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Last Updated: Wednesday, 24 May 2006, 15:06 GMT 16:06 UK
Voyager probes detail Sun domain
By Jonathan Amos
BBC News science reporter, in Baltimore

Voyager impression (Nasa)
The probes have plenty of power to keep their systems running
Twenty-eight years on and the Voyager spacecraft are still making discoveries as they hurtle out of our Solar System.

The probes are now more than 11.5 billion km from Earth and should leave the vast region of space under our Sun's influence in the next 10 years.

Scientists say this region, which they refer to as the heliosphere, has now been more closely defined thanks to data returned by Voyagers 1 and 2.

The probes' power packs should maintain their missions until at least 2020.

"When the power is lost, they'll still keep travelling outwards, of course," said Edward Stone, from the California Institute of Technology, US.

...we are going faster than anything that has ever been launched - a little over 17km/s...
Edward Stone, Voyager project scientist
"But it will be a long time before the Voyagers are closer to another star than to our own Sun - something like 40,000 years.

"Even though we are going faster than anything that has ever been launched - a little over 17km/s - we're still going very slowly compared with the distance between the stars," he told BBC News.

Stone and colleagues were here at the American Geophysical Union Joint Assembly to give an update on the spacecraft.

Upcoming shock

Both probes were launched in 1977. After studying the outer planets, they were sent on trajectories that would take them out of our Solar System in the general direction of the galactic centre.

Voyager 1 is flying above the plane of the planets and is now nearly 15 billion km (nine billion miles) from the Sun - nearly 100 times the Earth-Sun distance. Voyager 2 is south of the plane and a little under 12 billion km out (seven billion miles).

Both are investigating the extent of the heliosphere, the huge "bubble" within which the Sun dominates.

It contains electrically charged particles that have been blown off the Sun at high speed and which are now pressing up against matter from other stars.

Hubble view of L.L. Orionis (Nasa)
This Hubble picture shows how a star imprints itself on local space
Voyager 1 crossed a turbulent zone known as the termination shock in December 2004.

At the termination shock, the solar wind is slowed abruptly from its average speed of 300-700 km/s (190-430 miles per second) and becomes denser and hotter.

Scientists were able to report at the meeting that Voyager 2, although a good distance behind Voyager 1, is already seeing signs that it is coming up on the shock. It may even pass through it in the next year.

That Voyager 2 should get up to the shock so soon indicated the heliosphere was a different shape south of the planetary plane compared with the north, the meeting was told.

Long goodbye

Scientists believe that the observed discrepancies may be attributed to an interstellar magnetic field pressing inward on the southern hemisphere.

"The Voyager 1 terminal shock was about 90 astronomical units - 8.4 billion miles - from the Sun. Down south, the shock is somewhere between 80 and 84 astronomical units - 0.6 to one billion miles closer to the Sun than in the north," said Stone.

"Voyager 2 is now very close to the shock and should have crossed it by this time next year, if our estimates are correct."

Going beyond the termination shock takes the spacecraft ever closer to the "official" edge of the Solar System - the heliopause. Past this point, the Voyager probes would be in interstellar space and, by that stage, probably some 40 years on from their launches.

Schematic of the Solar System (BBC)
Glossary of terms that describe features of the heliosphere:
Solar wind: Stream of charged particles blown off the Sun and travelling at "supersonic" speeds
Termination shock: Area where particles from the Sun begin to slow and clash with matter from deep space
Heliosheath: A vast, turbulent expanse where the solar wind piles up as it presses outward against interstellar matter
Heliopause: The boundary between the solar wind and the interstellar wind, where the pressure of both are in balance
Interstellar wind: The gas and dust between the stars
Bow shock: The shock wave caused by the edge our Solar System travelling through deep space


Voyager 1 pushes for deep space
24 May 05 |  Science/Nature
Voyager probes in funding crisis
10 Mar 05 |  Science/Nature
Voyager 'at edge of Solar System'
06 Nov 03 |  Science/Nature
Mystery force tugs distant probes
15 May 01 |  Science/Nature
Distant probe phones home
30 Apr 01 |  Science/Nature

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