0:00:03 > 0:00:07We think of ourselves as a global species.
0:00:10 > 0:00:12But our bodies can only survive unsupported
0:00:13 > 0:00:15on a fraction of the Earth's surface.
0:00:15 > 0:00:20We've evolved to live in a temperate climate at sea level,
0:00:20 > 0:00:25yet our relentless desire to explore has pushed us higher and deeper,
0:00:25 > 0:00:29to every corner of the planet.
0:00:32 > 0:00:34'I'm Dr Kevin Fong.
0:00:34 > 0:00:36'I study the limits the human body can endure.
0:00:38 > 0:00:41'I've worked with NASA for 15 years,
0:00:41 > 0:00:43'finding ways to keep people alive in orbit.
0:00:45 > 0:00:49'And now I'm going to use my own body to demonstrate what happens
0:00:49 > 0:00:52'when we go to the most extreme environments on our planet.'
0:00:52 > 0:00:54Just stick with it.
0:00:54 > 0:00:55140.
0:00:55 > 0:00:57Feeling pretty heavy.
0:00:57 > 0:00:59DR FONG: 'Oh, ten more miles!'
0:00:59 > 0:01:01And that's gone.
0:01:01 > 0:01:04'In this programme, we're going to explore what happens to us
0:01:04 > 0:01:07'when we leave behind our comfortable sea-level home...'
0:01:07 > 0:01:10And that's weightlessness.
0:01:10 > 0:01:12'..and go up, through the atmosphere.'
0:01:12 > 0:01:15We live in a very narrow layer of air
0:01:15 > 0:01:17that's smeared across the surface of our planet.
0:01:17 > 0:01:20It extends to only a few kilometres above sea level
0:01:20 > 0:01:22and as we rise higher and higher
0:01:22 > 0:01:25it becomes harder and harder to survive
0:01:25 > 0:01:28and eventually, it becomes impossible.
0:01:28 > 0:01:32This is the story of what happens to our bodies
0:01:32 > 0:01:35as we ascend through the layers of the atmosphere
0:01:35 > 0:01:39and how we developed the technology that allowed us to explore
0:01:39 > 0:01:41its upper reaches,
0:01:41 > 0:01:43and eventually go beyond into space.
0:02:06 > 0:02:09Our natural habitat is just above sea level.
0:02:13 > 0:02:15But it's not in our nature to stay here.
0:02:16 > 0:02:19We've always dreamt of flying.
0:02:27 > 0:02:29But ever since we first took to the air,
0:02:29 > 0:02:34we've found that the higher we go, the harder it is for us to survive.
0:02:36 > 0:02:40The principle of all of this really couldn't be simpler.
0:02:40 > 0:02:43It hasn't changed in the best part of 250 years.
0:02:43 > 0:02:45You take a bag,
0:02:45 > 0:02:48you fill it with something lighter than the air around it
0:02:48 > 0:02:50and it rises up into the atmosphere.
0:02:50 > 0:02:53In this case, we're using heat.
0:02:53 > 0:02:55You heat the air inside the balloons,
0:02:55 > 0:02:59you make it lighter, less dense than the air around
0:02:59 > 0:03:02and it pulls these vast structures up into the sky.
0:03:08 > 0:03:14In 1783, the hot-air balloon became the first real flying machine
0:03:14 > 0:03:17and ever since, we've been enchanted by their ability
0:03:17 > 0:03:20to lift us effortlessly from the ground.
0:03:35 > 0:03:38It's a very beautiful way to fly, really.
0:03:38 > 0:03:40Apart from the occasional blast from the burner,
0:03:40 > 0:03:43it's pretty silent up here.
0:03:44 > 0:03:46And d'you know, you feel remarkably safe,
0:03:46 > 0:03:50given that you're in a basket underneath a bag of hot air.
0:03:56 > 0:03:59The balloon gave us a completely new perspective
0:03:59 > 0:04:02on the world in which we live.
0:04:02 > 0:04:06And crucially, it opened up a new frontier -
0:04:06 > 0:04:07the air.
0:04:10 > 0:04:13Pretty much as soon as people had mastered this -
0:04:13 > 0:04:15this idea that you can float in the air,
0:04:15 > 0:04:19in a basket, under a bag of hot air -
0:04:19 > 0:04:22people started to try and push the technology and themselves.
0:04:26 > 0:04:32In 1862, a pair of pioneers, James Glaisher and Henry Coxwell,
0:04:32 > 0:04:36took off from Wolverhampton gasworks for a world record attempt.
0:04:37 > 0:04:40They aimed to rise to the highest altitude
0:04:40 > 0:04:42any humans had ever reached.
0:04:42 > 0:04:45For the first 20 minutes the flight went brilliantly well.
0:04:45 > 0:04:47They were two miles up,
0:04:47 > 0:04:50they'd broken through that very simple boundary
0:04:50 > 0:04:52that we take for granted - the clouds.
0:04:52 > 0:04:55They were some of the first people to see them from above,
0:04:55 > 0:04:57and now, two miles in the air,
0:04:57 > 0:05:00they were in uncharted territory.
0:05:00 > 0:05:03As they rose higher, it got colder...
0:05:03 > 0:05:05they were expecting that.
0:05:05 > 0:05:10But there was another unseen and unexpected threat.
0:05:10 > 0:05:14After two hours, they were as high as the summit of Mount Everest
0:05:14 > 0:05:17and that's when things rapidly started to go wrong.
0:05:19 > 0:05:20At around 29,000 feet
0:05:20 > 0:05:24they started to experience extreme difficulty with their breathing.
0:05:24 > 0:05:27Glaisher began to lose his eyesight, he could no longer read his watch.
0:05:27 > 0:05:30Shortly after that, he lost the power of his limbs,
0:05:30 > 0:05:34became paralysed and slipped into unconsciousness.
0:05:36 > 0:05:40Something about the altitude was causing their bodies to fail -
0:05:40 > 0:05:41and they were still rising.
0:05:43 > 0:05:46As their open basket reached 37,000 feet -
0:05:46 > 0:05:49the height a modern airliner flies at -
0:05:49 > 0:05:51it seemed certain they would die.
0:05:52 > 0:05:55Coxwell too was now in trouble.
0:05:55 > 0:05:57His colleague was unconscious in the basket next to him,
0:05:57 > 0:05:59he was losing the power of his limbs,
0:05:59 > 0:06:01and at the last moment,
0:06:01 > 0:06:04he managed to pull the venting cord with his teeth.
0:06:04 > 0:06:09Slowly but surely, they started to drift back down to Earth.
0:06:10 > 0:06:14As they descended, escaping the lethal effects of high altitude,
0:06:14 > 0:06:16they spontaneously began to recover.
0:06:19 > 0:06:23They had flown higher than any other human would for nearly 80 years
0:06:23 > 0:06:26and had become the first people to experience
0:06:26 > 0:06:28the dangers of high altitude.
0:06:30 > 0:06:33To understand what happened to those brave but foolhardy balloonists
0:06:33 > 0:06:36we need to understand what happens to the body
0:06:36 > 0:06:38as it rises up through the atmosphere.
0:06:38 > 0:06:43And those changes start to happen remarkably close to the ground.
