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BBC The Planets Giants

where Voyager goals was Jupiter with his 4 moons (Io, Europa, Ganymede, Callisto) and Saturn In August 1977, two spacecraft called Voyager began an incredible journey. If they survived the hazards that lay across billions of miles of space, they would reach worlds so distant and strange; they defied the imagination - the gas giants. Voyager was heading for four vast planets that could swallow Earth thousands of times: Jupiter, with its strange bands of cloud and its great red spot, a world crackling with radiation that can be heard from Earth. Beyond Jupiter was Saturn. How did this planet alone come to have its spectacular array of rings? Stranger still was Uranus. It was known from the way its moons were orbiting, But Uranus had been tipped over on its back. Why should this be? Neptune was barely visible even through powerful telescopes. What kind of worlds were these? What could they reveal about the solar system, of which Earth is such a tiny part? Zero G and I feel fine. Capsule is turning around. Oh, that view is tremendous. In the early 1960s, sending a spacecraft all the way to the giants was unthinkable. Men had flown just a few hundred miles up in orbit around the Earth. Unmanned probes ventured further, but only to the nearest planets, Venus and Mars. Signal level's gone down, report continuing low signal level. Even this was pushing the very limits of science. In 1964, the first spacecraft, Mariner 4, flew by Mars. The spacecraft just barely made it there. Yes, it's there. And so the concept of going to Jupiter - which is almost a half a billion miles away, Saturn almost a billion, Uranus almost two, and Neptune almost three billion miles away and all twelve years in journey - was not even something one could very easily imagine. A solution came from a most unlikely source. A young student at NASA's Jet Propulsion Laboratory was asked to calculate trajectories for a flight to Jupiter. I was a summer student, working on my degree at that time. So when I was given the job of looking at the outer planets, I thought that was kind of a make-work project. I was being kind of kept down on the way while the really important business of getting to Mars was underway. Flandro discovered something that made the dream of a flight to the giants a real possibility. Obviously, the first thing is to determine when the planets are going to be in positions where we can 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, 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. Flandro's discovery was more than just a convenient planetary line-up. Rocket power alone could propel a spacecraft no further than Jupiter. But scientists knew that if a probe approached a planet at the right angle, it would be caught by the planet's momentum, and then pitched in a new direction, at a greater speed. The trick you use here is you fire your rocket with enough propulsion to get to Jupiter, which is, after all a long way's wait, 5 times the distance of the sun. That you get a gravitational slingshot effect. That propels you to Saturn. The same trick if Saturn's in the right place at the right time, which can propel you to Uranus, if Uranus is in the right place at the right time that happens about every 175 years. It happened in terms of launch date in 1977. In fact, a former administrator of NASA, Tom Paine, used to make the joke that the last time this happened, Jefferson was President, and he blew it. For NASA, this was too good an opportunity to miss. They announced a mission to the giants called Voyager. It marked a new era for astronomers who would struggle to understand the hazy views they could see through Earth-based telescopes. The man chosen to lead the imaging team was Brad Smith. I had started looking at Jupiter seriously back in the late 50s; we were frustrated by the difficulty of making such observations from the ground. And so, when the opportunity came up to be involved with Voyager, I realized that, for the first time, I was going to have that opportunity to see Jupiter up close; to see detail that we never could The hopes of astronomers now lay in the hands of the mission engineers, who faced what seemed an impossible task. The comment that I frequently heard from people I was working with was "That mission is never going to happen. It's just too complicated." We had to build a machine that could fly for, perhaps, 10 years without failing, and that was really pushing the limits of what we thought we could do with electronics at that time. We had to pass through the asteroid belt on the way. How can we get this large spacecraft through that very dangerous area beyond Mars without a collision? That seemed to be a very, very challenging task. One of the big worries was data transfer. Even if we could do this flight, could we get any useful data back to the Earth? So there a great deal of worry about that. The engineers had a decade to make the Voyager mission possible. Around the world, giant antennae were built - the Deep Space Network - to communicate with probes across billions of miles. But it still wasn't known if a spacecraft could survive hazards like the asteroid belt, a band of drifting rocks between Mars and Jupiter. Voyager would have only one chance. So two less elaborate probes