Postcards from the edge

Message from a craft at the solar system's final frontier holds surprises.

There are no signs to announce the edge of the solar system, but when the venerable Voyager 2 spacecraft approached this final frontier last Aug. 31 it was in for quite a shock. So were the scientists who analyzed the data that the craft radioed back to Earth, along with related observations by NASA’s twin Earth-orbiting STEREO spacecraft.

ON THE EDGE Location of the two Voyager spacecraft at the fringes of the solar system and the STEREO spacecraft. New data form the Voyager 2 craft and STEREO are providing fresh insight about the structure of the edge of the solar system. L. Wang/UC Berkeley

Voyager 2 travels nearly to the edge of the solar system, reaching the termination shock
OUTER REACHES An illustration of Voyager 2 at the solar system’s edge. The craft recently sent back surprising data as it approached this edge. JPL/NASA

The signals reveal that at a distance of 83.7 astronomical units (1 AU is the average Earth-sun separation), Voyager 2 had at least five encounters with a turbulent region known as the termination shock, the researchers report in the July 3 Nature. That’s the place where the solar wind — the sun’s hot supersonic wind of protons and other charged particles, which carves the heliosphere, a bubble in space extending well beyond the orbit of Pluto — slams into cold interstellar space and abruptly slows.

Analyzing the encounter is critical for understanding how the bubble interacts with surrounding space, and how the bubbles carved by other stars affect their surroundings, notes Voyager lead investigator Ed Stone of the California Institute of Technology in Pasadena, Calif.

Researchers had expected that Voyager 2 would have only one encounter with the shock. The multiple crossings indicate that “the shock is not the steady structure that is predicted by the simplest theory,” says Len Burlaga of NASA’s Goddard Space Flight Center in Greenbelt, Md. “It is like a wave approaching a beach, that grows, breaks, dissipates, and then re-forms closer to shore.”

Gusts in the solar wind may cause the shock to “come and go, re-forming itself and decaying,” Stone suggests.

Launched in 1977, Voyager 2 follows in the footsteps of its sister craft, Voyager 1, which headed toward the fringes of the solar system in the opposite direction, the northern celestial hemisphere, and passed through a single termination shock in 2004. The location of the shock detected by Voyager 2 — some 1.6 billion kilometers closer to the sun than the Voyager 1 shock — suggests that the solar system is lopsided. The bubble carved by the solar wind is pushed in on the southern side.

The dent may be due to extra pressure exerted by the Milky Way galaxy’s magnetic field. The field is generally uniform but could have become “tilted in such a way that it’s pushing more on the south than the north,” says Stone. A series of supernova explosions in the solar neighborhood about 10 million to 20 million years ago could have tilted the field, he notes.

Researchers studying the flow of energy at the solar system’s edge found another surprise. At the termination shock, the solar wind slows and dumps a large amount of energy into space. This energy must then exist in some form, such as heat. But John Richardson of the Massachusetts Institute of Technology and his colleagues found that the temperature of protons — a main constituent of the solar wind — in the slowed-down region is five to 10 times cooler than expected.

Using the STEREO spacecraft, Robert Lin and Linghua Wang of the University of California, Berkeley, and their colleagues trace the missing energy to a large group of “pickup protons”—particles that started out as neutral hydrogen atoms from interstellar space and then infiltrated the solar system. The solar wind ionized these atoms, turning them into protons that were then carried back out again by the wind, to the termination shock. About 80 percent of the energy released when the solar wind slows goes into accelerating the pickup protons, the researchers report.

The STEREO findings were a surprise because detectors on the twin craft are designed to detect energetic charged particles coming from the sun, which fluctuate in intensity due to variations in the solar magnetic field that pushes them around. But Wang’s team found that the sensors had detected a group of particles from the termination shock region that didn’t fluctuate in intensity and therefore must be neutral. These particles started out as pick-up protons.

The Berkeley-led team concluded that these particles came from the interstellar medium, providing the first map of particles from just beyond the solar system. This new map is especially important because material at the solar system’s edge is too tenuous and faint to be imaged by a visible-light telescope.

“Over the past few years, the stream of in situ and remote data from the outer reaches of the heliosphere has revolutionized our view of how the sun interacts with the galaxy,” comments J.R. Jokipii of the University of Arizona in Tucson. More is to come, he adds, as the two Voyager craft continue their journeys past the termination shock, to the very edge of the solar system during the coming decade.

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