0:06:59 > 0:07:02This is Chamonix in the heart of the Alps,
0:07:02 > 0:07:04Europe's highest mountain range.
0:07:04 > 0:07:06A mecca for mountain sports.
0:07:09 > 0:07:14Today, the mountains are an idyllic holiday destination.
0:07:14 > 0:07:19But until 150 years ago, they were regarded as a no-go zone
0:07:19 > 0:07:23and treated with fear and trepidation.
0:07:24 > 0:07:28Even now, three quarters of the world's population
0:07:28 > 0:07:30lives below 500m
0:07:30 > 0:07:34and there's a very good reason for that.
0:07:34 > 0:07:38We're up here at 2,300m, looking down the Chamonix Valley.
0:07:38 > 0:07:40Already, there's a very stark illustration
0:07:40 > 0:07:43of how much harder life is at altitude.
0:07:43 > 0:07:47The treeline there gives way to the sparse vegetation of the meadows
0:07:47 > 0:07:49and as you get right up the mountain,
0:07:49 > 0:07:52there's really nothing but rock and ice.
0:07:52 > 0:07:54All life finds it harder
0:07:54 > 0:07:57as you go higher and higher up the mountain
0:07:57 > 0:07:58and that includes us.
0:08:00 > 0:08:05Even at this very modest altitude, you feel much more out of breath
0:08:05 > 0:08:07than you would at sea level.
0:08:07 > 0:08:10And that is because the barometric pressure
0:08:10 > 0:08:13has dropped by about...25% here.
0:08:13 > 0:08:17And that means that every single breath I take
0:08:17 > 0:08:19has fewer molecules of oxygen in it,
0:08:19 > 0:08:23making the work of everything I do much harder.
0:08:28 > 0:08:31It's the decreasing amount of oxygen in the air
0:08:31 > 0:08:34that explains the effects of altitude on our physiology.
0:08:36 > 0:08:39The higher we go, the less oxygen there is to breathe.
0:08:39 > 0:08:42And with the help of a little technology,
0:08:42 > 0:08:44it takes only minutes to reach altitudes
0:08:44 > 0:08:47at which our bodies struggle to cope.
0:08:52 > 0:08:56Here, another thousand metres higher,
0:08:56 > 0:08:59the amount of oxygen in the air has dropped by a third.
0:09:01 > 0:09:04And even fairly light exertion -
0:09:04 > 0:09:06like running up a flight of steps -
0:09:06 > 0:09:08leaves me feeling physically sick.
0:09:10 > 0:09:12And it's all because my body's oxygen supply system
0:09:12 > 0:09:14is feeling the strain.
0:09:16 > 0:09:20As I breathe in, air rushes through my nose and mouth,
0:09:20 > 0:09:23down my airways and deep into my lungs.
0:09:25 > 0:09:27Here the oxygen dissolves in liquid
0:09:27 > 0:09:30that lines the walls of the millions of tiny sacs called alveoli.
0:09:32 > 0:09:35From here, the oxygen is picked up by red blood cells
0:09:35 > 0:09:39that squeeze through the tiny vessels that surround my lungs.
0:09:42 > 0:09:44Driven by the relentless beating of my heart,
0:09:44 > 0:09:48those red blood cells are pumped round my body
0:09:48 > 0:09:50where they release the oxygen to my cells,
0:09:50 > 0:09:54which use it to produce the energy that keeps me alive.
0:09:56 > 0:09:59This oxygen delivery system is incredibly efficient.
0:09:59 > 0:10:05It runs for 24 hours a day, 7 days a week, for all of our lives.
0:10:06 > 0:10:09For most of us, this whole system is calibrated
0:10:09 > 0:10:11to the oxygen levels we find at sea level.
0:10:13 > 0:10:15But at altitude, our bodies
0:10:15 > 0:10:18are constantly struggling to harvest enough oxygen.
0:10:20 > 0:10:21HE PANTS
0:10:21 > 0:10:25Now, you can get an idea of what that's doing to me using this thing.
0:10:25 > 0:10:28This gives you an idea of how much oxygen
0:10:28 > 0:10:30there currently is in my bloodstream.
0:10:30 > 0:10:33It's an oxygen saturation probe
0:10:33 > 0:10:36and it's reading round about 88% at the moment.
0:10:36 > 0:10:38It should read 100%.
0:10:38 > 0:10:42And that lack of oxygen - hypoxia - is bad for you.
0:10:42 > 0:10:44Oxygen is what keeps us alive.
0:10:44 > 0:10:47If I saw a patient in a hospital with this sort of number,
0:10:47 > 0:10:48I'd be worried.
0:10:48 > 0:10:51And in the same way as it makes a patient in hospital sick,
0:10:51 > 0:10:53it makes us at altitude sick
0:10:53 > 0:10:58and that we call, "Acute Mountain Sickness."
0:10:59 > 0:11:03Acute Mountain Sickness can be incredibly debilitating.
0:11:03 > 0:11:06The lack of oxygen causes headaches,
0:11:06 > 0:11:09lethargy, nausea and vomiting.
0:11:09 > 0:11:11And unchecked, the lack of oxygen
0:11:11 > 0:11:14would prevent us climbing much above 3,000m.
0:11:16 > 0:11:20But today - just a few days after arriving in the Alps -
0:11:20 > 0:11:23I'm going to try and climb to over 4,000m.
0:11:29 > 0:11:31It's a beautiful day out here.
0:11:32 > 0:11:35The Allalinhorn is just as beautiful, I reckon.
0:11:40 > 0:11:43The Allalinhorn is one of Europe's highest mountains
0:11:43 > 0:11:46and certainly the highest I've ever climbed.
0:11:48 > 0:11:51The only reason I stand any chance of reaching the summit
0:11:51 > 0:11:53is that since I arrived at altitude,
0:11:53 > 0:11:55my body has been recalibrating itself
0:11:55 > 0:11:59to survive on the lower levels of oxygen in the air.
0:12:03 > 0:12:07The first change was that my brain automatically increased
0:12:07 > 0:12:09the rate and depth of my breathing
0:12:09 > 0:12:11to shift more air through my lungs.
0:12:14 > 0:12:18But I've also undergone a series of more subtle physiological changes.
0:12:20 > 0:12:24My heart is beating faster to help pump more blood round my body.
0:12:27 > 0:12:30Non-essential functions - like digestion -
0:12:30 > 0:12:33have slowed down to prioritise the delivery of oxygen
0:12:33 > 0:12:37to more vital organs, like the heart and brain.
0:12:37 > 0:12:42As time goes on, my kidneys produce a hormone called erythropoietin,
0:12:42 > 0:12:46that stimulates the production of more red blood cells.
0:12:47 > 0:12:50It's a process called "acclimatisation"
0:12:50 > 0:12:52and it means that within a matter of days,
0:12:52 > 0:12:56my body has changed its physiology to become much better
0:12:56 > 0:12:59at harvesting oxygen from the thinner air.
0:13:04 > 0:13:05Ooh, yes!
0:13:06 > 0:13:08It definitely feels like harder work.
0:13:17 > 0:13:19Stunning views over Switzerland from here.
0:13:21 > 0:13:24- HE PANTS - I certainly feel the work.
0:13:30 > 0:13:35I think I've acclimatised a little over the last three or four days.
