March 2011 

NASA/TM-2011-217070 

NESC-RP-09-00592 

 

Independent Review of U.S. and Russian 

Probabilistic Risk Assessments for the 

International Space Station Mini Research 

Module #2 Micrometeoroid and 

Orbital Debris Risk 

Michael D. Squire/NESC 

Langley Research Center, Hampton, Virginia 
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March 2011 

NASA/TM-2011-217070 

NESC-RP-09-00592 

 

Independent Review of U.S. and Russian 

Probabilistic Risk Assessments for the 

International Space Station Mini Research 

Module #2 Micrometeoroid and 

Orbital Debris Risk 

Michael D. Squire/NESC 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 

The use of trademarks or names of manufacturers in the report is for accurate reporting and does not 

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National Aeronautics and Space Administration. 
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Independent Review of United States and Russian Probabilistic Risk 

Assessments (PRAs) for the International Space Station (ISS) Mini 

Research Module #2 (MRM-2) Micrometeoroid and Orbital Debris 

(MMOD) Risk 

February 3, 2011
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Approval and Document Revision History 

NOTE: This document was approved at the February 3, 2011, NRB. This document was 

submitted to the NESC Director on February 10, 2011, for configuration control. 

Approved

Version:

Original Signature on File 2/10/11

1.0 NESC Director Date

Version Description of Revision Office of Primary 

Responsibility

Effective Date

1.0 Initial Release Mr. Michael Squire, NESC 

Back-up Principal Engineer

2/3/2011
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Table of Contents 

Volume I: Consultation Report 

1.0 Notification and Authorization ........................................................................................ 5

2.0 Signature Page ................................................................................................................... 6

3.0 Team List ........................................................................................................................... 7

4.0 Executive Summary .......................................................................................................... 8

5.0 Assessment Plan .............................................................................................................. 10

6.0 Problem Description and Background ......................................................................... 10

6.1 MMOD Risk Assessment Methodology and Requirements ............................................. 10

6.2 MRM-2 Risk Assessment ................................................................................................. 11

7.0 Data and Evaluation ....................................................................................................... 13

7.1 Risk Estimates ................................................................................................................... 13

7.2 Impact of Divergent PNP and R-Factors on Assessed Risk ............................................. 14

7.3 Causes of PNCF Disparity: Finite Element Model (FEM) ............................................... 15

7.4 Causes of PNCF Disparity: R-factor................................................................................. 16

8.0 Findings, Observations, and NESC Recommendations .............................................. 22

8.1 Findings ............................................................................................................................ 22

8.2 Observations ..................................................................................................................... 23

8.3 NESC Recommendations.................................................................................................. 23

9.0 Alternate Viewpoints ...................................................................................................... 24

10.0 Other Deliverables .......................................................................................................... 24

11.0 Lessons Learned .............................................................................................................. 24

12.0 Definition of Terms ......................................................................................................... 24

13.0 Acronyms List ................................................................................................................. 25

14.0 References ........................................................................................................................ 26

Volume II: Appendices 

Appendix A. 5R Stage Operations Readiness Review ................................................................ 27

Appendix B. Questions to RSC-E with Replies .......................................................................... 40

Appendix C. ISS Probabilistic Risk Assessment: MMOD Assessment ...................................... 42

Appendix D. MRM-2 Ballistic Limit Equation Inputs ................................................................ 72

Appendix E. ISS Noncompliance Report NCR-RS-MRM2-01 .................................................. 75

Appendix F. Stakeholder Briefing for Independent Review of US and Russian PRAs for 

MRM2 MMOD Risk .............................................................................................. 81
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List of Figures

Figure 6.2-1. MRM-2 ................................................................................................................. 12

Figure 6.2-2. Location of MRM-2 on the Russian Orbital Segment of ISS ............................... 12

Figure 6.2-3. MRM-2 on Orbit ................................................................................................... 13

Figure 7.3-1. FEM used by RSC-E in Bumper Assessment Showing PID Assignments .......... 16

Figure 7.3-2. FEM used by NASA in Bumper Assessment Showing PID Assignments ........... 16

Figure 7.4-1. Comparison of RSC-E Empirical Predictions of Tip-to-Tip Crack Length for the 

