The Threat of Orbital Debris and Protecting NASA Space Assets from Satellite Collisions - 28 April 2009
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National Aeronautics and Space Administration
The Threat of Orbital Debris and
Protecting NASA Space Assets from
Satellite Collisions
28 April 2009National Aeronautics and Space Administration
Executive Summary
• Collision avoidance processes are in place for all NASA human space flight
missions and for maneuverable robotic assets in low Earth orbit and within
200 km of the geosynchronous orbit.
– Required by NASA Procedural Requirements 8715.6A (Section 3.4).
• DoD screens for close approaches (conjunction assessments) and provides
miss distance and uncertainty information to NASA.
• NASA computes the probability of collision, analyzes the risk, and makes
maneuver decision.
• During 2008, this process led to five collision avoidance maneuvers:
S p a c ec r a f t M a n e u v e r D a te O b je ct A v o id e d
A ura 2 6 - J u n -2 0 0 8 T R IA D 1 d e b ri s
C lo u d s at 2 0 - J u l- 20 0 8 D e l ta r o c k et b o d y d eb r is
IS S 2 7- A u g -2 0 0 8 C o s m o s 2 4 2 1 d e b r is
TD RS 5 1 -O c t-2 0 0 8 C os m o s 1 8 8 8
P A R A S O L (F ra n c e) 1 9- O c t-2 0 0 8 F e n g y u n -1 C de b r is
2National Aeronautics and Space Administration
Growth of the Satellite Population
1960 1965 1970 2005
1975 1980 1985
January 2009
>95% of Tracked Object
1990 1995 2000 Population are Debris
3National Aeronautics and Space Administration
What is Orbital Debris?
• Space debris encompasses both natural (meteoroid) and artificial
(man-made) particles.
• Meteoroids are in orbit about the Sun, while most artificial debris are
in orbit about the Earth. Hence, the latter are more commonly referred
to as orbital debris.
• Orbital debris is any man-made object in orbit about the Earth which
no longer serves a useful function.
• Non-functional spacecraft
• Abandoned launch vehicle stages
• Mission-related debris
• Fragmentation debris
• For most size regimes, the flux of orbital debris within 2000 km of the
Earth’s surface already exceeds the flux of meteoroids.
4National Aeronautics and Space Administration
Recent Growth of Satellite Population
in Low Earth Orbit
• The growth of the cataloged satellite population during the past 15 years
has been primarily influenced by China’s ASAT test in January 2007.
5.E- 08
Altitude of
Site of 10 Chinese 19 94
4.E- 08 February ASAT test
19 99
Spatial Density (objects per km3)
Collision
20 04
3.E- 08
20 09
2.E- 08 ISS
Altitude
Regime
1.E- 08
0.E+ 00
200 400 600 800 1 000 1 200 1 400 1 600 18 00 20 00
A ltitu de (km )
5National Aeronautics and Space Administration
Satellite Environment Characterization
• NASA and DoD cooperate and share responsibilities for characterizing the satellite
(including orbital debris) environment.
• DoD’s Space Surveillance Network discretely tracks objects as small as 5 cm in low
Earth orbit and about 1 m in geosynchronous orbit.
– Currently, ~14,000 officially cataloged objects are still in orbit.
– Total tracked objects exceeds 19,000.
• Using special ground-based sensors and inspections of returned satellite surfaces,
NASA statistically determines the extent of the population for objects less than 10
cm.
– Number of debris in Low-Earth Orbit (LEO)* 1 cm or greater exceeds 300,000.
• The combined results are used for spacecraft and launch vehicle design and
operations.
* Low-Earth Orbit (LEO) refers orbits ranging in altitudes from up to 2000 km above the Earth’s surface;
Geosynchronous Orbit (GEO) refers to orbits at ~36,000 km above the Earth’s surface; at that distance,
an orbits the Earth in a 24-hour period.
6National Aeronautics and Space Administration
Collision Risks
• Collision risks are divided into three categories depending upon size of
threat.
• ~ 10 cm and larger: Conjunction assessments and collision avoidance
maneuvers are effective in countering objects which can be tracked by the
U.S. Space Surveillance Network.
– Collisions of this type are potentially catastrophic.
• 1 – 10 cm: Objects in this category are usually too small to track and too
large to shield against.
– Collisions of this type can disable or disrupt a mission.
• < 1 cm: Debris shields can be effective in withstanding impacts of particles
in this category.
– Unshielded portions of satellite subject can lead to mission degradation or loss.
• The greatest risk to space missions comes from non-trackable debris.
