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Next Stop, Interstellar Space

Voyager journeys to the edge of the solar system

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11:19am, December 22, 2003
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On the interplanetary highway, there are no mile markers and no exit signs. Precious few clues indicate that you're nearing the edge of the solar system. Those clues, however, are revealing that the venerable Voyager 1 spacecraft, launched 26 years ago and now 90 times as far from the sun as Earth is, either has reached or will soon enter a turbulent region near the solar system's final frontier. There, the solar wind first slams into large numbers of atoms and molecules that have leaked into the solar system from interstellar space. The encounter puts the brakes on the solar wind, causing it to abruptly slow from supersonic speeds of 400 to 700 kilometers per second down to subsonic speeds of 100 km/sec, according to simulations.

Signals that the elderly spacecraft recently radioed to Earth indicate that it might have already encountered this bizarre region, known as the termination shock. If so, it would be the first time that a human-made object has reached that milestone, notes Stamatios (Tom) Krimigis of the John Hopkins Applied Physics Laboratory in Laurel, Md. The craft would then be on course to exit the solar system.

The solar system's outer boundary, which lies beyond the termination shock, is the edge of the heliosphere, the vast bubble of space filled by the wind of charged particles continuously blown by the sun. Contracting or expanding as the solar wind blows slower or faster, the heliosphere generously envelops all the planets. The radius of the heliosphere is more than three times that of Pluto's orbit. It will take Voyager 1 another 15 years or so to burst out of the bubble and begin exploring interstellar space.

Recent data from the spacecraft may be revealing previously unknown and complex features of the outer solar system, says Len A. Fisk of the University of Michigan in Ann Arbor. Information about the termination shock may also provide insight into the more powerful shocks generated by supernova explosions.

Fringe science

Launched in 1977, Voyager 1 took some spectacular images of the outer planets. As it journeyed billions of kilometers from Earth, through the outer reaches of the solar system, the information that Voyager relayed about the solar wind and energetic charged particles carried by the wind held no surprises. But for a 6-month period beginning in August 2002, the craft radioed data that didn't resemble any that planetary scientists had seen before.

One of the craft's detectors recorded a 100-fold surge in the intensity of certain charged particles–electrons greater than 350,000 electron volts and protons ranging in energy from 40,000 eV to more than 70 million eV. The particles were also moving in a different direction than were those the craft had recorded during the previous 25 years. Instead of riding the solar wind on an outbound trajectory, these particles were moving perpendicular to that direction.

For these changes to occur, "something must have happened to the solar wind," says Krimigis.

Those changes dovetail with other data that Krimigis rates as the clearest, albeit indirect, evidence that Voyager 1 had entered the termination shock. Voyager 1's solar-wind monitor stopped working in 1980, so scientists couldn't directly determine whether the solar wind had slowed.

Krimigis and his colleagues did the next-best thing. They estimated the wind's speed by using the spacecraft's charged-particle detector, which can measure the populations and directions of electrons and ions.

The detector is mounted on a motorized rotating platform–still powered by the spacecraft's nuclear batteries–that enables it to record information about the particles arriving from all directions. Krimigis and his coworkers compared the readings taken when the detector was facing the solar wind with ones taken when the detector was looking outward into deep space. The detector had always recorded substantially more particles in the wind-facing direction. That's just what would be expected when the wind, which boosts the movement of the charged particles, blows at high speeds. But beginning in August 2002 and continuing through January 2003, the detector recorded about equal numbers of particles in both directions.

Krimigis and his colleagues say that's strong evidence that Voyager 1 had entered a region in which the solar wind was moving much more slowly than it does in more-inward parts of the solar system.

In the Nov. 6, 2003 Nature, the team cites another telling piece of evidence that Voyager 1 had reached the termination shock. From August 2002 through January 2003, the relative abundances of hydrogen, helium, carbon, and oxygen ions recorded by Voyager 1 differed from all previous measurements. They no longer resembled the mix of ions found in interplanetary space within the solar system but instead mirrored the composition expected for interstellar atoms.

The data suggest that for 6 months, Voyager 1 was inside the termination shock, says Krimigis. His team conjectures that sometime in January 2003, a chance increase in pressure from the solar wind pushed the termination shock outward, leaving the spacecraft behind.

Edgy controversy

In the same issue of Nature, however, Frank B. McDonald of the University of Maryland in College Park and his colleagues rely on different Voyager 1 data to argue that the craft hasn't yet reached the shock region but may cross it shortly.

