Voyager 1's 26-year trip leads into deep space behind

Bumpy ride carries craft to edge of solar system

November 06, 2003|By Michael Stroh | Michael Stroh,SUN STAFF

Scientists have detected the first signs that Voyager 1, the most far-flung robotic explorer in NASA's fleet, has reached the fringes of the solar system and is closing on a milestone: becoming the first manmade object to breach interstellar space.

A team led by Johns Hopkins University scientists reports today in the journal Nature that unusual measurements recorded in recent months indicate that the 26-year-old probe has arrived at a turbulent, little-understood boundary near the rim of the solar system.

The region - dubbed the "termination shock" - is one of the final points Voyager must pass before sailing into interstellar space.

The precise location of the termination shock and the composition of space beyond are two of the great unanswered questions of astrophysics, says Stamatios M. Krimigis of Hopkins' Applied Physics Laboratory in Laurel, who led the team that made the observation.

"It's the Holy Grail," says Krimigis. "It means we're getting out of the cocoon of the solar system."

The Hopkins team's conclusions, however, have already triggered a spirited debate among astrophysicists, who have repeatedly attempted to predict the shock's location over the years. A companion paper in today's Nature, from a team led by the University of Maryland, argues that Voyager might still be a year or so away from the elusive boundary.

Either way, Voyager 1 - which hasn't made headlines for a discovery since Jimmy Carter was president - could be on the verge of a rare comeback in the scientific community. The closer the craft comes to the edge of the solar system, the more researchers want the data it's transmitting back, says Edward Stone of the California Institute of Technology, the project's scientific chief.

Twin explorers

One of identical twin robotic probes launched in the summer of 1977, Voyager 1 surveyed Jupiter and Saturn before peeling away from the planets in November 1980 and heading toward the stars. It's now 8.37 billion miles from the sun, so far that it requires nearly 13 hours for a speed-of-light radio signal to reach it.

Voyager 2, meanwhile, surveyed not only Jupiter and Saturn, but also Uranus and Neptune before beginning its voyage into deep space.

Rather than retiring the two hardy spacecraft, which were designed to last only five years, NASA devised a new mission for them in 1990: Discover where the solar system ends and interstellar space begins.

The solar system, contrary to popular belief, does not end at the orbits of Neptune or Pluto, the two outermost planets.

Instead, most scientists define the edge of the solar system as the place where particles rushing outward from the sun, known as the solar wind, collide with particles whooshing in from interstellar space.

When Voyager crosses this boundary - known as the heliopause - it officially enters interstellar space.

As the solar wind nears the heliopause, however, it suddenly slows drastically, creating a shock wave much as a jet does when it breaks the sound barrier. This is the termination shock.

Wind fluctuation

The first clues that Voyager 1 was nearing the termination shock came in August 2002. Krimigis, who oversees the spacecraft's low-energy charged particle detector (one of four instruments still operating), detected a fluctuation in the solar wind.

At one point Krimigis and his team watched as the speed plunged from 700,000 mph to less than 100,000 mph. By early February 2003, the wind had returned to its previous speed.

Krimigis says this reading can mean only one thing: The termination shock, which advances and retreats with the solar wind, appears to have washed over Voyager 1 briefly and then retreated.

To rule out the possibility that the strange readings might have been a result of some unusual solar activity, Krimigis and his team checked the solar wind instrument on Voyager 2, which is heading in a different direction and about 2 billion miles closer to the sun. Nothing.

Particles from space

Finally, Krimigis and his team detected particles from interstellar space, another hint that the spacecraft had passed beyond the termination shock.

"It's like we're piercing a hole in the curtain that separates us from the rest of the galaxy," says Merav Opher, a research scientist at the Jet Propulsion Laboratory in Pasadena who was not involved in the project.

But Frank B. McDonald, another long-time Voyager team member at the University of Maryland who works on the spacecraft's cosmic ray detector, says he's not ready to celebrate just yet.

In the neighborhood

"We're in the neighborhood of the shock, but we haven't crossed it," says McDonald, who lays out his arguments in the second Nature paper on the spacecraft.

The cosmic rays that his instrument measures aren't behaving as theory predicts they would at the termination shock. More importantly, he says, Voyager 1's equipment did not measure a jump in the magnetic field, as theory predicts it should.

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