The instinct to explore is one of the qualities that defines us as human beings. It's propelled us across vast oceans and to every corner of every continent. But far, far away from these shores, two tiny spacecraft are lifting this spirit of exploration to extraordinary levels. For three and a half decades, they've been investigating the outer reaches of our solar system. They are the Voyagers. Voyager was the right spacecraft at the right time, when a huge amount of stuff was waiting to be discovered and Voyager was capable of discovering it. Voyager was the seminal mission of the past 50 years. It represents the golden age of space exploration. The Voyager journey has been driven by remarkable human endeavour and achievement. They've been a window into worlds almost beyond imagination. And they've have helped unlock the secrets of our solar system. For many of us, they're probably best known for carrying a kind of message in a bottle. A record of humanity here on Earth, meant for any extra terrestrial civilisation that may find them. Each spacecraft carries a golden disc. It holds a snapshot of humanity, a dispatch to the stars. 'Hello from the children of planet Earth.' And now the Voyager mission is about to cross the final frontier. They are the first objects built by humans ever to pass beyond the solar system and into the galaxy beyond. This is the tale of the two most intrepid explorers in our planet's history. This is the Voyager story. Right, here we go. 1977, a good year for music. The question is, what do you start with? Do you go with a crowd pleaser or do you go with your favourite track? MUSIC: "Never Going Back Again" by Fleetwood Mac Brilliant. # She broke down and let me in... # It's 1977, and Fleetwood Mac have just released Rumours. The world feels like a different place. ..of the United States... Jimmy Carter is the new American president. And Elvis has just died. The cause of death is cardiac arrhythmia. And technologically, it's a million miles from today. A new company called Apple Computers has just been founded. # I was strolling on the moon one day. # And it's not long since the final Apollo mission landed on the moon. 'One of the most proud moments of my life.' And this new technological confidence has fuelled something else - a renewed interest in science fiction. The public has gone crazy for films like Star Wars and Close Encounters Of The Third Kind. And this combination of breakthrough technology and exciting science fiction has helped to inspire a surprising project. Because in August 1977, NASA began one of the greatest adventures in the history of spaceflight. "Three, two, one, zero." Here were two unmanned space probes, attempting something straight out of an Arthur C Clarke story. Their mission - to explore the outer planets of the solar system... ..Jupiter, Saturn, Uranus and Neptune. Their first encounter with Jupiter would be two years and half a billion miles away. The two spacecraft were now heading on their epic journey. But the story of the Voyager mission began almost 20 years earlier. To uncover its origins, I've come to California, to find out how it all started. Today, it's really easy to take the successes of these remarkable missions for granted. But in the years preceding Voyager, simply getting to the outer planets was thought to be impossible. The first object launched into orbit was Sputnik 1. And from then on, space scientists became obsessed with journeying ever further from Earth, exploring the far reaches of our solar system. Yet no spacecraft could get much further than Mars... and even that was a struggle. The simple fact was we just didn't have a rocket that was powerful enough to actually to be able to escape the gravitational pull of the sun and be able to get to the outer solar system. And even if we did, the vast distances involved would mean that a trip to Neptune would take half a lifetime - 30 or 40 years. The outer planets were simply out of reach. But back in 1961, here in California, one man thought he might know how to bring these planets into reach. He was a brilliant maths graduate, and his name was Michael Minovitch. My father taught me how to do arithmetic when I was like in 4th or 5th grade. And then I learned the language - the secret of science. And the secret of science is mathematics. At the age of only 25, while he was still studying for his PhD at UCLA, Minovitch set himself the challenge of solving the most difficult problem in space exploration. It was a puzzle that had stumped the world's greatest mathematicians for centuries. It's called the "three-body problem" - body one, body two and body three. And it involves the fiendishly complicated task of trying to plot the trajectory of a small object, i.e. a spacecraft, as it moves throughout the solar system, whilst at the same time being deflected by the gravitational pull of two much more massive objects, i.e. the sun and a planet. A solution to the three-body problem, the ability to predict exactly how a spacecraft passing a planet would have its path