The Disposal of Launch Vehicle Orbital Stages

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					National Aeronautics and Space Administration




                    The Disposal of
              Launch Vehicle Orbital Stages

                                  Nicholas L. Johnson
                            Chief Scientist for Orbital Debris



                                     28 October 2009
    National Aeronautics and Space Administration


                               Disposal Objectives

•   The responsible disposal of launch vehicle orbital stages is required to
    control the growth of the orbital debris population.

•   In addition to the U.S. Government Orbital Debris Mitigation Standard
    Practices, international orbital debris mitigation guidelines applicable to
    launch vehicle orbital stages have been adopted by the Inter-Agency Space
    Debris Coordination Committee (IADC) and the United Nations.

•   The three principal objectives for launch vehicle orbital stage disposal are

      – Passivation (currently addressed by FAA regulations)

      –   Limitation of time in high value orbital regions, i.e., LEO and GEO

      –   Limitation of risk of human casualty from reentries, i.e., risk < 1 in 10,000




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        National Aeronautics and Space Administration


                             Growth of Launch Vehicle Stages in Earth Orbit

                                                 1800



                                                 1600
                                                                               Abandoned launch vehicle stages
                                                                             continue to accumulate in Earth orbit.
Number of Launch Vehicle Stages in Earth Orbit




                                                 1400



                                                 1200



                                                 1000



                                                 800



                                                 600



                                                 400



                                                 200



                                                   0
                                                        1956

                                                               1958

                                                                      1960

                                                                              1962

                                                                                     1964

                                                                                            1966

                                                                                                   1968

                                                                                                          1970

                                                                                                                 1972

                                                                                                                        1974

                                                                                                                               1976

                                                                                                                                      1978

                                                                                                                                                 1980

                                                                                                                                                        1982

                                                                                                                                                               1984

                                                                                                                                                                      1986

                                                                                                                                                                             1988

                                                                                                                                                                                    1990

                                                                                                                                                                                           1992

                                                                                                                                                                                                  1994

                                                                                                                                                                                                         1996

                                                                                                                                                                                                                1998

                                                                                                                                                                                                                       2000

                                                                                                                                                                                                                              2002

                                                                                                                                                                                                                                     2004

                                                                                                                                                                                                                                            2006

                                                                                                                                                                                                                                                   2008

                                                                                                                                                                                                                                                          2010
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    National Aeronautics and Space Administration

                                           Long-term Consequences of Large Objects
                                                        in Earth Orbit
•   Without proper post-mission disposal, accidental satellite collisions will
    significantly increase the Earth satellite population.
                                                                           Objects ≥10 cm

                                           25,000

                                                          Non-Mitigation

                                                          PMD
                                           20,000
      Effective Number of Objects in LEO




                                           15,000




                                           10,000




                                            5,000




                                               0
                                                1950   1970        1990     2010       2030   2050   2070   2090   2110

                                                                                       Year


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                Launch Vehicle Stages as Sources of
                        Additional Debris
•   In addition to the direct contribution of intact launch vehicle stages to the
    orbital debris environment, smaller, more numerous debris can be
    generated via

      – Intentional or unintentional release of debris during normal operations

            •   e.g., multiple payload adaptors, small motor covers, degradation of vehicle elements


      – Explosions of derelict stages

            •   Prior to 2007, launch vehicle explosions represented the greatest source of hazardous
                orbital debris


      – Collisions of derelict stages with other resident space objects

            •   2005 collision of 31-year-old U.S. rocket body with another piece of debris




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     National Aeronautics and Space Administration


    USG Orbital Debris Mitigation Standard Practices

•    The USG Orbital Debris Mitigation Standard Practices were first developed
     in 1997, presented to industry in January 1998, and adopted via a USG
     Interagency process in early 2001.

•    Standard Practice 4-1 addresses launch vehicle disposal options.