0:13:37 > 0:13:41Still much more breathless on a slope like this...
0:13:42 > 0:13:43..than I would be down there.
0:13:47 > 0:13:50The effects of altitude are pretty obvious.
0:13:54 > 0:13:57I'm hoping my body will withstand the altitude.
0:13:57 > 0:14:00But it's by no means guaranteed.
0:14:01 > 0:14:04Although the changes my body has made
0:14:04 > 0:14:06are helping me cope with the thinner air,
0:14:06 > 0:14:10those changes can also seriously damage my health.
0:14:17 > 0:14:20Because the lower oxygen levels also cause physical changes
0:14:20 > 0:14:21to my circulation.
0:14:24 > 0:14:26My blood pressure rises, and the capillaries -
0:14:26 > 0:14:29the finest blood vessels in my body -
0:14:29 > 0:14:32can begin to leak, causing fluid to seep out
0:14:32 > 0:14:34into the surrounding tissues.
0:14:36 > 0:14:40If that happens in the lungs, the fluid builds up in the alveoli,
0:14:40 > 0:14:43causing blood oxygen levels to plummet.
0:14:44 > 0:14:47And if it happens in the brain, it can causing swelling
0:14:47 > 0:14:50that leads to headaches, confusion and paralysis.
0:14:50 > 0:14:53Exactly the symptoms suffered by Coxwell and Glaisher
0:14:53 > 0:14:57on their record-breaking balloon flight.
0:14:58 > 0:15:00Both conditions can be rapidly fatal
0:15:00 > 0:15:02if you don't return to lower altitude immediately.
0:15:04 > 0:15:09And they can occur at altitudes well below 4,000m.
0:15:09 > 0:15:11HE PANTS
0:15:18 > 0:15:21We're at 3,950m.
0:15:23 > 0:15:25This is about as high as I've ever been on a mountain.
0:15:28 > 0:15:29And I can honestly say...
0:15:30 > 0:15:35..it's one of the most beautiful places in the world I've ever seen.
0:15:40 > 0:15:43But by now, every step is a huge effort
0:15:43 > 0:15:45and I'm breathing as hard as I can
0:15:45 > 0:15:48but it still feels like I'm going to be out of breath forever.
0:15:50 > 0:15:5250 metres to go.
0:15:52 > 0:15:56It gets harder the higher you go.
0:16:08 > 0:16:10Here we go, on the summit.
0:16:16 > 0:16:19- Well done.- Thank you.
0:16:19 > 0:16:20Congratulations.
0:16:20 > 0:16:22Thank you. 4,000 metres.
0:16:22 > 0:16:254,000m and a bit, brilliant.
0:16:25 > 0:16:27Wow, now that is a view.
0:16:29 > 0:16:33It's amazing up here. I can see why you like this as your office.
0:16:33 > 0:16:34Yeah, it's an awesome place.
0:16:34 > 0:16:38Very, very amazing place to be.
0:16:39 > 0:16:40From up here,
0:16:40 > 0:16:46these high Alpine peaks seem to be the very roof of the world.
0:16:46 > 0:16:50And in many ways, for our species, that is exactly what they are.
0:16:53 > 0:16:56We're surrounded here by mountainous peaks
0:16:56 > 0:16:59running from 4,000 to nearly 5,000 meters.
0:16:59 > 0:17:03And you can climb up here, it's fantastic climbing up here.
0:17:03 > 0:17:06But that altitude, 5,000 meters,
0:17:06 > 0:17:09has special significance for our species.
0:17:09 > 0:17:11It's our high altitude limit.
0:17:11 > 0:17:15There are no permanent habitations above 5,000 meters.
0:17:15 > 0:17:21So for us, as the human race, this is it. This is our ceiling.
0:17:32 > 0:17:36It is, of course, possible to climb above 5,000 meters,
0:17:36 > 0:17:39but our stays at these heights can only be temporary
0:17:39 > 0:17:44because of the serious effects extreme altitude has on our health.
0:17:46 > 0:17:48Those effects are best seen
0:17:48 > 0:17:52in the climbers who take on the world's tallest mountain,
0:17:52 > 0:17:53Mount Everest.
0:17:55 > 0:17:58As soon as they get to base camp, which at 5,300 metres
0:17:58 > 0:18:03is already 1,000 metres higher than the Allalinhorn,
0:18:03 > 0:18:07even the fittest climbers start to suffer,
0:18:07 > 0:18:11losing weight as their bodies start to break down their muscles.
0:18:13 > 0:18:17From here it's another three and half kilometres vertically up
0:18:17 > 0:18:19to the summit.
0:18:19 > 0:18:20And the final ascent
0:18:20 > 0:18:24is one of the most demanding physical challenges
0:18:24 > 0:18:26we can put our bodies through.
0:18:26 > 0:18:28As soon as you go above 8,000 metres
0:18:28 > 0:18:32you enter what climbers call the death zone.
0:18:33 > 0:18:37One of the people who has made it to the top is my friend and colleague,
0:18:37 > 0:18:39Dr Dan Martin.
0:18:44 > 0:18:50So, Dan how does all this compare to your days climbing Everest?
0:18:50 > 0:18:51Well, it's a little different,
0:18:51 > 0:18:53little bit more vertical than Everest.
0:18:53 > 0:18:55We never had to do anything like this
0:18:55 > 0:18:57on the way up to the summit, thankfully.
0:18:57 > 0:19:00Ooh, ice!
0:19:00 > 0:19:03The physiological challenges are far greater
0:19:03 > 0:19:07than any other mountain on Earth.
0:19:07 > 0:19:10It's the highest and it really wears you down,
0:19:10 > 0:19:13all that time at high altitude.
0:19:18 > 0:19:22I can remember the final...trudge, is the only way I can think of it,
0:19:22 > 0:19:25to the summit, just this sort of very slow, one foot,
0:19:25 > 0:19:30then stopping and panting for ages, and then the other foot,
0:19:30 > 0:19:32and then the other foot.
0:19:32 > 0:19:36You kind of feel like you're never going to get there.
0:19:36 > 0:19:38By the time you reach the summit
0:19:38 > 0:19:42you're really at the end of what you can tolerate,
0:19:42 > 0:19:44both physically and mentally.
0:19:47 > 0:19:50On the summit of Everest, at 8,848 metres,
0:19:50 > 0:19:56the air contains only a third of the oxygen that it does at sea level.
0:19:56 > 0:19:58It's well into the death zone.
0:19:58 > 0:20:00You can't survive here for long
0:20:00 > 0:20:04because your body's deteriorating by the second.
0:20:06 > 0:20:09Hundreds of people have died trying to climb Everest.
0:20:12 > 0:20:15Dan's ascent was part of a medical expedition
0:20:15 > 0:20:19to find out why some people are so much better at coping
0:20:19 > 0:20:20in the tenuous atmosphere.
0:20:22 > 0:20:25What never ceases to amaze us when we go to the mountain
0:20:25 > 0:20:29is that very fit, sporty type people,
0:20:29 > 0:20:31who are really fit at sea level,
0:20:31 > 0:20:33can tend to do very badly at high altitude.
0:20:33 > 0:20:38We've learnt that the amount of oxygen you deliver around your body
0:20:38 > 0:20:42doesn't really give a good indication of how well you perform.
