Astronomy teams last year reported that the 28-year-old NASA spacecraft Voyager 1 has reached a new frontier by passing a boundary that separates our solar system from deep space.

The exact location of this boundary has long been the subject of scientific disputes. Called the “termination shock,” it is a region surrounding the solar system in which the solar wind-electrically charged gases ejected from the sun-drops below supersonic speeds of about 112,000 mph.

Evidence of crossing

Three years ago, NASA announced that Voyager 1, which has gone farther into space than any other spacecraft, showed signs of approaching the region. But this claim was not universally accepted.

Then four teams monitoring Voyager 1 reported that the spacecraft had passed the termination shock on or about December 16, 2004, about 8.74 billion miles from the sun. The scientists cite the detection of a sudden increase in low-energy-charged particles as evidence that the termination shock had been crossed. Such particles are rarely found in the solar system.

The data also suggest the termination shock was retreating toward the sun when Voyager 1 crossed it. “In fact, the shock probably crossed Voyager rather than Voyager crossing the shock,” says Ed Stone of NASA’s jet Propulsion Laboratory, which runs the mission.

Surprising discovery

“The recent Voyager 1 observations of the termination shock are a major achievement in defining the physical properties of the solar system,” says space science pioneer James Van Allen of the University of Iowa.

However, the termination shock also delivered on surprise, says space scientist Len Fisk of the University of Michigan. Scientists had theorized that high-energy cosmic rays would be accelerated by the solar wind at the boundary. This hasn’t happened. Instead, the unknown source of the rays seems to be deeper in space.

“For those of us who do this for a living, this is the fun part, where things we didn’t expect happen.”

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