Extreme Ultraviolet Light Mission Comes To An End

The Extreme Ultraviolet Explorer (EUVE) spacecraft surveyed the universe by observing extreme ultraviolet (EUV) light. "We opened a new window on the cosmos with EUVE," said Dr. Alan Bunner, Science Director for the Structure and Evolution of the Universe program at NASA Headquarters in Washington, DC. "No one had thoroughly explored the heavens in the extreme ultraviolet before, and EUVE filled significant gaps in our understanding."

Eventually, aerodynamic drag on the 7,000-pound spacecraft will cause it to reenter the Earth's atmosphere. Current predictions put EUVE's reentry sometime in late 2001 or early 2002.

Unlike the Compton Gamma Ray Observatory, which had a propulsion system to allow for its controlled reentry earlier this year, EUVE has no method for directing its impact to a specific entry point. However, extensive analysis by NASA reveals the amount of debris likely to survive reentry is extremely small and will likely fall harmlessly into the ocean.

EUVE was launched on June 7, 1992, aboard a Delta II rocket from Cape Canaveral, FL. EUVE was originally slated for three years of science observations, but NASA twice extended its mission.

Operated at NASA's Goddard Space Flight Center, Greenbelt, MD, during its early years, EUVE mission operations are now conducted by the University of California at Berkeley.

EUV light is invisible to the human eye, and is more energetic than ordinary ultraviolet light. Before EUVE was launched, many scientists feared it would not see much, because they believed the thin gas between the stars, known as the interstellar medium, would block extreme ultraviolet light. Just as only the closest streetlights are visible through a thick fog, only the closest and brightest EUV-emitting objects were expected to be seen through the absorption caused by the interstellar medium.

However, EUVE demonstrated the nature and density of the interstellar medium was different from what most expected; patches of it were ionized (electrons were removed from atoms, mostly hydrogen) rendering it transparent to extreme ultraviolet light. The transparent areas riddle the interstellar medium, like holes in Swiss cheese, and in some directions even remote EUV objects outside our galaxy can be seen.

"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 Dr. Yoji Kondo, Project Scientist for EUVE at Goddard.

Rather than seeing only a couple dozen nearby objects as many expected, EUVE observed more than 1,000 sources, including more than three dozen objects outside our galaxy.

EUVE observed the superheated, multimillion-degree outer atmospheres, or coronae, of stars. Astronomers studied various kinds of stars to determine how active and bright their coronae are; how hot, dense and variable they are; and what elements they are made of. Astronomers were able to compare these observations with detailed observations of the corona of the closest star, our Sun. This enabled them to obtain new insights into the processes that form coronae and cause these gases to be super-heated.

While no mission is currently planned to replace EUVE, Bunner said the Chandra X-ray Observatory, launched in July 1999, is capable of making observations of some of the same phenomena as EUVE, such as stellar coronae, X-ray binaries and active galactic nuclei.

More information on EUVE can be found on the Internet at: http://www.cea.berkeley.edu/