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<\/p>\nHoliday Reprise: Voyager to a Star
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As I write, Voyager 1 is almost 166 AU from the Sun, moving at 17 kilometers per second. With its Voyager 2 counterpart, the mission represents the first spacecraft to operate in interstellar space, continuing to send data with the help of skilled juggling of onboard systems not deemed essential. Despite communications glitches, the mission continues, and it seems a good time to reprise a piece I wrote on the future of these doughty explorers back in 2015. Is there still time to do something new with the two probes once the demise of their plutonium power sources makes further communications impossible? The idea is hardly mine, and goes back to the Sagan era, as the article below explains. It\u2019s also a notion that is purely symbolic, and for those immune to symbolism (the more practical-minded among us) it may seem trivial. But those with a poet\u2019s eye may see the value of an act that can offer a futuristic finish to a mission that passed all expectations and will inspire generations yet unborn.<\/em><\/p>\n After Voyager 2 flew past Neptune in 1989, much of the world assumed that the story was over, for there were no further planetary encounters possible. But science was not through with the Voyagers then, and it is not through with the Voyagers now. In one sense, they have become a testbed for showing us how long a spacecraft can continue to operate. In a richer sense, they illustrate how an adaptive and curious species can offer future generations the gift of \u2018deep time,\u2019 taking its instruments forward into multi-generational missions of interstellar scope.<\/p>\n Now approximately 24 billion kilometers from Earth, Voyager 1, which took a much different trajectory than its counterpart by leaving the ecliptic due to its encounter with Saturn\u2019s moon Titan, is 166 times as far from the Sun as the Earth (166 AU). Round trip radio time is over 46 hours. The craft has left the heliosphere, a \u2018bubble\u2019 that is puffed up and shaped by the stream of particles from the Sun called the \u2018solar wind.\u2019 Voyager 1 has become our first interstellar spacecraft, and it will keep transmitting until about 2025, perhaps longer. Voyager 2, its twin, is currently 138 AU out \u2014 20.7 billion kilometers from the Sun \u2014 with a round-trip radio time of 38 hours.<\/p>\n Throughout history we have filled in the dark places in our knowledge with the products of our imagination, gradually ceding these visions to reality as expeditions crossed oceans and new lands came into view. The Greek historian Plutarch comments that \u201cgeographers\u2026 crowd into the edges of their maps parts of the world which they do not know about, adding notes in the margin to the effect, that beyond this lies nothing but sandy deserts full of wild beasts, unapproachable bogs, Scythian ice, or a frozen sea\u2026\u201d But deserts get crossed, first by individuals, then by caravans, and frozen seas yield to the explorer with dog-sled and ice-axe.<\/p>\n Voyager and the Long Result Space is stuffed our imaginings, and despite our telescopes, what we find as we explore continues to surprise us. Voyager showed us unexpected live volcanoes on Jupiter\u2019s moon Io and the billiard ball-smooth surface of Europa, one that seems to conceal an internal ocean. We saw an icy Enceladus, now known to spew geysers, and a smog-shrouded Titan. We found ice volcanoes on Neptune\u2019s moon Triton and a Uranian moon \u2014 Miranda \u2014 with a geologically tortured surface and a cliff that is the highest known in the Solar System.<\/p>\n But the Voyagers are likewise an encounter with time. The issue raises its head because we are still communicating with spacecraft launched almost forty years ago. I doubt many would have placed a wager on the survival of electronics and internal mechanisms to this point, but these are the very issues raised by our explorations, for we still have trouble pushing any payload up to speeds equalling Voyager 1\u2019s 17.1 kilometers per second. To explore the outer Solar System, and indeed to travel beyond it, is to create journeys measured in decades. With the Voyagers as an example, we may one day learn to harden and upgrade our craft for millennial journeys.<\/p>\n New Horizons took nine years to reach Pluto and its large moon Charon. To reach another star? An unthinkable 70,000 years-plus at Voyager 1 speeds, which is why the propulsion problem looms large as we think about dedicated missions beyond the Solar System. If light itself takes over 23 hours to reach Voyager 1, the nearest star, Proxima Centauri, is a numbing 4.2 light years away. To travel at even a paltry one percent of lightspeed, far beyond our capabilities today, would mean a journey to Proxima Centauri lasting well over four centuries.<\/p>\n What is possible near-term? Ralph McNutt, a veteran aerospace designer at the Johns Hopkins Applied Physics Laboratory, has proposed systems that could take a probe to 1000 AU in less than fifty years, giving us the chance to study the Oort Cloud of comets at what may be its inner edge. Now imagine that system ramped up ten times faster, perhaps boosted by a close pass by the Sun and a coordinated shove from a next-generation engine. Now we can anticipate a probe that could reach the Alpha Centauri stars in about 1400 years. Time begins to curl back on itself \u2014 we are talking trip times as great as the distance between the fall of Rome and today. <\/p>\n The interesting star Epsilon Eridani, some 10.5 light years out, would be within our reach in something over three thousand years. Go back that far in human history and you would see Sumerian ziggurats whose star maps faced the sky, as our ancestors confronted the unknown with imagined constellations and traced their destinies through star-based prognostications. The human impulse to explain seems universal, as is the pushing back of frontiers. And if these travel times seem preposterous, they\u2019re worth dwelling on, because they help us see where we are with space technology today, and where we\u2019ll need to be to reach the stars.