Orbital debris mitigation measures have been developed to reduce the growth of the future debris population by the international space community over the past two decades. A major component in the overall mitigation strategy is postmission disposal (PMD). A key PMD element for the low Earth orbit (LEO, the region below 2000 km altitude) satellites is the 25-year decay rule. It is intended to limit the long-term presence of massive intact objects - rocket bodies (R/Bs) and spacecraft (S/C) in the environment. The effectiveness of the 25-year rule was well demonstrated and documented during the development of the mitigation measures. The orbital debris population in LEO, unfortunately, has significantly increased since that time. The objectives of this paper are to provide an updated assessment based on the 2012 LEO environment and to highlight the importance of the global compliance of the 25-year decay rule

Krisko, Paula

Liou, J.-C

NASA Technical Reports Server

 64th International Astronautical Congress, Beijing, China. Copyright ©2013 by the International Astronautical Federation. All rights reserved. 
 
 
IAC-13-A6.4.2          Page 1 of 4 
IAC-13-A6.4.2 
 
AN UPDATE ON THE EFFECTIVENESS OF POSTMISSION DISPOSAL IN LEO 
 
J.-C. Liou 
NASA Orbital Debris Program Office, Houston, TX 77058, USA, jer-chyi.liou-1@nasa.gov 
 
Paula Krisko 
JETS/Jacobs Technology, Houston, TX 77058, USA, paula.krisko-1@nasa.gov  
 
Orbital debris mitigation measures have been developed to reduce the growth of the future debris population by 
the international space community over the past two decades. A major component in the overall mitigation strategy 
is postmission disposal (PMD). A key PMD element for the low Earth orbit (LEO, the region below 2000 km 
altitude) satellites is the 25-year decay rule. It is intended to limit the long-term presence of massive intact objects - 
rocket bodies (R/Bs) and spacecraft (S/C) in the environment. The effectiveness of the 25-year rule was well 
demonstrated and documented during the development of the mitigation measures. The orbital debris population in 
LEO, unfortunately, has significantly increased since that time. The objectives of this paper are to provide an 
updated assessment based on the 2012 LEO environment and to highlight the importance of the global compliance of 
the 25-year decay rule. 
 
 
I. INTRODUCTION 
The accumulation of the orbital debris population 
and the threat from orbital debris to satellite operations 
have been recognized by the space community for 
decades. Figure 1 shows the historical increase of the 
objects tracked by the U.S. Space Surveillance Network 
(SSN) and placed in the satellite catalog. The top curve 
is the total and the population breakdown is represented 
by the four curves below the total. Fragmentation debris 
have dominated the population almost from the 
beginning and they are heavily influenced by major 
breakup events, such as the anti-satellite test conducted 
by China in 2007 and the accidental collision between 
the retired Cosmos 2251 and the operational Iridium 33 
in 2009, and, to a lesser extent, solar activity which 
modulates the atmospheric drag. Among the 3500 or so 
spacecraft in space, only about 1100 are operational and 
the rest are simply orbital debris. 
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
11000
12000
13000
14000
15000
16000
17000
19
57
19
59
19
61
19
63
19
65
19
67
19
69
19
71
19
73
19
75
19
77
19
79
19
81
19
83
19
85
19
87
19
89
19
91
19
93
19
95
19
97
19
99
20
01
20
03
20
05
20
07
20
09
20
11
20
13
N
um
be
r o
f O
bj
ec
ts
Year
Monthly Number of Objects in Earth Orbit by Object Type
Total Objects
Fragmentation Debris
Spacecraft
Mission-related Debris
Rocket Bodies
 
  Fig. 1: Monthly increase of the catalogued population 
between 1957 and 2013. The top curve is the total 
and the population breakdown is shown by the four 
curves below the total. About 57% of the 2013 
cataloged objects are fragmentation debris. 
 
The buildup of material in space is illustrated in 
Figure 2. The mass distribution is dominated by 
spacecraft (S/C) and rocket bodies (R/Bs). They account 
for approximately 54% and 42%, respectively, of the 
total mass in the environment. Although fragmentation 
debris dominate the number distribution, they only 
represent less than 3% of the total mass. The amount of 
material in Earth orbit exceeds 6500 tons as of 2013 and 
approximately 2700 tons of material is concentrated in 
low Earth orbit (LEO, the region below 2000 km 
altitude). 
0
1
2
3
4
5
6
7
19
57
19
59
19
61
19
63
19
65
19
67
19
69
19
71
19
73
19
75
19
77
19
79
19
81
19
83
19
85
19
87
19
89
19
91
19
93
19
95
19
97
19
99
20
01
20
03
20
05
20
07
20
09
20
11
20
13
M
as
s 
in
 O
rb
it 
(m
ill
io
ns
 o
f k
g)
Year
Monthly Mass of Objects in Earth Orbit by Object Type
Total Objects
Spacecraft
Rocket Bodies
Fragmentation Debris
Mission-related Debris
 
Fig. 2: Historical mass increase in the near-Earth space 
environment. The top curve is the total and the 
population breakdown is shown by the four curves 
below the total. 
 