affected, was still beyond science... until, that is, the young Minovitch came along. It would have been regarded as impossibility prior to what I did in 1961. I was gifted being at a university that had the 7090 computer, so that was the key. UCLA's state-of-the-art IBM computer was the fastest on Earth at the time, and Minovitch put it to good use. He began calculating thousands of alternative directions and speeds, in an attempt to home-in on the solution. It was a long shot - not only for the young student but also the university. Working on a 7090 was costing 1,000 an hour, so they were dumping bushels of money into a fantastic belief and what was the belief? Belief that a person that hadn't got his PhD solved the problem that all the most advanced mathematicians in history couldn't solve. That meant pressure on me, and so I thought to myself, "How could I...? "I can't live with myself, given this gift, knowing that there's "a very strong possibility that my trajectories were not correct." Minovitch went to the people with the most accurate data on the solar system at the time - NASA's Jet Propulsion Laboratory in Pasadena. They would decide if he had solved the three-body problem or just wasted a lot of the university's money. They ran the tests, about four or five different trajectories types, different encounters, and found every single one converged to the exact solution. It was a beautiful moment in mathematics. By solving the three-body problem, Minovitch had discovered a way to use gravity to propel a spacecraft further and faster than ever before. What Minovitch realised was, as a spacecraft approaches the planet, it gets pulled in by its gravity, and as long as it doesn't crash into the planet, because the planet is orbiting the sun at tens of thousands of kilometres an hour, that spacecraft can take some of that energy and use it to get catapulted out at an increased speed further out into the solar system. With his new slingshot technique, Minovitch had opened a gateway to the outer planets, at least theoretically. He identified hundreds of possible missions to the planets, meticulously drawing them up in his notebooks. The concept that I invented, and I can show you the printout, if you look here, you'll see there was no limit. I could have a sequence that was a 100 planets long, nonstop, planet to planet to planet, launched from Earth. And then you come to Jupiter. Jupiter, you get a nice big bounce and you use that to propel yourself to Saturn, and then Saturn is a pretty darn big planet, and that will catapult you out to Pluto. This concept could be used to explore the whole solar system with one launch vehicle at one time without any rocket propulsion at all. But buried amongst those hundreds of theoretical flight paths was one very special trajectory. And no-one, not even Minovitch, noticed its significance. In the summer of 1965, right here at NASA's JPL, another vacation student was hired to number-crunch the options for a mission to the outer planets. And his name was Gary Flandro. I was a summer student working on my degree at the time, so when I was given the job of looking at the outer planets, I thought that was kind of make-work project - I was being kind of kept out of the way. Flandro was a young engineer, grounded in the hard realities of spaceflight, and he began to look at whether a solution to the three-body problem could be put to practical use. Obviously, the first thing is to determine when the planets are going to be in positions where we could reach them. So I drew very careful maps of where the planets would be, and one of the most important drawings was one in which I drew the positions of the planets versus the date. And the thing that caught my attention immediately was that the lines for Jupiter, Saturn, Uranus and Neptune all crossed in about the 1975-76 time period. In other words, those four major planets were on the same side of the sun and in the same general position at the same time. So it gave me the idea immediately that we could do all of those planets with one flight. This narrow window - to slingshot from one planet straight to the next - would not open again for another 176 years. It was too good an opportunity to miss. And so was born the idea of a Grand Tour, the most ambitious space mission of its time. It would send two identical space probes to all four of the solar system's outer planets in one relatively short flight. Such encounters promised spectacular views of these distant worlds, planets we only knew as blurry objects through telescopes. The half billion miles to Jupiter would take two years. Then another two years to Saturn... ..five more to Uranus and a final three to reach Neptune. It meant the Voyagers would need to function for at least 12 years. Yet NASA had never built a spacecraft guaranteed to last longer than a few months. It was their biggest challenge yet. Hi, John, how are you? Oh, Dallas, I'm fine. Thanks for coming. And it was one which fell to a young engineer called John Casani. The issue was the