0:20:42 > 0:20:45Which goes against the classic teaching
0:20:45 > 0:20:47that we're all taught in physiology books,
0:20:47 > 0:20:50the more oxygen that goes round the better you are.
0:20:50 > 0:20:53So, just being an Olympic athlete isn't enough?
0:20:53 > 0:20:56No, sadly not, and it may count against you.
0:20:58 > 0:21:01It's believed that at very high altitude
0:21:01 > 0:21:05the most important thing is not how much oxygen you extract
0:21:05 > 0:21:08from the air, but how efficiently you use it.
0:21:10 > 0:21:15So, just below the summit Dan's team stopped to take blood samples
0:21:15 > 0:21:19to see just how little oxygen they were surviving on.
0:21:19 > 0:21:24The results surprised even the experts.
0:21:24 > 0:21:29We found some fairly astonishing numbers when we got back.
0:21:29 > 0:21:32When we calculated the oxygen saturation,
0:21:32 > 0:21:36which is comparable to what we see on the probe,
0:21:36 > 0:21:37mine was 34%.
0:21:37 > 0:21:4034%? Yeah.
0:21:40 > 0:21:41That's absolutely shocking.
0:21:41 > 0:21:44I've never seen those levels in a living patient....
0:21:44 > 0:21:45Me neither.
0:21:45 > 0:21:48..With those numbers.
0:21:48 > 0:21:51Dan's blood oxygen level
0:21:51 > 0:21:54was the lowest ever recorded in a living human being.
0:21:54 > 0:21:57Somehow, his body was surviving on levels of oxygen
0:21:57 > 0:22:00that would be fatal to most people.
0:22:03 > 0:22:06It's not clear how he was able to do this.
0:22:06 > 0:22:09But one intriguing possibility is that his body was remembering a time
0:22:09 > 0:22:14when it had to cope with much lower levels of oxygen.
0:22:16 > 0:22:18As an unborn baby,
0:22:18 > 0:22:22you had to rely on sharing the oxygen from your mother's blood.
0:22:22 > 0:22:25And there was much less of it to go around.
0:22:28 > 0:22:31In fact, the oxygen levels in the womb are almost exactly the same
0:22:31 > 0:22:34as those on the top of Everest.
0:22:36 > 0:22:40In order to cope, your body's physiology was modified,
0:22:40 > 0:22:43allowing it to use that tiny amount of oxygen
0:22:43 > 0:22:45as efficiently as possible.
0:22:47 > 0:22:49It may be that the only reason
0:22:49 > 0:22:52we're able to stand on the summit of the world's highest mountain
0:22:52 > 0:22:58is that our bodies are able to reactivate those metabolic pathways
0:22:58 > 0:22:59that we last used in the womb.
0:23:03 > 0:23:05But Everest is our limit.
0:23:05 > 0:23:08If it was just 100 metres higher,
0:23:08 > 0:23:10we wouldn't be able to survive on the summit
0:23:10 > 0:23:12without taking additional oxygen.
0:23:40 > 0:23:43Mount Everest is the highest point on Earth that we can climb to.
0:23:43 > 0:23:46But it is not the end of our upward ambition.
0:23:46 > 0:23:48In fact, it's just the beginning.
0:23:48 > 0:23:52The 20th century gave us technology that allowed us to leave the ground
0:23:52 > 0:23:56and conquer the air, taking us and higher and higher.
0:23:59 > 0:24:02The first aeroplane only flew in 1903.
0:24:02 > 0:24:04And those earliest machines
0:24:04 > 0:24:07could only stay in the air for less than two minutes.
0:24:11 > 0:24:14But within a few decades, we had mastered the skies.
0:24:19 > 0:24:23World War II brought a revolution in aviation
0:24:23 > 0:24:26with the demands of the military ushering in faster,
0:24:26 > 0:24:28more powerful planes,
0:24:28 > 0:24:32creating a whole new range of physiological challenges
0:24:32 > 0:24:34for the crews who flew in them.
0:24:36 > 0:24:39These aircraft are remarkable,
0:24:39 > 0:24:41marvels of engineering,
0:24:41 > 0:24:46that's the B17 flying fortress and just behind, the B24.
0:24:46 > 0:24:49They were manufactured with a specific purpose in mind,
0:24:49 > 0:24:53to deliver bombing loads during World War II.
0:24:53 > 0:24:55This changed aviation for ever -
0:24:55 > 0:25:01more air crew flying higher and faster than they ever had before.
0:25:12 > 0:25:16These planes were capable of flying higher than the summit of Everest,
0:25:16 > 0:25:23up to heights of 40,000 feet, over 12,000 metres.
0:25:23 > 0:25:25Just getting ready to take off here.
0:25:25 > 0:25:27Those engines, incredibly loud.
0:25:27 > 0:25:31We're just taxiing to get into position on the runway.
0:25:31 > 0:25:34This is the radio room of this B17...
0:25:35 > 0:25:38..very cramped, this was not built for comfort.
0:25:38 > 0:25:41It's almost like the crew were an afterthought.
0:25:52 > 0:25:56And if you were one of those World War II bomber crews,
0:25:56 > 0:25:59this would be the start of a day...
0:25:59 > 0:26:03..that would last a very, very, very, very long time.
0:26:05 > 0:26:09This remarkable plane, the 909,
0:26:09 > 0:26:12is one of only 13 B-17s still flying today.
0:26:12 > 0:26:15And it's been painstakingly restored
0:26:15 > 0:26:18to be exactly as it was in the Second World War.
0:26:20 > 0:26:23Well, there's so little room to move,
0:26:24 > 0:26:27and you're in amongst the clockwork of all of this
0:26:27 > 0:26:29all the way this all the way through.
0:26:32 > 0:26:35Wow, look at that!
0:26:35 > 0:26:38That is incredible.
0:26:40 > 0:26:44During the war, tens of thousands of B-17s and other bombers
0:26:44 > 0:26:47flew missions over Europe.
0:26:47 > 0:26:50While the British bombers flew at night,
0:26:50 > 0:26:53the American air force preferred to fly in daylight.
0:26:56 > 0:27:00But although it allowed them to see their targets,
0:27:00 > 0:27:04it also put them in plain sight of the enemy guns on the ground.
0:27:04 > 0:27:08On board were crew like 90-year-old Frank Tedesco,
0:27:08 > 0:27:12who in 1942 was just 20 years old.
0:27:12 > 0:27:15When we approached the target
0:27:15 > 0:27:21we could see up ahead large volume of explosions, called flak,
0:27:21 > 0:27:24and we knew we had to fly through it.
0:27:24 > 0:27:28I don't care how good a pilot you are, you can't avoid flak.
0:27:28 > 0:27:33So, hopefully, you don't get damaged.
0:27:34 > 0:27:37To avoid the dangers of the flak and enemy fighters,
0:27:37 > 0:27:41the planes were designed to fly higher and higher.
0:27:43 > 0:27:47But as they did so, they catapulted the crew deep into the death zone,
0:27:47 > 0:27:50to altitudes where their lives were in danger.
0:27:52 > 0:27:55That airplane has no form of heat for the crew.
0:27:55 > 0:27:58The temperatures we experienced
0:27:58 > 0:28:03were anywhere from 30 to 50 degrees below zero Fahrenheit.
