'I'll try.'

'I'm not inclined to make threats-'

'So don't.' He moved fractionally towards her. 'Trust me.'

She nodded. 'I suppose that'll have to be enough. Very well, you may go. Succeed and I'll grant you any reward it's in my power to give, within reason. Fail...' She shrugged. 'Fail, and I suppose you'll be dead, too.'

The Doctor asked for the times and locations of the attacks and stood over the Empress's map, his eyes half-closed, as he began his calculations to triangulate the most likely location for where the mosquito swarms were recharging.

After tripping over another exposed root on the forest floor, Clara stopped and folded her arms. 'How long are we going to have to walk around in circles before you admit we're in the wrong place?' she asked.

'All right, all right,' said the Doctor, 'we'll try the second most likely location.'

They moved through the trees as though they were tracking a wary fawn. From some distance away, Clara had spotted the clearing in the forest and its tiny silver occupants. Her heart was beating fast as they made their way forward. Her footsteps through the undergrowth sounded horribly loud. But the mosquitoes didn't seem to hear.

In the midst of the clearing stood a tall, twisted tower, soaring into the sky. Around it, there were thousands of shining mosquitoes, hanging in the air. Still and silent, as though they were captured in invisible amber.

'How do we get through?' asked Clara in a low voice.

The Doctor took out his sonic screwdriver and adjusted the settings. When he activated it, nearby mosquitoes began to buzz faintly and move away from them, slowly clearing a path to the centre of the clearing. 'Come on,' said the Doctor quietly, 'quick as you can.'

Clara followed the Doctor through the swarm. There were so many of them, and they were so close. Any moment, surely one would reach out and sting her... When they reached the central tower, she allowed herself a sigh of relief. There was a good clear gap of a metre or more between the swarm and the tower. She noticed the tower itself was emitting a low hum, like the sound of high-voltage electricity pylons.

'Interesting,' said the Doctor. 'This thing isn't just recharging, it's transmitting too.' He pocketed his sonic and pressed his palm against the tower. Instantly, the structure shifted, the silver metal reshaping itself into a control panel. As he tapped at the controls, the tower's hum changed pitch.

'Tell me that's a good sound,' said Clara.

'It's the sound of me trying to disconnect this tower from wherever the data's going, since that's also where the instructions for the mosquitoes are coming from.'

'Right. Good.'

'Ah,' said the Doctor.

'Ah? Good ah?'

'Bad ah. They're sending something back down the datastream.'

'Please don't let it be orders for all of these mosquitoes to eat us.'

The Doctor hesitated, then said, 'No, it's a message.'

'Oh.'

'They'd like to know who we are and why we're interfering with their scientific research,' he said, before he began tapping away again.

Clara was appalled. 'Then they are using the poor people! I hope you're giving them a suitably angry reply.'

'I am.'

'Is there swearing? Angry swearing?'

'There may be one or two pointed comments on their lack of ethical standards,' said the Doctor. The panel bleeped. 'It seems that their exploratory probe found no sentient life forms on the planet, and so they felt able to begin experimentation to search for a cure.'

'A cure for what?'

'A plague ravaging their own worlds.'

'So they decide to infect another planet? Brilliant plan.'

'Desperate people do desperate things,' said the Doctor. 'And a little compa.s.sion wouldn't go amiss.'

'It's not your planet they're sending alien diseases to.'

The Doctor shot her a look, then read on: 'However, since we're communicating with them, they accept there may have been an error and are prepared to terminate.'

'No sentient life?' said Clara. 'Seriously? Did they not notice all the people?'

'I think they did.'

She stared at him. 'You mean humans don't count as sentient?'

'Apparently not, not by their standards anyway.'

Clara narrowed her eyes. 'I don't like these aliens.'

'Well, they've seen reason so I-' The panel beeped again. 'Stand well back,' read the Doctor. The ground began to tremble, while the tower folded the communications panel back into itself and began to shake in an alarming way.

'Yeah,' said Clara, pulling on the Doctor's arm. 'I think that might be a hint to get out of here.'

Clara wasn't quite sure if the tower had taken off or exploded but, either way, it and the mosquitoes had gone, leaving a charred and smoking clearing in their wake.

'Well, all done here,' said the Doctor, sticking his hands in his coat pockets. 'How about we take another try at the Eternal City, hmm?'

'Hang on, what about all those sick patients?' He didn't answer. 'What? Doctor what?'

'No one else is going to get sick, not because of this. I did as much as I can.'

She held his gaze. 'They'll die.'

