'She was a brave woman,' said the Doctor. 'She gave her life for those people, in more ways than one.'

'But it could all be for nothing.'

The Doctor didn't say anything. Then he gave her a broad, impish grin. 'Let's say we nip forward a few thousand years to find out.' His hands moved swiftly over the TARDIS console and the engines gave a desperate, wracking wheeze. He pressed a b.u.t.ton and the doors burred open. 'Come along, Sarah. Chop, chop. No time to lose.' He was out the door before she'd had chance to protest.

Sarah expelled a long, heartfelt sigh. 'Doctor...'

'There are worlds out there where the sky is burning, where the sea's asleep, and the rivers dream. People made of smoke and cities made of song. Somewhere there's danger, somewhere there's injustice and somewhere else the tea's getting cold. Come on, Ace, we've got work to do!'

The Seventh Doctor, Survival (1989)

On 21 June 1969, in Episode Ten of The War Games, the Time Lords found the Second Doctor guilty of interfering in the affairs of other people. As punishment, the Doctor was exiled to Earth, without the use of his TARDIS, just as the planet faced a series of new perils. He was furious at the idea. Like us, he'd be trapped on a single world.

Except that a month later, on 20 July, a small box touched down on the Moon.

'That's one small step for [a] man,' said Neil Armstrong as he ventured out of it, 'one giant leap for mankind.' An estimated 600 million people back on Earth watched this first human footstep on another world as it was broadcast live thanks to new communications satellites, one of many technologies advanced by the race to the Moon.

According to the Doctor Who story Day of the Moon (2011), the clip of Armstrong's first moonwalk will become the most watched piece of footage in human history.

'The human race will spread out among the stars. You just watch them fly. Billions and billions of them, for billions and billions of years, and every single one of them at some point in their lives, will look back at this man, taking that very first step, and they will never, ever forget it.'

The Eleventh Doctor, Day of the Moon (2011)

But that giant leap didn't last long: just three and a half years later, on 14 December 1972, another small box blasted off from the Moon's surface carrying astronauts Harrison Schmitt and Eugene Cernan back towards the Earth. No one has been back to the Moon since. (Sixteen days later on 30 December, BBC One broadcast Episode One of The Three Doctors the story that ended the Doctor's exile. By strange coincidence, he was trapped on a single world for almost exactly the period we weren't.) At the time of writing, we are only in the early stages of plans to send people out beyond Earth orbit again but that won't happen until the 2020s, some fifty years after we last stood on the Moon. Why did we stop going into outer s.p.a.ce?

First, getting into s.p.a.ce is expensive. The Apollo programme that first got man to the Moon cost $24 billion at the time or $395 billion if we were to undertake the same project today. There are plenty of other worthy causes that need that kind of money. On the day of the Moon landing, protesters outside Mission Control on Earth demanded that the money spent on s.p.a.ce exploration should be used to help the poor.

Of course, that first Moon landing had to invent all the technology that got us to the Moon in the first place but, even with modern technology, s.p.a.ce travel is still not cheap. To escape the strong pull of Earth's gravity, you need a lot of thrust and that takes a lot of fuel. The more you send up into s.p.a.ce, the more fuel you need to get it there, so the amount of stuff or people you can take up is very strictly limited. One NASA estimate is that it costs about 14,000 for every kilogram sent into s.p.a.ce. Virgin Galactic offers members of the public the chance to fly, for a few minutes, more than 100 kilometres above the Earth the official definition of the height at which s.p.a.ce begins. Tickets are on sale for $250,000 each.

Besides the cost, s.p.a.ce is dangerous. The Amba.s.sadors of Death (1970) saw the Doctor on the trail of a missing s.p.a.ce capsule. Between the broadcast of its fourth and fifth episodes, events on screen were mirrored in real life as an oxygen tank ruptured on Apollo 13 on its journey to the Moon. With limited power, loss of heat and little water, the crew just got back to Earth but missed their chance to walk on the Moon. In fact, two days before Neil Armstrong took his first steps on the lunar surface, US President Nixon pre-recorded a TV address, in case something went wrong and the astronauts were stranded on the Moon, where they would have died. Luckily, the message wasn't needed and Armstrong and his crew got home safely.

