星系演化探测器准备发射

Galaxy Evolution Explorer ready for launch

BY JUSTIN RAY
SPACEFLIGHT NOW
Posted: April 24, 2003

An observatory that will survey 10 billion years of the Universe's history like never before will be hurled into space Monday aboard an air-launched Pegasus XL rocket.

                               
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The Pegasus rocket will be launched at 39,000 feet over the Atlantic. Photo: NASA

Dropped from the belly of the Stargazer carrier jet off the east coast of Florida at 8:00 a.m. EDT (1200 GMT), the 51,000-pound, three-stage Orbital Sciences launcher will deliver NASA's Galaxy Evolution Explorer into a 430-mile circular orbit inclined 29 degrees to the equator.

Fitted with a 19.7-inch telescope with near- and far-ultraviolet detectors, the 609-pound satellite is expected to begin scientific work by the end of May.

"This is going to be the first ultraviolet survey mission of the Universe," said James Fanson, the GALEX project manager from NASA's Jet Propulsion Laboratory. "Ultraviolet is really the last portion of the electromagnetic spectrum where we don't have a survey of the Universe. So it fills in that last gap."

"The Galaxy Evolution Explorer is crucial to understanding how galaxies, the basic structures of our universe, form and function," added Dr. Anne Kinney, director of astronomy and physics in the Office of Space Science at NASA Headquarters.

GALEX is expected to make observations for 28 months, although officials say funding is the only "life-limiting" constraint on the mission.

                               
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An artist's concept of GALEX deployed in orbit. Credit: NASA

"Being a survey mission, we are going to collect an enormous volume of data that will constitute a huge legacy for the entire scientific community. But from that data our scientists will be studying, specifically, evolution of galaxies over 80 percent of the age of the Universe," Fanson said in an interview.

"(GALEX) is designed specifically to measure the distances and star formation rate in a million galaxies. It is going to help us put together a picture of the Universe we see today -- how stars formed in galaxies, how galaxies evolved through time, what caused star formation and led to the development of heavy elements of the periodic table.

"There is evidence from surveys that have been done from the ground that star formation peaked at a particular point in history. But the data is not very precise, it is suggestive. This mission is designed to make a definitive measurement of what the star formation history has been in the evolution of galaxies."

The launch of GALEX was delayed last month after workers found a connector on the satellite wasn't attached, prompting a thorough inspection of connectors throughout the craft.

"In the course of performing some of our functional tests and final thermal blanket closeout, we discovered there was a connector on the spacecraft bus that pulled loose and had never been flight-mated," Fanson explained.

                               
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Workers install debris shields inside the GALEX spacecraft in the Kennedy Space Center clean room. Photo: NASA

"Each electrical connector has two screws. On the connector was still attached one screw but on the other side of the connector there was no screw. So it appeared as though the connector had never been flight-mated.

"It was supposed to have been mated some time ago. So our conclusion was that this missing fastener was probably never on the connector. But it was quite a surprise to discover that loose connector. So to be 100 percent certain that every other connector was properly mated, we did a 100 percent visual inspection both inside and outside the spacecraft."

The precautionary check revealed all of the other connectors were properly mated.

"We also took some additional time to put a thermal blanket over a portion of the instrument," Fanson said of the launch postponement. "There is an optics wheel in the instrument that is rotated by a mechanism. Just in the unlikely possibility that this missing fastener is still floating around somewhere inside, we wanted to make sure it could not float into the mechanism in any way."

So how the the loose connector escape earlier detection in pre-flight activities?

"Apparently, it was out of sight and didn't get caught in inspections. But that is the reason we do tests and people checking other people's work. What is also unusual is that apparently this connector was in a non-flight mated condition through the entire environmental tests...None of these environments were enough to pull it loose. It came lose when they were actually working on the blanket."