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Friday, November 07, 2003 - Page updated at 12:00 A.M.
By Usha Lee McFarling
As the robotic spacecraft continues to push far beyond the reach of the nine planets, two teams of scientists disagree whether it passed into the uncharted region of space where the sun's sphere of influence begins to wane.
The sun sends out a stream of highly charged particles, called the solar wind, that carve out a vast bubble known as the heliosphere around the solar system.
Beyond the bubble's ever-shifting boundary, called the "termination shock," lies a region where particles cast off by dying stars begin to hold sway. That region, called the heliopause, marks the beginning of interstellar space and the end of our solar system.
Whether Voyager 1 has reached that first boundary or is still on approach remains unclear.
A group of astronomers announced Wednesday that Voyager 1 had crossed the termination shock at the edge of the solar system where the solar wind drops from supersonic speeds to a relative whimper.
But another group argued that the spacecraft still has a journey ahead before it reaches this outer limit. They contend the strange readings collected by the aging craft in the past few months came from a "foreshock" and were nothing more than a brief hint of the exotic territory that lies ahead.
The one thing all agree on is that Voyager 1 has entered a final frontier far beyond our system's most distant planet, Pluto unlike anything humans or their space probes have encountered.
When Voyager I was launched in 1977 to study the outer planets, scientists hoped it would travel this far and possibly much farther. Along with its suite of scientific instruments, it carried with it photos of life on Earth, greetings in 55 languages and a collection of songs including Chuck Berry's "Johnny B. Goode" intended for an extraterrestrial audience that could encounter the craft in tens of thousands of years as it approaches other stars.
The spacecraft now is cruising through a turbulent realm where radiation pulses are 100 times more than normal, and the solar winds that speed past Earth at 1 million mph abruptly slow as they push up against the great celestial winds that travel between the stars. The pioneering spacecraft has entered a region with a different chemical mix and bathed in streams of peculiar cosmic rays.
"It's like we are piercing a hole in the curtain that separates us from the rest of the galaxy," said Merav Opher, an interstellar space expert at the Jet Propulsion Lab in Pasadena, Calif., who was not directly involved in the new research. "It's like stretching our arm and touching interstellar space."
The disagreement made public at a news conference Wednesday at NASA headquarters and in dueling papers published in yesterday's issue of the journal Nature highlights how little is known about the farthest reaches of our solar neighborhood. Despite crisp drawings that fill elementary-school textbooks, astronomers are not entirely sure where the solar system ends.
To astronomers, the termination shock at the solar system's edge lies 8 billion to 9.5 billion miles from the sun, where the sun's influence begins to weaken. Solar winds slow abruptly as they butt against winds of charged particles blowing toward us from distant stars.
The next boundary, the heliopause, is 10 billion to 15 billion miles from the sun where the pressure from solar wind is in balance with the interstellar winds. Even farther away, approximately 21 billion miles from the sun, is the "bow shock," a violent wave where the interstellar medium is pushed outward by the heliopause.
Pinpointing where these boundaries lie is a guessing game.
The heliopause breathes and pulses in a roughly 11-year cycle, a kind of solar heartbeat, said Edward Stone, Voyager's project scientist and one of the researchers who believes the craft has not yet left the solar system but is teetering on its fluctuating edge.
"We will likely be surfing the shock for three or four years," said Stone, the former director of the Jet Propulsion Laboratory and a physics professor at the California Institute of Technology.
The group that does believe Voyager 1 has left the solar system is headed by Stamatios "Tom" Krimigis, who also heads the space department at the Johns Hopkins University Applied Physics Lab.
Last August, when Voyager 1 was about 8 billion miles from the sun, it detected radiation levels 100 times more intense than normal. He believes this is when the craft crossed the termination shock. Nothing of the kind was recorded by Voyager 2, which lags more than a billion miles behind its sister craft, so "we knew something very special had happened," he said.
Indirect measurements also showed the solar wind had slowed to less than 100,000 mph, "which in this business is a slow wind," Krimigis said. The craft also detected different chemistry in its new environs: a mix of elements more akin to that found in the interstellar medium than that from the solar wind.
If Voyager 1 could measure the solar wind directly, astronomers would have a clear answer about whether it crossed the termination shock. But the solar-wind detector broke in 1990.
By this February, when Voyager 1 was 8.4 billion miles from the sun, all of the strange readings suddenly dropped off. Krimigis thinks the solar wind, fueled by solar activity, rushed out in front of the craft and enveloped it once again, bringing it back into the domain of the solar system where it currently remains.
The rival group, led by Frank McDonald of the University of Maryland used different instruments on Voyager to come to their conclusions. While McDonald's group agrees that Voyager encountered a large increase in high-energy particles, a drop in the solar wind and strange cosmic rays, the group did not see a high number of cosmic rays, precise patterns of streaming particles or high-intensity magnetic fields as they expected. They believe the termination shock still lies ahead.
Likening Voyager 1 to the Lewis and Clark of space exploration, McDonald said Voyager had not yet reached the mountains but was definitely "in the foothills."
What lies ahead
The question is likely to be resolved as Voyager 1 surges forward on its journey.
It is so distant now that the sun appears to it as only a dim spot in space, and its messages take more than 10 hours to reach Earth.
Further observations from Voyager 1 as well as Voyager 2, which trails its faster twin should resolve the dispute, as well as provide information about a never-before-probed region of space. The nuclear-fueled probes should last to 2020.
"We are beginning the exploration of a new frontier," said Voyager project scientist Stone.
Information from The Associated Press is included in this report.
On the Net: voyager.jpl.nasa.gov
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