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Voyager 1 on verge of interstellar space

Posted on: Thursday, 6 November 2003, 06:00 CST

The Voyager 1 spacecraft, our planet's most distant probe, has reached the solar system's edge and may have entered the uncharted reaches of interstellar space, scientists say.

At a NASA briefing Wednesday, two scientific teams released similar measurements recorded by the space probe since August 2002 but offered different interpretations. Both teams publish their results in today's Nature.

Launched in 1977 and now 8.37 billion miles from the sun, Voyager 1 reports changes in solar wind, magnetic fields and cosmic rays. But scientists disagree on whether the probe has actually crossed into interstellar space, where the effects of the sun's solar wind no longer predominate, or if it is merely on the cusp of this transition. The solar wind is a stream of atomic particles flowing out from the sun at speeds of nearly 2 million miles per hour.

The spacecraft, powered by radioactive batteries, is now three times farther from the sun than is Pluto, the system's most distant planet.

Measuring the properties of interstellar space has long interested astronomers, who want to understand the unique properties of the vacuum between stars. Where the solar wind dies out, space scientists have long expected to see a ''termination'' shock wave as solar particles bang into charged particles from the rest of the galaxy.

One team, led by physicist Stamatios Krimigis of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., reports the solar wind essentially came to a ''dead stop'' in August last year. Also, the team measured a jump in radiation 100 times larger than normal along with a spike in charged particles, indicating the probe had reached the solar system's edge.

The solar wind picked up again in February, moving the solar system's boundary farther into space, he says. The boundary shifts regularly according to the strength of the solar wind.

However, scientists led by physicist Frank McDonald of the University of Maryland disagree. Cosmic ray measurements would have been higher were the probe crossing the boundary, they argue. McDonald says the probe has quite a way to go to reach interstellar space. ''Half the fun is getting there,'' he says.

''The termination shock is expected to be a fascinating astrophysical object,'' space scientist Len Fisk of the University of Michigan in Ann Arbor says in a commentary accompanying the Nature reports. On balance, the data suggest that Voyager 1 has truly crossed, albeit briefly, into interstellar space, Fisk says.

Voyager 2, also launched in 1977, lags some 1.6 billion miles behind Voyager 1. Batteries on both craft, far too distant from the sun for solar panels, should last until 2020.

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