Astronomers tracking the little spaceship's 26-year journey from Earth believe Voyager 1 has gone through a region known as termination shock, some 14 billion kilometres from the Sun, and entered an area called the heliosheath.

"Voyager 1 has entered the final lap on its race to the edge of interstellar space," Edwad Stone, Voyager project scientist at the California Institute of Technology, said in a statement released on Tuesday.

Voyager watchers theorised last November that the craft might be reaching this bumpy region of space when the charged solar particles known as the solar wind seemed to slow down from a top speed of 2.4 million kilometres per hour.

Termination shock

This was expected at the area of termination shock, where the solar winds were expected to decelerate as they bump up against gas from the space beyond the solar system. It is more than twice as distant as Pluto, the furthest planet in the system.

By monitoring the craft's speed and the increase in the force of the solar wind, Voyager scientists now believe the craft has made it through the shock and into the heliosheath.

Predicting the location of the termination shock was hard because the precise conditions in interstellar space are unknown and the termination shock can expand, contract and ripple, depending on changes in the speed and pressure of the solar wind.

"Voyager 1 has entered the final lap on its race to the edge of interstellar space"

Edwad Stone,
Voyager project scientist at the California Institute of Technology

"Voyager's observations over the past few years show the termination shock is far more complicated than anyone thought," said Eric Christian, a scientist with Nasa's Sun-Solar System Connection programme.

Voyager mission

Voyager 1 and its twin spacecraft Voyager 2 were launched in 1977 on a mission to explore the giant planets Jupiter and Saturn. The pair kept going, however, and the mission was extended.

Voyager 2 went on to explore Uranus and Neptune, the only spacecraft to have visited these outer planets. Both Voyagers are now part of the Voyager Interstellar Mission to explore the outermost edge of the Sun's domain.

Both Voyagers are capable of returning scientific data from a full range of instruments, with adequate electrical power and attitude control propellant to keep operating until 2020.

Wherever they go, the Voyagers each carry a golden phonograph record which bears messages from Earth, including natural sounds of surf, wind, thunder and animals. There are
also musical selections, spoken greetings in 55 languages, along with instructions and equipment on how to play the record.