'Everyone dies. When you go back home, everyone here will've been dead over a thousand years. We helped, Clara, but I'm not your planet's medical service: I can't cure everything.'

'And we're not going back to tell Ulpia what's happened? She did promise you a reward; you could get your own villa.'

'The problem with despots, Clara, is that when they find someone useful, they tend to want to keep them. I'm sure she'll figure it out soon enough.'

'Right, I'll keep that in mind. So which direction is it back to the TARDIS?'

The Doctor spun around, and then pointed off into one patch of trees that looked much the same as all the others. 'Forwards, of course.'

'Have you ever thought what it's like to be wanderers in the fourth dimension?'

The First Doctor, An Unearthly Child (1963)

We've discussed different ideas about how time travel might be possible, but if we could really travel through time, what skills and knowledge should we take with us?

It's not easy becoming an astronaut. First, it's not often that a job comes up in s.p.a.ce. Since the European s.p.a.ce Agency was founded in 1975, it has recruited new astronauts on just three occasions: in 19781979, 19911992 and 20082009.

There was a lot of compet.i.tion for those jobs when they came up. On the last occasion, there were 8,413 applications from people who could meet all the initial requirements, which included supplying a JAR-FCL 3 Cla.s.s 2 certificate the same kind of medical certificate you need when applying for a private pilot's licence. Further medical, psychological and cognitive tests, plus interviews, helped ESA narrow the 8,413 applicants down to just six people, who then began their astronaut training. The rejected 8,407 candidates could try applying to the limited number of other s.p.a.ce agencies round the world, or give up on their dreams of going into s.p.a.ce.

For the six candidates who made it, basic training at ESA takes about sixteen months. There are lessons in the practical skills needed to live and work in s.p.a.ce: engineering, orbital mechanics, propulsion, human physiology, biology, astronomy and ways to study the Earth. Scuba-diving lessons help prepare astronauts as much as possible for being and working while weightless. They are taught how to operate the systems on the International s.p.a.ce Station where they will work, plus all the different s.p.a.cecraft that dock with it. There is focused tuition on operating robotic systems, and rendezvous and docking in orbit.

Trainee astronauts at ESA also study psychology and human behaviour to help them deal with the stress and weirdness of being out in s.p.a.ce. One training programme includes living underground in a system of caves for a week to better understand human behaviour and performance in extreme conditions. They are taught about national and international laws and policies that affect s.p.a.ce, and learn about all current and planned s.p.a.ce missions planned by different countries and organisations. They even learn to speak Russian, since ESA often launches and lands its astronauts from Russia.

That's just the basic training. There is then a year's advanced training on technical skills needed to live and work on the International s.p.a.ce Station, including being able to run science experiments there. After that, the candidate astronauts are a.s.signed to a flight into s.p.a.ce and have eighteen months of training specific to their particular role and mission. In total, astronaut training takes about four years and many astronauts are selected because they already have skills and experience they can bring to the job. For example, one of the six astronauts recruited by ESA in 2009 was Major Timothy Peake, who was already a graduate of both the British Army Air Corps and Empire Test Pilots' School, and had a degree in flight dynamics and evaluation all good preparation for becoming, in 2015, the first British ESA astronaut to visit the International s.p.a.ce Station.

All this effort is to equip astronauts for the difficulties of work in s.p.a.ce and to prepare them for the various hazards they might face there. As we discussed in Chapter 2, missions can go wrong. In 1970, when an oxygen tank exploded on the Apollo 13 s.p.a.cecraft while on its way to the Moon, the crew and their team back on Earth knew their equipment and the science so well that they were able to improvise a number of brilliant fixes that got the astronauts back to Earth alive.

In Doctor Who, the Doctor's companions who face the same hazards plus monsters and explosions have to develop their skills and state of mind as they go. The Doctor doesn't expect his companions to provide a JAR-FCL 3 Cla.s.s 2 certificate, but he clearly has criteria for what makes a good companion. Even when he's not travelling alone, he's on the lookout for people with the 'right stuff' to join him on his travels he offers 'all of time and s.p.a.ce' to Rita in The G.o.d Complex (2011) and Osgood in Death in Heaven (2014). He's also been known to dump would-be companions after one trip if they don't measure up as he does with Adam in The Long Game (2005).