The trend of the top curve in Fig. 2 shows a steady 
increase of mass over time and there is no sign of 
slowing down. This is a serious problem because as 
https://ntrs.nasa.gov/search.jsp?R=20140002497 2019-08-29T14:42:52+00:00Z
 64th International Astronautical Congress, Beijing, China. Copyright ©2013 by the International Astronautical Federation. All rights reserved. 
 
 
IAC-13-A6.4.2          Page 2 of 4 
more material is added to the environment, the chances 
of accidental collisions among existing objects will 
increase in the future. More frequent collisions will 
generate more small debris to threaten the safety of 
operational satellites. 
The international space community has attempted to 
address the orbital debris problem since the 1970s. 
Postmission disposal (PMD) mitigation measures, such 
as end-of-mission passivation to reduce potential 
explosions and the 25-year decay rule to limit the long-
term presence of massive S/C and R/Bs in LEO, have 
been adopted by many space agencies, including 
members of the Inter-Agency Space Debris 
Coordination Committee (IADC), as part of the overall 
mitigation strategy to manage the orbital debris 
problem. The 25-year rule simple means to place a S/C 
or R/B at the end of its mission in an orbit where the 
atmospheric drag will limit its orbital lifetime to 25 
years or less. The selection of 25-year was based on 
“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.”1 The 
effectiveness of using the 25-year rule to reduce the 
future debris growth was well documented during the 
development of the measure in the 1990s2-4. The 
objectives of this paper are to provide an updated study 
based on the 2012 environment and to highlight the 
importance of the global compliance of the 25-year 
decay rule. 
 
II. STUDY SCENARIOS 
Reliable future debris environment projection is an 
impossible task. Major uncertainties in environment 
simulations include the characteristics of future 
launches (orbit, mass, material, mission lifetime, etc), 
solar activities, explosions, and options for mitigation 
and potential remediation. There are two general 
approaches to handle the uncertainties. The first is to 
examine the extreme cases and then use the results to 
bound the problem. Examples of this approach include 
the assumptions of no future launch, no future 
explosion, or no mitigation measure. The second is to 
analyze nominal cases based on reasonable 
assumptions, such as repeating a typical launch cycle or 
repeating an average solar activity cycle. The choice 
really depends on the objectives of the simulations. Any 
projection results should only be considered as a 
benchmark to guide the expectation of the future 
environment under the assumed scenario5. 
Future environment simulations usually follow a 
Monte Carlo (MC) approach where collision events are 
selected based on the collision probability of each likely 
event and the draw of a random number to determine if 
such an event would occur. Multiple MC simulations, 
ranging from tens to hundreds, are performed for the 
projection period. The MC results can be processed to 
obtain the average and the standard deviations, but 
individual MC predictions can also be analysed for their 
distribution to gain additional insights of the projected 
environment. 
The simulation tool for this study is NASA’s state-
of-the-art orbital debris evolutionary LEGEND (LEO-
to-GEO ENvironment Debris) model6,7. The model 
simulates the historical R/Bs, S/C, mission-related 
debris, and fragmentation debris based on documented 
launches and breakup events (explosions and 
collisions). For future projection, the model propagates 
the historical populations forward in time and uses a 
pair-wise collision probability evaluation algorithm and 
a MC method to simulate future collision activities. The 
NASA Standard Breakup Model is used to generate 
fragments after each collision. The model can adopt 
user-specified future launch traffic, post-mission 
disposal, and active debris removal options. 
The study scenarios include the following. The 
simulations focus on the 10 cm and larger population in 
LEO. Historical simulation covers the period from 1957 
to the end of 2011 and the future projection covers 2012 
through 2212. The 2004 to 2011 8-year launch cycle is 
assumed to be representative of future traffic so this 8-
year cycle is repeated during the projection period. All 
future S/C is assumed to have 8-year mission lifetime. 
No stationkeeping and no collision avoidance maneuver 
for operational S/C is allowed. Future solar activity is a 
repeat of an average cycle derived from Solar Cycles 18 
to 23. The PMD scenarios include the following. No 
explosion for R/B and S/C is allowed in the future. Each 
R/B is moved a 25-year decay orbit after launch. Each 
S/C is moved to a 25-year decay orbit or a LEO 
graveyard orbit (depending on the least V required for 
the two options) after 8-year of mission lifetime. The 
25-year decay rule success rates for R/Bs and S/C are 
set to 0%, 10%, 50%, 75%, and 95%, respectively. A 
total of 100 MC runs are carried out for each success 
rate scenario and the results are analysed and presented 
in the next section. 
 