0:28:04 > 0:28:09It got to the point where you almost used to dread the cold
0:28:09 > 0:28:11as much as the anti-aircraft fire.
0:28:19 > 0:28:22But it wasn't just the cold that threatened the crew.
0:28:22 > 0:28:25There was so little oxygen at those altitudes
0:28:25 > 0:28:27that even the engines struggled to function.
0:28:28 > 0:28:32If you look out the window there, that engine is turbo charged.
0:28:32 > 0:28:36It uses the exhaust gases to drive a turbine
0:28:36 > 0:28:40to compress air to enough of a mixture to get it to burn
0:28:40 > 0:28:45at 40,000 feet, the upper limit of operation of this aircraft.
0:28:45 > 0:28:49The engines found it difficult to breathe and so did the crew.
0:29:09 > 0:29:12This is the oxygen system for this aircraft, very basic.
0:29:12 > 0:29:15You draw your oxygen off that compressed cylinder there
0:29:15 > 0:29:17into this demand valve.
0:29:17 > 0:29:21That's all that protected the crew from the lethal hazards
0:29:21 > 0:29:25of the thin atmosphere at the huge altitudes these aircraft flew at.
0:29:26 > 0:29:28The failure of these oxygen systems
0:29:28 > 0:29:32was a major cause of death on those bombing missions.
0:29:32 > 0:29:34Many crew died in their seats
0:29:34 > 0:29:37without realising anything was wrong.
0:29:39 > 0:29:40And to find out why,
0:29:40 > 0:29:44I'm going to put myself through the same experience.
0:29:45 > 0:29:48These are the laboratories of QinetiQ in Hampshire,
0:29:48 > 0:29:53operated by Dr Henry Lupa and his team.
0:29:53 > 0:29:56The room I'm sitting in, is a decompression chamber.
0:29:56 > 0:30:00Any second now, all the air is going to be sucked out of it,
0:30:00 > 0:30:05leaving me at an equivalent altitude of 25,000 feet.
0:30:05 > 0:30:08All military pilots have to do this training
0:30:08 > 0:30:13so they become familiar with the effects of sudden hypoxia.
0:30:13 > 0:30:15Just confirm you're happy to proceed.
0:30:15 > 0:30:16Yeah, very happy, thank you.
0:30:16 > 0:30:23Stand by for rapid decompression in 5, 4, 3, 2, 1.
0:30:27 > 0:30:31As the pressure drops, the chamber suddenly fogs up
0:30:31 > 0:30:34as the moisture condenses out of the air.
0:30:36 > 0:30:41A lot colder in here now, some condensation in the cabin.
0:30:41 > 0:30:4225,000 feet.
0:30:43 > 0:30:45While I've still got my breathing mask on,
0:30:45 > 0:30:47I can function fine at this altitude.
0:30:47 > 0:30:49So still on oxygen at this point
0:30:49 > 0:30:56and the oxygen saturation probe there is showing me 99% saturated.
0:30:56 > 0:30:59OK, you're feeling all well?
0:30:59 > 0:31:00Feeling good.
0:31:00 > 0:31:03'But when the time comes to take my mask off,
0:31:03 > 0:31:05'I'll be suddenly exposed to air
0:31:05 > 0:31:08'with only a third of the amount of oxygen in it.'
0:31:08 > 0:31:10What I would like you to do is drop your mask
0:31:10 > 0:31:13and then switch the regulator off.
0:31:13 > 0:31:17'This is rapid decompression. Unlike in the mountains,
0:31:17 > 0:31:21'there's no time to acclimatise to the lack of oxygen
0:31:21 > 0:31:23'and the effects are very different.'
0:31:26 > 0:31:30'Whilst initially I can easily perform normal functions
0:31:30 > 0:31:33'like writing my name and address,
0:31:33 > 0:31:35'my blood oxygen levels are falling dramatically.'
0:31:51 > 0:31:53'And within a couple of minutes,
0:31:53 > 0:31:56'I'm struggling to perform even simple tasks.'
0:32:06 > 0:32:10'But although I was struggling to perform these tasks,
0:32:10 > 0:32:12'I really didn't care.
0:32:12 > 0:32:14'The over-riding sensation I was feeling
0:32:14 > 0:32:18'was one of euphoria and well-being.
0:32:18 > 0:32:21'But after three minutes, I can't even write properly.'
0:32:22 > 0:32:23'I'm now in great danger
0:32:23 > 0:32:27'and not even registering anything that's said to me.'
0:32:44 > 0:32:49'Without help with that mask, I would've passed out in my chair.
0:32:49 > 0:32:52'I'd been without my oxygen supply for less than four minutes.'
0:33:34 > 0:33:37How was that? Did you enjoy the experience?
0:33:37 > 0:33:40You do feel quite euphoric during it.
0:33:40 > 0:33:43That's one of the biggest dangers of hypoxia,
0:33:43 > 0:33:46in that it is actually quite an enjoyable...
0:33:46 > 0:33:48People enjoy the euphoria of it.
0:33:48 > 0:33:52Certainly watching your face, you seemed quite happy,
0:33:52 > 0:33:54jovial for most of the time.
0:33:54 > 0:33:55Towards the end,
0:33:55 > 0:33:58you could see that you were slumping forward in the chair.
0:33:58 > 0:34:00I was a bit concerned you were going to collapse.
0:34:00 > 0:34:02I don't think I would've guessed
0:34:02 > 0:34:05that you get that hypoxic that quickly.
0:34:05 > 0:34:08It's very easy to get to a state where even if you recognise it,
0:34:08 > 0:34:09you just don't care.
0:34:09 > 0:34:12You lose your self-criticism, you lose your judgement
0:34:12 > 0:34:14and you don't do anything about it.
0:34:14 > 0:34:15And then it'll kill you.
0:34:15 > 0:34:20The first bit of writing seemed like a bit of trivial nonsense.
0:34:20 > 0:34:22The second bit, I didn't care what it looked like,
0:34:22 > 0:34:25I didn't realise it looked quite that bad.
0:34:25 > 0:34:28I mean, there's me, normal bad doctor's handwriting,
0:34:28 > 0:34:30there's me, hypoxic doctor's handwriting.
0:34:30 > 0:34:31Barely intelligible,
0:34:31 > 0:34:34didn't get to the second and third line of the address.
0:34:34 > 0:34:38'It was exactly this creeping mental deterioration
0:34:38 > 0:34:42'that affected so many of the bomber crew during the Second World War.
0:34:42 > 0:34:47'A brief period of euphoria followed by unconsciousness and death.'
0:34:49 > 0:34:51And because they flew higher, at 40,000 feet,
0:34:51 > 0:34:54when their oxygen failed,
0:34:54 > 0:34:59they would've slipped into unconsciousness within 30 seconds.
0:34:59 > 0:35:03It casts into sharp relief that video you always ignore
0:35:03 > 0:35:04at the start of a commercial flight,
0:35:04 > 0:35:07where they show the yellow masks dangling from the ceiling
0:35:07 > 0:35:12and explain to you how to put them on and to put them on quickly.
0:35:12 > 0:35:14If you don't get them on quickly, you've got seconds.
