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This article is from the In-Depth Report Robotic Exploration of the Solar System
See Inside March 2008

Voyagers to the End

The solar system may be dented at the bottom

Last year, after 30 years in space, Voyager 2 plowed through waves of charged particles as it crossed the termination shock, the first boundary marking the end of the solar system. As it joins its twin, Voyager 1, which is on a more northerly route to the stars, researchers have announced that, based on the two spacecraft readings, the solar system is "squashed" on one side—specifically, that the solar wind does not go as far to the south as it does to the north before being blunted and changing direction.

Astronomers define the termination shock zone as the place where the solar wind can no longer hold its own against the vast ocean of interstellar space particles. The solar wind consists of a supersonic river of charged particles traveling radially out away from the sun at 400 kilometers per second, faster than any other wave along the sun’s magnetic field. (Space can transmit sonic waves, which travel in the solar system at speeds of about 50 to 70 kilometers per second; even so, in space no one can hear you scream because it is such a rarefied medium that the amplitude of any sound waves will be extremely weak.) Only as the solar wind approaches the termination shock does it begin to slow down, to 300 kilometers per second—the result of cosmic-ray particles swimming upstream into the solar wind from the heliosheath, which is defined as the immediate area on the other side of the shock.

At the termination shock, the solar wind cuts its speed almost in half as it falls to 150 kilometers per second and mixes with wisps of plasma coming from the wind of other stars. The result, as Voyager 2 discovered, is a surf of energetic ions. The undaunted spacecraft crested five waves of high-speed charged particles between August 30 and September 1 as it crossed the termination shock into the lull of the heliosheath. There the slower and diluted solar wind eddies back and trails in the wake of the sun’s own orbit through the Milky Way galaxy. Before Voyager 2, astronomers had categorized the speed of the solar wind on the other side of the termination shock as subsonic. “One of the surprises was that the solar wind doesn’t slow down as much as we expected,” says Voyager mission scientist Ed Stone of the California Institute of Technology.

Like dolphins on either side of the prow of a ship, the Voyagers flank the ecliptic, cruising the front wave of the solar system. Voyager 2 hit the termination shock at a distance of 84 astronomical units (AU) from the sun—one billion miles closer to the sun than Voyager 1’s encounter with the termination shock at 94 AU in 2004 (1 AU is the average distance from Earth to the sun). The asymmetry of the termination shock indicates that for some reason the solar system is heeling to the north, exposing more of its south-facing hull to the interstellar wind. “We need to know why,” Stone says.

As any good sailor knows, the winds encountered play a significant role in how a ship handles at sea. Voyager magnetometer specialist Leonard Burlaga of the NASA Goddard Space Flight Center interprets the short distance as an indication that the interstellar magnetic field is pushing harder against the southern hemisphere of the solar system. At the same time, the sun is also trimming the sails a bit through variations in the solar cycle.

Voyager 2, which unlike its twin still has a functioning plasma detector, also reported another surprise. The reduced velocity of solar wind at the termination shock should have been converted to heat. “We expected to find ions in the heliosheath with temperatures upward of one million kelvins,” Stone says, “and instead the numbers were around 100,000 to 200,000—
a factor of five to 10 cooler than we expected.” Astronomers suspect cosmic rays may have pirated the energy for their own accelerating purposes. As Burlaga puts it, “ions bounce off the magnetic perturbations from the wind, and the energy of the solar wind ends up in these ions.” How far away from the termination shock and into the heliosheath such accelerations take place remains unknown.

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