An artist's concept of EUVE. Photo: NASA

A chapter in the history of astrophysics quietly came to an end this week when NASA shut down the Extreme Ultraviolet Explorer (EUVE) spacecraft, a mission that gave astronomers their first glimpse of the universe at a key chapter of the electromagnetic spectrum.

Spacecraft controllers put EUVE into a safe hold Wednesday at one second before midnight GMT (6:59:59 pm EST), four days after its last scientific observations were performed, effectively turning off the otherwise-healthy orbiting spacecraft.

Launched into low-Earth orbit June 7, 1992 on a Delta 2 rocket from Cape Canaveral, EUVE was the first spacecraft able to observe the universe at extreme ultraviolet (EUV) wavelengths, a region of the spectrum at wavelengths between 70 and 700 angstroms that lies between ultraviolet and more energetic x-rays. (By comparison, visible light lies between approximately 3500 and 7000 angstroms.) EUVE was part of NASA's Explorer class of spacecraft that dates all the way back to America's first satellite, Explorer 1, in 1958.

Delta 2 rocket lifts off with EUVE. Photo: NASA

EUV wavelengths were once considered uninteresting by astronomers because hydrogen, the primary component of the interstellar medium, effective absorbs light at those wavelengths. Trying to observe at those wavelengths, some feared, would be like trying to observe through a fog, with only a few bright, nearby sources of EUV radiation visible. However, bits of data collected from other missions suggested that the interstellar medium might not be as dense and as uniform as once thought, which, coupled with advances in detector technology, made it worthwhile for the space agency to pursue a mission like EUVE.

Data from EUVE showed that the interstellar medium was far more transparent to EUV radiation than anyone had previously thought, thanks in part to ionized regions that are transparent to light at those wavelengths. Instead of cataloging just a few dozen sources of radiation, according to the most pessimistic estimates, EUVE detected more than 1,000 sources of EUV radiation, including more than three dozen outside our galaxy.

EUVE provided insights into a wide range of astronomical phenomena. EUVE observations of several comets detected soft x-ray emissions caused by the interaction of charged particles from the solar wind with neutral atoms and molecules from the comets. Observations of distant stars allowed astronomers to study their extremely hot outer atmospheres, or coronae, and compare them with the Sun's own corona in an effort to understand how they are heated. EUVE was also used in joint observations with the Chandra X-ray Observatory to help calibrate some of Chandra's instruments.

"EUVE opened up one of the last frontiers of astronomy, closing the crucial gulf between the two probed regions of electromagnetic radiation, gamma-ray and x-ray at the high energy end and far-ultraviolet to visible light, infrared and radio at lower frequencies, thus making our view of the cosmos more complete," said EUVE project scientist Yoji Kondo of NASA's Goddard Space Flight Center.

Although EUVE remained in excellent health throughout its mission, earning two mission extensions, issues of cost and scientific merit led NASA to decide late last year to terminate the mission. A senior review panel of scientists concluded last summer that the science being returned by the aging spacecraft was "not compelling", and recommended that EUVE be shut down.

A false-color image of the Cygnus Loop supernova remnant. The extreme ultraviolet (EUV) light seen in the image originates from gas that was heated to very high temperatures by a supernova explosion. The temperature of the gas is between 1 and 5 million degrees Celsius. Photo: CEA/UCB/NASA

Mission supporters made a brief effort to keep the mission alive last fall, trying to drum up Congressional support for extending EUVE's mission one more year. They argued that the amount of money needed to continue EUVE operations was minimal -- only about $1 million -- and that the additional funding would permit long-term science projects using the spacecraft to continue.

NASA, though, countered that even though the amount of funding requested was small, it was money that could be better spent elsewhere. "It's not a money issue, it's based on the science," Ed Weiler, NASA associate administrator for space science, told members of Congress during a hearing last fall. "I would rather take that money and put it into Chandra, Mars Global Surveyor, or the Hubble Space Telescope, because I believe the [scientific] community would tell us the money would be better spent."

Efforts to raise Congressional interest in saving EUVE failed, though, and project officials conceded defeat last October. NASA provided funding to keep EUVE operational through January. "Unfortunately, we were unable to mobilize sufficient support to obtain Congressional help," said project director Roger Malina.

EUVE will remain in orbit, silent, for about one more year as its orbit gradually decays. NASA predicts the spacecraft will reenter the Earth's atmosphere late this year or early next year. The 3,200-kg spacecraft is not expected to pose a threat to life or property upon reentry: NASA estimates that the amount of material expected to survive reentry is "extremely small" and would likely land in the oceans.

NASA has no plans for the foreseeable future for a follow-on mission to EUVE, depriving astronomers a view of the universe at extreme ultraviolet wavelengths for potentially many years. "It will be decades before astronomers have access to the EUV band," said Malina. "The unobservable ultraviolet will soon be unobservable again!"