So what makes the 'right stuff'? Quite often, the Doctor's companions are scientists of different kinds (there are many different ways to pursue a career in science). One of the first companions in Doctor Who, Ian Chesterton, teaches science at Coal Hill School. The Second Doctor's companion Zoe Heriot is an astrophysicist with a pure mathematics major useful for her work on a s.p.a.ce station. The Third Doctor's companion Liz Shaw, we're told, has 'degrees in medicine, physics and a dozen other subjects'. Later companions Romana and Nyssa, being aliens, don't hold qualifications from Earth but they're clearly both scientists: in The Pirate Planet (1978), we learn that Romana studied the life cycle of the Gallifreyan Flutterwing; in Terminus (1983), Nyssa synthesises an enzyme on the table in her bedroom. Adric, another alien, wears a gold star for mathematical excellence. In Planet of Fire (1984), Peri is meant to be working on an ecology project; we learn later that she is a botanist. Harry Sullivan, Grace Holloway and Martha Jones are all medical doctors. Ace failed her chemistry exams at school but the Seventh Doctor calls her an 'expert' in explosives. Other companions are from the future, which means they understand science and technology we have not yet developed.

River Song is depending when we meet her a student, doctor and then professor of archaeology. We'll talk more in Chapter 10 about whether history and archaeology count as 'science' as we understand the term, though the word derives from the Latin word 'scientia' knowledge. But is a knowledge of history any use to the Doctor? Another of the first companions in Doctor Who, Barbara Wright, teaches history in the same school as Ian. But few companions since Barbara have had particular qualifications in history. Sarah Jane Smith was a journalist, another job that involves researching, a.n.a.lysing and organising information. She knows a surprising amount about ancient Egyptian mythology in Pyramids of Mars recalling the number of G.o.ds listed in the tomb of Thutmoses III but generally it's the Doctor who provides any historical detail that a story requires.

For all the Twelfth Doctor says he doesn't like soldiers, many of his companions have had military experience. Harry Sullivan was on active service when he joined the Doctor. Steven Taylor, Jamie Macrimmon and Vislor Turlough had all fought in wars. Leela is a warrior of the Sevateem. Ian Chesterton would presumably have done National Service. Rose, Mickey, Martha, Amy and Rory all learn to become soldiers, using guns to battle Daleks, Cybermen, Sontarans and the Silence.

A general level of fitness is also important to being a good companion: as Donna complains in The Doctor's Daughter (2008), 'Seriously, there's an outrageous amount of running involved.' The Doctor doesn't need to travel with athletes, but Rose's bronze in gymnastics is crucial to her helping him in her first story.

Knowing about science and history, and being able to fight monsters and run are all useful skills in a series that travels in time with plots involving monsters and chases. (We could think of it another way: it's useful for the writers of Doctor Who if the companions are able to help explain and drive the plot.) Yet some of the most successful of the companions in Doctor Who are not scientists or soldiers. The Doctor doesn't offer trips in the TARDIS to Rose Tyler, Donna n.o.ble, Amelia Pond and Clara Oswald to name just a few of them because they have particular skills or qualifications (though Donna's ability to type a hundred words a minute comes in handy in Journey's End (2008)). So what do they offer that makes them the 'right stuff' for being a companion?

Before we try to answer that question, it might help to think about the sorts of problems and hazards we would face if we could travel in time. The crew of Apollo 13 had to deal with an oxygen cylinder exploding. We've seen the TARDIS get into difficulties, such as when the police box exterior shrinks in Flatline (2014) or collides with a salvage ship in Journey to the Centre of the TARDIS (2013). Just as the crew of Apollo 13 improvised a solution, Clara and the Doctor are able to work out how to save the TARDIS by giving it an energy source or sending a message back in time.

In Journey to the Centre of the TARDIS, it's the Doctor who works out the solution. Clara works out the solution in Flatline but several times in the story we're told she has started to think like the Doctor and in Death in Heaven (2014) she fools the Cybermen into thinking she is him. The implication is that it's his influence what she's learnt while she's been travelling with him that helps her solve the problem. Just as some companions learn to operate at least some of the TARDIS controls, these are skills learnt on the job not something the companions arrive with.

A more common problem in travelling in the TARDIS is not knowing where or when you've arrived. The Doctor might aim for London but end up in Glasgow as he did at the end of Deep Breath (2014). Being able to work out your location would be a useful skill.

In fact, for centuries before global positioning satellites allowed us to work out our position on the Earth's surface with a high degree of accuracy, people used the stars. In the northern hemisphere, we can use Polaris, also known as the Pole Star because it appears in the sky almost directly over the North Pole.

To find Polaris, you first need to look for a distinctive group of seven stars called the Plough (it's also known in America as the Big Dipper, among other names). Imagine a line between the two stars at the right end of the Plough they're called Merak and Dubhe. Extend that line up away from the 'rectangle' of the Plough, going five times the distance of the gap between Merak and Dubhe. The bright star you come to is Polaris.

