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Voyager 1's intrepid mission into the heliosheath

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As Voyager 1 spacecraft speeds through the outermost boundary of our solar system it is tearing clods out of our understanding of the universe, say mission scientists.

Researchers at the University of Maryland's Institute for Physical Science and Technology confirmed in Science last week that Voyager 1 had crossed the termination shock, the unexplored area of space that marks the beginning of the heliosheath, where our sun's solar wind starts to run out of steam as it comes smack against the swirling currents of interstellar space. (The date this occurred had been disputed, but it happened on 16 December 2004, said the Maryland team).

When Voyager got to the shock it discovered that things were not quite as predicted. Scientists expected it to find there the source of the little understood Anomalous cosmic rays (ACRs). Instead, it found that they come from somewhere further out there on its trajectory.

Matthew Hill, an anomalous cosmic ray expert at Johns Hopkins University Applied Physics Laboratory (who was at Maryland till this week), said the findings will help us understand other shocks, such as those created by the solar flares that wreck our communications systems and endanger our astronauts.

ACRs were thought to be created in the termination shock after electrically neutral particles had been netted from the interstellar space clouds through which our solar system hurtles on its course around the galaxy. On hitting the solar wind, the interstellar particles pick up a charge, becoming pickup ions. The ions are then accelerated with energy, it was thought, by the shock, to become anomalous cosmic rays.

ACRs are, as their name would suggest, a bit weird. They are weaker than cosmic and galactic rays, and tend to get buffeted around like clouds of manic dandelion spores on the solar wind. They exist in high densities at the edge of the solar system and spread farther apart the closer they get to the sun. They pass through the solar system without causing any bother beyond a headache for puzzled astrophysicists.

It appears they are now causing a little more turmoil than their low charges would warrant. Hill said 25 years of research on the termination shock and ACRs would now have to be revised.

"We'd now like to find out where these particles are coming from. It's either in the heliosheath or beyond," he said. "We don't know where they are coming from...But they are probably still created in the heliosphere because with their energy it would be difficult for them to come from too far away."

Next up on Voyager 1's intrepid mission into the heliosheath, in addition to discovering the source of anomalous cosmic rays, will be a greater understanding of what happens when our solar wind goes phut phut against the pressures of interstellar space - and there are probably some more theories to be debunked as well. ®

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