The Voyager 1 spacecraft, now venturing into the unknown world beyond our solar system, has collected abundant data about the outer threshold of the solar system, scientists reported on Thursday.

Four papers based on the spacecraft's findings were published in the Sept. 23 issue of the journal Science to depict new features of the solar system's boundary. And these findings provide surprise, scientists said.

Voyager 1, launched on Sept. 5, 1977 by US space agency NASA, was designed to investigate planets and possible life in other star systems. The spacecraft and its twin, Voyager 2, are two man-made celestial bodies most distant away from Earth.

Around Dec. 16, 2004, Voyager 1 passed an important milestone: it crossed the "termination shock" which is the point where solar wind from our Sun begins the process of merging into the interstellar medium, the gas and dust that exists between the stars.

At the termination shock, solar wind moving at ultrasonic speed slows down so that its speed reaches subsonic region. The termination shock surrounds the entire solar system, and its location changes based on changes in properties of the solar wind.

The location of the termination shock had been debated for many years until Voyager 1 actually passed the point. The spacecraft crossed at 94 astronomical units (an astronomical unit is the distance from the Sun to Earth), or 14.1 billion km, from the Sun, scientists said.

Early in the space age, the termination shock was predicted to be relatively close to the Sun, within the orbits of the planets. Scientist later converged on predictions that the shock would lie between 90 and 100 astronomical units from the Sun.

As predicted, Voyager 1 found the intensity of low-energy particles increased abruptly at the termination shock, two papers in the Science journal said. But contrary to earlier predictions, the spacecraft found high energy particles did not accelerate.

This finding from the termination shock fulfills "the mantra of space exploration," according to Len Fisk, a professor of space science at the University of Michigan.

Much of the solar system is composed of very hot gas called plasma made of negatively charged electrons and positively charged ions. Voyager 1 sporadically detected oscillations in the plasma from Feb. 11, 2004 until the shock crossing around Dec. 15, 2004.

But after this time, no further electron plasma oscillations were observed, according to scientists' third paper. This observation is consistent with the spacecraft having crossed the termination shock around Dec. 15, 2004.

Magnetic fields measured by Voyager 1 also show that the spacecraft crossed or was crossed by the termination shock in the middle of December, 2004, according to scientists' fourth paper.

Voyager 1 observed magnetic fields characteristic of the distant solar wind prior to Dec. 17, 2004. The magnetic field increased abruptly between Dec. 16 and Dec. 18 after which the magnetic field strength was unusually large for the next 125 days, the paper said.

"Voyager is like those intrepid British explorers of the 1800s in search of the source of the Nile," Fisk commented in Science. " When we go somewhere that is new, we find the unexpected, and that's what makes it so exciting."

Once it passed the shock, Voyager 1 moved into the heliosheath, the most distant region of the solar system, and then it will enter another star's scope, scientists said.