Voyager 1 Approaching Edge of the Solar System
After nearly 28 years of touring the giant planets and beyond, NASA's Voyager 1 has now reached the outermost antechamber of the solar system, a final interlude before it departs.
"We're now in the final lap of the race to get to interstellar space," said Dr. Edward C. Stone, the project scientist for the two Voyager spacecraft, which were launched in 1977 and continue operating on their plutonium power sources.
In New Orleans last week, at a meeting of the American Geophysical Union, scientists reported that last December Voyager 1 passed through a boundary, called the termination shock, that is similar to a sonic boom. Although silent to human ears, the tenuous gas particles in outer space do bounce back and forth in fast-moving sound waves. At the termination shock, the speed of particles streaming out from the Sun suddenly drops from supersonic - 700,000 to 1.5 million miles per hour - to subsonic, slowed by the pressure of interstellar particles pushing on the solar system.
Voyager 1, about 8.7 billion miles from the Sun and more than three times as far as Pluto, has entered a region of space known as the heliosheath.
Dr. Stone said that a phenomenon similar to a termination shock could be seen when water flowing out of a faucet hits the flat bottom of a sink. Where the water hits, "It's very thin and very fast," he said. But as the flow spreads outward, it becomes too sparse to continue pushing the water in front, and the water suddenly piles up.
"You'll notice that a ring forms around the water in the bottom of the sink," Dr. Stone said. "The water gets very thick and slow. That thick region is the heliosheath."
The crossing occurred Dec. 16, the one day last year that no listening time on NASA's Deep Space Network was devoted to the Voyagers. On Dec. 15, "We're clearly in the solar wind," said Dr. Norman F. Ness, a professor at the University of Delaware's Bartol Research Institute and the principal investigator for Voyager's magnetometer. On Dec. 17, "we're clearly in the heliosheath," he said.
Two years ago, a team led by Dr. Stamatios M. Krimigis of the Applied Physics Laboratory at Johns Hopkins University had made a similar claim. Dr. Krimigis and his colleagues described a drop in the speed of the solar wind in August 2002 and a sharp rise in the number of high-energy particles bombarding the spacecraft, suggesting that Voyager 1 had crossed the termination shock. The solar wind appeared to speed back up six months later. The acceleration could have occurred if stronger solar winds had pushed the termination shock boundary outward, back beyond Voyager 1.
Other scientists disagreed. Dr. Ness referred to the earlier claim as "the alleged crossing of the termination shock" and said that a crucial characteristic of a crossing - a large, sustained rise in the magnetic fields, caused by the piling up of solar wind particles - did not occur. The controversy arose in part because the instrument that directly measured the solar wind was knocked out in 1980 as the spacecraft passed through the radiation belts of Saturn.
Voyager 2 is expected to pass through the termination shock in a few years. Its solar wind instrument is still working.
Dr. Krimigis said last week that he now agreed that the 2002 event was not a termination shock crossing. "We thought it may be the termination shock, but we were quite guarded in our interpretation," he said.
What appears to have happened, Dr. Stone said, is that the shock boundary was expanding outward at about the same speed as Voyager 1 and that particles accelerated by the boundary and funneled inward by magnetic fields were occasionally hitting the spacecraft. "We were sort of surfing the shock, but not crossing it," he said.
In mid-2004, after the maximum of the Sun's 11-year cycle, the boundary started collapsing inward again, he said, allowing Voyager 1 to finally catch up.
Other puzzles remain. Astrophysicists had thought that the termination shock boundary provided the kick for certain high-speed particles seen in the inner solar system. But Voyager 1 detected no change in the numbers of these particles after it crossed the termination shock. "There must be another source," Dr. Stone said. "We'll try to understand where that source is."
In the heliosheath, the average velocity of the solar wind particles continues to slow until, at the edge of the solar system, they are overwhelmed by the interstellar wind - made up of particles that flow between stars. The heliosheath may stretch several billion miles, so Voyager 1 is expected to take about a decade to reach that final crossing into the interstellar medium.
Voyager's plutonium power source is expected to last until 2020, but whether NASA will still be listening when that moment comes is uncertain. As part of the shift in priorities toward sending people back to the Moon and then to Mars, a proposed budget for NASA next year cuts about $20 million of the $70 million spent for about a dozen missions, including the Voyagers, that continue past their original designed lifetimes. The Voyagers cost about $4 million a year.
Dr. Richard R. Fisher, deputy director of NASA's Earth-Sun division, said last week that he was looking to transfer some money from other areas to the extended missions and that an independent review later this year would prioritize which missions should be continued. A final decision will be made next year, he said.
Dr. Ness said NASA would be foolish to cut the financing. "The risk is very low," he said. "The return is very high."
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David Constantine/The New York Times |
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