Others were not so lucky. The following year, the three-man crew of Soyuz 11 were found dead when their capsule arrived back on Earth, due to a mechanical error. The s.p.a.ce shuttle Challenger exploded in 1986, as did Columbia in 2003 in each case killing the whole crew. Just training to get into s.p.a.ce could be dangerous: a sudden fire in 1967 killed the crew of Apollo 1 as they practised their launch sequence, their s.p.a.ce capsule still on the launch pad. The following year, Yuri Gagarin who, in 1961, became the first man to go into s.p.a.ce was killed in a plane crash while training for a s.p.a.ce mission. People working on s.p.a.ce rockets have been killed in explosions, as have people living nearby when rockets have gone off course.

Astronauts go into s.p.a.ce well aware of the risks. In fact, in 2004 (after the Columbia accident) NASA thought it was too dangerous to send astronauts on a repair mission to the Hubble s.p.a.ce Telescope but the astronauts lobbied to be allowed to go, and NASA ultimately let them. In 2014, one of Virgin Galactic's s.p.a.cecraft crashed during a test flight, killing one pilot and injuring another but the company vowed to learn any lessons and continue with its programme to get members of the public into s.p.a.ce.

Once you are up there, s.p.a.ce presents lots of problems for the human body. Being weightless weakens your bones and tissues because they don't need to work as hard in the low gravity. The circulation of your blood and lymphatic fluid can also be affected. Gravity helps food pa.s.s through our bodies, so digestion is more difficult in s.p.a.ce. Weightlessness causes problems when you go to the toilet or if anyone is sick astronauts have had to deal with stinking matter floating round their s.p.a.cecraft.

It's a three-day trip to the Moon, so with a day on the Moon's surface you need a week's food for each astronaut all of it packed into your s.p.a.cecraft when you launch (because you can't just nip out to the shops if you run out of anything). You also need oxygen, s.p.a.cesuits, washing things, changes of clothes... To get to Mars takes at least 150 days. Think of all the food and equipment you'd need to take, even for a small crew and Mars is the second closest planet to us.

There's more. Astronaut James Irwin was the eighth person to walk on the Moon. While there he suffered something like a heart attack. The stress of the mission might have worsened a pre-existing condition, but other astronauts have shown disturbances in the rhythm of their heartbeats while in s.p.a.ce. They also suffer sickness from decompression, the effects of pressure in the body's tissues, weakened immune systems, and there are effects on sleep, balance and eyesight, to name but a few. Astronauts might be exposed to dangerous radiation in s.p.a.ce or even from the ultrasound imaging tools they use on their s.p.a.cecraft.

There are psychological effects, too. It's not only the stress of going into s.p.a.ce and facing those dangers. Returning to Earth, some astronauts have struggled to fit back into everyday life it just seems boring after you've been in s.p.a.ce.

'You took me to the furthest reaches of the galaxy, you showed me supernovas, intergalactic battles, and then you just dropped me back on Earth. How could anything compare to that?'

Sarah Jane Smith, to the Tenth Doctor, School Reunion (2006)

If it's extremely expensive and very dangerous, is it worthwhile going into s.p.a.ce? What do we get in return? The answer to that question says a lot about our relationship with s.p.a.ce since we first walked on the Moon.

We learnt a lot by going to the Moon. For example, Neil Armstrong and Buzz Aldrin set up a series of mirrors on the lunar surface. Scientists on Earth then fired a laser a very accurate beam of light at these mirrors, and measured the precise time it took for the laser to bounce back to them: it took 2.4 seconds. A laser beam moves at the speed of light (299,972,458 metres per second). Halve the time it takes to get to the Moon and back, and multiply that time by 299,972,458 and you have a very accurate measurement of the distance from the Earth to the Moon: 359,966,949.6 metres. Repeating the experiment over many years, we now know the Moon is getting further away from us, by about 3 cm every year (the opposite of what happens in the year 2049, during Kill the Moon!).

