Mitigating Orbital Debris via Space Vehicle Disposals

Editor’s note: Orignally published in the July 2004 issue of NASA JSC’s Orbital Debris Quarterly News.

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.