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Planetary News: Voyager (2006)Voyager 1 Sailing Past 100 AU en route to Interstellar SpaceBy A.J.S. RaylAugust 15, 2006
Voyager 1 logs yet another milestone in space history August 17 when it crosses an invisible boundary that marks 100 astronomical units (AU) from the Sun -- about 15 billion kilometers (9.3 billion miles) out there -- farther away than any human-made object has ever gone in space. It's headed now for interstellar space. Voyager 2, at 80 AU, is about six years behind. Nearly 30 years after the twin Voyager spacecraft took off from Cape Canaveral, the mission has become a legend in its own time, rewriting the planetary science books, and introducing us to our own diverse neighborhood. The twins, meanwhile, have become the poster children of space exploration, still communicating after all these years and sending data home regularly via the Deep Space Network. “One of our objectives was to explore interstellar space, and following the successful Saturn flyby in 1981, the mission was renamed the Voyager Uranus Interstellar Mission," said Voyager Chief Scientist Ed Stone, professor of physics and director of the Space Radiation Laboratory at the California Institute of Technology (Caltech), and a former director of the Jet Propulsion Laboratory (JPL), where the mission was designed and is being managed. "This was a boldly optimistic goal because we knew neither how long it would take to reach interstellar space nor how long the spacecraft would continue operating beyond their original 4-year mission to Saturn which is only 10 AU from the Sun. It is indeed remarkable that the Voyager spacecraft have already operated 7 times longer and 10 times further from the Sun than originally planned," he said. "With some luck, the two will reach interstellar space while they still have electrical power," continued Stone, who has been the project scientist on the mission since 1972, overseeing the efforts of 11 teams of scientists in their studies of Jupiter, Saturn, Uranus, and Neptune. "Whether they reach interstellar space or not under power, they will be -- and [Isaac] Newton tells us this -- humankind's first interstellar probes, the first objects launched from Earth to reach interstellar space. Crossing into interstellar space will be a major milestone in our journey from Earth into the Milky Way."
Voyager 1 and Voyager 2 took off from Earth in 1977 on a Grand Tour, flying by every outer planet but Pluto. "Many of us hoped back then that we would reach interstellar space, but the space age itself was only 20 years old," Stone remembered. "We had no way of knowing if the spacecraft could last this long, or whether they would be able to communicate and operate so far from the Sun, and we had no idea how long the journey would be." They still don't know just how far Voyager 1 or 2 will have to go to reach interstellar space, but they know they're getting closer. And in recent months, the two venerable spacecraft have returned data loaded with hints of what's really out there. The Grand TourThe Voyager mission blazed the trail to the outer planets -- Jupiter, Saturn, Uranus, and Neptune, introducing us to the multi-ethnic beauty of our neighborhood. "Voyager revealed the complex diversity of the solar system, and that is really the most important thing we learned from the Grand Tour," Stone said. "Each planet is unique and that's because they've had a different history, a different evolution," Stone continued, as he considered some of the highlights. "Jupiter, with its great red spot, features dozens more giant hurricane-like storm systems. And the Jovian moon, Io, has 100 times more volcanic activity than Earth, while another Jovian moon, Europa, has an ice crust probably over a liquid water ocean. At Saturn, we found that the rings are riddled with wakes from moons, which orbit both inside and outside the rings. One of Saturn's moons, Enceladus, is the whitest, brightest object in the solar system and has a very fresh surface. And then, there's the moon Titan, which has a thick atmosphere in which liquid natural gas rains on the surface. At Uranus, we discovered that the magnetic pole is all the way down near its equator, and we found a moon, which was only 300 miles across, but featured one of the most complex surfaces we've yet seen. Neptune, the furthest planet from the Sun that we've visited, has the fastest winds, strangely, with the least energy from the Sun to drive them, and the surface of its moon Triton is only 40 degrees above absolute zero, yet we found geysers erupting from its polar ice cap." In addition to the wealth of planetary findings and an ahhh-some treasure trove of images, Voyager created the opportunity for Galileo to return to Jupiter to explore the gas giant more closely from orbit, and Cassini to return to Saturn, where it dropped the Huygens probe into Titan's, atmosphere, and is currently in orbit studying the Ringed Planet. Through it all, Voyager 1 and Voyager 2 sailed on.