Russian Research Module, Normal Impact at 6.5 km/s ........................................ 21

List of Tables

Table 7.1-1. Risk comparison from October 2009 .................................................................... 13

Table 7.1-2. Risk Comparison from December 2009 ............................................................... 14

Table 7.2-1. Risk Values Uses as Baseline for Sensitivity Study ............................................. 14

Table 7.2-2. Case I: Effect of PNP Values on Risk Assuming a Constant R-factor ................. 15

Table 7.2-3. Case II: Effect of R-factor Values on Risk Assuming a Constant PNP ................ 15

Table 7.4-1. R-factor Comparison (N/C = Not Calculated in Analysis Provided) ................... 17

Table 7.4-2. Coefficients for Equation 7.4-2 ............................................................................. 20
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1.0 Notification and Authorization 

Mr. Michael Suffredini, International Space Station (ISS) Program Manager, requested an 

independent review of the separate micrometeoroid and orbital debris (MMOD) catastrophic risk 

assessments for the Mini-Research Module-2 (MRM-2) performed by both the Russian Federal 

Space Agency and NASA. The risk assessments produced by the two organizations differed by 

roughly one order of magnitude. 

This NESC assessment was approved as an out-of-board activity on November 2, 2009 by the 

NESC Director. Mr. Michael Squire, Back-up Principal Engineer at the NASA Langley 

Research Center (LaRC), was selected to lead this assessment. The stakeholder requested a short 

duration for this activity, so the requirement for an assessment plan was waived. The results of 

the assessment were presented to the stakeholders on December 17, 2009. The final report was 

presented and approved by the NESC Review Board on February 3, 2011.

The stakeholders for this assessment were Mr. Michael Suffredini and the ISS Program. 
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2.0 Signature Page 

Submitted by:

Team Signature Page on File – 3/10/11

Mr. Michael Squire Date 

 

Significant Contributors: 

Dr. Joel Williamsen Date Mr. Dana Lear Date 

Dr. William Schonberg Date Dr. Fayssal Safie Date 

 

Signatories declare the findings and observations compiled in the report are factually based from 

data extracted from Program/Project documents, contractor reports, and open literature, and/or 

generated from independently conducted tests, analyses, and inspections. 
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3.0 Team List 

Name Discipline Organization

Core Team

Michael Squire

Lead/NESC Principal Engineers 

Office LaRC/NESC

Dana Lear MMOD

JSC/Human Exploration 

Science Office

Hank Rotter

Technical Fellow for Life Support 

and Active Thermal Control JSC/NESC

Fayssal Safie

Technical Fellow for Reliability 

and Maintainability

MSFC/NASA Safety 

Center

William Schonberg MMOD Protection System Design Independent Consultant

Joel Williamsen MMOD Survivability

Institute for Defense 

Analysis

Pamela Throckmorton MTSO Program Analyst LaRC/NESC

Administrative Support

Pamela Sparks Project Coordinator LaRC/ATK

Donna Gilchrist Planning and Control Analyst LaRC/ATK

Christina Williams Technical Writer LaRC/ATK
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4.0 Executive Summary 

The Mini-Research Module-2 (MRM-2), a Russian module on the International Space Station 

(ISS), does not meet its requirements for micrometeoroid and orbital debris (MMOD) probability 

of no penetration (PNP). To document this condition, the primary Russian Federal Space 

Agency ISS contractor, S.P. Korolev Rocket and Space Corporation-Energia (RSC-E), submitted 

an ISS non-compliance report (NCR), NCR-RS-MRM2-01 (Appendix E), which was presented 

at the 5R Stage Operations Readiness Review (SORR) in October 2009. In the NCR, RSC-E 

argued for waiving the PNP requirement based on several factors, one of which was the risk of 

catastrophic failure was acceptably low at 1 in 11,100 (0.009 percent). However, NASA 

independently performed an assessment of the catastrophic risk resulting in a value of 1 in 1380 

(0.07 percent) and believed that the risk at that level was unacceptable. The NASA Engineering 

and Safety Center (NESC) was requested to evaluate the two competing catastrophic risk values 

and determine which was more accurate. 