•    The basic options for launch vehicle stages are

       – Leave stage in orbit which will result in reentry within 25 years, taking into account
         potential human casualty risks on Earth

       – Leave stage in disposal orbit above LEO and below GEO, avoiding the GPS altitude
         regime

       – Leave stage in disposal orbit above GEO




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                           Standard Practice 4-1
“4-1. Disposal for final mission orbits: A spacecraft or upper stage may be disposed of by one of three methods:

    a. Atmospheric reentry option: Leave the structure in an orbit in which, using conservative projections for
    solar activity, atmospheric drag will limit the lifetime to no longer than 25 years after completion of mission. If
    drag enhancement devices are to be used to reduce the orbit lifetime, it should be demonstrated that such
    devices will significantly reduce the area-time product of the system or will not cause spacecraft or large
    debris to fragment if a collision occurs while the system is decaying from orbit. If a space structure is to be
    disposed of by reentry into the Earth’s atmosphere, the risk of human casualty will be less than 1 in 10,000.

    b. Maneuvering to a storage orbit: At end of life the structure may be relocated to one of the following
    storage regimes:

        I. Between LEO and MEO: Maneuver to an orbit with perigee altitude above 2000 km and apogee altitude
            below 19,700 km (500 km below semi-synchronous altitude
       II. Between MEO and GEO: Maneuver to an orbit with perigee altitude above 20,700 km and apogee
            altitude below 35,300 km (approximately 500 km above semi-synchronous altitude and 500 km below
            synchronous altitude.)
       III. Above GEO: Maneuver to an orbit with perigee altitude above 36,100 km (approximately 300 km above
            synchronous altitude)
       IV. Heliocentric, Earth-escape: Maneuver to remove the structure from Earth orbit, into a heliocentric orbit.

            Because of fuel gauging uncertainties near the end of mission, a program should use a maneuver
    strategy that reduces the risk of leaving the structure near an operational orbit regime.

    c. Direct retrieval: Retrieve the structure and remove it from orbit as soon as practical after completion of
    mission.”

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            IADC Space Debris Mitigation Guideline
•   The IADC Space Debris Mitigation Guidelines are essentially derived from
    the U.S. Government Orbital Debris Mitigation Standard Practices and were
    adopted in 2002 by the 11 IADC member agencies.

•   Guideline 5.3 addresses launch vehicle stage post-mission disposal.

     – For objects passing through LEO:

         “Whenever possible spacecraft or orbital stages that are terminating their operational
         phases in orbits that pass through the LEO region, or have the potential to interfere
         with the LEO region, should be de-orbited (direct re-entry is preferred) or where
         appropriate manoeuvred into an orbit with a reduced lifetime. Retrieval is also a
         disposal option.

          “A spacecraft or orbital stage should be left in an orbit in which, using an accepted
         nominal projection for solar activity, atmospheric drag will limit the orbital lifetime after
         completion of operations. A study on the effect of post-mission orbital lifetime limitation
         on collision rate and debris population growth has been performed by the IADC. This
         IADC and some other studies and a number of existing national guidelines have found
         25 years to be a reasonable and appropriate lifetime limit …”
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IADC Space Debris Mitigation Guideline (continued)

  – For objects near GEO:

 “Spacecraft that have terminated their mission should be manoeuvred far enough away
 from GEO so as not to cause interference with spacecraft or orbital stage still in
 geostationary orbit. The manoeuvre should place the spacecraft in an orbit that remains
 above the GEO protected region.

 “The IADC and other studies have found that fulfilling the two following conditions at the
 end of the disposal phase would give an orbit that remains above the GEO protected
 region:

   1. A minimum increase in perigee altitude of 235 km + (1000·CR·A/m)
       where CR is the solar radiation pressure coefficient,
             A/m is the aspect area to dry mass ratio (m2kg-1)
            235 km is the sum of the upper altitude of the GEO protected region (200 km)
              and the maximum descent of a re-orbited spacecraft due to luni-solar &
              geopotential perturbations (35 km).

   2. An eccentricity less than or equal to 0.003”

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    National Aeronautics and Space Administration


United Nations Space Debris Mitigation Guidelines

•   The United Nations Space Debris Mitigation Guidelines were developed
    within the Committee on the Peaceful Uses of Outer Space and adopted in
    2007, followed by endorsement of the General Assembly the same year.

•   Guideline 6 addresses launch vehicle stage disposal in LEO:

      – “Spacecraft and launch vehicle orbital stages that have terminated their operational
        phases in orbits that pass through the LEO region, should be removed from orbit in a
        controlled fashion. If this is not possible, they should be disposed of in orbits which
        avoid their long-term presence in the LEO region.

          “When making determinations regarding potential solutions for removing objects from
          LEO, due consideration should be given to ensure that debris which survives to reach
          the surface of the Earth does not pose an undue risk to people or property, including
          through environmental pollution caused by hazardous substances.”




