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Planetary News: Voyager (2007)Voyager 2 Finds Solar System "Dented" as it Crosses Termination Shock on Way to Deep SpaceBy A.J.S. RaylDecember 11, 2007
Voyager 2 has followed Voyager 1 into the heliosheath, a vast region the far edge of our solar system, and surprised its team with new discoveries, Voyager scientists reported yesterday at the annual fall meeting of the American Geophysical Union (AGU) meeting being held in San Francisco. Now both of the legendary spacecraft are headed for the real Final Frontier -- interstellar space. “Voyager 2 crossed the termination shock at least five times as it entered the outermost layer of the giant heliospheric bubble surrounding the Sun and joined Voyager 1 in the last leg of the race to interstellar space,” announced Voyager Project Scientist Edward C. Stone, of the California Institute of Technology Caltech, who has been with the mission since the beginning, or, for more than half his life. Since both Voyager spacecraft are traveling about a million miles a day through this realm where Sun's influence begins to 'fade to the blackness' of space, the current models predict that Voyager 1 will cross into interstellar space in "about seven to 10 years," Stone told The Planetary Society. "Voyager 2 will follow Voyager 1 out of the heliosheath and into interstellar space about three years later" or in 2017-2020, he said. Unlike Voyager 1, which crossed the termination shock in December 2004 and is leaving the solar system rising above the ecliptic plane, at an angle of about 35 degrees, at a rate of about 520 million kilometers (about 320 million miles) a year, Voyager 2 took a different path into this boundary region. Diving below the ecliptic plane at an angle of about 48 degrees Voyager 2 cruised into the area where the solar wind hits the thin gas between the stars, at a rate of about 470 million kilometers (about 290 million miles) a year. The path less traveled in this case revealed something Voyager scientists had been suspecting for a while now -– that that our solar system is asymmetrical or "squashed," that it is dynamic, changing shape and structure as it moves through space, and not a static sphere as some theories long held. What all this means is -- we -- everything on Earth and in our solar system -- resides in a shape-shifting bubble that zips through interstellar space. The Voyagers are about to break on through the bubble -- to the other side. Since the solar wind, the thin gas of electrically charged particles or plasma blown into space by the Sun, spreads in all directions at 1-2 million miles per hour, it actually creates this "bubble" that extends past the orbit of Pluto. The bubble is the heliosphere and its outermost area, the last boundary to interstellar space, is the heliosheath.
Voyager 1 is the first spacecraft to explore the heliosheath. As it made its historic passage three years ago, it encountered the shock wave that scientists now know surrounds our solar system. Scientifically known as the solar wind termination shock, or termination shock for short, this is the area where the solar wind is abruptly slowed by pressure from the gas and magnetic field in interstellar space. "The solar wind is supersonic -- it blows radiation outward, but before it can contact interstellar space, it has to go through a supersonic shock [kind of like a sonic boom] and abruptly slow down, so the wind can turn around and go down the comet-like tail that's created out of the heliosphere by the interstellar wind," Stone explained to reporters. "The solar wind is moving at one million miles per hour and suddenly slows down. All that energy has to go somewhere. It didn't go into heating the wind itself, but into accelerating these cosmic ray particles," he expounded. "So we have this bubble around the Sun filled with matter from our Sun, and outside the bubble and outside the heliopause, where matter has come from other stars." Since Voyager 2 crossed the shock on August 30, 2007, around 20 billion kilometers (10 billion miles) away from Voyager 1 and almost 2 billion kilometers (1 billion miles) closer to the Sun, it confirmed that the termination shock along the southern hemisphere at least is closer to the Sun than model indicated, meaning, basically, our solar system – the heliosphere -- is asymmetrical or "squashed." "With Voyager 2, we crossed the termination shock 84 astronomical units (AU) about 7.8 billion miles from the Sun -- about 1 billion miles closer to the Sun than Voyager 1 crossed it," Stone informed. "There's something outside pushing in on southern hemisphere of heliosphere." They think it's "an interstellar magnetic field that is distorting the otherwise more or less symmetrical sphere," created by the Sun's energy. "Once we get there, we'll measure it directly, but we can infer it now with our models and we will now be able to refine that model."
Even though Voyager 2 is the second spacecraft to cross the shock, its accomplishment marks a scientific milestone on a couple of other fronts. With this Voyager, scientists were able to directly measure the solar wind, because its plasma science instrument, which directly measures the velocity, density and temperature of the solar wind, is working, noted John D. Richardson, principal investigator for plasma science, of the Massachusetts Institute of Technology (MIT). Since the same instrument on Voyager 1 was and is not working, estimates of the solar wind speed were made indirectly or inferred and those estimates indicated that the spacecraft had only a single shock crossing, which occurred during a gap in data. Voyager 2 crossed the shock multiple times, at least five times over a couple of days. "The shock is turbulent," said Stone. "It's a little bit like a tide coming in. The spacecraft crossed it, then it receded, and we crossed it five times, and maybe more," he explained. Better still, three of the five crossings detected are clearly evident in the data. "This is the first time we have direct observations of the shock," Stone said. And what a weird shock it seems to be. "The shock didn't stay the same and in fact over a period of hours it changed drastically," reported Leonard F. Burlaga, of NASA Goddard, the principal investigator of the magnetometer instrument on the spacecraft. In a normal shock wave, fast-moving material slows down and forms a denser, hotter region as it encounters an obstacle. Voyager 2, however, found a much lower temperature beyond the shock than was predicted. This probably indicates that the energy is being transferred -- not to the solar wind -- but to cosmic ray particles that were accelerated to high speeds at the shock. "It's sort of like a cosmic ping-pong game," Stone offered. "Some of the ions are reflected back and forth across the shock and then become cosmic rays. Over a period of year or so, these ions can gain enough energy to have 10% the speed of light. It's a cosmic ray accelerator." As for how much energy goes into that accelerator, "it turns out a fair fraction of solar wind ends up in these ions that are ping-ponged back and forth across the shock," Stone said. "We expected to find ions from solar wind would be at a million degrees, instead we them at 200,000 degrees," he pointed out. That missing thermal energy most likely went into accelerated particles, as showed at the press conference by Robert B. Decker, of the Applied Physics Lab at Johns Hopkins University, the principal investigator of the low-energy charged particle instrument. What they did not find and expected to find in the nose of the heliosphere, where the Voyagers crossed the shock crossed was a/the source of these cosmic rays "It's somewhere else along the shock, not near nose region where Voyager 1 and Voyager 2 crossed, either further out or back on in the flanks," said Stone.