Because the outcome of this activity was requested to be complete within approximately 6 

weeks, the analysis of the risk assessments of each organization was necessarily performed at a 

high level. The sources of divergence between RSC-E and NASA catastrophic risk assessments 

were identified as were further areas of analysis that would likely result in a convergence of risk 

assessment values. During the course of the activity, RSC-E and NASA refined their 

assessments and produced risk values that were within a factor of 2 of each other instead of the 

factor of 10 that was initially observed, so determining which assessment was more accurate 

became less urgent. 

The MMOD catastrophic risk is calculated using the risk factor (R or R-factor), which is defined 

as the ratio of catastrophic1 MMOD impacts to total MMOD impacts. The probability of no 

catastrophic failure (PNCF) is defined as PNCF = PNPR. The catastrophic risk as a percentage is

defined as 1-PNCF. To explore the influence PNP and R have on catastrophic risk, the NESC 

team performed a rudimentary sensitivity study varying PNP and R in PNCF calculations. The 

results show that while both PNP and R affects the PNCF, the effect of PNP is more pronounced. 

One source of disparity in PNP values (and catastrophic risk) was the MRM-2 finite element 

models (FEM) used by the MMOD risk assessment application Bumper II (Bumper). The FEM 

used by RSC-E was an older version and contained differences from the FEM used by NASA. 

Differing values for R was another contributor to the disparate risk assessments. The RSC-E and 

NASA R-factors used in initial risk assessments were 0.010 and 0.090, respectively. Each Rfactor is the summation of individual R-factors for each identified risk, and when the individual 

R-factors were compared, differences between NASA and RSC-E became evident. For example, 

RSC-E considered the loss of crew due to hypoxia and a docking unit failure to be higher risks 

 

1

 A catastrophic failure is one that causes the loss of a crew member or loss of the spacecraft. 
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than NASA (i.e., the RSC-E R-factors for each of those risks are greater than NASA’s). 

Similarly, NASA considered the risks due to fragmentation and internal equipment penetration to 

be higher than RSC-E, and those corresponding R-factors were greater for NASA than RSC-E.

For NASA, R-factors are calculated using the Manned Spacecraft and Crew Survivability 

(MSCSurv) program. In MSCSurv, the effects of impact velocity are incorporated into the hole 

diameter and crack length predictions using a momentum scaling factor for hole diameter and an 

energy scaling factor for crack length. In contrast, no velocity effects are evident in the hole 

diameter and crack length equations used by RSC-E, so the equations are valid only for a single 

impact velocity (6.5 km/s) from which the equations were empirically derived. In addition, there 

is some ambiguity as to the origin of the values for two of the coefficients used in the RSC-E 

crack length equation, where the values given in one reference do not match those provided in 

another. This may be another source of divergence between risk assessments. 

Although the PNCF values converged during the course of this assessment, both the NASA and 

RSC-E PNCFs indicate that additional shielding should be installed on the MRM-2 to bring the 

PNP into compliance and reduce the catastrophic risk. In addition, because the PNCFs are still 

different by a factor of 2, the NESC team recommends that the two agencies continue to 

collaborate and decrease the discrepancy. As a part of this, the uncertainties associated with the 

R-factors and PNCFs should be defined. Finally, NASA’s plans to reevaluate MSCSurv will 

provide R-factors that are a more accurate reflection of the current ISS configuration and 

operation. 
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5.0 Assessment Plan 

The NESC team relied on documentation review (see References and the Appendices for specific 

documents) and interviews with NASA and Russian MMOD organizations to arrive at findings, 

observations and NESC recommendations. Information from NASA was provided by the NASA 

Astromaterials Research and Exploration Systems/Human Exploration Science (HES) Office. 

Questions for the primary Russian Federal Space Agency ISS contractor, RSC-E, were relayed 

through the HES to their RSC-E counterparts. 

6.0 Problem Description and Background 

6.1 MMOD Risk Assessment Methodology and Requirements 

The PNP is used to assess the ability of a spacecraft to resist MMOD impacts. PNP for a given 

spacecraft is a function of the shield properties, the MMOD environment (flux, directionality, 

mass, size), configuration (e.g., the deployed position of radiators and solar panels may shadow 

areas of the spacecraft), flight attitude, and failure criteria. Bumper II (Bumper) is the tool used 

by NASA for calculating PNP. While the PNP gives the probability that an MMOD particle will 

inflict damage exceeding defined failure criteria, it does not describe the potential for the loss of 

life or the loss of the vehicle. The PNCF gives the probability that an MMOD penetration will 

not cause the loss of a crew member or the loss of the vehicle. 

