Voyager craft exits the Solar System

An artist's impression of Voyager 1 as it passes across the Milky Way

The Voyager-1 spacecraft has left the Solar System, the first man-made object to do so.

Launched in September 1977, the probe was sent initially to study the outer planets, but then just kept on going.

The US space agency (Nasa) reports that Voyager has now entered a realm of space beyond the influence of our Sun.

This interstellar region is calculated to be more than 18 billion km from Earth, or 123 times the distance between our planet and the Sun.

Voyager-1 is on course to approach a star called AC +793888, but it will only get to within two light-years of it and it will be tens of thousands of years before it does so.

Confirmation of the probe's exit from the heliosphere - the bubble of gas and magnetic fields originating from the Sun - was confirmed on Tuesday in a release by the American Geophysical Union.



The organisation has accepted a scholarly paper on the topic from Nasa scientists that will be published shortly in the house journal Geophysical Research Letters.

The announcement had been expected for some time.

Cosmic ray detection
Voyager had been monitoring changes in its environment that suggested it was about to cross the Solar System's border - the so-called heliopause.

It had been detecting a rise in the number of high-energy particles or cosmic rays, coming towards it from interstellar space, while at the same time recording a decline in the intensity of energetic particles coming from behind, from our Sun.

A big change occurred on 25 August last year, which scientists said was like a "heliocliff".

"Within just a few days, the heliospheric intensity of trapped radiation decreased, and the cosmic ray intensity went up as you would expect if it exited the heliosphere," explained Prof Bill Webber from New Mexico State University in Las Cruces.

Voyager-1 was launched on 5 September 1977, and its sister spacecraft, Voyager-2, on 20 August 1977.

The probes' initial goal was to survey the outer planets Jupiter, Saturn, Uranus and Neptune - a task they completed in 1989.

They were then despatched towards deep space, in the general direction of the centre of our Milky Way Galaxy.

Their plutonium power sources will stop generating electricity in about 10-15 years, at which point their instruments and transmitters will die.

The Voyagers will then become "silent ambassadors" from Earth as they move through the Milky Way.

- BBC.co.uk