time. It takes time to cover that distance. You're going a long ways, and that takes time, and the time is...can you make all these machines operate without human intervention or adjustment? Well, I mean... At that point in time, that was a mind-blowing thought - how you build a spacecraft that can survive failures and still keep on chugging? Five years of testing and redesigning followed, as NASA's engineers grappled with the task of building a spacecraft capable of the job. And they needed to do it before 1977, when the launch window for this Grand Tour would close... at least for another 176 years. The thing that was scary was that it was going to be based on a lot of new technology, so it was a technological leap. We thought we could do it - nobody else did. They'd cracked the mathematics, they were confident tackling the technology, but there was one more thing they needed - money. NASA still lacked the funding to support the mission beyond Saturn. To ensure further funding, the public and Congress would need regular reminders of their achievements. The Voyagers needed a voice, someone who could turn their saga of celestial exploration into something that all Americans could share. They turned to a young member of the Voyager team with a passion for storytelling - his name was Carl Sagan. Wouldn't it be lovely to make contact with another civilisation that has arisen and evolved independently? Aware that the Voyagers would head away from us forever, Sagan proposed an extraordinary idea. On board each spacecraft, he suggested placing a message from Earth - an idea which would capture public imagination. Attached to each spacecraft is a fairly elaborate message in the form of a phonograph record and instructions for playing. It was a gold-plated copper record - a gift of recordings and greetings from the inhabitants of this planet to those of some other. 'Bonjour, tout le monde...' GREETINGS IN JAPANESE AND RUSSIAN Each disc contained a combination of sounds and pictures and above all music - from Chuck Berry to Azerbaijani bagpipes and Johann Sebastian Bach. Sagan argued that sometime, somewhere, another civilisation may find one of the Voyagers. The record's purpose was to tell them what kind of creatures had sent it. How much will they know about us, what we're really like? To communicate that, music is a way of expression of human feelings, desires, passions, hopes. In some sense, all the performers and composers on this record will live forever. With their golden records on board, and the public's imagination fired up, the Grand Tour was underway. But no-one could know if the mission was going to deliver results until the Voyagers reached their first planet, and that would take two long years. April 1979, and two years after launch, Mission Control was steering the Voyagers towards their first rendezvous. It was with the largest planet in our solar system - Jupiter. Before Voyager, the best images astronomers had of Jupiter and its moons were fuzzy photographs. Could the Voyagers change all that? I think we all felt that we were in the tradition of Galileo, who was the first to see the moons of Jupiter, and the first to apply an instrument to increase our ability to observe the universe. Voyager was just the latest tool which we, as a civilisation, had managed to devise. And, of course, the tool was so powerful that we saw things nobody had seen before and that nobody had imagined we would see. For the man who'd first proposed the mission, it was a thrilling moment. That first encounter with Jupiter was a marvellous... time for me, especially the approach shots showing the planet revolving and watching the great red spot revolving getting closer and closer till finally we could see that indeed this was the top of a large storm. As a child, I had studied that and wondered if that was a storm or was that an island floating in an ocean - it was very difficult to know - and, finally, the answers were there before our eyes. At the time, Voyager scientist Andy Ingersoll revealed these discoveries to a BBC Horizon crew. The movie here shows pictures of Jupiter taken every 10 hours. The shutter was snapped, then this is played in a sequence over and over again so you can see motion. And this rapid mixing makes the existence of permanent different-coloured, different chemical features even more mysterious. See, I'm a weather man, I'm an atmospheric scientist, and we knew about the 300-year-old storms, the great red spot, because everyone had been looking at it from Earth, and for me the surprise was, when we got up close, we saw that the atmosphere was just churning and turbulent, and it made this 300-year-old storm all the more mysterious, cos how could it go on in the midst of all this turbulence? We all approached Jupiter with great expectation and we all had our grandiose theories about what we were going to see, but, of course, Jupiter fooled us all. There was some bizarre behaviour. Little clouds moving along and being swept up in the great red spot and then being... it would spit them