Several US space missions have recently demonstrated their commitment to curtailing the growth of the orbital debris environment by following vehicle disposal recommendations set forth in NASA Safety Standard 1740.14, Guidelines and Assessment Procedures for Limiting Orbital Debris, and in the US Government Orbital Debris Mitigation Standard Practices. The principal goals are to prevent debris generation by explosions and collisions. The former can be achieved by passivating the vehicle, i.e., depleting sources of stored energy, while the latter can be satisfied by removing the vehicle from highly congested regions of space.

NASA's Gravity Probe B mission began on 20 April 2004 with the launch of the spacecraft into an operational orbit near 640 km altitude. Following release of the spacecraft, the second stage of the Delta 2 launch vehicle (International Designator 2004-014B, US Satellite Number 28231) performed a maneuver to eliminate residual propellants and pressurants and to reduce dramatically the orbital lifetime of the stage. By lowering the stage's perigee to approximately 185 km, operators were able to limit the stay of the stage in Earth orbit from decades to only five weeks. Reentry of the Delta 2 second stage occurred uneventfully over a broad ocean area on 27 May 2004.

The NOAA 11 meteorological spacecraft (International Designator 1988-089A, US Satellite Number 19531), orbiting the Earth at an altitude of approximately 840 km, completed nearly 16 years of service on 16 June 2004. Decommissioning procedures included disconnecting the battery charge and discharge paths to prevent an accidental battery overcharge and subsequent explosion. Since NOAA 11 was designed and launched in the 1980's, prior to the establishment of formal orbital debris mitigation guidelines, the spacecraft was unable to maneuver into a shorter-lived disposal orbit. The next generation of polar-orbiting environmental spacecraft (POES) will have the capability for endof- mission maneuvers which will significantly reduce their time in Earth orbit and the chances of actual collisions with other resident space objects.

For spacecraft in high altitude geosynchronous orbits (GEO), the recommended disposal strategy is to maneuver the satellite into a storage orbit above GEO where it cannot interfere with operational spacecraft. NASA and other US Government agencies currently recommend placing retired spacecraft into an orbit at least 300 km above GEO, in accordance with a 1993 recommendation of the International Telecommunication Union (ITU). In 1997 the Inter-Agency Space Debris Coordination Committee (IADC) proposed a formula for determining the minimum initial perigee for the storage orbit, based upon spacecraft characteristics, to prevent future gravitational and solar radiation pressure perturbations causing the spacecraft later to come within 200 km of GEO. The ITU, NASA, and other US Government agencies are considering or in the process of adopting the IADC GEO disposal recommendation.

During 5-6 May 2004 the 10-year-old GEOS 8 spacecraft (International Designator 1994-022A, US Satellite Number 23051) reached the end of its useful life and was maneuvered into a disposal orbit of approximately 375 km by 400 km above GEO, satisfying all current US and international recommendations. The three maneuvers employed also consumed all remaining propellant in the spacecraft to prevent a later accidental explosion.

Two US commercial GEO communications spacecraft were retired during the first six months of 2004, and both were maneuvered into storage orbits more than 300 km above GEO. The first was the GSTAR 4 spacecraft (International Designator 1990- 100B, US Satellite Number 20946). During the period 29 January - 2 February, the spacecraft conducted a series of maneuvers to place it in a nearly circular orbit about 315 km above GEO. In March the PAS 6 spacecraft (International Designator 1997-040A, US Satellite Number 24891) was decommissioned prematurely due to power system difficulties. Since the spacecraft still contained a significant amount of propellant, the vehicle was placed into a moderately elliptical orbit with a perigee of about 450 km above GEO.

Finally, NASA's Advanced Communications Technology Satellite (ACTS) (International Designator 1993-058B, US Satellite Number 22796) was decommissioned on 28 April after more than 10 years of service. Unfortunately, a 1998 reassessment of propellant reserves revealed a much lower amount than expected, rendering the spacecraft incapable of performing a planned disposal maneuver. In August 2000 ACTS was moved to the stable point near 105o West to ensure that it would not drift around the GEO ring after termination and become a collision hazard.

The events cited above clearly indicate the commitment of the US Government and a growing number of commercial operators to prevent the generation of unnecessary orbital debris by properly disposing of spacecraft and launch vehicle orbital stages at the end of their useful lives. Many other countries and international organizations are following similar procedures to preserve the near-Earth environment for future generations.