At the termination shock, McDonald notes, charged particles are accelerated to 1,000 times their initial energy, creating a group of energetic particles known as anomalous cosmic rays. A detector on Voyager 1 recorded an increase in intensity of these cosmic rays from August 2002 through January 2003, but the spike was nowhere as large as would be expected for a craft within the termination shock region, McDonald notes. As he sees it, the slight rise in cosmic ray intensity indicates that the spacecraft is approaching the shock region but is not quite there.

Bolstering that view, simple models of the heliosphere require that the magnetic field intensifies in regions where the solar wind slows. In the Oct. 30, 2003 Geophysical Research Letters, Leonard F. Burlaga of NASA's Goddard Space Flight Center in Greenbelt, Md., and his colleagues report that Voyager 1 didn't detect a rise in the magnetic field.

McDonald and Krimigis have been arguing about their data since last April, when Krimigis presented his findings in Nice, France, at a joint meeting of the European Geophysical Society, the American Geophysical Union, and the European Union of Geosciences. "Perhaps he had too much French wine to drink," McDonald says with a chuckle.

Notes astrophysicist Fisk, who studies the solar wind: "There's no question that the sets of data are in conflict." But that's where things start to get interesting, he adds.

"Maybe the termination shock is not what would be expected . . . from the simplest models," he says. "Assuming everyone did their job correctly [analyzing the Voyager 1 data], you may have to make up a new complex model to reconcile all these data sets."

Fisk notes that in more-complex models the magnetic field at the termination shock may not be significantly greater than in more central parts of the solar system.

Fisk also suggests that the termination shock may have a ragged shape. Instead of being a perfectly spherical shell, the termination-shock region has inwardly directed protrusions of material, he speculates. If Voyager 1 passed through one of these protrusions, it might show a decrease in the speed of the solar wind, just as the detector used by Krimigis' team indicated.

Because the bulk of the termination shock would still have been quite a distance away, however, the spacecraft wouldn't have encountered a large upturn in the abundance of anomalous cosmic rays. That scenario is consistent with McDonald's finding.

"We may have to rethink how [anomalous] cosmic rays are accelerated," says Fisk. He notes that the termination shock is the closest known analog to the supernova shock wave generated when material hurled from an exploded star rams into the low-density region of space surrounding it.

"If the termination shock is more complex than we had thought, this may have direct application to supernova shock waves," says Fisk.

Future shock

Scientists won't have to wait long for direct observations of the termination shock. If Voyager 1 hasn't entered that region yet, it is expected to do so within the next few years and beam clear-cut evidence back to Earth, says McDonald.

If the craft at that time is merely catching up with the termination shock that it had already encountered, the new data ought to be identical to those recorded in August 2002, notes Krimigis.

But there's an even simpler test. The Voyager 2 spacecraft is also heading out to the solar system's edge. Some 29 million km farther from that edge than Voyager 1 is, Voyager 2 has a working solar-wind-speed detector. A few years from now, that detector is expected to provide unequivocal proof that Voyager 2 is passing through the termination shock, says Fisk.

The ultimate milestone remains the exit from the solar system into the vastness of interstellar space, and researchers now have new information about exactly where that edge lies. Guided by strong bursts of radio waves that Voyager 1 has detected since November 2002, scientists now calculate that the edge, known as the heliopause, resides between 153 and 158 times as far as Earth does from the sun.

The bursts were probably generated when clouds of solar material, hurled by the sun during an unusually stormy period in April 2001, rammed into cold interstellar ions at the edge of the solar system, says Donald A. Gurnett of the University of Iowa in Iowa City. Radio bursts from that collision then traveled back toward the sun and were intercepted by Voyager 1. From the speed of the clouds and the time it took them to reach the heliopause, Gurnett's team calculated the distance to the edge. He reported the findings, which are consistent with his team's analysis of similar radio bursts recorded by Voyager 1 a decade ago (SN: 5/29/93, p. 343), last month at the fall meeting of the American Geophysical Union in San Francisco.

Trying to find the solar system's edge, Gurnett cautions, is a search for a moving target because its location depends on how strongly the solar wind is blowing. Nonetheless, the best estimate is that Voyager 1 could reach that border around 2020, just as the aging craft is expected to run out of power. Researchers will be anxious to learn whether Voyager 1 broadcasts its swan song to Earth or travels silently into the abyss.

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