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    National Aeronautics and Space Administration

United Nations Space Debris Mitigation Guidelines
                   (continued)

•   Guideline 7 addresses launch vehicle stage disposal in GEO:

      – “Spacecraft and launch vehicle orbital stages that have terminated their operational
        phases in orbits that pass through the GEO region, should be left in orbits which avoid
        their long-term interference with the GEO region.


      – “For space objects in or near the GEO region, the potential for future collisions can be
        reduced by leaving objects at the end of their mission in an orbit above the GEO region
        such that they will not interfere with, or return to, the GEO region.”




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      National Aeronautics and Space Administration

LEO Commercial Communications Satellite Missions

  •      Iridium (operational orbit 780 km)
           – 95 spacecraft launched using four different launch vehicles from three countries
           – 28 orbital stages inserted, but only one remains in orbit (due to malfunction)
                 •   Proton orbital stages de-orbited over Pacific Ocean
                 •   Delta, Long March, and Rokot orbital stages moved to lower disposal orbits


  •      Globalstar (operational orbit 1415 km)
           – 52 spacecraft launched using Delta and Soyuz launch vehicles
           – 19 of 21 orbital stages have already decayed
           – Eight additional Soyuz stages were de-orbited into Pacific from altitude near 900
             km


  •      Orbcomm (operational orbit 670-820 km)
           – 41 spacecraft launched as primary or secondary payloads using Pegasus and
             Cosmos launch vehicles
           – Nine orbital stages used for dedicated missions (37 spacecraft); 1-2 orbital
             stages will fail to meet 25-year guideline: one due to lower stage malfunction

           At least 95% compliance with 25-year rule, excluding two malfunctions
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    National Aeronautics and Space Administration


      Recent U.S. Experience: Delta 2 second stage

•   From January 1990 through September 2009, 133 Delta 2 second stages were
    left in Earth orbits.
      – 97 have already reentered
      – 1 more expected to reenter within 25 years of launch
      – 2 were left in disposal orbits above LEO
      – 29 were left in orbits within LEO exceeding 25 years (all but 2 launched in 1990’s*)
      – 4 have no publicly available data

      Overall rate of compliance with 25-year rule beginning in 2001: 100%, excluding one
        mission in 2003 with a secondary DoD mission


•   For NASA missions, the Delta 2 second stage routinely performs a post-
    payload delivery maneuver, leaving stage with a lifetime of less than 1 year.
      – Example: NOAA 19 inserted into orbit of 847 km by 866 km in 2009
                  NOAA 19 Delta 2 second stage maneuvered to 177 km by 819 km;
                         reentered within 2.5 months
      – Two Delta 2 second stages (one in 2008 and one in 2009) were left in disposal orbits
        above LEO (payload destinations were 1330 km circular and Earth escape, respectively)

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                                Delta 4 and Atlas 5

•   The Delta 4 and Atlas 5 vehicles have flown relatively few missions to date.

•   Of 24 cataloged stages in Earth orbits:

     – 4 have already reentered

     – 6 were left in orbits above LEO

     – 9 have no publicly available orbital data

     – 5 left in orbits with perigee in LEO




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    National Aeronautics and Space Administration

               Predicted Lifetimes of Orbital Stages
              Traversing LEO: Missions in Year 2000
•    Orbital Stages Launched in 2000 (last year before USG Standard Practices)
                                                     <25 Year Lifetime   >25 Year Lifetime
                 USA**     Atlas Centaur                    3                    4
                           Delta 2 and 3                    9                    2
                           Minotaur                         1                    1
                           Pegasus                          1                    1
                           Taurus                           1                    0
                           Titan 4                          2                    0              USA: 68%
                 Russia    Dnepr                            0                    1
                           Kosmos                           2                    1
                           Proton                           12                   0
                           Rokot                            1                    0
                           Soyuz                            12                   1
                           Start                            1                    0
                           Zenit                            1                    1
                 ESA       Ariane 4 and 5                   5                    7
                 China     Long March 3 and 4               4                    1
                 Other     Sealaunch                        1                    1
                                             Total          56                   21             All: 73%
                 * Includes highly elliptical orbits with initial perigee in LEO
                 ** Two USA stages, for which no orbital data are available, are not included


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                            Recent NASA Study for FAA

•   During the summer of 2009, the NASA Orbital Debris Program Office
    collected statistics on the compliance of all launch vehicles with the 25-year
    rule for the period January 2004 through June 2009.