With the Grand Tour complete in 1990, the Voyagers headed toward the far reaches of the solar system. At the urging of Carl Sagan, co-founder of The Planetary Society, Voyager 1 turned its camera back inward toward the slowly receding Sun and snapped one last set of images of the planets, a sort of family portrait, before its cameras were shut down. But the image of Earth, a tiny, tiny, pale blue dot, is the one that draws us in and compels us. "That's home. That's us," Sagan wrote on seeing the image. "On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives." It was a "postcard" home that has no equal. Each Voyager carries it own kind of "postcards" in the form of a golden record that Sagan was instrumental in creating. The 12-inch, gold-plated copper discs carry spoken greetings in 55 languages from people all around Earth, along with 115 images and hundreds of sounds representing our home planet. As the Voyagers journey out into the Milky Way, members of another spacefaring civilization that may be out there might some day come across the lonely mechanical travelers -- and perhaps humanity will still inhabit the Earth should they decide to seek out the "sender." The Termination Shock and BeyondIn December 2004, Voyager 1 crossed the Termination Shock, the last major signpost before interstellar space. By the following spring, it was officially confirmed. All the anticipated signs were there. "We know Voyager 1 crossed the Shock, because the magnetic field, which is carried by the wind from the Sun, was compressed along with the wind itself as the wind abruptly slowed down. We saw the magnetic field intensity go up by a factor of three," Stone explained. In early 2005, however, NASA considered pulling the plug on the mission, because of budgetary cuts in the agency's science program. But the mission costs only about $5 million annually to keep online and with public uproar looming, agency officials apparently thought better of the ill-made decision. They scheduled a review -- in which Voyager passed at the top of its class -- and managed in short order to find the money to keep the mission alive. Since then, both Voyager 1 and Voyager 2 have continued returning good data from the nether regions of our solar system.
How is it that the spacecraft built with early 1970s technology can still be functioning all the way out there? The simple answer is -- excellent engineering and a good, solid power source. "The Voyager spacecraft are powered by radioisotope thermoelectric generators, that feature a plutonium 238 heat source, which is converted to electricity via thermocouples," expounded Stone. "It's a very simple, robust power supply, which should last us until at least 2020." At that point, Voyager 1 will be at 150 AU, and he added, "could have flown well into interstellar space." Currently, Voyager 1 is cruising at about 61,000 kilometers (38,000 miles) per hour, traveling nearly 1 million miles per day through the heliosheath, the boundary region at the far edge of the solar system where the interstellar wind meets the heliosphere, In essence, the heliosheath is the outermost layer of the heliosphere or "bubble" formed by the solar wind blowing one million miles per hour radiating outward in all directions. Hence, it can be considered an outer boundary region of our solar system. The heliosphere, however, is not so much a sphere. In fact, it’s shaped more like a comet, because it is moving through space. “Since the heliosphere is created by the supersonic wind, as the wind approaches contact with interstellar wind, it must abruptly slow down and turn back around and head down the tail,” Stone explained. “Where it abruptly slows down, there is a standing shock wave, which we call the Termination Shock. It's like a supersonic shock wave just like in front of a supersonic aircraft." If you want to visualize the Shock for yourself, go to your kitchen sink, said Stone, turn on the water, and watch what happens when the water hits the sink bottom. It flows out generally in a circular manner, with a thick ring or bump forming at the edge. That thick ring of water is, in essence, analogous to the heliosheath, with its inner edge corresponding to the Termination Shock.
"Voyager 1 has crossed that Termination Shock for the first time and is in the heliosheath where the wind has slowed down and is turning around to head down the tail of the heliosphere," Stone continued. Since the Shock is fluid and ebbs and flows much like the waves of an ocean, the Shock “breathes” in and out as the solar wind varies over the 11-year cycle of solar activity. As solar activity subsides over the next few years, the Shock should move inward toward Voyager 2. The outer edge of the heliosheath is the heliopause or surface of the heliosphere. "That is the edge of interstellar space," Stone pointed out. "While there are some interstellar dust particles and interstellar atoms that drift into the heliosphere -- in fact the Stardust spacecraft captured some of that dust in 2004 -- essentially all the material inside the heliosphere is coming from the Sun. That's where all the planets are, that's where all the spacecraft have been, that's where Voyager 1 still is. The edge of that bubble is the edge of the solar system,” he explained. “Beyond the heliopause or bubble is interstellar space and Voyager 1 will eventually exit this bubble and enter interstellar space where it will be surrounded by material from other stars." Voyager 2 will join its twin in exploring the heliosheath as soon as it crosses the Termination Shock, which, Stone said, it could do "anytime now." The New World of Interstellar SpaceAlthough we have some ideas of what the outer heliosphere and interstellar space are like, Voyager 1 and Voyager 2 are the pioneers that are giving us the first glimpse in the data they're sending home, data that challenges our understanding and is forcing the scientists to come up with new ideas and explanations for what they see as the spacecraft instruments take in what’s out there.