were sent ahead to test the way. Pioneer 10 and 11 were launched to Jupiter and Saturn. Pioneer 10 and 11 were a very important part of astrology exploring the outer solar system. It was critical to have the pioneers lead the way, telling us about the environment and making some very important discoveries which would then allow us with the Voyagers James Van Allen was a veteran of missions to Mars and Venus. Now he led a team of scientists on Pioneer. Pioneer 10 was the first venture beyond Mars. First time we were going at such enormous distances. First time we were about to cross the asteroid belt which lies between the orbits of Mars and Jupiter. So it was a very hazardous and high-risk mission. And we had a keen sense of its historic possibilities in blazing the trail to the outer planets. Pioneer 10 successfully got through the asteroid belt. And it was still performing beautifully. And so we now knew that, to get to the outer planets safely. Another key question which Pioneer 10 had to address was how intense was the radiation environment around Jupiter? Since the '50s, radio emissions had been detected coming from Jupiter, suggesting there might be intense radiation around the planet. The man whose task was to investigate this was James Van Allen, who had discovered bands of radiation around Earth with the very first American satellite. They were later called the Van Allen belts. We've encountered a very high intensity of radiation, which is of the order one thousand times as intense as could be attributed to cosmic rays as ordinarily understood. Van Allen predicted greater radiation belts around Jupiter. But even he did not anticipate what Pioneer found. As we went in, the radiation intensity got greater and greater... And there was a very strong apprehension about the survival of the electronic equipment in the spacecraft. But it went up to a maximum, and then it started back going down again so we of course breathed the sigh of relief that we had finally made it. The radiation belts of Jupiter turned out to be 10,000 times more intense than Earth's. And Pioneer also encountered a vast magnetic field, stretching 7 million miles out from the planet. One thing that Pioneer 10 discovered was that Jupiter's magnetic field is the largest structure in the solar system. If you could see it from Earth in the sky it would appear to be as large as the sun, even it was five times further away. But, of course, it's invisible, and this is very damaging to spacecraft and can damage electronics. The Voyager probes had to be hurriedly redesigned to survive the intense magnetic fields and radiation belts of Jupiter. The Pioneer saved our lives. Had we flown into that, unknowingly, with a more sophisticated electronics and more sophisticated mechanisms than we had on Voyager, we would have died right on the spot. We were well into the design of that spacecraft when the Pioneer results came in and we had to do a lot of redesign. As the launch date drew near, final preparations were made for Voyager. For any curious extraterrestrials they might meet, the probes carried a disc showing where they had come from and a collection of images from Earth. This was a moment that I had imagined and thought about for years. And there was happening, there was our spacecraft, on the way to Jupiter, and Saturn, Uranus and Neptune. A very marvelous feeling really. This was the world where Voyager was heading. It was nearly 4 centuries since Galileo turned the first telescope on Jupiter, and discovered four points of light moving around the planet. These were four moons. The first proof that not everything in the universe revolves around the Earth. For Voyager's chief scientist, the mission promised a new era of discovery. 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 is just the latest tool which we as a civilization 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. The probe was still 50 million miles from Jupiter when it sent back the views everyone had been waiting for. We all approached Jupiter with great expectation and we all had grandiose theories about what we were going to see. But of course Jupiter fooled us all. There was bizarre behavior. Little clouds moving along, and being swept up in the great red spot, then being... spit them out again. Other clouds would roll along next to one another, coalesce into a single cloud and break apart again. Those kinds of details are not understood, not even now, not 20 years later. The first encounter with Jupiter was a marvelous time for me, especially the approach shots showing the planet revolving, watching the great red spot revolving and getting closer and closer, till finally we could see that indeed this was the top of a large storm. As a child, I'd 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. Voyager revealed an atmosphere of hydrogen and helium gas, whose clouds were much more dynamic than had been imagined. Jupiter's winds gust at hundreds of miles an hour. And the red spot alone is three times the size of the Earth. The greatest storm in the solar system. Voyager hinted at why this should be. Scientists now believe that at the heart of this massive planet the gases are compressed until they become a metallic liquid. This hot churning core could be the powerhouse that drives Jupiter's winds, and, like a