                                               25-Year Compliance / Non-compliance                     % Compliance

                                                    LEO      Elliptical with LEO Perigee   Total No.

     USA              Atlas 2AS                                         2/ 1                   3

                      Atlas 3A                                          1/ 0                   1

                      Atlas 5                       1/ 0                2/ 2                   5

                      Delta 2 second stage          24 / 0              4/ 0                  28

                      Delta 2 third stage                              10 / 1                 11

                      Delta 4                       1/ 0                                       1

                      Minotaur                      5/ 1                                       6

                      Pegasus                       3/ 1                0/ 1                   5

                      Taurus 1                      0/ 1                                       1

           Subtotal                                 34 / 3             19 / 5                 61           87%

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           Recent NASA Study for FAA (continued)

                                          25-Year Compliance / Non-compliance                           % Compliance

                                                 LEO          Elliptical with LEO Perigee   Total No.

Russia          Cosmos                           5/ 7                                          12

                Dnepr                            1/ 7                                           8

                Molniya 3rd stage                4/ 0                                           4

                Molniya 4th stage                                        3/ 1                   4

                Proton 3rd stage                15 / 0                                         15

                Proton Breeze stage                                      0/ 2                   2

                Rokot                            2/ 2                                           4

                Shtil 1                          1/ 0                                           1

                Soyuz 3rd stage                 48 / 0                                         48

                Soyuz Fregat stage               1/ 0                    3/ 0                   4

                Start 1                          1/ 0                                           1

                Tsyklon 2                        2/ 0                                           2

                Tsyklon 3                        2/ 0                                           2

                Zenit 2                          0/ 2                                           2

     Subtotal                                   82 / 18                  6/3                  109           81%


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         Recent NASA Study for FAA (continued)


                                          25-Year Compliance / Non-compliance                         % Compliance

                                                LEO         Elliptical with LEO Perigee   Total No.

China          CZ-2C 2nd stage                  5/ 2                                          7

               CZ-2C 3rd stage                                         0/ 1                   1

               CZ-2D                            5/ 0                                          5

               CZ-2F                            2/ 0                                          2

               CZ-3A                                                   6/ 2                   8

               CZ-3B                                                   5/ 1                   6

               CZ-3C                                                   1/ 1                   2

               CZ-4B                            6/ 2                                          8

               CZ-4C                            1/ 0                                          1

    Subtotal                                    19 / 4                12 / 5                 40           78%




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             Recent NASA Study for FAA (continued)

                                                  25-Year Compliance / Non-compliance                       % Compliance

                                                      LEO         Elliptical with LEO Perigee   Total No.

ESA/France         Ariane 5                           1/ 1                  3 / 20                 25           16%

India              PSLV                               5/ 1                   1/ 0                   7

                   GSLV                                                      2/ 0                   2

        Subtotal                                      5/1                    3/0                    9           89%

Japan              H-2/2A                             3/ 2                   0/ 3                   8

                   M-5                                3/ 0                                          3

        Subtotal                                      6/2                    0/3                   11           55%

SeaLaunch          SeaLaunch Block-DM SL stage                               5/ 7                  12

                   SeaLaunch Block-DM SLB stage                              0/ 1                   1

        Subtotal                                                             5/8                   13           38%

Israel             Shavit                             1/ 0                                          1          100%

Iran               Safir                              1/ 0                                          1          100%



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         Recent NASA Study for FAA (concluded)



                                           25-Year Compliance / Non-compliance

                                                  LEO      Elliptical with LEO Perigee   Total No.

     Total       All Launch Vehicles            149 / 29             48 / 44             197 / 270

                                                 84%                  52%                  73%




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                     Origin of Reentry Risk Metrics

•   NASA Safety Standard 1740.14 (August 1995) first established the guideline
    for all LEO spacecraft and launch vehicle orbital stages to remain in orbit
    for no more than 25 years after end of mission for the purpose of protecting
    the space environment for future operations.

•   The most practical and cost-effective strategy for compliance is disposal of
    the vehicle in a low altitude orbit from which a natural, uncontrolled reentry
    will occur within the allotted time.

•   However, such uncontrolled reentries shift on-orbit satellite collision risks
    to human casualty risks on Earth. To limit human casualty risks from
    surviving satellite debris, NASA developed a specific risk criterion and risk
    assessment process.