III. STUDY RESULTS 
The projected orbital debris population increases 
from the five test scenarios are presented in Figure 3. 
The effective number is defined as the fractional time, 
per orbital period, an object spends below 2000 km 
altitude. Each projection curve is an average of 100 MC 
simulations. Because of the averaging effect, sharp 
increases due to major collision events, such as the 
increase from the Iridium 33 and Cosmos 2251 collision 
shown in Figure 1, are smoothed out. Nevertheless, the 
average curves provide a good way to quantify the 
differences in the expected population growth from 
different projection scenarios. As expected, the no 
future explosion and no 25-year decay rule scenario 
projects the highest growth and the increase is nonlinear 
 64th International Astronautical Congress, Beijing, China. Copyright ©2013 by the International Astronautical Federation. All rights reserved. 
 
 
IAC-13-A6.4.2          Page 3 of 4 
for the next 200 years. The no explosion and 95% 
compliance of the 25-year decay rule leads to the 
slowest population increase in 200 years. 
0
10,000
20,000
30,000
40,000
50,000
60,000
1950 1970 1990 2010 2030 2050 2070 2090 2110 2130 2150 2170 2190 2210
Ef
fe
ct
iv
e 
N
um
be
r o
f O
bj
ec
ts
 (≥
10
 c
m
) i
n 
LE
O
Year
LEGEND Simulations (averages of 100 Monte Carlo runs per scenario)
Reg launches +  0% PMD
Reg launches + 10% PMD
Reg launches + 50% PMD
Reg launches + 75% PMD
Reg launches + 95% PMD
 
Fig. 3: Projected average population growth in LEO. 
Each projection curve is the average of 100 MC 
simulations. Implementation of the 25-year PMD 
rule can significantly reduce the future debris 
population increase. 
 
When the 25-year decay rule is implemented at a 
50% level, the average population increase is cut 
significantly. When it is implemented at a 95% level, 
the projected average population increase is limited to 
about 54% in 200 years. Table 1 summarizes the 
average 10 cm and larger objects in LEO at a 50-year 
interval for the five scenarios. 
 
 2012 2062 2112 2162 2212 
0% 12,866 15,088 23,720 38,294 58,692 
10% 12,866 14,728 21,931 33,820 50,919 
50% 12,866 13,610 17,633 24,730 32,767 
75% 12,866 13,560 15,509 19,446 24,315 
95% 12,866 13,319 14,954 17,225 19,795 
Table 1: Average population increase for the 10 cm and 
larger objects in LEO at a 50-year interval from each 
study scenario. 
 
A different parameter to characterize the future 
orbital debris environment is collision activities. There 
are two types of collisions – catastrophic and non-
catastrophic. A catastrophic collision occurs when the 
ratio of impact energy to target mass exceeds 40 J/g. 
The outcome of a catastrophic collision, such as the 
collision between Iridium 33 and Cosmos 2251, is the 
total fragmentation of the target or targets involved and 
the generation of thousands of ≥10 cm debris and tens 
of thousands of millimetre-to-centimetre debris. A non-
catastrophic collision occurs when the energy mass ratio 
involved is less than 40 J/g. The outcome of a typical 
non-catastrophic collision is minor damage to the target 
and the generation of a small amount of debris that has 
minimal contribution to population growth. The average 
catastrophic collisions projected by the five scenarios 
are shown in Figure 4. 
0
10
20
30
40
50
60
70
2010 2030 2050 2070 2090 2110 2130 2150 2170 2190 2210
C
um
ul
at
iv
e 
N
um
be
r o
f C
at
as
tr
op
hi
c 
C
ol
lis
io
ns
 in
 L
EO
Year
LEGEND Simulations (averages of 100 Monte Carlo runs per scenario)
Reg launches +  0% PMD
Reg launches + 10% PMD
Reg launches + 50% PMD
Reg launches + 75% PMD
Reg launches + 95% PMD
 
Fig. 4: Projected catastrophic collision activities in LEO 
from the five study scenarios. Each curve is the 
average of 100 MC simulations. 
 
The numbers of catastrophic and non-catastrophic 
collisions from the study are summarized in Table 2. 
For the no future explosion and no 25-year decay rule 
scenario, an average of 71 catastrophic collisions are 
expected in LEO for the next 200 years. The no future 
explosion and 95% compliance of the 25-year decay 
rule scenario has the lowest number of catastrophic 
collisions. On average, only 26 such events are expected 
in LEO for the next 200 years. Although the number is 
significantly lower than 71, it still means an average of 
one catastrophic collision every 7.7 years. The threat 
from small debris (millimetre to centimetre) to 
operational S/C from such events on a regular basis 
cannot be overlooked. 
 
