'We're somewhere in the Garn Belt.

There's an atmosphere. Early indications suggest that-'

'We're on Alfava Metraxis, the seventh planet of the Dundra System. Oxygen-rich atmosphere, all toxins in the soft band, eleven hour day and chances of rain later.'

River Song and the Eleventh Doctor, The Time of Angels (2010)

In fact, a companion's skills and experience are unlikely ever to match the Doctor's. We can leave flying the TARDIS and fixing it when it goes wrong largely to him. Instead, the biggest risk of travelling with the Doctor is what to do if we get separated from him which happens a lot in the series. So the 'right stuff' required to be a companion means practical things like self-reliance, bravery, the ability to adapt quickly and improvise.

But there's more than that, too. Time travel may be commonplace in the universe of Doctor Who, but its consequences can still be very hard for his companions to deal with. When the Doctor first takes Rose Tyler into the future in The End of the World (2005), she finds all the aliens and being able to understand their languages a bit overwhelming. To help, the Doctor fixes Rose's mobile phone so that she can call her mum, back in the distant past. For a moment, Rose is happy just to hear Jackie's voice. But the moment the call is over, the truth hits her hard:

'That was five billion years ago so she's dead now.

Five billion years later, my mum's dead.'

Rose Tyler, The End of the World (2005)

We're repeatedly shown in Doctor Who the odd side effects of travelling in time. Rose doesn't just travel to the future where her mum is dead but also in Father's Day (2005) goes to her parents' wedding in the 1980s and meets herself as a baby.

In The Power of Three (2012), Amy and Rory know that the amount of time they've been travelling with the Doctor for doesn't match the amount of time that has pa.s.sed for their friends and family on Earth. Their friends don't know that Amy and Rory travel in time, but we see that it still affects their relationships: Amy's friend Laura isn't sure about asking her to be a bridesmaid because Amy's not always around. We don't know if Amy makes Laura's wedding, but we do know Amy and Rory end up back in time at the end of The Angels Take Manhattan (2012), separated from their friends and family for ever. How companions survive after they've travelled with the Doctor how they return to ordinary life after all of time and s.p.a.ce is a worry, too.

Companions have to be tough psychologically. To travel with the Doctor, they must ask questions and solve problems, but they have to be ready for answers that can be surprising and unsettling. They are often threatened with death and they often see people die. There are times when they can stop and help and save people, and there are times when they must walk away. It's OK to be scared, it's OK to make mistakes, and it's OK to be ordinary. What gives a companion the 'right stuff' is an open mind, being willing even eager to explore all the strangeness of time and s.p.a.ce, to puzzle out how it works and fits together, to face the consequences of discovery.

But that, after all, is what scientists do every day. Rather than a collection of facts, science is really an att.i.tude a way of looking and thinking about the world. It's a quest for knowledge, no matter how counterintuitive or strange, with a readiness to accept in the face of new evidence that everything you thought you knew is wrong. That's what makes science such a powerful tool for understanding the universe around us.

In Battlefield (1989), the Seventh Doctor takes Ace with him when he visits UNIT, handing her an old security pa.s.s so she can get past the soldiers on guard. The pa.s.s is for Liz Shaw, the companion of the Third Doctor with degrees in physics and medicine. Ace who we later learn couldn't pa.s.s her school chemistry exams is worried. But the Doctor rea.s.sures her: it's not qualifications that matter but att.i.tude.

'Who's Elizabeth Shaw? I don't even look like her.'

'Oh, never mind. Just think like a physicist.'

Ace and the Seventh Doctor, Battlefield (1989)

Here we are now, in the park. With the pram beside her and the little one asleep, Tilly Pilgrim is watching a father pushing a child on a swing. Up curves the swing, and then, because this is how swings work, down comes the swing, and then it comes back, then down, then up and the child opens his mouth to laugh...

And then Tilly comes unstuck in time...

A cold red Sun curves across a darkening sky in a great arc rising, peaking, and plunging down again behind the horizon. A bare Moon rises and speeds on its path. Then the Sun again, and the Moon, and the Sun, and the Moon never-ending, and all the time Tilly can hear the cries of the d.a.m.ned trapped in this h.e.l.l-time. Months, it seems, pa.s.s years, and the chorus of screaming is ceaseless.

And then a single voice comes, rising steadily above the rest, breaking through...

'Can you hear us? You must hear us! We are coming! Coming through...'

And down comes the swing. And back and down and up and down and back and 'h.e.l.lo,' says the man now sitting beside her. 'Your daydreams. They're very disruptive. There should be a warning sign. Something of a nuisance for you, too, I should think.'