0:35:14 > 0:35:16After that, you're not going to care,
0:35:16 > 0:35:18the people around you aren't going to care
0:35:18 > 0:35:21and everyone's going to do pretty badly.
0:35:24 > 0:35:27A reliable oxygen supply has been a must for pilots
0:35:27 > 0:35:29for the last 70 years.
0:35:31 > 0:35:33Since then, aviation technology
0:35:33 > 0:35:36has produced jets that fly even higher and faster,
0:35:36 > 0:35:40breaking the sound barrier and climbing to over 15,000 metres.
0:35:42 > 0:35:46But flying these machines exposes the crew to other hazards,
0:35:46 > 0:35:48not least G-forces,
0:35:48 > 0:35:51the immense forces of acceleration the pilots endure
0:35:51 > 0:35:54while manoeuvring these planes.
0:36:00 > 0:36:03And now I'm going to experience those forces for myself.
0:36:07 > 0:36:10This centrifuge was built in the 1950s
0:36:10 > 0:36:15to recreate the high G-forces experienced by fighter pilots.
0:36:15 > 0:36:19It's capable of generating up to 9G,
0:36:19 > 0:36:21a level that leads to unconsciousness
0:36:21 > 0:36:24and risk of fatal accidents in pilots.
0:36:24 > 0:36:27But it's what fighter crews need to tolerate.
0:36:27 > 0:36:29What's going to happen to me now?
0:36:29 > 0:36:32What we're going to do is gradually spin you up
0:36:32 > 0:36:35with an acceleration of 0.1g per second.
0:36:35 > 0:36:38We keep going until your vision begins to fail,
0:36:38 > 0:36:43which is one of the early signs of inadequate oxygenation of the brain.
0:36:43 > 0:36:46How am I going to know when enough is enough?
0:36:46 > 0:36:49In front of you are three lights.
0:36:49 > 0:36:53As soon as you start to see the peripheral lights disappear,
0:36:53 > 0:36:56I'd like you to press that button.
0:36:56 > 0:36:59- This button here. - That will stop the centrifuge.
0:36:59 > 0:37:07All right, I guess I sort of look forward to it, see how we go.
0:37:07 > 0:37:09Stand by, 0.1g per second onset rate.
0:37:48 > 0:37:49As the G load increases,
0:37:49 > 0:37:54blood moves away from my head down into my legs.
0:37:54 > 0:37:57The force is so strong that my heart is pulled
0:37:57 > 0:37:59a couple of inches down inside my chest,
0:37:59 > 0:38:03making it even harder for it to pump blood to my head.
0:38:03 > 0:38:07And among the first organs to suffer will be my eyes.
0:38:07 > 0:38:12Starved of oxygen, my colour vision will fail as I go through greyout,
0:38:12 > 0:38:15which for most people, happens at between 3 and 4G.
0:38:17 > 0:38:20What happens shortly afterwards is G-loc -
0:38:20 > 0:38:23G induced loss of consciousness.
0:38:23 > 0:38:27It's consequences when flying a plane are predictably disastrous.
0:38:27 > 0:38:30And I'd rather avoid it even in this centrifuge.
0:38:30 > 0:38:33So I'm going to bail out before it happens to me.
0:38:36 > 0:38:394G.
0:39:02 > 0:39:06You actually did very well, you got up to about 4.7G,
0:39:06 > 0:39:08something like that. Very good, in fact.
0:39:12 > 0:39:16I did better than average, but I could still only stand 4.7G.
0:39:17 > 0:39:21Fighter pilots need to be able to withstand much higher forces.
0:39:21 > 0:39:26And the only way they can do that is with the help of technology
0:39:26 > 0:39:29and with the invention of some very special trousers.
0:39:29 > 0:39:32- How does that feel? - That feels great actually, yeah.
0:39:32 > 0:39:34So what do these do, Henry?
0:39:34 > 0:39:36What they do is inflate
0:39:36 > 0:39:42and stop blood going down into the legs whilst you're pulling G.
0:39:42 > 0:39:45They also pressurise the abdomen and stop the heart descending,
0:39:45 > 0:39:48because it does descend a few centimetres
0:39:48 > 0:39:50while pulling high levels of G.
0:39:51 > 0:39:55This is the flight suit worn by the pilots of the RAF's
0:39:55 > 0:39:57most advanced plane, the Typhoon.
0:39:57 > 0:39:59And with its protection,
0:39:59 > 0:40:03I'm going to see if I can really push my G-tolerance.
0:40:36 > 0:40:372G.
0:40:38 > 0:40:44My suit automatically inflates as it detects the increasing G force,
0:40:44 > 0:40:46squeezing my legs and abdomen
0:40:46 > 0:40:49and forcing the blood back towards my heart and brain.
0:41:05 > 0:41:065G.
0:41:06 > 0:41:10With the gondola spinning at nearly 70mph,
0:41:10 > 0:41:15we reach 5G, a level that would render most people unconscious.
0:41:15 > 0:41:176G.
0:41:17 > 0:41:21But with the suit's help, I'm still going strong.
0:41:21 > 0:41:24In response to the rising G, the breathing system
0:41:24 > 0:41:27detects the G-force and increases the pressure of air
0:41:27 > 0:41:32pushed into my lungs, enhancing my G-tolerance even further.
0:41:53 > 0:41:56Well, you did fantastically well - that was 8G.
0:42:04 > 0:42:06So it's quite incredible.
0:42:08 > 0:42:10You can feel that really squeezing
0:42:10 > 0:42:13and the pressure comes up from your legs,
0:42:13 > 0:42:17pushing all the way up into your abdomen and, I guess,
0:42:17 > 0:42:22trying to help you return blood to your brain.
0:42:22 > 0:42:25And, you know, it definitely works.
0:42:25 > 0:42:28It took you well above 3G,
0:42:28 > 0:42:30above what I'd experienced before, more than 3G.
0:42:30 > 0:42:33There's not a chance I could have got there
0:42:33 > 0:42:35relaxed and unprotected before
0:42:35 > 0:42:37so the suit definitely works.
0:42:37 > 0:42:40They really are magic trousers.
0:42:41 > 0:42:44It's protective clothing like these anti-G suits
0:42:44 > 0:42:48that has helped pilots fly high into the atmosphere.
0:42:49 > 0:42:52By the 1950s, some jets were flying above 70,000 feet,
0:42:52 > 0:42:56over 20,000 metres, where the air is so thin
0:42:56 > 0:43:01that the pressure is less than 10% of that at sea level.
0:43:01 > 0:43:05At these heights, the curvature of the Earth spreads out below us
0:43:05 > 0:43:08and above, we can see the blackness of space.
0:43:16 > 0:43:19But we still hadn't reached the top of our atmosphere.
0:43:19 > 0:43:23In fact, we weren't even halfway there.
0:43:23 > 0:43:26And in our quest to go higher, we had to go back
0:43:26 > 0:43:29to the oldest flying technology of all.
0:43:34 > 0:43:38In 1960, Joseph Kittinger, a military test pilot,
0:43:38 > 0:43:43boarded a gondola strapped under a massive helium balloon.
0:43:44 > 0:43:47It took him up through the atmosphere, and up and up
0:43:47 > 0:43:50to 30,000 metres.
0:43:50 > 0:43:53With 99% of the atmosphere below him,
0:43:53 > 0:43:56he was at the very edge of space.