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                             Reentry Risk Criterion

•   In NASA Safety Standard 1740.14, a total debris casualty area metric was

                                                               
    established:              N
                                  D A   0.6 
                                              2
                                                           Ai
                                           i 1

    where N is the number of objects that survive reentry and Ai is the area of the surviving piece in m2. The
    term 0.6 represents the square root of the average cross-sectional area of a standing person, as viewed
    from above. Debris with impacting kinetic energies less than 15 Joules are no longer considered.


•   Total human casualty expectation, E, can then be defined as

                                           E = DA x PD
    where PD is equal to the average population density for the particular orbital inclination and year of
    reentry.

•   A fundamental human casualty risk threshold of 1 in 10,000 per reentry
    event was adopted by NASA in 1995, which was equivalent to a debris
    casualty area of no more than 8 m2 averaged over all inclinations for that
    year.


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                  Reentry Risk Evaluation Process

•   Reentry risk assessments are required for all NASA programs and projects
    in conjunction with the Preliminary Design Review (PDR) and Critical
    Design Review (CDR) milestones.

     •    Applies to spacecraft, launch vehicle orbital stages, large mission-related debris, and
          objects intentionally released from the International Space Station.


•   NASA maintains two levels of reentry risk assessment software:

         DAS (Debris Assessment Software)
                        and
         ORSAT (Object Reentry Survival Analysis Tool)

•   DAS is publicly available and can be used by program/project personnel.

•   ORSAT is a higher fidelity, more capable model run by trained specialists at
    the NASA Johnson Space Center.
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    National Aeronautics and Space Administration

                   ORSAT Summary Table for Terra Spacecraft:
                           Surviving Components

                                                                                                       Max.               Debris
                                                 Thermal                                             Demise Downrang Casualty       Impact     Kinetic
#         Object Name                        Qty Mass (kg)                    Material             Factor (%) e (km)     Area (m2) Mass (kg) Energy (J)
1.4       Bay 1 Nodes                         6     9.48     Ti-6Al4V                                   97      1173        0.63      1.66      776.3
2.4       Bulkhead 2 Sleeve Fittings          6     0.76     Ti-6Al4V                                   99       708        0.48      0.07        5.4
2.6       Bay 2 Nodes - Large                 4     7.77     Ti-6Al4V                                   98      1053        0.64      0.69      128.3     *
3.7       Bay 3 Nodes                         4     6.98     Ti-6Al4V                                   99      1078        0.60      0.59      126.8
5.4       Bulkhead 5 Sleeve Fittings          8     0.69     Ti-6Al4V                                   99       826        0.46      0.06        6.6
5.8       Bay 5 Nodes - Large                 2     7.20     Ti-6Al4V                                   99      1090        0.59      0.60      134.6     *
6.5
9.1.3
          Bulkhead 6 Sleeve Fittings
          DMU
                                              8
                                              2
                                                    0.67
                                                   69.91
                                                             Ti-6Al4V
                                                             Al 6061-T6
                                                                                                        99
                                                                                                        94
                                                                                                                 871
                                                                                                                1097
                                                                                                                            0.44
                                                                                                                            1.15
                                                                                                                                      0.06
                                                                                                                                     13.13
                                                                                                                                                  7.4
                                                                                                                                               5872.3     *
9.1.5     SFE                                 1    21.09     Al 6061-T6                                 97       513        3.10      4.21      114.4
10.3.4    Power Distribution Unit             1    37.73     Al 6061-T6                                 95       561        1.19      7.23     1982.7
11.1.2             Base Panel (w/fittings) 1       21.03     Al 5056-H39 core, M46J/7714A FS            97       465        2.94     13.70     1675.6
11.3      Propellant Tank                     1    31.94     Ti-6Al4V                                   66       816        2.13     31.94    13410.7
12.1.8.1 Rotor Assembly                       4     9.40     Stainless Steel                            73      1160        0.59      9.40    21800.1
13.1.10.3
20.1
                   Kinematic Mount 3 Axis 1
          CERES Panel                         1
                                                    0.77
                                                   27.88
                                                             Ti w/ teflon lined stainless bearings
                                                             Al 5056-H39 core, M46J/7714A FS
                                                                                                        99
                                                                                                        90
                                                                                                                 894
                                                                                                                 541
                                                                                                                            0.47
                                                                                                                            2.71
                                                                                                                                      0.10
                                                                                                                                     14.02
                                                                                                                                                 13.4
                                                                                                                                               2004.8     *
21.1.2    MOPITT Power Module                 1    28.00     Al 6061-T6                                 99       963        1.01      5.30     1568.9
21.3.1                       corner fittings 6      1.00     Ti-6Al4V                                   30       814        0.44      1.00     2078.1
21.3.2                    MAGE fitting (1) 1        0.80     Ti-6Al4V                                   33       803        0.44      0.80     1328.0
21.3.3                    MAGE fitting (2) 2        0.30     Ti-6Al4V                                   42       755        0.44      0.30      185.9
21.5      1 Axis KM Ftg                       1     2.44     Ti-6Al4V                                   88       992        0.64      2.44     2445.2
21.9      3 Axis KM Ftg                       1     4.63     Ti-6Al4V                                   73       944        0.81      4.63     3908.6
22.1.1    MISR Power Module                   1    34.00     Al 6061-T6                                 87       921        1.17     13.18     6852.2
22.1.2    Electronics Module                  1    18.00     Al 6061-T6                                 94       807        1.10      3.50      572.4
23.1      Instrument                          1   115.37     Al 6061-T6                                 95       885        2.20     22.56     5974.2
25.1      Instrument                          1   148.48     Al 6061-T6                                 95       908        2.46     29.04     8244.1
26.1      Instrument (w/alignmnet plate)      1   230.66     Beryllium                                  26      1046        3.40    230.66 301213.5
27.2      Release Mechinism Brackets          4     2.48     Ti-6Al4V                                   99      1029        0.46      0.39      177.0
27.3.1    HGA Gimbal                          2    21.77     Ti-6Al4V                                   59      1304        0.63     15.43    64144.5
27.3.2                             ACON 1          16.78     Ti-6Al4V                                   62       937        1.08     16.78    13302.9
28.1.1            SAD Shaft and bearings 1          8.20     Ti and Stainless                           17      1313        0.88      8.20     6021.3
                                                                                Total DCA for Objects Impacting < 15 J =   48.48
* Less than 15 joules
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    National Aeronautics and Space Administration