The man is funny-looking, limbs awkward like a baby giraffe. Tilly owns a lot of giraffes these days or, rather, the little one does but the point is Tilly knows what giraffes look like when she sees one. And this man looks like one a baby giraffe that is or he would, if baby giraffes wore bow ties.

'And tweed,' Tilly says, and giggles.

'Tweed?'

'My daydreams.'

'Your daydreams are tweed?'

'Don't be silly,' says Tilly, and giggles again. Then the man looks at her with such compa.s.sion that she thinks she might start crying. 'How do you know about my daydreams?'

'Well, I could hardly miss them, could I? They almost knocked me off course!'

'Off course? Well, of course!' Tilly says gaily. Very little about her life makes sense at the moment what with the daydreams that seem more real than everyday life, and the thinking that the world is ending, and the being unstuck in time so why not something else? Why not a tweedy baby giraffe with a bow tie?

He is looking at her now, intensely, and the urge to weep is very strong again. This man, she thinks, has seen whole worlds become unstuck. He has seen monstrosities, and he has judged, and sometimes he has forgiven. And Tilly who doesn't know what is to be done, but knows that something must be done and quickly makes a decision.

'Let's go for a coffee,' she says.

The man with the bow tie stares at her in horror. 'A coffee?'

'A coffee!' He looks around, enchanted, and his hands flap about, like a dodo's wings might have done when failing to fly. 'Me! Going for a coffee! In a cafe! Me!'

'It's nowhere special,' Tilly says apologetically. 'Just one of those chain ones.' But the staff are nice and they don't hurry her out, and the mums in the indie cafes are slim and blonde and judge.

'Can I have one of those giant chocolate biscuits?' says her companion.

'You can have whatever you like. Your dollar-'

'Ah.' He fidgets. 'Yes. You see, I don't have any money.'

Tilly sighs. 'Two double espressos and a big bourbon biscuit, please,' she says to the nice young woman at the counter. Bow-tie man, happy again, capers off to find a seat, and she adds, 'Could you make one of those espressos decaffeinated?'

'So,' says the man-boy, after she's wheeled the pram round and sat down opposite him, 'These dreams.' He's not flapping or fidgeting now. 'Serious business, dreams. Particularly ones that can knock a time machine off course.'

The baby stirs, but rolls her head to the other side and goes back to sleep. 'Time machine,' Tilly says, meditatively.

'Don't worry about it,' he urges. 'Tell me about the dreams.'

She ponders how to explain. 'Have you ever read the Narnia books?'

'Read them? I built the wardrobe!'

'It's a made-up wardrobe-'

'Believe that, by all means, if it helps.'

Tilly decides, sensibly, that this is the least of her worries. 'The Pevensie children. They went into the wardrobe, and became kings and queens of Narnia, and reigned for years. Then they went on a hunt, and they chased the white hart through a waste land-'

He is nodding. He knows the story. 'And they fell out of the wardrobe no older than they were when they went in.' He picks up a little packet of brown sugar from the table and twists it around between thumb and forefinger. All this nervous energy, Tilly thinks, could make you feel really rather tired, if you weren't very tired already from the sleepless nights and the dream-filled days. 'Is that what happens, Tilly? Do you fall into the wardrobe and become a queen of Narnia, and then fall out again no older than you were when you went in?'

'I didn't tell you my name.'

'No, you didn't. I'm the Doctor. There equal footing. Is that what happens?'

'Yes,' Tilly says, with great relief. 'That's how it happens. How it's always happened.'

He's very intent now. 'They've not just started, then?'

'No, I got them as a child...'

The Most Beautiful Place in the World Here she is now, in her home away from home, back again in the magic land, the daydream land, the garden, the place where a child can roam safe and free. This is Middle-earth, this is Narnia, this is where the wild things are and the white flowers bloom at night. This is where colours are brighter, sounds sharper, and she is still young enough that she misses nothing, records all, remembers all.

Here she quests, and explores, and wanders, and sometimes just sits by running water and watches it ripple over her toes. Here she becomes a hero, a captain, a navigator, a gardener, a king and a queen. Here she leads many long lives of adventure, and peace and, later, love. But when she returns home, she is still on the bed, and the clock still shows the same time.

And the places she visits are always various and new, and the people are generous and kind, and the summers are warm but not unbearable, and the winters are beautiful but not cruel, and spring is full of life, and autumn full of promise, and time, relentless time, is pa.s.sing and then, and then, and then...

'And then?'

'And then it... stopped.'

'Why?' says the Doctor, urgently. 'What happened?'

Tilly sighs. 'I guess I fell out of the wardrobe,' she says with regret.

'So the dreams?'

'Went away.'

'Where did they go?'