Meeting the Solar System’s Final Frontier

Heliosphere diagram
This artist's concept shows structures in the tenuous gas where the expanding solar wind meets the interstellar medium far beyond Pluto. Voyager 1 has apparently crossed the first boundary layer, as described in the text. Voyager 2 still has a way to go.
Courtesy NASA / Walt Feimer.
NASA's farthest-ranging spacecraft has begun encountering the first physical boundary between our solar system and the interstellar space beyond.

Voyager 1, which is now about three times farther from the Sun than Neptune and Pluto are, has sent back evidence that it may have reached the "termination shock" in the solar wind. This is the first sign of the thin solar wind piling up against the even thinner interstellar gas beyond. "Voyager is beginning to explore the final frontier of the solar system," said Edward Stone, project scientist for Voyager, at a NASA press conference today called to coincide with reports published in the November 6th Nature.

Voyager 1 was launched in 1977 and flew by Jupiter and Saturn in 1979 and 1980. It is now 90 astronomical units (13.5 billion kilometers) into deep space, with its radioactive power source still feebly alive and some of its instruments still sensing the surrounding particles and fields.

Starting in July 2002, say Stamatios Krimigis (Johns Hopkins University Applied Physics Lab) and seven colleagues, the craft sensed a dramatic change in the surrounding radiation levels, detected signs that the solar wind had greatly slowed, and found changes in the wind's composition suggesting that interstellar matter was beginning to be mixed in with it. All this suggests that Voyager 1 was encountering the termination shock, the boundary where the solar wind changes from supersonic to subsonic flow.

You can see an example of a termination shock where water from a faucet hits the bottom of a sink. The water spreads out smoothly until reaching a thicker, turbulent ring, where it slows down and piles up. The ring marks the change from supersonic to subsonic flow, with the "speed of sound" being the speed that ripples travel across the water.

In fact, say Krimigis and his associates, the termination shock seems to have swept back and forth across Voyager three times, moving in and out in response to changes in the strength of the solar wind. After Voyager penetrated it in mid-2002, the boundary seemed to retreat outward past the spacecraft in February 2003, leaving it back in the supersonic solar wind. The boundary moved inward again past the spacecraft last August. Said Stone, "We will probably be surfing the termination shock for the next three or four years."

However, another group of scientists studying the same Voyager data comes to a different conclusion. Appearing at the same press conference and publishing in the same issue of Nature, Frank B. McDonald (University of Maryland) and five colleagues including Stone say that the evidence points only to Voyager coming near to the termination shock, not yet actually penetrating it. They call the observed change in Voyager's surroundings "an expected precursor event." Said McDonald, "We're in the foothills [of the termination shock] and will soon be in the mountains."

Crossing the termination shock is, or will be, only the first of several milestones before Voyager 1 reaches true interstellar space. The craft should remain in the heliosheath — the turbulent region corresponding to the water outside the smooth circle in the sink — for perhaps 12 or 14 years. Then it should cross the heliopause, the boundary where solar material gives way to pure interstellar matter. Stone thinks Voyager will still be alive at that time.

Even farther out it may cross a bow shock, where interstellar gas first piles up against the disturbance created by the solar system moving through it. Beyond that lies untouched interstellar space.

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