dynamo, creates the enormous magnetic field around the planet. Voyager then turned its cameras towards Jupiter's moons. At the time of the encounter, Bruce Murray was head of JPL. Just before the mission, the interest was on the planet, on those bands. The things that you could see through a telescope. There was hardly any interest at all in the satellites of the planets, because they at most were little spots, some of them we couldn't see of course. I had to lead a one-man crusade to have them listed as targets for Voyager when it went there. Scientists had expected the moons of Jupiter to be cold, dead, and covered in craters, like our Moon. What they found was an array of worlds as different and surprising as the planets themselves. Io, the closest of Jupiter's large moons, turned out to be more geologically active than Earth. Jupiter's enormous gravity stretches and squeezes Io, heating it up, so it stays molten inside. We found that Io had eight active volcanoes on it, the most volcanically active body in the solar system and it's just as 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 characterized for us the sense of seeing things that we really hadn't thought about. And that was, in fact, very characteristic of the rest of the mission. As Io orbits close to Jupiter, it is constantly brushing against the planet's magnetic field. The little moon builds up a huge electrical charge, which discharges onto Jupiter in a continuous flow of three million amps, causing storms on the surface of the planet. The rest of it is red, reddish. Underneath this icy crust, scientists believe there are oceans of warm water. Its landscapes of rock and ice reminded Voyager's geologist Laurence Soderblom of Earth. Ganymede turned out to be really exciting. We found a broken surface, complex patterns. It's kind of a cross between ice floes in the Arctic and continental drift on the Earth. Sheared, twisted, and broken, something we didn't expect. The last major moon, Callisto, was different again. Like our moon, it was covered with craters. Its cold icy crust preserved a record of a violent age, when meteorites crashed into its surface. What Voyager found at Jupiter's moons transformed the rest of the mission. The first thing that strikes one is "My Lord, everything is different." So the diversity is overwhelming. Because this is a mission discovery, this is Captain Cook. This is really in the solar system, seeing really new things. ..Well, Io and Europa, there's a twin, a pair there, and there's a pair... What about the relief from the cracks? So I would have thought there'd be enough heating... All the scientists, with the exception of me, were atmospheric scientists and astronomers. And, in fact, it wasn't until we really recognized the exotic variety and diversity of the satellites that geologists were really added to the Voyager team. ...Just rotate it a little bit ... And in fact, the satellites, in my view, became the star of the whole Voyager experience. Jupiter's moons are a solar system in miniature. As Jupiter formed, its immense gravity must have attracted a cloud of dust and gas, from which its moons were born, just as the planets around the sun. Close to Jupiter are small, dense, active worlds, Io and Europa. A mirror of the inner rocky planets, Venus, Earth and Mars. Further out, Ganymede and Callisto are larger, icy worlds, the giants of the Jupiter system. Voyager's next goal was Saturn. For earlier astronomers, Saturn was the last planet in the solar system. The first to observe the rings, Giovanni Cassini, saw a flat disc with just one gap. Scientists hoped Voyager would reveal clues to the origin of these mysterious rings. This time, after Voyager's success at Jupiter, the press and the public were queuing up for a glimpse of the first images of everyone's favorite planet. 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, with gaps, and there were all sorts of dynamical phenomena that we didn't understand. So we very hurriedly reprogrammed Voyager 2, to take a much closer look at the rings. When I began my work in about 1964, I had suggested that one thing we could do with this particular mission was to fly between the planet and the rings. Very fortunately, we didn't do that, because as we approached Saturn we saw that the region there that we would have to have flown through in 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 detail revealed by Voyager 2. They saw delicate rings that were intertwined, and rings that were held in place by tiny moons 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. 