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Discovery Newsletter

Volume 13Discovery NewsletterIssue 1Discovery NewsletterOctober 2001

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Satellite with Montana Connections Celebrates a Decade of Discovery


by Annette Trinity-Stevens

Loren Acton A spacecraft with a telescope designed and built by Montana native Loren Acton turned the ripe old age of 10 last month.

The Yohkoh spacecraft, launched August 30, 1991, has captured more than six million images of the sun and ushered in a decade of discovery for solar scientists around the globe. "The world of solar physics will never be the same again, thanks to Yohkoh," said Acton, the Fergus County native whose team built the telescope while at Lockheed-Martin Solar and Astrophysics Laboratory in Palo Alto, Calif.

In 1985, Acton flew aboard the space shuttle Challenger to observe the sun from space and to head up other in-space science projects.

Not long after Yohkoh's launch, Acton retired from Lockheed, moved to Bozeman and began a solar physics research group at Montana State University.

Japanese for "sunbeam," Yohkoh is a mission of Japan's Institute of Space and Astronautical Sciences. Acton's team supplied the telescope, which uses lower energy X--rays, or soft X-rays, to capture images of the sun's hottest regions.

The United Kingdom also is involved in the mission. Acton heads the U.S. part of the international project from MSU.

Yohkoh revolutionized studies of the sun by taking continuous digitized X-ray images of earth's closest star through nearly an entire 11-year sunspot cycle. During that cycle, the sun goes from relatively calm with few flares and related outbursts to a time of intense storms and sunspots and back again.

"Before Yohkoh, observations of the sun were a few weeks at most," said Acton. "So the long-term observation of the sunspot cycle is really a tremendous difference." MSU research scientist David McKenzie said Yohkoh's main contribution has been in the study of the sun's outer halo, called the corona. Scientists have long been stumped over why the corona burns nearly 1,000 times hotter than the surface of the sun itself.

"In the last several years, Yohkoh has allowed us to rule out some theories and reinforce the strongest and best theories," McKenzie said.

Yohkoh also has helped scientists understand solar flares, huge explosions in the atmosphere of the sun caused by a violent burst of magnetic energy. A typical solar flare can release in less than one hour as much as 10,000 times the annual energy consumption of the U.S.

Acton and McKenzie don't even try to hide their enthusiasm, even affection, for the satellite still going strong despite 10 years spent in the harsh, uncaring environment of outer space. "The spacecraft is in near perfect health," McKenzie said. NASA has agreed to fund two more years of its operation.

Realistically, though, Yohkoh is expected to last another decade--long enough to observe another complete sunspot cycle and to complement solar images taken by another NASA spacecraft, named HESSI, scheduled for launch in October.

McKenzie is one of several scientists who operate the telescope on a rotating basis from Japan. The job, he said, is a blast.

"As part of my job, I'm one of the first people on the planet to see what our star has done in the last few hours, and that is just a kick in the pants," McKenzie said. "The sun is changing all the time...and to be the first to see it is really thrilling."

Others from MSU who take turns operating the telescope are Piet Martens, Richard Canfield and Jun Sato.

Yohkoh's birthday will be celebrated next January when Acton, McKenzie and about 150 other scientists from around the world meet for a conference in Kona, Hawaii. The group will discuss Yohkoh's latest discoveries.

Annette Trinity-Stevens is the director of research communications.

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