US: After a storied, 28-year odyssey, Nasa's venerable Voyager 1 spacecraft appears to have reached the edge of the solar system, scientists have reported.
This is a turbulent zone of near-nothingness where the solar wind begins to give way to interstellar space in a cosmic cataclysm known as "termination shock".
"This is a historic step in Voyager's race," said the California Institute of Technology physicist Edward Stone, the mission's chief scientist since Voyager 1 and its twin, Voyager 2, were launched in the summer of 1977. "We have a totally new region of space to explore, and it's a once-in-a-lifetime opportunity."
Dr Stone said project scientists finally agreed that data from Voyager 1's tiny 80kb computer memory showed that the spacecraft had passed through termination shock to the "heliosheath", a frontier of unknown thickness that defines the border with interstellar space.
Stamatios Krimigis, another Voyager scientist, said the spacecraft may remain in the heliosheath for perhaps 10 years but should easily survive, going dark only when its plutonium power source finally expires around 2020.
Of far greater concern to scientists was the possibility that Nasa could kill the $4.2 million-a-year project to free money for President Bush's initiative to send humans back to the moon and eventually to Mars.
Voyager's "design mission" to Jupiter and Saturn lasted five years, then simply kept on going. Voyager 2 flew by Uranus and Neptune to complete a "grand tour" of the major planets and is now about seven billion miles from the sun travelling at 63,000mph.
Voyager 1 broke away from the tour at Jupiter and headed for interstellar space. When it entered the heliosheath it was 8.7 billion miles away - the farthest any man-made object has ever travelled.
Dr Krimigis said the solar wind, comprised of fast-moving electrons, protons and other charged particles, has a magnetic field that prevents the interstellar wind from breaching the solar envelope, known as the "heliosphere", as the solar system travels through space.
The heliosheath is the heliosphere's outer frontier, a zone where the solar wind begins to dissipate. This process begins at termination shock and is marked by a sudden drop in the speed of the solar wind and a corresponding build-up in heat and the strength of the magnetic field. The effect is like traffic piling up on a motorway during rush hour.
Dr Stone stressed, however, that this clash of cosmic forces has little effect on Voyager, since the amount of matter in the heliosheath is so meagre.