Other lunar experiments included studies of moonquakes, the composition of solar wind and the lunar atmosphere, the strength of the Moon's magnetic field, heat flow from and electrical currents through the lunar crust, the properties of the regolith (the loose dust and rock covering the Moon's surface), variations in surface gravity and levels of cosmic rays. Some 385 kg of moonrock was returned to Earth for further tests. All this study has given us new ideas about the formation of the Moon and Earth and the history of the Solar System.

But we didn't just go to the Moon for the science. Only one of the twelve people to walk on the Moon was actually a scientist (Harrison Schmitt, the second-last person there, was a geologist). The race to the Moon between the USA and the then USSR was as much about politics as science and once one side had won that race, it was much harder for any country to justify the expense. Ten more flights had been planned to the Moon after Apollo 11; only six were launched (one, Apollo 13, didn't get there).

Yet the effort of going to the Moon benefited us closer to home. The technology invented to get there led to improvements in everything from computers to non-stick saucepans. As we saw, communications satellites allowed those 600 million people around the world to watch the Moon landing live on TV. Satellite pictures improved weather forecasts, and a.s.sessments of erosion and irrigation. We now have maps on our mobile phones that use live connections to satellites to tell us exactly where we are. (Our new reliance on satellite navigation systems was used against us in The Sontaran Stratagem / The Poison Sky (2008)). For the time being, we can justify the cost and risks of going into orbit to launch and service satellites, and even have an international s.p.a.ce station circling the Earth. We just don't go any further.

That may change as we can see in Doctor Who. In Colony in s.p.a.ce (1971), Jo Grant is told why humans have settled on the bleak planet Uxarieus in the year 2471. On Earth, there is apparently, '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.' (We'll return to Jo and her feelings about s.p.a.ce travel in Chapter 5.) If it's currently difficult to justify sending people further than a near-Earth orbit, computers and robots don't need the same volumes of food and water, and don't have the same risks of disease or injury. An unmanned rocket or craft still costs a lot, but nowhere near as much as sending people. And something going wrong isn't quite so disastrous as when there's a loss of life.

We've sent robots and probes to all the planets in our Solar System. The failure of Beagle 2 to land on Mars on Christmas Day 2003 inspired the loss of Guinevere One at the start of the Doctor Who story The Christmas Invasion (2005). Gadget, the friendly robot in The Waters of Mars seems influenced by exploration rover Opportunity, which landed on Mars in 2003 and is still sending us back data from the Martian surface. (Opportunity has since been joined by another rover, Curiosity, and both are now searching for evidence of life on Mars. So far they've not found any, meaning the known population of Mars consists entirely of robots.) Even the look of outer s.p.a.ce in Doctor Who all brightly coloured nebulae and star systems is the result of unmanned technology we've sent into s.p.a.ce. The Hubble telescope's view of outer s.p.a.ce isn't distorted by Earth's atmosphere, so it's captured the brightest, most detailed images ever seen using visible light (rather than ultraviolet or other spectra).

These probes and s.p.a.cecraft have taught us lots about other planets, s.p.a.ce and even the origins of the universe. As we'll see in Chapter 5, they've taught us a lot about our own planet, too. They've even shown that Doctor Who gets its science right if entirely by accident. In Planet of the Daleks (1973), the Doctor stops an army of Daleks by detonating a kind of volcano. On this particular planet, volcanoes don't burst with hot lava but with molten ice.

In 1973, that was a fun idea dreamt up by writer Terry Nation. But on 25 August 1989, s.p.a.ce probe Voyager 2 flew by Triton, largest moon of Neptune, and spotted real ice-canos. It's now thought there is cryovolcanic activity the scientific name for ice-canos on several other moons in the Solar System.