The new data on the termination shock are still being pondered. But with the new data come new questions and so much more for the Voyagers to learn as they sail on through the heliosheath and into interstellar space. "One thing they do know now is that the heliosheath, which is a rather thick outer layer where the wind is hot and slow as described above, is "a very important barrier against the entry of low energy galactic cosmic rays," said Stone, so it provides a shield to reduce irradiation that gets in from outside." This boundary region, then, is an important part of our greater or solar system shield. "By better understanding its properties and characteristics, we'll better understand how that might change over time, because this bubble will change as the interstellar medium outside changes." Our solar system is now in a cloud caused by the explosion of a supernova 10-20 millions years ago. "When we exit that cloud, the external conditions will change and the size of the heliosphere will change, and the amount of shielding the heliosphere provides will change, so on a reasonable timescale, it affects radiation even deep inside the heliosphere, even at Earth," Stone explained. It was 30 years ago last summer that the twin Voyager spacecraft took off from Cape Canaveral. Between 1979 and 1989 Voyager 1 and Voyager 2 flew by Jupiter, Saturn, Uranus and Neptune and since then has become the planetary mission by which all others are measured. That the spacecraft have survived is something that still has people marveling. JPL engineers designed the hardware for the long haul, installing a system that would allow for enhanced remote control programming, allowing the spacecraft even greater capability than they possessed when they left Earth if they did fly farther across the solar system. As the mission’s first project manager Harris ‘Bud’ Schurmeier, has so often noted over the decades: “The versatility we built in the Voyagers really paid off." "In 1977 when these spacecraft launched, the space age was only 20 years old, so there was no basis of experience to know that we could build anything that could last so long or go so far from the Sun," noted Stone. "But that doesn't mean we weren't planning for it." The Voyager team selected launch dates that would allow Voyager 2 to fly by Saturn on a trajectory that would swing it off toward Uranus and Neptune, if Voyager 1 succeeded at both Jupiter and Saturn. With the Grand Tour complete in 1990, the Voyagers headed toward the far reaches of the solar system. "We planned a stepwise mission, Stone continued. "Since then, about every 3 years we go through a review process with NASA which determines whether to continue operating Voyager." In 2002, Voyager 1 and Voyager 2 celebrated their 25th anniversary continuing the journey that is taking them where no spacecraft has gone before Both Voyager 1 and Voyager 2 have continued phoning and sending data Home nearly every single day, communicating via the Deep Space Network, operating now on a lean $4.0-4.5 million annual budget.
The mission became a legend in its own time, rewriting the planetary science books, and introducing us to our own diverse, complex, and beautiful neighborhood. Despite its achievements and accolades, a little more than two years ago, not long after Voyager 1 crossed the termination shock, NASA was going to pull the plug and shut down all operations in order to cut costs for President Bush's new Vision for Space Exploration. The Planetary Society campaigned to help agency officials to see the error of that plan. The idea of turning off these veteran spacecraft as they were beginning to actually head into interstellar space seemed unconscionable given the no frills annual budget, a proposal that was simply not thought through. When the ramifications of turning off two of the most successful robotic explorers in history were realized, the funding to keep Voyager alive was found. The two Voyager spacecraft will be the only source of local observations of this distant, highly interesting region for years to come. Next summer, NASA will be launching a mission specifically designed to globally image the termination shock and heliosheath remotely from Earth orbit. The Interstellar Boundary Explorer (IBEX), led by David McComas of the Southwest Research Institute in San Antonio, will use energetic neutral atoms to create all-sky maps at various energies of the interaction of the heliosphere with interstellar space. IBEX will detect some of the particles that happen to be headed towards Earth, and the number and energy of the particles coming from all different directions will tell us much more about the overall structure of the interaction between the heliosphere and interstellar space. IBEX will not have enough power to get out as far as Voyager, Stone said. "Voyager will be first interstellar probes. Newton tells us that. The question is will they still have electric power?" Now, at the edge of the solar system, Voyagers 1 and 2 will continue to explore the unknown, going forth on behalf of all humanity. If everything goes on as it has been, the two hardy spacecraft should continue to return rich data through 2020, perhaps as long as 2025 when they will exhaust their plutonium power sources and their radioisotope thermonuclear generators will run no more. Meanwhile, for the Voyager team members, it's that time again already. "We are now preparing a proposal for next three year segment of this journey of discovery," Stone said. "Does it look promising?" asked a reporter. "Yes," Stone said, stating the obvious amidst chuckles. "We think so." |
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