At The Solar System's Edge

Hopkins lab uses instrument on aging Voyager 2 to examine collisions that halt wind from the sun

July 21, 2008|By Dennis O'Brien | Dennis O'Brien,Sun reporter

Physicist Rob Decker is obsessed with a region of space that has an ominous name: the termination shock.

It's out there, at the very edge of the solar system, 90 times as far away as Earth is from the sun. It is the region where solar wind comes to a halt.

There you will find huge, constant collisions as solar wind - the waves of hydrogen and helium plasma that shoot out from the sun at 1 million mph - crashes into a dense haze of charged particles flowing through interstellar space.

Decker and his team at the Johns Hopkins Applied Physics Lab in Laurel used an instrument aboard the venerable Voyager 2 spacecraft this month to give astronomers the first detailed look at that smashup - in a region long shrouded in scientific mystery.

Decker's group published findings in the July 3 issue of the journal Nature based on measurements taken with the spacecraft's Low Energy Charged Particle Instrument. It gave astronomers a better picture of how the sun - and stars like it - interact with the space that surrounds them.

"Every star is doing this, but the sun is the only one we can see doing it," said Eugene Parker, a physicist at the University of Chicago who was not involved in the research.

The report, with others published this month on Voyager's trip into the termination shock, could compel astronomers to revise their theories about what goes on at the edge of the solar system, an area known as the heliosheath.

"Hundreds of papers have been published about this topic over the past 20 years. Now we have to start all over again," said APL's Stamatios "Tom" Krimigis, a co-author of the article.

Parker described the work as a major step toward understanding solar wind and how it behaves. "Things are always more complicated than your first groping theories would indicate," he said.

The existence of solar wind was not confirmed until the early 1960s - a few years after Parker, now 81, published a paper predicting its existence. The discovery was greeted skeptically, but researchers say it opened the door to a new field in astrophysics.

"The question since then has been, 'Where does the solar wind stop? Where is this termination shock?' " said Krimigis, who has worked with Voyager 2 since its launch 31 years ago. "What we've learned is, it certainly goes a lot farther out than anyone thought and it behaves differently than almost anyone believed."

Another question has been why solar wind cools as it reaches the termination shock.

"Everyone's been asking, 'Where did the energy go? Where did the energy go?' " said Krimigis "Now we know."

Solar wind shoots from the sun like water flowing into a pan from a tap, radiating outward until it reaches a turbulent edge.

When Voyager reached termination shock last August, its detectors showed the same effect as water in a sink, with hydrogen particles from space slowing down the solar wind and robbing it of heat.

"These hydrogen ions dominate what happens out there," said Decker, an APL physicist since 1980.

The two Voyager probes were designed to last just five years when NASA launched them in 1977 to study Jupiter, Saturn, Uranus and Neptune. But they were still operating in 1990, when NASA sent them on to explore the solar system's outer reaches.

Voyager 1 had a central role in the first Star Trek movie, released in 1979. In the film, the probe is a menacing interstellar force known to the starship's crew as the mysterious "V'GER." Its identity becomes clear only at the end of the movie.

Voyager 1 passed through the termination shock in late 2004, heading out of the solar system in a different direction. But its data were not as precise - astrophysicists debated whether the probe had reached the edge of the solar system or was still a year away. Voyager 2 answered many of their questions.

Engineers say the plutonium power generators on the twin probes could keep them in business till 2020.

"They could be making discoveries another 10 years or longer," Krimigis said. "We'll just to have to wait and see."

dennis.obrien@baltsun.com

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