Triton was Voyager 2's last meeting with one of the planets and their moons...o...b..ting our Sun. Launched on 20 August 1977, it sent us back data from Jupiter, Saturn, Ura.n.u.s and Neptune before venturing on to study the edge of the Solar System there's some debate among scientists about whether it's got there yet. That gives us some idea of the vast size of the Solar System: Voyager 2 is moving at 15.5 kilometres per second 55,800 kilometres or 33,673 miles per hour. Even so, it took it 12 years to reach Neptune and it's only just escaping the furthest reach of our Sun's gravity after 38 years. (That's more than seven whole lifetimes for the Doctor when Voyager 2 was launched, the Fourth Doctor was about to face the Horror of Fang Rock).

Voyager 2 and its sister craft, Voyager 1 are journeying out of the Solar System with a message to the stars. Each carries a record of sounds and images from Earth. It's a sign of how long ago these craft were built that the sounds and images are contained on phonograph records what we now think of as old technology. These records are a greeting to any alien life that the Voyager craft might encounter, the sounds and images chosen to represent the diversity of Earth, with greetings in 55 languages. The idea was that part of us part of everyone would travel with these s.p.a.cecraft out into deep s.p.a.ce.

It's a nice idea, but Voyager 2's 38-year journey to the edge of the Solar System is almost nothing compared to the distance to the nearest star to our Sun. Proxima Centauri is 4.2 light years away that is, it would take 4.2 years to get there travelling at the speed of light, 299,792,458 metres per second (or 1,079,253,000 kilometres or 670,616,600 miles per hour). Voyager 2 isn't going anything like that fast: even at 15.5 kilometres per second, it would take 81,236 years for it to reach Proxima and that's our nearest neighbour!

s.p.a.ce is enormous. It's hard for us to get our heads round just how vast it is. So how can we possibly get out there to explore it?

One solution might be s.p.a.ceships that can travel close to or faster than the speed of light. There are several examples of craft that can apparently do this in Doctor Who, but it still comes at a cost. A ship would take hundreds of years of Earth time to cross hundreds of light years, though due to relativistic time dilation (which we'll talk about more in Chapter 6), people on board would experience only a few weeks or years of travel time. However, if they returned to Earth they would discover that hundreds of years even millennia had pa.s.sed, and everyone they'd left behind at home would be long dead.

Alternatively, people might journey to other stars in ships that take thousands of years just like in the story that preceded this chapter. Astronauts setting off from Earth would never see their final destination, and neither would generation after generation of their descendants. Civilisations might rise and fall on those s.p.a.cecraft as they do in the Doctor Who story The Ark (1966). Over 80,000 years, it's possible that gradual evolution would mean that by the time people arrived at Proxima Centauri, they'd be a different species from the people back on Earth. (For more on evolution, see Chapter 11.) Or perhaps they'd arrive to find people already there supposing that new, faster s.p.a.cecraft had been invented on Earth in the millennia since they left. In fact, it's possible they'd arrive at Proxima Centauri to find the ruins of a civilisation that left Earth long after they did.

There are still two ways we can reach the stars. First, we can use ever better telescopes and technology to explore s.p.a.ce in greater detail we stay on Earth but let information about the distant universe come to us in the form of light and other kinds of radiation. As we saw in Chapter 1, in just the last twenty years we've discovered thousands of planets...o...b..ting other suns. We might yet find life on one of these worlds, even if we can't physically get there. And it's just possible that that alien life will already know all about us because of the other way we can reach the stars.

In 1888, Heinrich Hertz sent a pulse of electromagnetic radiation to a receiver: the first undisputed man-made radio signal. Radio waves travel at the speed of light. In about a second, a radio transmission spans the gap between the Earth and the Moon. In less than five minutes it pa.s.ses the orbital path of the planet Mars. As we know, in 4.2 years it reaches Proxima Centauri the nearest star to the Sun. In less than ten years, it pa.s.ses Sirius the bright Dog Star binary system, which the Doctor points out to Captain Avery in The Curse of the Black Spot (2011).