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 just, Into one pile I put all those images that seemed to have a lot of spokes in them, into another pile for 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 just 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. Carolyn Porco 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. But where did the rings come from? A possible answer came to light when Voyager encountered Saturn's moons, a collection of icy worlds scarred by great impact craters made by meteorites long ago. The Saturnian system was more like we'd actually expected, small, cold, icy moons which were heavily cratered; a set of objects. But there were some real surprises there as well. The innermost of the large satellites is Mimas. We found large impacts. This crater, called Herschel, is a fourth the size of the object - nearly large enough to blast it apart. We find a similar impact crater on Tethys, the next moon out, and roughly a third the size of the object. So it's clear that in their early history, they were being blasted by things that were large enough to have torn them apart. If Mimas nearly got bashed up to bits, then it's very likely there were other satellites that did get smashed up to bits. And the rings of Saturn probably came from a satellite that was close into the planet, got smashed up; the debris of the collision shards got strewn out into a planetary ring system. Saturn held mysteries of its own. Saturn is smaller and colder than Jupiter. It generates less heat within, and receives less energy from the sun. Yet Voyager recorded even faster winds on Saturn than on Jupiter: a thousand miles an hour. Why this should be was not yet understood. And as Voyager left Saturn, there was one final enigma. Saturn's largest moon, Titan, is the only moon in the solar system with a thick atmosphere. Voyager's cameras could not penetrate the orange haze to see what lay beneath. I found myself alone in the Voyager imaging area, late in the evening, about ten o'clock in the evening. It was just me and the television monitor. And this was the monitor that had showed us all these tremendous pictures that people were gathered around and ... now was just showing the image of Saturn that Voyager I had as it receded from the planet. And I was mesmerized by this whole thing, thinking about how human kind had never seen Saturn from this perspective before, 'cause we had never been on the other side of Saturn before. And I was so moved by me and Saturn alone in this room that I was completely swept away by the whole thing. Voyager's next goal was Uranus, a world discovered just 200 years before and still barely seen. Would this strange planet, tipped over on its back, resemble its great neighbors, Jupiter and Saturn? Even travelling at over 50,000 miles an hour, it would take some five years for Voyager to reach Uranus. The engineers needed every moment to prepare for their most difficult challenge yet. Voyager was planned to operate at one billion miles at Saturn, it was now being asked to operate at two billion at Uranus, where the sun was very dim. We had to do several things, for instance after much longer exposures on the camera. And if you have too long exposure, the spacecraft is moving very rapidly, things become smeared. So we had to learn how to program the spacecraft to turn it just the right rate so that it would compensate for the motion of the spacecraft. But when Voyager reached Uranus, its cameras found little to photograph. We'd been so spoiled by the glamour, and the color and the intricacies of what we saw in the atmospheres of Jupiter and Saturn that Uranus was a little bit of a let-down, 'cause it was so bland. There's more atmosphere and more haze above the clouds, and so it's hard to see the features. Even at its closest approach, Voyager revealed little detail in the atmosphere of Uranus. Uranus is different than Jupiter and Saturn in the sense that it has no internal heat source. That both Jupiter and Saturn are radiating more energy than they receive from the sun, because there's heat inside those planets. For some reason, in Uranus, that heat source had shut down; it was not driving the atmosphere, so the atmosphere was much blander. Voyager had found a very different kind of giant. A world many times smaller and colder than Jupiter and Saturn. It was shrouded in different gases mostly methane and ammonia - under which scientists believed there might lie oceans of water and ice. What exactly is that? Is that in here? You think...? If Uranus had been something of a disappointment, the imaging team found plenty of surprises in the planet's moons. Most striking of all was the tiny moon Miranda. Miranda looks like a 3D jigsaw puzzle, in which 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 coarse pieces stayed intact, and then they were glued back together, so you get this hodgepodge. Perhaps it was just such a collision, on a much grander scale, that knocked Uranus itself over on its back in the earliest days of the solar system. From Uranus onwards to Neptune, some three billion miles away from Earth, the probe would have to take a very precise trajectory over North Pole of the planet, to get the best possible view of Neptune and its large moon Triton. The challenge at Neptune was the most difficult one we'd have. We had to know within one second when we were going to fly over the north pole of Neptune. Now was a major navigational challenge. We had never delivered that kind of accuracy before. And if we were right, it worked. If we were wrong, we had no second chance. After 12 years in flight, Voyager arrived at Neptune. Brad Smith and his team feared, after the bland face of Uranus, that they would see little when they got to the last giant. They need not have worried. ..Is that a rugged surface or what? The final encounter I was able to witness here at JPL was 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, and in this case, white clouds floating in its atmosphere. And these things unfolded before our very eyes. And what a wonderful surprise.