0:43:56 > 0:43:59For a few minutes, he was further from the Earth
0:43:59 > 0:44:02than any human had ever been before.
0:44:03 > 0:44:05And then...
0:44:07 > 0:44:08..he jumped out.
0:44:18 > 0:44:22But Kittinger's altitude record was only to last a few weeks.
0:44:24 > 0:44:28Because mankind was about to enter the Space Age,
0:44:28 > 0:44:31developing the most impressive machines ever built
0:44:31 > 0:44:35in an attempt to leave the planet altogether.
0:44:35 > 0:44:37'50 seconds and counting.'
0:44:41 > 0:44:42'45 seconds.'
0:44:45 > 0:44:48This is the Saturn V.
0:44:48 > 0:44:51It's the most powerful rocket ever built.
0:44:53 > 0:44:57This vast structure, 110 metres long,
0:44:57 > 0:45:00is a symbol of how much effort and energy we need
0:45:00 > 0:45:04to extend our reach into the extremes of space.
0:45:07 > 0:45:09This entire vehicle,
0:45:09 > 0:45:12everything up until here, is just fuel and engines.
0:45:12 > 0:45:15'We have the power transfer.'
0:45:15 > 0:45:18There's a simple reason why rockets like these need so much fuel.
0:45:18 > 0:45:22And that's because to escape the Earth's gravity
0:45:22 > 0:45:25they need to create a huge amount of thrust.
0:46:17 > 0:46:22All told, the Saturn V carried 2,500 tonnes of fuel.
0:46:24 > 0:46:26Enough for the vast engines
0:46:26 > 0:46:29to produce the seven million pounds of thrust required
0:46:29 > 0:46:32to send the rocket to the moon.
0:46:37 > 0:46:41But delivering enough power to escape the Earth
0:46:41 > 0:46:42was only half the problem.
0:46:42 > 0:46:47A much bigger challenge was keeping the crew alive on the journey.
0:46:49 > 0:46:52Project Apollo was about more than just brute force.
0:46:52 > 0:46:56The first problem was that your crew were sitting on top of a rocket
0:46:56 > 0:46:59with the explosive capacity of a small nuclear weapon
0:46:59 > 0:47:02and if they survived the launch, the engineers then had to find
0:47:02 > 0:47:05a way to protect them against the void of space
0:47:05 > 0:47:09and provide them with something in the way of life support.
0:47:09 > 0:47:12The success of human space flight
0:47:12 > 0:47:16depended on finding ways for people to survive in space.
0:47:16 > 0:47:19Completely removed from the safety of the Earth's atmosphere,
0:47:19 > 0:47:23astronauts would have to be provided
0:47:23 > 0:47:26with everything they needed to stay alive.
0:47:27 > 0:47:32And from the start, it was a process of trial and occasional error.
0:47:34 > 0:47:37In April 1960, Russian Yuri Gagarin became the first man
0:47:37 > 0:47:40to successfully orbit the Earth.
0:47:41 > 0:47:44He was followed into space just three weeks later
0:47:44 > 0:47:47by the American astronaut Alan Shepard
0:47:47 > 0:47:49on his Mercury-Redstone rocket.
0:47:51 > 0:47:55Shepard was in space for just 15 minutes, but he and Gagarin
0:47:55 > 0:47:59had proved that mankind could survive in the vacuum of space
0:47:59 > 0:48:02by taking an artificial atmosphere with them.
0:48:05 > 0:48:08This is a real Mercury capsule.
0:48:08 > 0:48:12This carried Gordon Cooper into space in 1963
0:48:12 > 0:48:13and look at it, it's tiny.
0:48:13 > 0:48:17There's about the space of a telephone box in there,
0:48:17 > 0:48:21carrying an astronaut around the Earth.
0:48:21 > 0:48:24To survive in space, you need to bring enough oxygen with you
0:48:24 > 0:48:27with enough pressure behind it to make it breathable.
0:48:27 > 0:48:30The Mercury system solved this
0:48:30 > 0:48:34by filling the capsule with 100% oxygen.
0:48:34 > 0:48:37And that worked well, that kept the astronauts alive.
0:48:37 > 0:48:39But it came with an enormous risk.
0:48:39 > 0:48:44The use of pure oxygen systems survived into the Apollo programme,
0:48:44 > 0:48:48the mission that was to land men on the moon.
0:48:49 > 0:48:53The first stage of that mission was due to launch in February 1967,
0:48:53 > 0:48:55carrying three astronauts -
0:48:55 > 0:48:59Gus Grissom, Edward White and Roger Chaffee.
0:49:00 > 0:49:05They would orbit the Earth to test the Apollo hardware.
0:49:05 > 0:49:07But just three weeks before launch,
0:49:07 > 0:49:10the crew entered the craft for a full systems check.
0:49:12 > 0:49:15It's not clear what happened next,
0:49:15 > 0:49:19but it's thought a stray spark from the electrical system
0:49:19 > 0:49:21started a small fire,
0:49:21 > 0:49:26which in the 100% oxygen atmosphere quickly turned into an inferno.
0:49:27 > 0:49:29Trapped in their capsule,
0:49:29 > 0:49:32the astronauts burned to death in seconds.
0:49:42 > 0:49:45Pure oxygen atmospheres were never used again.
0:49:50 > 0:49:55By the time the next manned mission, Apollo 7, launched in October 1968,
0:49:55 > 0:49:59the cabin was filled with a mixture of oxygen and nitrogen,
0:49:59 > 0:50:01more like the air we breathe on Earth.
0:50:02 > 0:50:06Only when in orbit in the vacuum of space
0:50:06 > 0:50:09was the atmosphere replaced with pure oxygen,
0:50:09 > 0:50:13but at such a low pressure it didn't pose the same fire risk.
0:50:13 > 0:50:16But for these longer Apollo missions
0:50:16 > 0:50:20where the crew would live in their capsules for more than a week,
0:50:20 > 0:50:22there were other problems to solve.
0:50:23 > 0:50:25If you want to survive in space,
0:50:25 > 0:50:27it's not enough just to provide oxygen.
0:50:27 > 0:50:29You need to remove the waste gases,
0:50:29 > 0:50:32the gases of respiration, carbon dioxide.
0:50:32 > 0:50:35If that accumulates, it causes drowsiness, confusion,
0:50:35 > 0:50:38later coma, and eventually death.
0:50:38 > 0:50:40In 1970, the crew of Apollo 13,
0:50:40 > 0:50:43Jim Lovell, Fred Haise and Jack Swigert,
0:50:43 > 0:50:47encountered a catastrophic failure of their life support system,
0:50:47 > 0:50:50an explosion which left them short of oxygen and without the ability
0:50:50 > 0:50:53to remove enough carbon dioxide from their atmosphere.
0:50:53 > 0:50:56And that would lead to one of the greatest dramas
0:50:56 > 0:50:59in the history of human space exploration.
0:50:59 > 0:51:02'Houston, we have a problem here.'
0:51:02 > 0:51:05- Say again, please. - 'Houston, we've had a problem.'
0:51:05 > 0:51:08The crew of Apollo 13 were on their way to the moon
0:51:08 > 0:51:11when an explosion in an oxygen cylinder
0:51:11 > 0:51:14caused the command module to lose power and life support.
