The scientists and engineers who operate FUSE, the Far-Ultraviolet Spectroscopic Explorer, will celebrate a special anniversary today as the satellite-observatory-that-could officially moves beyond its original mission time and into a three-and-a-half year mission extension granted by NASA last summer.

The party for FUSE begins at 3:15 p.m. in the lobby of the Bloomberg Center for Physics and Astronomy and will include several brief presentations in the adjoining Schafler Auditorium.

The shift to FUSE's extended mission is cause for reflection and much rejoicing among the FUSE staff at Johns Hopkins and elsewhere, according to FUSE principal investigator Warren Moos, professor of physics and astronomy in the Krieger School of Arts and Sciences.

"We're going to celebrate all we've accomplished and look forward to the future," Moos said.

FUSE received tentative approval for the mission extension, which more than doubles the satellite observatory's lifespan to seven years, in 2002; a panel of scientists assembled by NASA gave the extension final approval last summer.

"The facility was oversubscribed by a factor of three in the last go-round, and I'm anticipating that there will continue to be high demand," said Bill Blair, chief of observatory operations for FUSE and a research professor of physics and astronomy in the Krieger School. "This, along with the science we produced, was one of the keys to getting the mission extension. People want to use FUSE."

Joel Bregman, a professor of astronomy at the University of Michigan, has been using FUSE as a guest investigator since the first round of observations. He said he found FUSE so useful that he thought of it as "almost a given" that FUSE would win extended mission support.

"They've just been one of the most important missions in this waveband for a while," Bregman said. "It's turned out to be possibly a more valuable mission than even the planners had envisioned."

Bregman, who uses FUSE to study hot gas in early galaxies and galaxy clusters, noted that FUSE has even proved itself essential for astronomy problems that became high-priority only after the FUSE mission began.

"For example, one big topic FUSE is going after is missing normal matter in the universe," Bregman said. "Well, that wasn't really a central goal when the mission started, but it's now clear that this issue is a very important one, and one that FUSE is very well-suited for."

Blair cited a range of impressive statistics accrued by FUSE in its prime mission.

"We have observed 1,700 unique targets and obtained just shy of 29 million seconds of data in the first three years, so it's been a very productive prime mission," Blair said. "The astronomers who have used FUSE had already produced 146 refereed papers by our last tally."

According to Alex Fullerton, a visiting associate research scientist at Johns Hopkins, a key contributor to FUSE's high publication rate has been the fact that many investigators have put together proposals for observing targets never thought of by the project's principal investigator team.

"I've used FUSE to study hot stars that live fast and die young and are important because they create many of the heavier elements," said Fullerton, whose permanent appointment is at the University of Victoria in Canada. "FUSE looks at just the right portion of the spectrum to let us really test the reliability of our models for how hot stars work."

In addition to the new and sometimes unexpected work guest investigators have produced, FUSE's principal investigators have made significant progress on its primary scientific missions, according to Moos.

"Some of our big projects are mature and have been published, and some have a fair amount of data analysis left and maybe even some observations," Moos said.

For example, FUSE astronomers have assembled a picture of the outer edges of the galaxy, where they've detected signs of a halo of hot gas that has never been seen directly. Scientists using FUSE were able to infer the existence of this corona of hot gas by showing that cold gas falling into the Milky Way has a hot edge produced by its impact with the corona, not unlike a meteor falling into Earth's atmosphere. The new data should help astronomers seeking to develop a better understanding of how the galaxy formed.

On another front, astronomers have begun using FUSE to look back in time to a critical period in the universe's formation when elements began to regain electrical charges. They also have used FUSE to characterize the presence of deuterium, a form of hydrogen produced by the Big Bang. They've been studying its presence in Earth's cosmic backyard and beyond, an important step toward learning more about the Big Bang, the great explosion that created the universe.

As FUSE switches over to its extended mission, FUSE investigators, formerly guaranteed a share of observing time, will have to compete with the rest of the astronomical community for use of the satellite.

"It's inevitable that in looking back, we'll shudder a little," Moos added about the upcoming celebration, and laughed. "We've had some tough experiences. But we're beyond the learning phase now and mainly looking at taking advantage of its unique capabilities."

FUSE has survived challenging times both before its launch and after. Budget cuts nearly forced the mission's cancellation prior to its launch in June 1999, but the FUSE leadership team was able to save it through several cost-saving measures, including a unique plan for placing satellite operations at a facility on the Johns Hopkins Homewood campus. In late 2001 and early 2002, failure of a critical component for maneuvering the satellite caused FUSE to shut itself down. After weeks of intense efforts to come up with a solution, engineers and scientists managed to conduct a daring in-orbit reprogramming of FUSE that adapted a part originally used for other purposes to push and pull on the Earth's magnetic field, supplying the necessary maneuvering power.

FUSE is headed for another milestone possibly as soon as this month. Blair explained that engineers are planning a sort of computerized "brain transplant" for the satellite that will help insure its longevity. He and Moos promised more details to come.

"When the capability that's provided by FUSE is gone, it's gone," Blair noted. "There's nothing else coming down the line that can make these kinds of observations, so we're doing what we can to make sure it stays running as long as possible."

FUSE, which is operated for NASA by Hopkins, is an international collaboration among NASA, the Canadian Space Agency and the Centre National d'Etudes Spatiales (France).