'Minimal' risk from deorbiting spacecraft

After almost a decade in space it is heading home

By BBC News Online science editor Dr David Whitehouse

The American space agency's Extreme Ultraviolet Explorer (EUVE) spacecraft is due to re-enter the Earth's atmosphere and break-up sometime during the next 24 hours.

The probability of the few EUVE surviving pieces falling into a populated area and hurting someone is very small

Ronald Mahmot, EUVE project manager

Engineers at the Goddard Space Flight Center in Maryland, US, say the 3,100-kilogram (7,000-pound) spacecraft could come down as early as 0400 GMT on 31 January or as late as 1300 GMT that day.

EUVE is currently 200 kilometres (120 miles) above the Earth with a descent rate of 25 km (15 miles) a day. Because EUVE is in a 28.5-degree orbit, it could re-enter in any location that much north or south of the equator.

When it breaks up, the estimated debris field is expected to be 800 to 1,000 km (500-620 miles). Several large chunks will reach the ground but scientists say the danger is minimal.

"The probability of the few EUVE surviving pieces falling into a populated area and hurting someone is very small," says Ronald Mahmot, Project Manager for Space Science Mission Operations at Goddard. "It is more likely that the small pieces will fall into the ocean or fall harmlessly to the ground."

Uncertain impact zone

Unlike the Compton Gamma Ray Observatory, which was safely de-orbited in June 2000, EUVE does not have an on-board propulsion system so engineers cannot control its re-entry.

Based on past experience, much of EUVE will burn up in the atmosphere before ever reaching the ground. However, estimates show that up to nine objects ranging from approximately four to 450 kg (1,000 pounds) may survive re-entry. Much of this debris is made of titanium and stainless steel.

EUVE: On its way up

EUVE will start to break up when it falls to within 80 km (50 miles) of the Earth. At this point, EUVE will have only four or five 90-minute orbits left before re-entering the Earth's atmosphere. Engineers will not know the re-entry point until just a few hours before impact.

Scientists will be sad to see EUVE destroyed. Since it was launched in June 1992 until its science operations were ended in December 2001, it has opened a new window in the extreme ultraviolet region of the spectrum.

Rather than seeing about 24 nearby objects as many astronomers had predicted, EUVE observed more than a thousand nearby sources, as well as more than three dozen objects outside our galaxy.