NASA'shas begun to study what on Earth to do with the Upper Atmosphere ResearchSatellite (UARS).

Thisheavy weight spacecraft is due to de-orbit on its own in years to come, due tonatural forces. However there's a problem: UARS was not designed for acontrolled re-entry. That increases the chance it might not safely crash intoremote ocean waters. Due to its size, prospects are high that leftover chunksof the spacecraft could reach Earth.

NASAis studying whether plucking the craft from orbit via the shuttle is an option,albeit an expensive choice. Alternatively, the satellite may be allowed toaugur in naturally.

Rightnow, UARS remains hard at work delving into the workings of Earth's atmosphere.It was deployed from space shuttle Discovery in September 1991. UARS, among itsduties, is helping develop techniques for early detection of harmful changes inthe ozone layer.

UARShad a design life of 18 months. But after a decade of service the spacecraftcontinues to collect Earth science data. The craft weighs some 13,000 pounds(5,909 kilograms), with eight of its ten instruments still functioning.

Not toworryfor now

Astudy was done last month by the NASA Johnson Space Center's (JSC) orbitaldebris office. They assessed the probability of human casualty resulting froman uncontrolled reentry of UARS.

However,don't worry, at least for now.

UARSisnt due to take a tumble from above for another decade, according to currentestimates.

DavidSteitz, a spokesman for the Earth Science Enterprise at NASA Headquarters inWashington, D.C., told SPACE.com that the UARS mission is to continue tooperate through the end of September.

UARSis on tap to gather information that might one day complement data from theEuropean Space Agency's ENVISAT, due for liftoff on an Ariane 5 booster thisThursday, Steitz said.

"Thereis no definitive decommissioning date for UARS," Steitz said. "NASAwill conduct periodic assessments to evaluate the cost expenditures of UARSversus the return of science and our science needs. The next assessment of themission will most likely occur in late spring, looking toward next year,"he said.

"UARSwas designed in the 1980s, a time when the agency created 'expendable' missionswithout the same level of attention to reentry concerns that we have today.UARS was not designed for a controlled reentry," Steitz said.

Nodecision has been made on how UARS will come home - on its own, or tucked inwithin a shuttle's cargo bay, Steitz said. "We have ample time tothoughtfully look at all options, and are still considering all options, withsafety as NASA's first priority," he said.

Anon-compliant newcomer

Whiledecisions are still forthcoming on what to do with any incoming UARS, another satellitehas also gained the attention of orbital debris experts.

Ateam led by William Rochelle of Lockheed Martin's Advanced Systems Group inHouston, Texas, reports that NASA's soon-to-be-launched Aura spacecraft hasbeen found to be "non-compliant" with a NASA safety standard onreentering satellites.

Aurais part of NASA's Earth Observing System (EOS) program. The spacecraft isslated for launch by a Delta booster in the June-July 2003 time frame, and isunder management of the space agency's Goddard Space Flight Center inGreenbelt, Maryland. Builder of the craft is TRW Inc. of Redondo Beach,California.

TheEOS Aura satellite is based on TRW's EOS Common Spacecraft, and is a sibling ofNASA's EOS Aqua mission. That spacecraft has just arrived at Vandenberg AirForce Base in California for launch this spring.

Oncelobbed into a polar, sun-synchronous orbit, Aura's mission is to study theEarth's ozone, air quality and climate, and is to operate for five or moreyears.

Rochellereports that Goddard engineers first performed a reentry analysis of Aura,making use of NASA Johnson Space Center's (JSC) Debris Assessment Software.That specialized software can assess the chances of a spacecraft's componentssurviving a dive through Earth's atmosphere. Data showed that pieces of Aurawould likely reach the surface.

Byutilizing a higher fidelity Object Reentry Survival Analysis Tool (ORSAT) --developed by specialists at JSC and Lockheed Martin Space Operations -- it wasfound that Aura hardware would, indeed, survive a fiery fall from space.

Survive thedive

TheORSAT assessment found that Aura would break-up on reentry, with most of thespacecraft consumed by heat during the plunge. However, due to spacecraft mass,size and materials used, an "increased possibility" exists thatpieces of the over two-and-a-half ton (2,400 kilograms) satellite could surviveand pose a safety risk to people on the ground.

SeveralAura components were found likely to endure the hellish plunge through thin tothick atmosphere. Among them: A titanium propulsion module, four steel reactionwheel assemblies; scan mirror assemblies in one Aura instrument, as well as aberyllium gimbal that's part of the spacecraft's Tropospheric EmissionSpectrometer.

Rochelleand other reentry debris experts point out that titanium, steel, and berylliumtend to have higher survival tendencies. That's due to the high temperaturesneeded to melt those material types.

UsingORSAT, a simulated Aura reentry found the debris footprint to be some nearly175 miles (280 kilometers) in length.

Forthe last several years, NASA has dealt with several deorbiting scientificsatellites.

TheCompton Gamma Ray Observatory was finessed from orbit in 2000 using a series ofdelicate maneuvers. That craft safely splashed down in the Pacific Ocean.

Latelast month, NASA's Extreme Ultraviolet Explorer (EUVE) spacecraft returned toEarth on its own. Pieces of that satellite are thought to have landed somewherein Egypt. There were no reports of debris being recovered.