Until recently, most models of the heliosphere assumed that the distance to the Termination Shock was the same in the northern and southern hemispheres. Last March, Merav Opher, assistant professor of physics and astronomy at George Mason University, and her colleagues showed that an interstellar magnetic field could press the Shock closer to the Sun in the south. At the meeting of the American Geophysical Union (AGU) last May, the Voyager teams reported that the Shock is closer in the south than in the north. “Since the heliosphere is a comet-shaped object, it has a blunt nose, and it turns out that Voyager 1 is in the northern hemisphere of that blunt nose, and Voyager 2 is in the southern hemisphere," Stone said. "The big surprise is that Voyager 2 has found that the Shock, where the wind slows down, is about 9 astronomical units, about 900 million miles closer to the Sun than it is in the north. That is, the front nose of the heliosphere is actually pushed in and is distorted in the south by, evidently, an interstellar magnetic field which is pressing against it preferentially in the south." Because of that, he pointed out, "Voyager 2 may reach interstellar space sooner than expected." One thing the team has discovered recently with Voyager 1 is that the magnetic field in the heliosheath has "potholes" and "bumps" in it. "Unlike the supersonic wind, when the wind slows down, holes appear in the magnetic field where the field almost goes away, but at other times the field is stronger than average," informed Stone. "So it's a very bumpy magnetic field in the heliosheath.” Earth logic would incline one to believe that means Voyager 1 is being tossed and turned out there, but it's not. "They would be buffeted around, except that this is a better vacuum than any here on Earth, so the spacecraft itself is totally unaware of any of this," Stone explained. "Only our sensitive instruments can detect the bumps and potholes in the magnetic field." In contradiction to many predictions that the Termination Shock was the source of medium energy anomalous cosmic rays, their intensity did not peak there as expected. Their intensity has, however, increased with Voyagers 1's increasing distance beyond the Termination Shock, indicating their origin remains to be discovered. However, the Shock is a strong source of low energy ions, leading the University of Michigan's Len Fisk, a member of the National Academy of Sciences and professor of planetary science, and George Gloeckler, professor in the department of Atmospheric, Oceanic and Space Sciences, to suggest that the Shock energizes particles in a different way than anybody ever thought. Generally, physicists theorized that the particles at the Shock bounce back and forth from one side to the other like a ping-pong ball and slowly, but surely, gain energy over the period of a year or so until they become anomalous cosmic rays. Fisk and Gloeckler, however, are now theorizing that at lower energies the particles are already there in the solar wind that carries them into the Termination Shock where they are compressed and heated. “These are the sorts of things that mean the field is now grappling with new ideas,” said Stone. The Journey ContinuesWhen Voyager 1 and 2 do cross into interstellar space, the events will be somewhat dramatic. "I expect that the interface, that contact surface between the solar wind and interstellar wind, will be quite turbulent and we will find a period of turbulence before Voyager finally reaches steady interstellar conditions," said Stone. As Voyager 1 does make the crossing, it will find an interstellar magnetic field that is likely stronger than the heliosheath magnetic field. "When that happens," he noted, "we will know we've crossed into interstellar space."
Since scientists don't know exactly how large this heliosheath region is, no one knows how long it will take Voyager 1 or 2 to actually cross the much-anticipated threshold. "It may be about 10 years more before Voyager 1 reaches the edge of interstellar space," projected Stone. But once that happens, it may not be all that long before Voyager 2 follows suit. "Because the interstellar magnetic field is pushing the heliosphere inward in the south where Voyager 2 is, it may reach interstellar space at about the same time, even though it's closer to the Sun than Voyager 1," he adds. That is one of the new discoveries." The mission has more than 6.4 billion kilometers (4 billlion miles) to go before the robot spacefarers go silent, and the team is obviously anxious to find out what's outside. "That's because what is outside has come from supernovas, the explosion of large stars, 5 to 10 million years ago. And for the first time, we will be embedded in material and winds from other stars and magnetic fields that have come from other objects in the galaxy. So it would be our first direct measure of what's outside, pressing inward on this bubble in which we all live.” Over the years, the world witnessed no end of historical events down on Earth -- the Industrial Age gave way to the Information Age . . . Star Wars put the magic back in movies . . . the Space Shuttle took flight . . . an actor became President . . . a new disease called AIDS appeared out of nowhere . . . the worst disaster in the history of nuclear power took place at Chernobyl. . . the Berlin Wall came smashing down . . . the Cold War ended . . . two Beatles died . . . CDs replaced vinyl record albums and music downloads threatened to replace CDs . . . hippies turned grey and Gen X came of age . . . cell phones became all the rage . . . personal computers and the World Wide Web shrunk the world to a global village . . . thousands of protesters died at the hands of the Chinese government in Tiananmen Square . . . Americans went to war in the Middle East . . . console wars broke out in the world of gaming . . . terrorists took down the World Trade Center. . . America invaded Iraq over weapons of mass destruction that didn't exist . . . the Europeans went to Mars, the Chinese sent a man into space, and NASA put 2 rovers on Mars that are still roving 2.5 years later . . . the Boston Red Sox won a World Series . . . and global warming became obvious. Through it all, Voyager 1 and Voyager 2 have sailed onward, boldly going where no spacecraft have ever gone before. "It's really been a wonderful journey of a lifetime, there's no question about that, and all of us on Voyager have really been very fortunate to have been on the frontier of discovery for so many years," Stone reflected. "And here it is, almost 30 years after launch, and we're still on the frontier of discovery. I don’t know what surprises await us, but crossing the edge of interstellar space will be an historic first step into the Milky Way and lead the way for future generations of robotic explorers. For a scientist, this is really a wonderful, wonderful mission.” For the rest of humanity, Voyager continues to be a space adventure like no other. |
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