable. But that they'd decided to test their prototype on him... Well, that had come as a bit of a shock.

'I should be flattered!' he told it, as the drone turned a glowing blue sensor grid on him. 'But seriously. There are better ways to get on my good side...'

He trailed off as the machine gave a shiver and rose up. Panels snapped open and produced the maws of lethal energy cannons, all turning to train on him.

'Ah,' he said, as the cave lit up with flashes of atomic lightning.

'You're handling this well,' said the other Martha, brushing a strand of hair out of her eyes.

Martha's hand was halfway raised, about to do the same thing, and she suddenly felt self-conscious. 'Um. I suppose. But I've seen a lot of strange stuff lately. Judoon, Plasmavore, Carrionites... Travel in s.p.a.ce and time, yeah? So the idea of meeting an older me isn't that weird.' She paused; it was very weird, to be honest. 'h.e.l.lo.'

'Hi.' Her elder self gave a sad smile. Martha could see her better now; a fair bit older, with wrinkles at the corners of her eyes, a bit of grey in her hair, and the way she moved spoke of someone who had seen a lot, done a lot. 'Look, I'm really sorry to do this,' she said, 'but this is the only place where you're not with him, and we can talk. He'd be furious if he saw us together.'

'The Doctor?' In the distance, there was a crash of steel on rock.

Older Martha nodded. 'It's because of him I came back in time to talk to you. Because of what happens in the future.' A melancholy look crossed the other woman's face. 'If you stay with him, you're going to see a lot of amazing things... But he'll break your heart.'

Martha stiffened. 'I don't know what you're on about. It's not like that.'

'Yes it is. Remember who you're talking to?' said the other woman, in a tone like the one Martha's mother used. 'But it's more than that. Terrible things will happen if you travel with the Doctor. People will get hurt, people you love. You'll see stuff that will change you for ever, and not for the better. Right now, you're free of all that. You haven't been... tainted.'

'What are you saying?' Martha gave her future self a hard look.

'Go back to the TARDIS, right now. It can take you home. Forget you ever met the Doctor, or saw any of this.' The older woman gestured around her. 'It's for the best. Please, you must believe me!'

'You want me to change my future,' said Martha. 'Your past?'

'That's right. Make sure it never comes to be. The Doctor will go on and do what he does, but you'll be safe. You and everyone you care about.'

Martha shook her head, her thoughts reeling. Could it be true, she wondered? So I've seen some scary monsters... But what if there's worse to come? 'If I do what you say,' she said haltingly, 'won't that mess up history? I mean, for me what you say hasn't happened yet, but for you it has. Isn't that a, what's it called? A paradox?'

Another thunderous crash echoed down the stony tunnels. 'That doesn't matter. I know what I'm doing. If you can't trust me,' said the other Martha, 'who can you trust? I don't want you to have the terrible life I had! To end up alone, on a devastated Earth...'

Martha's blood ran cold. 'What about my family?'

'You can make sure they're safe. Mum and Dad, Leo and Let.i.tia...'

'What did you say?' Suddenly, something very unpleasant occurred to Martha Jones.

'I get why you're doing this,' said the Doctor, skipping from stone to stone across a pool of oily water. He ducked a laser bolt that chopped off a dangling stalact.i.te. 'You want to be important!'

The octopoid drone drew back its weapon-arms and stalked around the edge of the cave, scanners humming as it considered its next attack.

The Doctor guessed that whatever nasty sort had set this thing running was probably watching him through its sensor eyes, so when he talked, that was who he was addressing. 'Because you don't want to go on being second-banana scourges of the galaxy, do you?' He gave a shrug. 'I mean, when they're handing out the Cosmic Menace of the Year Awards, you don't want to be sitting in the audience. Again.' The Doctor made a mock-sad face. 'Who wins this year? Oh, the Daleks, what a surprise. Runners-up? Those silver fellas, always the CyberBridesmaid, never the CyberBride. Even the Draconians get a look in, but not you, eh? I bet that stings.' He puffed himself up. 'You want the Karadax to be feared. For the very stars themselves to quake at the sound of your name!'

And then he opened his hands, and gave a compa.s.sionate look. 'Why have you got to be like that? It never turns out well. Why can't we all just get along?'

For long seconds, the drone paused, and the Doctor started to entertain the idea that it might actually be considering his words; but then it leapt at him with a snarl, extruding blades like spreading flower petals.

'How do I know you're really me?' Martha jabbed a finger at her older self. 'You could be some kind of shape-changer or something!'

'That's ridiculous,' said the older woman, tapping her chest. 'It's me. I mean, you. I mean, us.'

'But you would say that, wouldn't you?' Martha shook her head. 'When the Doctor comes back-'

'You can't wait for him,' she insisted. 'You've got to decide. Him or your family. His future or yours.'

Martha took a step closer, her eyes narrowing. 'My sister's name is Tish. Never Let.i.tia. She can't stand being called that. Too posh, she always says.' She glared at her other self. 'If you were me you'd know that.'

'I did. I do. But she... Got used to it.' The older woman blinked, her tone becoming evasive. 'You know Tish. So changeable!'

'Who was the first boy we ever kissed?' Martha said, in a low voice.

'You're testing me? You don't trust yourself?'

'What was his name?' she demanded.

The older woman blinked, as if she was going to cry. 'I'm sorry. I don't remember that, it was a long time ago. At my age, you forget some things... But you have to listen to me, Martha. It's important! You have to do what I say! We don't have any choice. You have to abandon the Doctor, and you have to do it right now!'

'Why are you so determined to make me leave?' Martha reached out to grab the older woman, and her other self shrank backward. 'Tell me-'

She grabbed Older Martha's wrist, and something strange happened. Instead of encountering warm skin, her hand sank through the flesh like it was smoke and grasped a thin, cold rod concealed beneath.

A flicker ran through the other woman, as if she were an image on a TV screen pixelated by interference. 'Visitors must not touch the exhibits,' said Older Martha, and her words had a metallic echo to them.

'What... are you?' Martha gasped, s.n.a.t.c.hing back her hand.

'I'm sorry,' said her duplicate, her face shimmering. 'I didn't want to do this. And now I've failed, they're going to come here.'

'OK,' said the Doctor, between panting breaths. 'I'm getting peeved now.' He kicked aside the slushy ice the drone had made, using a cryotherm cannon in an attempt to flash-freeze him. 'Stop trying to kill me. We hardly know each other. It's rude!'

He worked the tiny, complex controls on his sonic screwdriver, eyeing the octopoid as it went through yet another reconfiguration cycle. What was it going to produce this time? An electro-charged restraint web? Photonic lance? Bio-shredder? The machine had the templates of millions of different weapons in its memory, and the will to use them.

'I know why you're doing this,' he told it. 'My reputation precedes me, doesn't it? I've beaten pretty much every big bad in the universe at one time or another, if I do say so myself. And you think if you can take down the Doctor, everything else will be child's play.' He raised the sonic. 'Fair enough. Bring it on.'

For a second, Older Martha's body flashed and then it was gone. Revealed beneath was a skinny, shop-window dummy made of coppery metal, covered in glowing, gem-like projector lenses. 'This is really me,' she said, still speaking with Martha's voice. 'I am exhibitoid UF-032, an artificial simulacrum from the Museum of the Doctor on planet Reldeen, chronolocation forty-fifth century by your measurement system.'