Point with your finger to where Polaris is lowest in the sky and you are pointing north. That's useful for navigation, but you can also use Polaris to work out your lat.i.tude. Lat.i.tude is a measure of how far north or south you are on the Earth's surface: at 0 you are on the Equator, an imaginary line running round the Earth's middle, exactly halfway between the South Pole and the North Pole, and running through countries such as Brazil, Kenya and Indonesia.

Every 111.2 km (or 69 miles) north or south of the Equator, there's another imaginary line of lat.i.tude, measured in degrees either plus or minus, depending if you're heading north or south. The North Pole is at +90 (sometimes called 90 north), and the South Pole is at -90 (sometimes 90 south). On a map with north at the top, the lines of lat.i.tude run horizontally.

The Western Rocks on the Isles of Sicily are the most southerly part of the United Kingdom and are at +4951' (like hours, degrees divide up into 60 'minutes' indicated by the inverted comma symbol). The southernmost part of the British mainland is Lizard Point in Cornwall, at a lat.i.tude of +4957'. Southampton is more or less at +51 and London at +5130'. Milton Keynes is roughly at +52, Stoke-on-Trent, Derby and Nottingham are all close to +53. York is close to +54, Newcastle upon Tyne and North Shields close to +55, Edinburgh close to +56 and Aberdeen a little above +57. The northernmost part of Britain, Dunnet Head in Scotland, is at +5840'. Out Stack in the Shetland Islands the northernmost part of the United Kingdom is at +6051'.

Here's the clever bit. You used the Plough to find Polaris, and by pointing to Polaris you are now pointing north. But the angle your arm is making to the ground will give you your degree of lat.i.tude, too. If you're pointing straight up at Polaris, then you must be directly underneath it which means you're at +90 that is, the North Pole. If you're pointing directly ahead, Polaris is on the horizon which means you're at a lat.i.tude of 0C and standing on the Equator. Anywhere in between those two reference points (which should make a right angle), the angle of your arm will match your degree of lat.i.tude.

This was extremely useful in navigation, but knowing how far north or south you are is only half the problem. To fix your location on the Earth's surface you also need to know how far east or west you are and this is much harder to do. Again, a solution lies in the stars. For more than 200 years, the annual Nautical Almanac has provided navigational tables and charts of the Sun and Moon along with 58 of approximately 6,000 stars that are visible to the naked eye, with details for another 115 stars. These tables can be used to calculate your position not just in terms of north and south, but also east and west and they have enabled sailors and other travellers to make their journeys safely and quickly, while also allowing cartographers to greatly improve their maps of the Earth.

However, pinpointing your position by the stars like this is tricky. Britain's Royal Observatory was founded in Greenwich in 1675 to find a way to do it, and astronomers there took almost a century to solve the problem and produce the first Nautical Almanac. Making north-south measurements is the easy part: Polaris is unique among celestial objects because it is almost directly above the Earth's North Pole, which is the axis on which the planet rotates. That means that as the Earth turns, Polaris appears to stay in the same place in the sky, hovering always above the pole. But, as we saw in Chapter 1, all other stars appear to move through the sky as if fixed to a wheel though in fact it's we who are turning.

For example, take the brightest star in the sky the Sun. As we also saw in Chapter 1, the Sun is at its highest position in the sky at midday. But because the Earth is turning, 'midday' is different in different places round the Earth. While it's midday in the United Kingdom, it's the middle of the night in Australia. But we also know that the Sun and other stars move through the sky in a regular cycle which we can use to fix our east-west position or 'longitude'.

Longitude, like lat.i.tude, is a series of imaginary lines measured in degrees. Whereas lines of lat.i.tude run parallel to one another (and so never touch), lines of longitude all run between the two poles dividing up the Earth like the segments of an orange. Because of its long history of using astronomy to perfect the art of navigation, the line of longitude that runs through the Royal Observatory Greenwich in London is designated as 0. This is known as the Prime Meridian and it is the reference line from which all other longitudes are measured.

Because there are 360 degrees in a full circle we divide up the Earth into 360 lines of longitude: 180 heading east of Greenwich and 180 heading west. On the opposite side of the Earth from Greenwich is the antimeridian, a line of longitude that is 180 both east and west of the prime meridian. It cuts through parts of eastern Russia, the Fiji Islands and the middle of the Pacific Ocean.

