Voyager probe 'leaves Solar System'

The Voyager-1 spacecraft has become the first manmade object to leave the Solar System.

Scientists say the probe's instruments indicate it has moved beyond the bubble of hot gas from our Sun and is now moving in the space between the stars.

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

Today, the veteran Nasa mission is almost 19 billion km (12 billion miles) from home.

This distance is so vast that it takes 17 hours now for a radio signal sent from Voyager to reach receivers here on Earth.

"This is big; it's really impressive - the first human-made object to make it out into interstellar space," said Prof Don Gurnett from the University of Iowa and the principal investigator on the PWS.

On 25 August, 2012, Voyager-1 was some 121 Astronomical Units away. That is, 121 times the separation between the Earth and the Sun.

Breaching the boundary, known technically as the heliopause, was, said the English Astronomer Royal, Prof Sir Martin Rees, a remarkable achievement: "It's utterly astonishing that this fragile artefact, based on 1970s technology, can signal its presence from this immense distance."

Although now embedded in the gas, dust and magnetic fields from other stars, Voyager still feels a gravitational tug from the Sun, just as some comets do that lie even further out in space. But to all intents and purposes, it has left what most people would define as the Solar System. It is now in a completely new domain.

Voyager-1 left Earth on 5 September 1977, a few days after its sister spacecraft, Voyager-2.

The pair's primary objective was to survey the planets Jupiter, Saturn, Uranus and Neptune - a task they completed in 1989.

They were then steered towards deep space. It is expected that their plutonium power sources will stop supplying electricity in about 10 years, at which point their instruments and their 20W transmitters will die.

Voyager-1 will not get near another star for another 40,000 years, even though it is moving at 45km/s (100,000mph).

"Voyager-1 will be in orbit around the centre of our galaxy with all its stars for billions of years," said Prof Stone.

The probe's work is not quite done, however. For as long as they have working instruments, scientists will want to sample the new environment.

The new region through which Voyager is now flying was generated and sculpted by big stars that exploded millions of years ago.

There is indirect evidence and models to describe the conditions in this medium, but Voyager can now measure them for real and report back.

The renowned British planetary scientist Prof Fred Taylor commented: "As a young post-doc, I went to [Nasa's Jet Propulsion Laboratory] and worked for a while with the team that was doing the science definition study for the Outer Planets Grand Tour, which later became Voyager.

"It seemed so incredible and exciting to think we would see and explore Jupiter and Saturn close up, let alone Uranus and Neptune.

"The idea that the spacecraft would then exit the Solar System altogether was so way out, figuratively as well as literally, that we didn't even discuss it then, although I suppose we knew it would happen someday. Forty-three years later, that day has arrived, and Voyager is still finding new frontiers."