Voyager 1 Craft Nears Edge of Solar System

Jahnava Nitai Das · November 6, 2003

Scientists Differ on 'Termination Shock'

By Kathy Sawyer

Washington Post Staff Writer

Thursday, November 6, 2003; Page A03

The spacecraft Voyager 1, launched in 1977, has become the first human emissary to approach the boundary region where the sun's domain ends and the vastness of interstellar space begins.

The first signs came in August, when onboard instruments detected evidence that the spacecraft had entered a new environment fraught with bizarre cosmic rays and other characteristics not seen in its previous 26 years of space exploration, scientists said yesterday.

But they disagreed on whether the 1,600-pound spacecraft has already felt the effects of "termination shock" -- a long-awaited milestone zone that the craft must cross as it leaves the solar system. One team of researchers concludes in today's issue of the journal Nature that it definitely has, while a second says not quite yet.

Either way, "we will likely be surfing this termination shock over the next three or four years," said Edward Stone of the California Institute of Technology, lead Voyager project scientist. He called the observations "unusual and surprising. . . . It means there is something new to be learned as we begin exploring this final frontier of the solar system."

Stone and several other scientists discussed the findings at a NASA briefing yesterday.

The venerable Voyager continues to send back news of its remarkable travels even as it cruises more than 8 billion miles from Earth -- 90 times Earth's distance from the sun. Traveling at a velocity of 38,000 mph relative to the sun, the spacecraft is about three times farther out than the outermost planets, Pluto and Neptune. Transmissions from the craft take almost half an hour, traveling at the speed of light, to reach mission controllers at the Jet Propulsion Laboratory in Pasadena, Calif., which manages the mission for NASA.

"If nothing breaks on the spacecraft, we can continue until 2020" when its power generators will die, Stone said.

Termination shock is produced when the supersonic "wind" that flows continuously from the sun clashes with the distinctly different medium that fills the vastness of interstellar space beyond, said scientists who have eagerly anticipated their first opportunity to study this frontier.

The solar wind -- a thin flow of electrically charged (ionized) gas -- expands continuously outward at velocities of about 1 million mph or more. It hollows out a sort of bubble in which the sun and its planets plow through the interstellar medium.

When the solar wind runs into the thin cloud of ionized gas and dust that fills the space between the sun and other stars, it slows abruptly.

As this supersonic flow goes subsonic, it creates a shock zone similar to the one that produces sonic booms when high-speed aircraft break the sound barrier. Astronomers estimate that the region of termination shock surrounds the solar system in a giant sphere at distances ranging between 85 and 120 times the Earth's 93 million-mile distance from the sun. The size of this zone presumably fluctuates with the sun's activity.

The most obvious sign that Voyager has encountered termination shock would be evidence that the solar wind speed has dropped, the scientists said. Another clue would be high numbers of unusual cosmic rays and other particles. This is because, in theory, the shock wave should accelerate energetic particles in the zone by a factor of 10,000, forming a population of unusual cosmic rays.

Voyager has sent back information that bears on these and other predicted phenomena, but the researchers differ on what it means.

The spacecraft's main instrument for measuring the solar wind speed -- the plasma detector -- has been broken since 1980, but a team led by Stamatios M. "Tom" Krimigis of Johns Hopkins University's Applied Physics Laboratory in Laurel worked out another, less direct way to assess the solar wind speed using one of the four working instruments.

"To our total amazement," Krimigis said, the data showed the speed had dropped by a factor of seven -- from about 700,000 mph to much less than 100,000 mph.

A trailing sister ship, Voyager 2, registered no similar effects. This indicated that the measurements were most likely related to the environment around Voyager 1 and not to a fluctuation in the sun's activity, Krimigis said. Launched in the same year more than a quarter-century ago, Voyager 2 is traveling a different path and is almost 2 billion miles behind in the race toward the outskirts of the solar system.

The region of termination shock enveloped Voyager for six months before expanding outward and leaving the spacecraft back in the flow of solar wind, the group said.

However, a team led by Frank McDonald of the University of Maryland at College Park argued that Voyager was never in the shock zone. The group did detect increased numbers of weird cosmic rays and other particles, but said the patterns did not fit predictions and were instead a precursor event indicating that Voyager is "in the vicinity" of the shock zone.

Stone said the dispute will soon be resolved by more data.

"Getting there," McDonald noted, "is half the fun."