Eight billion miles away, Voyager exits Solar System

It was launched in the year in which Elvis died of a heart attack, Donna Summer hit number one with "I Feel Love" and a punk band called the Sex Pistols were taking Britain by storm.

Since 1977, the Voyager 1 space probe has witnessed at close quarters the violent "red spot" of Jupiter, a permanent storm on the planet's equator, and taken stunning photographs of its four biggest moons.

In 1980, a year after passing Jupiter, Voyager 1 made a dramatic fly-by of Titan, the largest of the 31 known moons of Saturn, and in 1991 its camera pointed briefly back towards Earth to capture an historic photograph of nearly allof the Solar System's nine planets.

In February 1998, Voyager 1 overtook the Pioneer 10 probe, launched in 1972, to become the most distant man-made object in space.

Now scientists are wondering if it has broken the ultimate record of space endurance by becoming the first probe to reach the outermost boundary of the Solar System.

Stamatios Krimigis of Johns Hopkins University in Laurel, Maryland, says the latest data from Voyager 1's enfeebled instruments suggests the probe has left the Solar System for the icy depths of interstellar space.

In a study published today in the journal Nature, Dr Krimigis and his colleagues argue that Voyager 1, which is now more than 8 billion miles from Earth, is going where no space probe has gone before.

The edge of the Solar System is defined as the point at which the high-velocity solar wind - a stream of charged particles from the Sun travelling at up to 467 miles per second - finally peters out to be replaced by the interstellar winds of deep space.

Scientists call this boundary the "termination shock" because the sudden drop in velocity of the solar wind from supersonic to subsonic speeds causes a transition similar to the sonic boom caused as an aircraft travels faster than the speed of sound.

The instrument on board Voyager 1 that could measure the speed of the solar wind directly has long since broken down, but scientists have invented an ingenious alternative based on the study of lower-energy particles.

Dr Krimigis and his team have interpreted their analysis as confirmation that Voyager 1 has finally begun the transition into interstellar space, but other scientists, led by Frank McDonald of the University of Maryland, believe that the probe has yet to reach the transition boundary.

To complicate matters further, the scientists accept that the edge of the Solar System is a moveable feast, with the termination shock boundary rapidly pulsating.

Dr Krimigis said that although he believed Voyager 1 passed through the boundary, it only did so for about 200 days before the boundary rebounded and enveloped the probe once more with a supersonic solar wind.

Len Fisk, an astronomer and commentator at the University of Michigan at Ann Arbor, said that the argument over whether Voyager 1 had gone through the boundary was important because the termination shock was a fascinating astrophysical object that had never been properly studied.

"I tend to agree with Krimigis et al that their data can most readily be explained if the termination shock had been crossed," Dr Fisk said. "And once the termination shock has definitely been passed, the adventure enters a new phase."