 0% 10% 50% 75% 95% 
cat  71 63 39 30 26 
non-cat 76 65 37 24 19 
total 147 128 76 54 45 
Table 2: Average catastrophic and non-catastrophic 
collisions in LEO for the next 200 years as predicted 
by the five study scenarios. 
 
Another important parameter to assess the 
seriousness of the orbital debris problem is the total 
mass of material in the environment. As shown in 
Figure 2, the historical mass increase between 1957 and 
2013 has been quite steady. The no future explosion and 
no 25-year PMD rule projection follows a similar trend 
(Figure 5). The projected mass increase for this scenario 
is a direct reflection of the repeated traffic cycle (i.e., 
2004 through 2011), especially the masses of the R/Bs 
and S/C, their mission altitudes, and their area-to-mass 
ratios. For the no future explosion and 95% compliance 
of the 25-year decay rule scenario, the mass is more or 
 64th International Astronautical Congress, Beijing, China. Copyright ©2013 by the International Astronautical Federation. All rights reserved. 
 
 
IAC-13-A6.4.2          Page 4 of 4 
less maintained at a constant level, although still higher 
than the beginning of the 2012 environment. 
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
1950 1970 1990 2010 2030 2050 2070 2090 2110 2130 2150 2170 2190 2210
Ef
fe
ct
iv
e 
M
as
s 
(to
n)
 in
 L
EO
Year
LEGEND Simulations (average of 100 Monte Carlo runs per scenario)
Reg launches +  0% PMD
Reg launches + 10% PMD
Reg launches + 50% PMD
Reg launches + 75% PMD
Reg launches + 95% PMD
2012
 
Fig. 5: Projected mass increase in LEO from the five 
study scenarios. The 2012 environment is indicated 
by the arrow. 
 
IV. SUMMARY  
Predicting the future debris environment is very 
difficult. The results are always sensitive to key 
assumptions adopted by the model, including future 
launches, future explosions, and solar activity. 
Nevertheless, one can make reasonable assumptions, 
define nominal scenarios, and then draw conclusions 
from the average results for effective environment 
management. This updated study again illustrates the 
effectiveness of using the 25-year decay rule to limit the 
future debris population growth in LEO. Mitigation 
measures, especially passivation and the 25-year decay 
rule are the first and the most cost-effective defense 
against future population growth in LEO. The benefits 
of compliance are clearly quantified in the study. The 
current global compliance rate of the 25-year decay rule 
for LEO R/Bs and S/C is significantly lower than 95%. 
It is critical for the international space community to 
fully implement the mitigation measures to better 
protect the future environment. On the other hand, the 
study results also show that even with no future 
explosion and a global 95% compliance of the 25-year 
decay rule, the LEO debris population is expected to 
slowly increase and collision activities are expected on a 
regular basis in the next 200 years. To better manage the 
orbital debris problem and improve the safety of 
operational S/C in LEO, more aggressive measures, 
such as active debris removal, should be considered by 
the international space community. 
 
REFERENCES 
1. IADC Space Debris Mitigation Guidelines, IADC-
02-01, available at http://www.iadc-online.org. 
2. Krisko, P.H., N.L. Johnson, J.N. Opiela, EVOLVE 
4.0 orbital debris mitigation studies, Adv. Space 
Res.  29, 1385–1390, 2001. 
3. Opiela, J.N. and P.H. Krisko, Evaluation of orbital 
debris mitigation practices using EVOLVE 4.1, in: 
J. Bendisch (Ed.), Space Debris 2001, 2002, pp. 
209–219. 
4. Walker, R., C.E. Martin, P.H. Stokes, J.E. 
Wilkinson, H. Klinkrad, Studies of space debris 
mitigation options using the debris environment 
long term analysis (DELTA) model, in: J. Bendisch 
(Ed.), Space Debris 2000, pp. 305–314, 2001. 
5. Liou, J.-C., A statistical analysis of the future 
debris environment, Acta Astronautica 62, 264-271, 
2008. 
6. Liou, J.-C., D.T. Hall, P.H. Krisko, and J.N. Opiela, 
LEGEND – A three-dimensional LEO-to-GEO 
debris evolutionary model. Adv. Space Res. 34, 5, 
981-986, 2004. 
7. Liou, J.-C., Collision activities in the future orbital 
debris environment, Adv. Space Res. 38, 9, 2102-
2106, 2006. 
 


An Update on the Effectiveness of Postmission Disposal in LEO

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server