0:51:14 > 0:51:16'We had a pretty large bang
0:51:16 > 0:51:19'associated with the caution and warning.'
0:51:19 > 0:51:24It forced the crew to evacuate to the lunar landing module, the LM.
0:51:25 > 0:51:27'I want you to get some guys
0:51:27 > 0:51:31'figuring out minimum power in the LM to sustain life.'
0:51:31 > 0:51:34Although the LM was capable of making the return journey to Earth
0:51:34 > 0:51:38and the crew had enough oxygen and food,
0:51:38 > 0:51:40there was a crucial flaw in the plan.
0:51:40 > 0:51:43The LM didn't have enough filters
0:51:43 > 0:51:46to remove the carbon dioxide from the air.
0:51:46 > 0:51:49And the spare filters from the command module
0:51:49 > 0:51:51wouldn't fit the LM's systems.
0:51:52 > 0:51:56If they couldn't find a way to remove the CO2,
0:51:56 > 0:52:01the crew would suffocate and die long before they reached home.
0:52:04 > 0:52:08What followed was the greatest piece of DIY in space history.
0:52:09 > 0:52:12The command module filters were adapted
0:52:12 > 0:52:14with cardboard and sticky tape.
0:52:16 > 0:52:19Around the world, millions watched,
0:52:19 > 0:52:22breathlessly waiting to see if the improvised filters
0:52:22 > 0:52:25would work well enough to bring the crew safely back to Earth.
0:52:29 > 0:52:32'Houston, you're looking good.'
0:52:32 > 0:52:35- 'Welcome home.'- 'Thank you.'
0:52:37 > 0:52:41Since then, we've got better and better at sending people into space.
0:52:43 > 0:52:46We can now not just keep people alive for a week or so,
0:52:46 > 0:52:50we can keep a presence in space permanently.
0:52:51 > 0:52:55For more than ten years, astronauts have been living and working
0:52:55 > 0:52:57on the International Space Station,
0:52:57 > 0:53:02supported by an artificial atmosphere just like the Earth's.
0:53:05 > 0:53:0950 years after we first started going into space,
0:53:09 > 0:53:11we now live and work there,
0:53:11 > 0:53:13we explore and we do science, and to do that,
0:53:13 > 0:53:17we've had to create much more sophisticated life support systems.
0:53:17 > 0:53:20The International Space Station does a pretty good job
0:53:20 > 0:53:23of creating what we're used to down here on Earth.
0:53:23 > 0:53:27One atmosphere of pressure, enough light, heat, water,
0:53:27 > 0:53:3021% oxygen - something that looks an awful lot like home.
0:53:30 > 0:53:34And to have achieved that in orbit,
0:53:34 > 0:53:37in an environment so uniquely hostile to human life,
0:53:37 > 0:53:39is a tremendous achievement.
0:53:41 > 0:53:44The International Space Station is a home away from home.
0:53:44 > 0:53:48But although we can replicate an Earth-like atmosphere,
0:53:48 > 0:53:51living in space for extended periods
0:53:51 > 0:53:55presents another serious challenge to our physiology.
0:54:03 > 0:54:05Simulating the effect of being in orbit
0:54:05 > 0:54:08is almost impossible on the Earth.
0:54:08 > 0:54:13But there is one place we can go, where for just a few seconds,
0:54:13 > 0:54:16we can experience what it's like to be in space.
0:54:18 > 0:54:24This Airbus 300 is used by the European Space Agency.
0:54:24 > 0:54:28It's been specially modified for what's known as parabolic flight.
0:54:38 > 0:54:42First, the pilots fly the plane steeply upwards,
0:54:42 > 0:54:47before guiding it over the top of the arc into an equally steep dive.
0:54:51 > 0:54:53As the plane falls away...
0:54:58 > 0:55:01..it's as though gravity disappears.
0:55:10 > 0:55:12(LAUGHS)
0:55:24 > 0:55:26And that's weightlessness.
0:55:26 > 0:55:29'It doesn't last long, but for just 20 seconds,
0:55:29 > 0:55:33'it's as though we've escaped the pull of the planet.'
0:55:41 > 0:55:43Now that's fun!
0:55:44 > 0:55:47So fantastic and really hard to describe.
0:55:47 > 0:55:53All of those dreams you ever had of flying are just suddenly for real.
0:55:58 > 0:56:03'Nothing in our evolutionary past has prepared us for this.'
0:56:08 > 0:56:12So this looks like pretty good fun. And it is.
0:56:12 > 0:56:15When gravity disappears temporarily, it's fantastic.
0:56:15 > 0:56:18But like anything else, you can have too much of a good thing.
0:56:18 > 0:56:21When you have to live in weightlessness,
0:56:21 > 0:56:24it can have serious consequences for your body.
0:56:31 > 0:56:34For astronauts on the International Space Station,
0:56:34 > 0:56:38who are weightless for months at a time,
0:56:38 > 0:56:39there are many potential hazards.
0:56:41 > 0:56:44For a start, simple necessities like eating,
0:56:44 > 0:56:48washing and going to the toilet become fraught with difficulty.
0:56:48 > 0:56:53But removed from gravity, our physiology also changes.
0:56:53 > 0:56:56Released from the constant pull of the Earth,
0:56:56 > 0:56:59our muscles start to waste away
0:56:59 > 0:57:02and our bones, no longer bearing weight, lose calcium,
0:57:02 > 0:57:06becoming weaker and brittle.
0:57:06 > 0:57:10And since the heart doesn't have to work so hard to pump blood
0:57:10 > 0:57:12to our head, it too becomes weaker.
0:57:16 > 0:57:19Because of these changes, the astronauts on the Space Station
0:57:19 > 0:57:24have to exercise for hours every day, to retain their strength.
0:57:25 > 0:57:28In time, we hope to travel further into space,
0:57:28 > 0:57:33and we'll need more elegant ways to cope with the lack of gravity.
0:57:33 > 0:57:37But in the meantime, our colonisation of orbit
0:57:37 > 0:57:39is a remarkable stepping stone.
0:57:39 > 0:57:44It's extended our habitable range to hundreds of miles
0:57:44 > 0:57:48above the Earth's surface and made us a space-faring species.
0:57:49 > 0:57:52A remarkable journey when you consider
0:57:52 > 0:57:57that not even 250 years ago, no-one had even left the ground.
0:58:01 > 0:58:04That journey that takes us away from the surface of the Earth
0:58:04 > 0:58:08out into the atmosphere and beyond into space
0:58:08 > 0:58:12is the purest expression of our desire to explore.
0:58:12 > 0:58:15And it's that urge that makes us want to find ways to explore
0:58:15 > 0:58:17new, extreme environments.
0:58:17 > 0:58:21It's extended our reach from the narrow but comfortable envelope
0:58:21 > 0:58:25of sea-level to the deepest reaches of our ocean,
0:58:27 > 0:58:29to the top of our atmosphere and eventually, out into space.
0:58:29 > 0:58:33And in the future, who knows where that will take us?
0:58:33 > 0:58:36Possibly to completely different worlds.
0:58:57 > 0:59:00Subtitling by Red Bee Media Ltd
0:59:00 > 0:59:03E-mail subtitling@bbc.co.uk