Our broadcast radio signals are not particularly powerful by cosmic standards and their strength decreases still further as they pa.s.s through Earth's atmosphere and then spread out across the vastness of s.p.a.ce. But over the past century, the sheer number of radio signals that we produce has increased enormously as we've invented things like radar systems and television. For around a hundred years, a bubble of radio noise has been expanding around our Solar System, spreading out across s.p.a.ce at the speed of light.

On 23 November 1963, the first episode of Doctor Who was broadcast from transmitters in the UK. The signals would have reached into s.p.a.ce. It's possible they kept going, and in early 1968 4.2 years later they pa.s.sed Proxima Centauri. As Doctor Who celebrated its tenth anniversary on Earth, that first episode had reached Sirius. As the Ninth Doctor made his debut in Rose (2005), the first episode reached the stars Rho Cancri (which we know to have four planets) and HR3259 (which we know to have three planets). As The Day of the Doctor was broadcast on 23 November 2013, the show's first episode had covered 473,026,420,000,000 kilometres or 293,924,990,500,000 miles.

The Need for Speed Doctor Who has suggested lots of ways to cut down travel times in s.p.a.ce though they don't always conform to the laws of physics as we currently understand them.

Sleep through it The Ark (1966) The Earth's population is miniaturised and stored in trays, 1 million people to a cabinet, to be returned to normal size on arrival hundreds of years later. It's not said on screen, but presumably they're unconscious for that time.

Skip it Frontier in s.p.a.ce (1973) Earth ships 'jump' into hypers.p.a.ce as a shortcut and almost collide with the TARDIS suggesting it travels through hypers.p.a.ce, too. Yet in The Stones of Blood (1978), the Doctor says hypers.p.a.ce is a 'theoretical absurdity' (though a theory can be both absurd and true). He also calls it a different dimension another kind of s.p.a.ce.

Bend it Nightmare of Eden (1979) The interstellar cruise liner Empress and other ships can 'warp' across long distances, presumably bending s.p.a.ce to make the journey quicker. In Planet of the Spiders (1974), a starship travels by 'time jump' which suggests s.p.a.ce is being warped, too.

Surf it Boom Town (2005) A tribophysical waveform macro-kinetic extrapolator allows the user to 'surf' across s.p.a.ce and time.

Go faster The Waters of Mars (2009) The Doctor says Susie Fontana-Brooke will pilot the first lightspeed ship to Proxima Centauri in about 2089.

Make a hole Quantum tunnels in School Reunion (2006) are used to join distant bits of s.p.a.ce. Matter transmission (or 'transmat') technology can also move people quickly though in Doctor Who it's usually for relatively small distances such as the Travel-Mat or T-Mat system from the Earth to the Moon (The Seeds of Death (1969)). The Time Lords have 'open-ended' transmats (The Five Doctors (1983)), but we don't know what range that might mean.

Go backwards With time travel, you can arrive on a far distant planet before you set off but it gets complicated (as we'll see in Chapters 6 to 10).

There are at least 2,000 stars within fifty light years of Earth and the majority of them almost certainly have planets. If alien creatures live on any of them, they could just could be watching Doctor Who. We might be stuck for the moment on a single world, but not the Doctor. He might be a fictional character, but he really is travelling on our behalf, far out into s.p.a.ce.

The vast scale of outer s.p.a.ce is boggling. But at the very small scale, s.p.a.ce is even weirder...

'Oh, I wouldn't open that door,' said the man.

Carefully not showing my surprise, I reached instead for the door on the right. He tutted.

'And not that one either.'

My hand froze on the plain white handle. I turned to the little man who'd come from nowhere. He really was remarkably odd, with his mop of untidy black hair and crumpled suit. A cloud of anxiety hovered around him, as though he'd borrowed someone else's clothes and now found himself expected to make their decisions. It probably wasn't helping that I was shouting at him.

'But there are no other doors!'

His drooping face managed to be both supercilious and nervous. 'Ah, well, are you quite sure?'

Ludicrous! We were in a large white room. It had two doors at the end and that was it. No windows. No furniture. Just a huge whiteness that gave nothing away.