out again. Other clouds would roll along next to one another, coalesce into a single cloud and then break apart again. Voyager's pictures suggested that Jupiter's wildly churning atmosphere seemed to be driven by heat from deep within the planet. Scientists speculated that it was came from a hot, high-pressure core of metallic hydrogen. Such a centre also seemed to be powering an immense magnetic field, And for the Voyagers, that was a problem. Because this magnetism creates lethal radiation belts, which can scramble the computers of any spacecraft that gets too close. Yet getting close was exactly what was needed. The Voyager team wanted to send Voyager 1 to explore Io, one of Jupiter's four largest moons. And it was the nearest of all of them to the planet. The spacecraft was designed to withstand a certain total dose of radiation, and fully 50% of that expected dose was going to occur as we approached and flew by Io. As Voyager 1 approached, it sent back recordings of the radio signal generated by the radiation. LOUD, DISTORTED WHISTLES These are the real sounds of the onslaught. Back at JPL, the Voyager team worried whether it could withstand such an assault, and whether the gamble would pay off. Voyager navigation engineer Linda Hyder was the first to find out. I came in about nine o'clock that morning to the navigation area, and the tape with the pictures the spacecraft had taken the day before was on my desk. I put them on the computer system and I displayed them. And I could see that Io, the moon of Io, was a crescent, as very often our own moon is a crescent in the night sky, and I went and enhanced the brightness, and there appeared beside Io an object, a huge object, and completely captured my attention. It looked like another moon peeking out from behind Io. But there was no other moon... and no fault in the camera. Linda decided this object had to be part of Io. And, in fact, that was very hard to accept, because the size of this object was enormous. And when I explored it, I was able to find that this large, strange object, it was exactly coincident and fell over a heart-shaped feature on Io. What I had discovered was the huge plume of a volcanic eruption, arising 270km over the surface of Io and raining back down onto it. So I had discovered the first ever volcanic eruption ever seen on another world besides the Earth. The gamble of being exposed to such radiation had paid off. Voyager 1 had revealed that Io, the closest of Jupiter's large moons, was more geologically active than the Earth. Jupiter's enormous gravity stretches and squeezes the moon, forcing its core to heat up and its interior to stay molten. We found that Io had eight active volcanoes on it, the most volcanically active body in the solar system, and it's just a small moon, and that was so unexpected. And it was such a shift in our paradigm about what was going on in the outer solar system where it's very cold and, presumably, we thought very dead. So in that sense, it characterised for us the sense of seeing things that we hadn't really thought about, and that was in fact very characteristic of the rest of the mission. And that wasn't all. As the Voyagers flew by Jupiter's other moons, more discoveries began pouring in. These exotic satellite worlds of rock and ice needed a new expertise to interpret them. The Voyager team had to react quickly, bringing on board more planetary geologists. There's a twin, a pair there, and then there's... What about the relief from the cracks? Shouldn't the cracks... In order for there... 'All of the scientists, with the exception 'of me, were atmospheric scientists and astronomers.' And, in fact, it wasn't until we really recognised the exotic variety and diversity of the satellites, that geologists were really added to the Voyager team. And in fact the satellites, in my view, became the star of the whole Voyager experience. Voyager's encounter with Jupiter was a triumph, and Carl Sagan hosted a televised evening to celebrate. It's impossible to look at these pictures with only a scientific cast of mind, because they are simply exquisite. And this is part of the remarkable historical transition, which is happening in the late 20th century in which we are, for the first time, learning the realities, not the myths, of our little swimming hole in space. On a night like tonight, our eyes, our minds, our souls, our blood are moving out through the universe. We're part of history, and that means that we have to replace the old myths with new ones. With Jupiter behind them, the two Voyager spacecraft headed further out into inter-planetary space. It would be more than two years before they reached the next destination on their Grand Tour - the planet Saturn, almost a billion miles away. The technology and engineering needed to accomplish such long-distance, long-duration spaceflight, was truly remarkable. The spacecraft needed to be designed to cope with anything their multi-billion-mile journey would throw at them. Luckily, you don't need to travel 11 billion miles to get up