An artist's concept of EUVE. Photo: NASA

A defunct NASA astronomy satellite, in orbit for nearly a decade, is predicted to reenter the Earth's atmosphere by Thursday but poses little risk to people on the ground, space agency officials said Tuesday.

The Extreme Ultraviolet Explorer (EUVE) spacecraft is scheduled to come down between 10 p.m. EST Wednesday and 7 a.m. EST Thursday (0300 to 1200 GMT Thursday). The spacecraft, currently 200 kilometers (125 miles) above the Earth, is losing altitude at 25 km (16 mi.) a day, a rate that will accelerate as the spacecraft dips deeper into the upper atmosphere.

Most of the 3,275-kilogram (7,220-pound) spacecraft is expected to burn up harmlessly in the Earth's atmosphere. However, NASA officials said that nine objects, made of titanium and stainless steel and weighing between 2 and 45 kilograms (4 and 100 pounds), could survive reentry and hit the ground.

"The probability of the few EUVE surviving pieces falling into a populated area and hurting someone is very small," said Ronald Mahmot, project manager for space sciences missions at NASA's Goddard Space Flight Center. "It is more likely that the small pieces will fall into the ocean or fall harmlessly to the ground."

EUVE is in an orbit inclined 28.5 degrees to the equator, which means the spacecraft overflies regions of the Earth between 28.5 degrees north and south latitudes. This includes much of the state of Florida as well portions of South and Central America, the Caribbean, much of Africa, south Asia, Indonesia, and northern Australia.

Unlike NASA's Compton Gamma Ray Observatory, which was deliberately deorbited in June 2000, there are no thrusters on EUVE to control its reentry. The exact reentry time and location will not be known until about 12 hours beforehand, a few hours before the spacecraft reaches an altitude of 80 kilometers (50 miles) and begins to break up.

The reentry marks the final chapter in the long history of EUVE. The spacecraft, an Explorer-class mission, was launched June 7, 1992 on a Delta 2 from Cape Canaveral. From its location in Earth orbit, above the atmosphere, the spacecraft was the first to study the universe in detail at extreme ultraviolet (EUV) wavelengths of light, between 70 and 760 angstroms. While experiments flown on previous missions, including Apollo-Soyuz in 1975, had suggested that at least a handful of objects could be seen at these wavelengths, it was EUVE that observed over 1,000 objects, opening a new field of astronomy.

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 was originally intended to operate for only three years, but the success of the spacecraft caused NASA to extend the mission twice, eventually transferring control of the mission from Goddard to the University of California Berkeley. In 2000 a NASA scientific advisory panel recommended that the mission be terminated, even though the spacecraft was still in good health, in order to better spend limited space science funds. Although scientists involved with EUVE petitioned NASA to keep the mission alive for one more year, EUVE was shut down in January 2001.