         Launch Vehicle Orbital Stage Assessments

•   NASA has evaluated the uncontrolled reentry risk potential for several
    launch vehicle orbital stages:

      – Pegasus third stage (Orion 38): Compliant with 1 in 10,000 reentry risk

           (Orion 38’s used as final stages for Taurus and Minotaur-4 are also likely compliant)

      – Delta 2 second stage: Slightly non-compliant reentry risk (limited number left to launch)

      – Delta 4 second stage: Highly non-compliant reentry risk

      – Atlas 5 Centaur stage: Highly non-compliant reentry risk


•   NASA has not yet evaluated the Falcon series of launch vehicles.




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    National Aeronautics and Space Administration


                     Options for Delta 4 and Atlas 5

•   To avoid the uncontrolled human casualty reentry risks associated with
    Delta 4 and Atlas 5 second stages, three basic options are available:

      – Leave stage in long-lived disposal orbit with perigee above 2000 km

            •   Utilized by NASA Delta 4 launches of GOES spacecraft (GOES 13 and 14) to GEO;
                to be utilized by NASA Atlas 5 launches of TDRS spacecraft (TDRS K and L)


      – Send stage into Earth escape trajectory

            •   Natural consequence for NASA Atlas 5 deep space missions, e.g., to Mars and Pluto


            •   Utilized by DoD Atlas 5 launch of DMSP 18 to LEO


      – Execute a controlled reentry

            •   Utilized by DoD Delta 4 launch of DMSP 17 to LEO

            •   Being planned for a future NASA Atlas 5 mission

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    National Aeronautics and Space Administration

                                         Summary

•   International consensus exists for the proper disposal of launch vehicle
    orbital stages, including passivation, short-term presence in LEO and GEO,
    and limitation of reentry risks.

      – All topics are addressed by the U.S. Government Orbital Debris Mitigation Standard
        Practices.


•   Overall, U.S. launch vehicle operators are doing a good job in all three
    areas.

      – Formal evaluations for all commercial space missions would be beneficial.


•   The greatest challenge is the reentry risk for Delta 4 and Atlas 5 orbital
    stages.




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