Space observatory shut down
BY JEFF FOUST
Posted: February 2, 2001
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.
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.
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!"