Next stop, interstellar space: Voyager journeys to the edge of the solar system
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.
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