<\/p>\n A certain humility settles in. While we work to improve propulsion systems, ever mindful that breakthroughs can happen in ways that no one expects, we also have to look at the practicalities of long-haul spaceflight. Both Voyagers have become early test cases in how long a spacecraft can last. They also force us to consider how things last in our own civilization. We have buildings on Earth \u2014 the Hagia Sophia in Constantinople, the Pantheon in Rome \u2014 that have been maintained for longer than the above Alpha Centauri flight time. A so-called \u2018generation ship,\u2019 with crew living and dying aboard the craft, may one day make the journey.<\/p>\n Engagement with deep time is not solely a matter of technology. In the world of business and commerce, our planet boasts abundant examples of companies that have been handed down for centuries within the same family. Construction firm Kongo Gumi, for example, was founded in Osaka in 578, and ended business activity only in 2007, being operated at the end by the 40th generation of the family involved. The Buddhist Shitennoji Temple and many other well known buildings in Japanese history owe much to this ancient firm.<\/p>\n The Japanese experience is instructive. Hoshi Ryokan is an innkeeping company founded in Komatsu in 718 and now operated by the family\u2019s 46th generation. If you\u2019re ever in Komatsu, you can go to a hotel that has been doing business on the site ever since. Nor do we have to stay in Japan. Fonderia Pontificia Marinelli has been making bells in Agnore, Italy since the year 1000, while the firm of Richard de Bas, founded in 1326, continues to make paper in Amvert d\u2019Auvergne, providing its products for the likes of Braque and Picasso.<\/p>\n Making Missions that Last<\/strong><\/p>\n We have long-term thinking in our genes, as the planners of the Pyramids must have assumed. The Long Now Foundation, which studies issues relating to trans-generational thinking and the long-term survival of artifacts, has pointed out that computer code has its own kind of longevity. Enduring like the Sphinx, deeply planted software tools like the Unix kernel may well be operational a thousand years from now. Jon Lomberg and the team behind the One Earth Message \u2014 an attempt to transmit a kind of digital \u2018Golden Record\u2019 to the New Horizons spacecraft as a catalog of the human condition \u2014 estimate that the encoded data will survive at least one hundred thousand years, and perhaps up to a million if given sufficient redundancy. <\/p>\n \u2018Deep time\u2019 takes us well beyond quarterly stock reports, and even beyond generational boundaries, an odd place to be for a culture that thrives on the slickly fashionable. It\u2019s energizing to know that there is a superstructure that persists. The Voyagers are uniquely capable of keeping this fact in front of us because we see them defying the odds and surviving. Stamatios \u201cTom\u201d Krimigis (JHU\/APL) is on record as saying of the Voyager mission \u201cI suspect it\u2019s going to outlast me.\u201d <\/p>\n Krimigis is one of the principal investigators on the Voyager mission and the only remaining original member of the instrument team. His work involves instruments that can measure the flow of charged particles. Such instruments \u2014 low-energy charged particle (LECP) detectors \u2014 report on the flow of ions, electrons and other charged particles from the solar wind, but because they demanded a 360-degree view, they posed a problem. Voyager had to keep its antenna pointed at the Earth at all times, so the spacecraft couldn\u2019t turn. This meant that the tools needed included an electric motor and a swivel mechanism that could swing back and forth for decades without seizing up in the cold vacuum of space. <\/p>\n The solution was offered by a California company called Schaeffer Magnetics. Krimigis\u2019 team tested the contractor\u2019s four-pound motor, ball bearings and dry lubricant. The company ran the motorized system through half a million \u2018steps\u2019 without failure. The instruments are still working, still detecting a particle flow that is evidently a mix of solar and interstellar particles, one that is moving in a flow perpendicular to the spacecraft\u2019s direction of travel, so that it appears we\u2019re just over the edge into interstellar space, a place where the medium is roiled and frothy, like ocean currents meeting each other and rebounding.<\/p>\n One Last Burn<\/strong><\/p>\n Although the spacecraft are expected to keep transmitting for several more years, we\u2019ll continue to see both Voyagers suffering from power issues. But there is a way to keep them alive, if not in equipment then as a part of our lore and our philosophy. They will take about 30,000 years to reach the outer edge of the Oort Cloud (the inner edge, according to current estimates, is maybe 300 years away). Add another 10,000 years and Voyager 1 passes some 100,000 AU past the red dwarf Gliese 445, which happens to be moving toward the Sun and will, by this remote date, be one of the closest stars to the Solar System. As to Voyager 2, it will pass 111,000 AU from Ross 248 in roughly the same time-frame, at which point the red dwarf will actually be the closest star to the Sun. <\/p>\n Carl Sagan and the team working on the Voyager Golden Record wondered whether something could be done about the fact that neither Voyager was headed for another Solar System. Is it possible that toward the end of the Voyagers\u2019 active lifetimes (somewhere in the 2020s), we could set up a trajectory change that would eventually lead Voyager as close as possible to one of these stars? Enough hydrazine is available on each craft that, just before we lose radio contact with them forever, we could give them a final, tank-emptying burn. Tens of thousands of years later, the ancient craft, blind, mute but still more or less intact, would drift in the general vicinity of a star whose inhabitants, if any, might find them and wonder.<\/p>\n A trajectory change would increase only infinitesimally the faint chance that one of these spacecraft would someday be intercepted by another civilization, and neither could return data. But there is something grand in symbolic gestures, magic in the idea that these venerable machines might one day be warmed, however faintly, by the light of another sun. Our spacecraft are our emissaries and the manifestations of our dreams. How we conceive of them through the information they carry helps us gain perspective on ourselves, and shapes the context of our future explorations. Giving the Voyagers one last, hard shove toward a star would speak volumes about our values as a questioning species determined to confront the unknown.<\/p>\n<\/p>\n
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