Since we know the Earth rotates every 24 hours, we can work out that midday on the antimeridian will be 12 hours different to midday in Greenwich, halfway round the world. In fact, if we divide the 360 of longitude by 24 hours, we see that every 15 of longitude means an hour's difference in time. That calculation has been used as the basis for the different time zones on Earth with some variation for political and historical reasons.

For most of us, this difference in time only affects us when we try to call someone in another country and must factor in that our mid-afternoon is their middle of the night, and so on. When we travel to other countries in another time zone, we must change the time on our watches to give local time.

Knowing the difference between the local time that is, the apparent position of the Sun and other stars in the sky from where we are and the time back in Greenwich means we can calculate our longitude east or west of Greenwich. For example, if the Sun has just reached its highest position in the sky at our location but a watch set to Greenwich time is reading 1 p.m., we must be 15 west of Greenwich. It sounds simple in principle, but before it could be used in practice astronomers had to map the positions and motions of the Sun, Moon and stars with extreme precision, and clockmakers had to design and build clocks which would accurately keep Greenwich time even on long and arduous sea voyages lasting many months.

It still takes training and practice to make the necessary measurements, but knowing where on Earth you are is such a useful even lifesaving piece of information that for centuries people were willing to make the effort. Today, of course, anyone with a smartphone can use the satellites of the Global Positioning System to tell them their location. But this technology is in many ways a development of those early measurements of longitude using clocks and stars: the GPS satellites form an artificial constellation and computer software does all the difficult calculations for us, but the basic principles remain the same.

Being able to find out where you are your position in s.p.a.ce is something that we all find useful, but for time travellers like the Doctor and his companions finding out when you are your position in time is also a skill that can come in very handy. Surprisingly the stars could also prove very useful here, too. As the Earth spins on its axis, everything in the sky appears to move around us once every twenty-four hours and in particular the position of the Sun relative to the horizon is what tells us whether it's morning or afternoon, day or night. As the Moon orbits round the Earth, we see different portions of it lit up by the Sun. This 29.5-day cycle of lunar phases new Moon, waxing crescent, Full Moon, waning crescent, and then back to new Moon is the origin of our calendar system of months. The Earth also orbits the Sun, so that throughout the year the Sun appears to move once around the sky, relative to the background groupings or 'constellations' of stars. Days, months and years are therefore all astronomical measurements, marked out by the motions of the Earth and the apparent movements of the Moon and Sun in the sky. By understanding how their positions change, we can use the heavens as a giant clock, telling us hours, days and months.

One thing that doesn't seem to change is the constellations of stars. Night after night they appear to rise, move across the sky and set, but their familiar shapes the Plough, Orion the Hunter, Taurus the Bull and dozens of others are rea.s.suringly constant. However this constancy is an illusion and if we compared their positions after fifty or a hundred years, we would see small changes as each individual star slowly shifted its position in the sky. This is because the Sun, along with every other star we can see, is...o...b..ting round the centre of the Milky Way galaxy. It takes our Sun, and the entire Solar System with it, between 225 and 250 million years to travel once around the galaxy. The last time we were in this part of the Milky Way we're in now, the dinosaurs had just evolved and the Earth had only one continent. The Sun has made this epic journey about eighteen times since it formed 4.5 billion years ago and will probably make another twenty orbits before it finally dies.

Our galaxy is vast and, even moving at typical speeds of about 230 kilometres per second, the changes in the positions of the stars are barely noticeable in a human lifetime at least to human eyes. But with instruments like the European s.p.a.ce Agency's Gaia s.p.a.ce telescope, astronomers are beginning to measure and track the motions of around a billion stars in the Milky Way. Using computer simulations we can fast-forward these motions millions of years into the future or rewind them into the past, making the night sky seethe and swirl like a swarm of bees as each star follows its own unique path through the spiral arms, clouds of gas and billions of other stars which make up our galaxy.

The constellations which hung above Tyrannosaurus Rex and Velociraptor were not Orion or the Plough, and our own descendants will need to invent new names and stories to describe the patterns that the stars make in the unfamiliar skies of the distant future. Perhaps a time traveller with enough knowledge of galactic dynamics would be able to take one look up and know exactly when they had arrived in the Earth's long journey through the Milky Way.

We'll talk about other ways we might work out the year in which we'd arrived in Earth's history in Chapter 10, which would be useful for a time traveller. But in Doctor Who, the Doctor rarely needs his companions to deduce where and when the TARDIS lands. He can do that himself. For example, in The Time of Angels (2010), River checks the TARDIS controls for information while the Doctor just pokes his head out the door.