close and personal and really appreciate the extraordinary engineering of Voyager, because there's another one a little bit closer to home. When JPL built the Voyagers, they also assembled a couple of extra models from flight spares, as an Earth-bound reminder of their visionary 1970s technology. Dominating the entire structure is this great communications dish that's beaming back to Earth all that data that the Voyager spacecrafts collect across billions of miles of empty space. Incredibly, the power of this signal was designed to be a mere 20 watts - about the same as a fridge light bulb. And situated on this arm, quite sensibly far away from the spacecraft, is Voyager's power supply. It's a plutonium-fuelled generator that can power the spacecraft in deep space when solar power just isn't an option. And over on the other side, sticking out on another boom, perhaps most excitingly, this is Voyager's eyes. This great collection of cameras that revealed new worlds for the first time, and let us see the solar system with greater clarity than ever before. the public were queuing up to get their first clear views of the mysterious ringed planet, Saturn. MUSIC: "Don't Stop Believing" by Journey The Voyager team had prepared in meticulous detail for the encounter, as they knew they had just a tiny window to get it right. Each spacecraft would fly by so quickly, on such a close approach, there was almost no time to gather data. The closest approach fly-by sequences are a matter of hours. Really, the tightest closest approach activity is within a 12-hour span. In particular, the team needed to decide where to point the cameras. The scan platform, which included the cameras and spectrometers, am I going to point it at the moon and which moon, or am I going to point it at the planet? Which way am I going to point it? And so you have to argue with your colleagues. Blue-ish. Blue-er than grey. But it was the rings of Saturn which stole the show. We thought we knew it all, but, once again, we were looking at a very, very complex situation. The rings were broken up into mini-rings. There were gaps in there, there were all sorts of dynamical phenomena that we didn't understand. When I began my work, I had suggested that one thing we could do with this particular mission was to fly between the planet and the rings, and, very fortunately, we didn't do that, because, as we approached Saturn, we saw that the region there we would have had to flown through with the spacecraft was filled with more rings. There was no question that spacecraft would not have survived trying to go through that gap. The imaging team could barely cope with all the new data. What I remember...it wasn't really stressful, but it was just chaotic and hectic and exciting. Right in the few days around the encounter, trying to keep up with the discoveries as they poured in. Eventually, no-one got any sleep, because we were just overwhelmed with new stuff. Voyager revealed delicate rings that were intertwined and rings that were held in place by tiny moons they called shepherds. There were strange features called spokes, patches of dust particles, slightly raised above the rings. These caught the eye of one young graduate student in particular. I got involved in the study of the spokes, which were these ghostly features that were seen to come and go, and it just came to my head to kind of categorise the pictures. Into one pile, I put all those images that seemed to have a lot of spokes in them, and into another pile, I put those images that seemed to have virtually no spokes at all. And I made an intermediate category. And, of course, each image was tagged with a time, and I basically did an analysis on the computer of this and found that the spokes actually weren't just sporadic but, in fact, they came and went with a certain period. Remarkably, Carolyn Porco had discovered that the spokes followed Saturn's magnetic field as it rotated with the planet. I made my very first scientific discovery, and just knowing that I had found something that nobody else on the face of the planet knew at that time was just such an exhilarating experience. Well, I think Saturn has not disappointed us. I really expected that since we had such a rudimentary... knowledge of Saturn system that we would be seeing many surprises, but, as usual, our imaginations were not nearly up to what nature provided. Four years since launch, the Voyagers had, so far, been a wild success. But now came the mission planner's biggest gamble. Here at Saturn, the twin spacecraft would part company. Voyager 1 would be diverted towards Saturn's largest moon, Titan. It was an enticing target. It was clear that the composition of Titan's atmosphere makes it kind of an analogue with the Earth, which is terribly surprising, because no-one expected years ago you'd find an analogue of the Earth out at the distance of Saturn. With an atmosphere of similar density to Earth's, it was believed Titan might even harbour primitive life. But the manoeuvre came at great cost. To fly past