7National Aeronautics and Space Administration
U.S. Space Surveillance Network
Organized as:
• Dedicated = primary
THULE
GLOBUS II • Collateral = secondary
• Contributing = contracted
CLEAR
FYLINGDALES services
CAVALIER
LSSC MOSS
BEALE CAPE COD COBRA DANE
AFSSS
SOCORRO EGLIN
MAUI &
MSSS Kwaj
ASCENSION
DIEGO
GARCIA
Tracking Radar
Detection Radar LSSC = Lincoln Space Surveillance Center
Millstone, Haystack, HAX
Imaging Radar
MSSS = Maui Space Surveillance System
Optical Telescope (former AMOS/MOTIF site)
AFSSS = Air Force Space Surveillance System
Passive Receiver
8National Aeronautics and Space Administration
Space Shuttle Vulnerabilities
Potential Shuttle Damage 6.5
Window Replacement
EVA Suit Penetration 5.5
Radiator Penetration
RCC Penetration
4.5
TPS Tile Penetration
Cabin Penetration
3.5
Cargo Bay Damage
Goldstone Radars Space Surveillance Network
2.5
Haystack Auxiliary Radar
Spacecraft
Surface
1.5
Inspections
Haystack Radar
0.5
0.001 0.01 0.1 1 10 100 1000
Debris Diameter in Centimeters
9National Aeronautics and Space Administration
Debris Environment for International Space Station
1.E+03
1.E+02
SSN
Catalog
1.E+01
ne arly
complete for
1.E+00 objects
Flux (no/m2/year
2009 greater than
1.E-01 No SSN 10 cm
tra cking data
for objects
1.E-02
smaller than 5
cm
1.E-03
Some SSN
1.E-04 data on
ob jects 5 -
Imp acts by debris 1 0 cm
1.E-05
smaller than 0.3 cm
not critical to ISS
1.E-06
1.E-07
0.00 1 0.01 0.1 1 10 100
Particle Diameter (cm)
10National Aeronautics and Space Administration
Evolution of NASA Collision Avoidance Process
• NASA implemented a conjunction assessment and collision avoidance
process for human spaceflight beginning with STS-26 in 1988.
– Initially based upon simple miss distance and a 4-km by 10-km by 4-km ellipsoid
(picture a protected football-shaped volume [keep out] around the Shuttle).
• Before launch of the first element of ISS in 1998, NASA and DoD jointly
developed and implemented a more sophisticated and higher fidelity
conjunction assessment process for human spaceflight missions.
– Also adopted by other USG national space assets.
• In 2005, NASA implemented a similar process for selected robotic assets,
e.g., the Earth Observation System satellites in LEO and TDRSS in GEO.
• In 2007, NASA Procedural Requirements 8715.6 extended the conjunction
assessment process to all NASA maneuverable satellites within LEO and
within 200 km of GEO.
11National Aeronautics and Space Administration
Basic Conjunction Assessment and
Collision Avoidance Process
• DoD maintains high accuracy satellite catalog on objects which pose a
threat to designated NASA space assets.
– Lower fidelity, publicly available data (“two-line element sets”) are NOT used.
• DoD’s Joint Space Operations Center (JSpOC) is responsible for performing
conjunction assessments for all designated NASA space assets in
accordance with an established schedule, i.e., every 8 hours for human
spaceflight vehicles and daily Monday through Friday for robotic vehicles.
– All objects tracked by SSN are considered: cataloged and uncataloged.
• JSpOC notifies NASA (JSC for human spaceflight and GSFC for robotic
missions) of conjunctions which meet established criteria.
– Data are exchanged 24/7 via direct links and telecon between JSpOC and JSC/GSFC.
• JSpOC tasks SSN to collect additional tracking data on threat object to
improve conjunction assessment accuracy.
12National Aeronautics and Space Administration
Basic Conjunction Assessment and
Collision Avoidance Process (continued)
• NASA computes the probability of collision, based upon miss distance and
uncertainty provided by JSpOC.
• Based upon specific flight rules and detailed risk analysis, NASA decides if
a collision avoidance maneuver is necessary.
• If a maneuver is required, NASA provides planned post-maneuver orbital
data to JSpOC for screening of near-term conjunctions. This process can
be repeated if planned new orbit puts the NASA vehicle at risk of future
collision with the same or another space object.
• In the case of a NASA robotic satellite, a second maneuver might be
required for the vehicle to resume its mission. This maneuver also would
be coordinated with JSpOC.
• NASA also informs JSpOC prior to normal operational maneuvers to aid
future conjunction assessments.
13National Aeronautics and Space Administration
Debris Avoidance Maneuver Planning for
Human Spaceflight Operations
• Debris avoidance maneuvers are planned when the probability of collision
from a conjunction reaches limits set in the Shuttle and ISS flight rules:
– Probability > 1 in 100,000: Maneuver if it will not result in significant impact to mission
objectives.
– Probability > 1 in 10,000: Maneuver unless it will result in additional risk to crew (re-
flight, additional spacewalk, etc.).
• Debris avoidance maneuvers are usually small and occur from one to
several hours before the time of the conjunction.
– Shuttle can plan and execute a debris avoidance maneuver in a matter of hours.
– ISS requires around 30 hours to plan and execute a debris avoidance maneuver,
mainly due to dependence on Russian propulsion assets.
• Both the Shuttle and ISS have conducted several collision avoidance
maneuvers during the past 10 years.
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