  BBC The Planets where Voyager goals was Jupiter with his 4 moons (Io, Europa, Ganymede, Callisto) and Saturn
 
A mission to the giants called Voyager
A mission to the giants called Voyager
  Jupiter gives out twice as much energy as it receives from the sun
Jupiter gives out twice as much energy as it receives from the sun
  The moon Europa has a surface of water ice, frozen as hard as rock
The moon Europa has a surface of water ice, frozen as hard as rock
  Ganymede is bigger than the planet Mercury
Ganymede is bigger than the planet Mercury
 
Neptune for me was a great surprise. There was something strange and eerie about Neptune because here the last planet, you know, the sentinel at the outer edge of our solar system looks like Earth, with its beautiful deep blue color and its white clouds floating in the atmosphere. We were back with a really exciting planet, again with Neptune. There were fast-moving clouds, clouds moving in different directions, some of them almost at sonic speeds. The complexity of the planet's atmosphere was far beyond our expectations. Neptune turned out to have the strongest winds of all. Here in the furthest extremes of the solar system, where the sun barely penetrates, the last giant defied all expectations. You might expect the further you got from the sun where there was less energy to drive the winds, the winds would be slower. Winds on Jupiter were already hundreds of miles per hour. It turned out rather than seeing slower winds, we found faster winds, and we found winds over 1,000 miles per hour at Neptune. We now understand why that's the case. And that is: If you have enough energy, it creates a lot of turbulence, and that slows the wind down. At Neptune there was so little energy that the wind basically got started and would just go and go and go. Neptune's atmosphere was more dynamic than Uranus, but it was made of the same gases and ices. The last two giants were very different from their more massive cousins. Uranus and Neptune are not gas giants but ice giants. From Earth, nobody had seen a full set of rings at Neptune. Some scientists believed they had seen incomplete segments of a ring, which they called arcs. By the time we got to Neptune, I was leading the small group of individuals on the imaging team; we were responsible for the rings and the ring arcs. In fact, there were some people on the Voyager imaging team who just completely doubted the existence of these things; they thought we were crazy, they thought we were wasting precious spacecraft resources. And so it was gratifying to finally see that one image come down, where we captured, finally caught them in the act, the Neptune ring arcs. So it was a tremendous achievement. Neptune is indeed surrounded by ring arcs.
How they got there and why the ring is incomplete is not yet understood. The impossible mission was almost over. Neptune's moon Triton was the final encounter. Triton is a large moon; it's about the same size as the planet Pluto actually. So if it were out in orbit around the sun we would call it a planet, but it's in orbit around Neptune. But unlike all the other satellites which orbit in the same direction as the planet rotates, Triton is in retrograde orbit, it's going around Neptune backwards, which told us it was not likely formed around Neptune, but had been captured by Neptune. Triton is a moon that might have been a planet. It strayed too close to Neptune and was caught by its gravity. Triton was one of the strangest worlds encountered. This is too much. This is too much to believe. Oh-ho. God. Look at the tire tracks. Tyre tracks! Right. There you go. Triton was a world unlike any we had seen before. It was the coldest surface we had seen in the solar system, 40 degrees above absolute zero. 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 had survived to reach the extremes of the solar system and reveal not just the giants themselves, but whole systems of rings and moons unlike anything imagined. As the planets moved out of their alignment, Uranus and Neptune once again drifted out of our reach. It's unlikely that they'll be visited again in our lifetime. But Voyager was not the last mission to the gas giants. In 1994, a probe named Galileo returned to Jupiter and its moons. It found that Io's surface had been covered by fresh eruptions of sulphurous lava. Europa, which had looked so smooth, turned out to be covered in great ridges and chasms. Fresh detail was revealed in Ganymede's alien landscapes. And there was more destruction than had been imagined on Callisto. In October 1997, a mission called Cassini set off for Saturn. It would spend four years sending back high-resolution images of the great planet, its rings and its many satellites. Head of the imaging team is Carolyn Porco. We are interested, for scientific purposes, in taking images of Titan and all the satellites, too. So how many images do you think will you need in that block? Four is what we normally thought about. Four images? In 30 minutes? OK. We have designed the camera system specifically, among many other things; we've decided to see down to the surface of Titan, which of course is something we weren't able to do on Voyager. And we'll be able to see things that are on the scale of office blocks. We'll be collecting data from Cassini at least for a period of four years, so we'll have a chance to monitor changes and, you know, it'll be a new era. But it will never have the same feeling or even... even significance, historical significance as Voyager had because that experience can never be duplicated... The next awards are the Exceptional Scientific Achievement Medal. The first award goes to Gary Flandro, for seminal contributions to the design and engineering of missions, including the grand tour opportunity for the epic Voyager explorations. We got it started right in that office. Is that right? Building 180 is where you worked? I sat looking out that window through those trees up towards the sun, and said "We could be there. Four planets in one flight." With the planets behind it, Voyager carried on searching for the edge of the solar system, where the sun's influence runs out, and interstellar space begins. The Voyager mission is not over. We hope we can continue listening to it for at least another 20 years before we finally lose power on the spacecraft. In 2015, Voyager I will be 130 times as far from the sun as the Earth, about 12 billion miles from the Earth, and perhaps in interstellar space for the first time. So we listen to the two Voyager spacecraft every day, looking for some signal we're getting close to interstellar space. The greatest voyager in history is still travelling.