Titan, Voyager 1's Grand Tour would have to be sacrificed. To visit this intriguing moon, it needed to be put on a different path, throwing it up at an angle, out of the plane of the solar system. Beyond Titan, there would be no more planetary encounters for Voyager 1. In the end, the Titan fly-by was a disappointment. Voyager 1's cameras couldn't penetrate its atmosphere to offer further clues to whether life might lie beneath. Titan was the first major setback for the Voyager team. It meant Voyager 1 had been sacrificed for very little and was now speeding away from the solar system. The rest of the Grand Tour would have to rely on one single spacecraft - Voyager 2. Now on its own, it was heading across the solar system towards the outermost planets. But then, just as it left Saturn, another setback - the team noticed Voyager 2's camera platform had started to jam. Without the crucial ability to pan its cameras, there would be few pictures of the other outer planets. It was a potential disaster, and the team struggled to find the cause. In the case of the stuck scan platform, the expectation was that there was a piece of debris, which is not likely. I mean, we're so careful when we put these machines together. So then it goes down to, well, maybe it's the lubricant, the way the lubricant has distributed itself. So how do you fix a spacecraft that's over a billion miles away? What we decided to do was to exercise it very carefully, moving the gears train back and forth slowly over this spot. We could see that we were making progress and we said, "OK this is it. We can work through it". But without any target to focus the cameras on, they had no way to know if their fix was successful. They'd only know that when Voyager 2 reached its next destination - Uranus. Even travelling at 50,000 miles an hour, this encounter was five years away. Half a decade of uncertainty and anxiety. Well, just about two minutes ago, Voyager 2 passed through its closest approach to Uranus. APPLAUSE Despite their fix to the scan platform, with the limited light this far from the sun, the Voyager team knew their cameras would struggle. Voyager was planned to operate at 1 billion miles at Saturn. It was now being asked to operate at 2 billion miles at Uranus, where the sun was very dim. And we had to do several things. For instance, you have to have much longer exposures on the camera, and, if you have too long an exposure, the spacecraft's moving very rapidly, things become smeared. So we had to learn how to program the spacecraft to turn at just the right rate, so that it would compensate for the motion of the spacecraft. They had to basically re-program the brains of the spacecraft. It didn't have very many brains by today's standards, but they had to re-program it. Those were fantastic achievements. As the first images of Uranus arrived back on Earth, it became clear the engineers' ingenuity had once again paid off. But the extraordinary, pin-sharp pictures of this distant planet, two billion miles from Earth, revealed tantalisingly little. After all the waiting, it was a reminder that with Voyager, nothing could be taken for granted. Uranus is different than Jupiter and Saturn in the sense that it has no internal heat source. Both Jupiter and Saturn are radiating more energy than they receive from the sun, because there's still heat inside those planets. For a reason, at Uranus, that heat had been shut down and was not driving the atmosphere, so the atmosphere was much blander. Check... If Uranus itself was something of a disappointment, once again, the team found plenty of surprises in its moons. Most striking of all was the tiny moon, Miranda. Miranda looks like a three-dimensional jigsaw puzzle. We see regions looking like giant, complex racetracks, almost as if it's put together by a committee. There are pieces stuck on the surface that look like they belong to different planets, and one idea was that it was busted apart and these core pieces stayed intact, and then they were glued back together, and so you get this hodge-podge. From Uranus, Voyager 2 faced it's final challenge - the journey to Neptune, over a billion miles further out and three years more space travel to survive. The last major planet in the solar system, this most mysterious world had resisted investigation from even the most powerful telescopes. To maintain it's trajectory, Voyager 2 needed to make a low pass over Neptune's north pole. But this brought its own problems. Because of increased speed and approach angle, Voyager 2's window of opportunity would be the narrowest yet. The challenge at Neptune was the most difficult one we had. We had to know, within one second, when we were going to fly over the north pole of Neptune. That was a major navigational challenge - we had never delivered that kind of accuracy before. If we were right, it worked. If we were wrong, we had no second chance. Not only did the team need to position a spacecraft to within a second of accuracy, after a flight of 12 years, but to ensure scientific success, they also had to forecast the weather on a planet three billion miles away from Earth. We had to forecast where to point the cameras, two weeks in advance, where those interesting features were going to be. And we said, "Well, they're moving around. "There are storms in the atmosphere of Neptune." And this was August of 1989, and there was a big hurricane off the coast of Florida. And weather forecasters here were saying, "Well, 12 hours from now, we think it's going to veer right "or we think it's going to go left, but we're not sure." It may be starting to turn a little bit more towards the northwest or west-northwest... And, meanwhile, we were confidently issuing weather forecasts for Neptune two weeks in advance and telling the engineers, "OK, two weeks from now, "point your camera there and there will be a storm there." And we were right. It was glorious! The fly-by was approaching. Would the software rewrites and running repairs hold together to give humanity its only close encounter with Neptune? There was nothing more to do but wait and hope. After 12 years of flight, and decades of anticipation, the giant blue planet began to loom in Voyager 2's lenses. On the 25th August 1989, the spacecraft passed within 3,000 miles of Neptune's north pole. The craft had survived the three billion mile journey to the edge of the solar system. APPLAUSE The final encounter I was able to witness, here at JPL with my youngest son, and we watched with fascination as the pictures of Neptune unfolded. Suddenly things that no-one had imagined were there. Here was a planet that was vibrant with life. It had its own great spot, a dark spot in this case, white clouds floating in its atmosphere, and these things unfolded before our very eyes. What a wonderful surprise. Neptune, for me, was a great surprise. There was something strange and eerie about Neptune, because here, the last planet, the sentinel at the outer edge of our solar system, looks like Earth, with its beautiful deep blue colour and its white clouds floating in the atmosphere. We were back with a really exciting planet again at Neptune. There were fast-moving clouds, clouds that moved in different directions, some of them almost at sonic speeds. The complexity of the planet's atmosphere was far beyond our expectations. The Grand Tour was almost over, but Voyager 2 had one more surprise in store. Neptune's moon, Triton. This is too much... too much to believe. - Look at the tyre tracks. - Yeah. Tyre tracks. Triton was a world unlike any we had seen before. It was the coldest surface we had seen in the solar system, So cold that nitrogen, which forms most of the atmosphere on Earth, is frozen, solid ice, and the polar caps on Triton are frozen nitrogen, not frozen water. Even so, we found geysers on the surface of Triton, nitrogen geysers miles high. So even at the very deepest part of our solar system, there is geologic activity. It is everywhere. The solar system is alive, evolving, and that's what makes it so exciting. And makes it so much to learn. Voyager 2 had survived to reach the extremes of the solar system. It had revealed not just the planets themselves but whole systems of rings and moons unlike anything we'd imagined. Suzanne Dodd captured a final image from the flight. One of the images I took and helped design was the one where you have... it's actually one taken when you're going away. You have Neptune, the crescent of Neptune, and then you have the crescent of Neptune's moon, Triton, in the background, and you're taking that as the spacecraft is travelling out of the solar system. That's the last image that Voyager 2 is going to take, and that's the last image that spacecraft is going to remember of those planets. Voyager 2 delivered its final images in 1989. More data on the outer planets had been collected by the two Voyager spacecraft than in the rest of human history. But let's not forget Voyager 1, heading out of the plane of the solar system. Although it hadn't been able to have any more encounters with planets, there was one last, special task its makers asked of it. Because it was high above the solar system rather than in its plane, Voyager 1 had a view of all the planets that its twin could never have. Carl Sagan and Carolyn Porco began discussing an idea. Voyager was going to be in a location that no other spacecraft had been before, equipped with, you know, sophisticated instrumentation so that it could turn around and take a picture of all the planets in the solar system. And I thought that this would be a riveting collection of images, you know, a first. And they said, "Well, there's really no scientific justification "for this," and I couldn't argue with that, because there wasn't. The planets were going to be just pinpoints, they were going to be just pixels. They couldn't see it. On Valentine's Day 1990, 13 years after leaving Earth, Voyager 1 was asked to turn its cameras back towards the planets. Now 3.7 billion miles away, by the time Voyager's pitifully weak signal reached the dishes on Earth, it was just a millionth of a billionth of a watt of power. It was then boosted and sent on to Pasadena, where the image was assembled, here in the Deep Space Control Room. A unique family portrait, the ultimate snapshot of our solar system. And this is it! There's actually only six planets visible, because Mercury and Mars were obscured by the sun's glare. But the picture that captured everybody's imagination was that of Earth, only a tenth of a pixel in size. And here it is blown up. Here is the mosaic... For Carl Sagan, the symbolic value of the photograph was a gift. He held a press conference to publicise it around the world. The portrait of the planets has now been taken. This looks more than a dot, but it is in fact less than a pixel. In this colour picture, you can see it is slightly blue, and this is where we live, on a blue dot. With this final historic image captured, and nothing more to photograph, Voyager 1's cameras were switched off to save power. But that wasn't the end of the mission. Over 35 years on, as they hurtle away from us at over 10 miles a second, their cutting-edge 1970s technology keeps on chugging. And remarkably, they continue to send back new information about the space they're now travelling through, 11 billion miles from Earth. Even travelling at the speed of light, their messages take quite a while to get home. The journey time now is about 15 hours one way from Voyager 1 back to Earth, so you send a signal up, and the next day, you come back and you have some indication that the spacecraft heard the signal and responded. There are five instruments that are still operating on the spacecraft. And we're starting to see the evidence now in the data that we are crossing into interstellar space. We're seeing things that would lead us to believe that we are on that boundary. Now, at the end of 2012, our planetary explorers are crossing this boundary of the sun's influence. They're travelling beyond the limits of our solar wind and into the galaxy beyond. It's the first time any object built by humans has achieved this. A new chapter in human exploration is beginning. We have enough power to get us to about 10 more years, maybe out to 2025, but we will, over the course of those years, have to turn off things so that we continue to have enough power to run the transmitter to send the data back to Earth. The fact that Voyager's still alive and there's still a signal from it and it's about to leave the solar system, I think that's wonderful. That it hasn't just given up or that we haven't given up on it. It's a tribute to what Voyager means to us that we've kept it going. Really, it's wonderful, as a scientist, to be still exploring, still going somewhere no spacecraft has been before. events in their lives running parallel to the Voyager's encounters. When I started on the Voyager, my two daughters were young. By the time they were in college, we already had passed Saturn and were on our way to Uranus. They got married, and the Voyager just kept going, we had grandchildren, and Voyager just kept going, and so now our grandchildren are aware of what's happening to Voyager, just like our children were. Long after all their power has gone, the Voyagers will continue to rush away from us. One and a half tonnes of 1970s engineering, monuments to human endeavour and exploration, heading out towards the stars. I believe the next encounter with the closest star is something like 40,000 years from now. The two Voyager spacecraft are the furthest that we've ever sailed, but for all their amazing science and new worlds that they've found and data that they've collected, the Voyager mission is still an incredibly symbolic mission. Because those two golden discs are still fixed to the sides of each spacecraft. And in the benign, empty environment of deep space, they will outlive the pyramids, they're likely to outlive us, and perhaps even the Earth itself - the only record of our existence. 'Hello from the children of planet Earth.' Yet, despite the ambition, given the vastness of space, it's almost inconceivable that these two tiny spacecraft will ever be intercepted by other beings. It's a little bit like throwing a bottle into the cosmic ocean. But Sagan was clever enough to realise this. He knew it wasn't what the golden record said to other civilisations that mattered, more significant was what it said to our own. You might think that it is a hopelessly quixotic project to launch this message in a bottle into interstellar space and expect anyone will find it, but there are really two kinds of recipients of the message on the Voyager records. One is the extra-terrestrial audience. The other one is the audience down here down on Earth. Here is a moment when we have to suddenly think, "What is there about our culture "that we would want others to know about, "that we would be proud of?" The record should represent the human species as an entirely. The unity of the human species, seen down here, is a fact that is essential for the human future. MUSIC: "Over The Rainbow" by Israel Kamakawiwo'ole