The ratio of catastrophic impacts to total impacts is the risk factor (R-factor or R). The PNCF, 

PNP, and R are related as shown in the equation: 

PNCF = (PNP)R Eq. 6.1-1

Like PNP, PNCF is expressed as a decimal numeral with a value less than 1. The catastrophic 

risk is typically expressed as a percentage and is defined as: 

Catastrophic Risk = 1 – PNCF Eq. 6.1-2

The R-factor for a spacecraft (or module) is achieved by summing the individual R-factors for 

each failure mode. This summative R-factor is then used to determine the PNCF and 

catastrophic risk for the overall spacecraft or module.

The MMOD environment models used for the PNP calculation will affect the resultant PNP. 

The Russian ISS requirements, specified in the Space Station Natural Environment Definition for 

Design (SSP-30425) [ref. 1], state that the orbital debris model used for calculation of PNP is the 

NASA 1991 Orbital Debris Model. A newer model, ORDEM2000, is available and can also be 

used for PNP calculations, but is not used to meet requirements. The micrometeoroid model 

specified by Reference 1 used for MMOD risk analysis is the 1991 Meteoroid Model. The 
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newest meteoroid model is called the Meteoroid Engineering Model (MEM) but also is not used 

to meet requirements.

The NASA-desired PNP requirement for the MRM-2 is PNP ≥ 0.995 based on an exposure 

duration of 15 years. This is the value specified in the Russian Segment Specification [ref. 2] for 

the Docking Compartment #2 (DC-2), which is structurally identical to the MRM-2. A 

notification of document change (NDC) has been submitted (NDC-41163-52 Revision B) to 

change “DC-2” to “MRM-2.” However, RSC-E rejected this change and countered with a 

proposal that the MRM-2 PNP requirement should reflect as-launched MMOD capability (i.e., a 

PNP requirement of 0.9734 for 15 years). The MMOD requirement for MRM-2 has not been 

resolved, but for this assessment, the requirement of PNP ≥ 0.995 was assumed. 

6.2 MRM-2 Risk Assessment 

The MRM-2 (Figure 6.2-1) was launched aboard a modified Progress spacecraft in November 

2010. It provides a docking port for Progress and Soyuz spacecraft, adds additional space for 

experiments, and contains data-transmission interfaces for external payloads. It is attached to the 

Russian ISS Segment at the zenith port of the Service Module Transfer Compartment, and 

extends in the zenith direction 4 m (see Figures 6.2-2 and 6.2-3). The maximum hull diameter is 

2.5 m. Two EVA hatches oriented 180 degrees apart are on the forward and rear faces of the 

module. Also located on the MRM-2 exterior is a Strela extendable cargo boom, an equipment 

box, a multipurpose work station, and EVA handrails. The orientation of the module exposes the 

forward face, including most of the forward hatch, to the highest MMOD risk. 

Separate risk assessments performed by RSC-E and NASA concluded that the MRM-2 MMOD 

PNP was less than the requirement of PNP ≥ 0.995. Using the 1991 meteoroid and orbital debris 

environments, RSC-E assessed the PNP as 0.985. Similarly, NASA independently assessed the 

PNP as 0.983 using the same environment models. The violation was documented in the 

noncompliance report (NCR), NCR-RS-MRM2-01 [Appendix E]. Within the NCR and the 

discussions it generated, there was agreement between RSC-E and NASA that the MRM-2 PNP 

was not in compliance with a PNP ≥ 0.995 requirement. In the NCR, RSC-E proposed waiving 

the requirement based on several factors, one being that the catastrophic risk for the MRM-2 was 

at an acceptable value of 0.009 percent (1 in 11,100/PNCF = 0.9999). NASA calculated the 

catastrophic risk as 0.0725 percent (1 in 1380/PNCF = 0.9993) and judged this to be 

unacceptable (there is no requirement for catastrophic risk or PNCF at the module level). NASA 

further recommended a risk mitigation strategy that included augmenting the MRM-2 MMOD 

shielding. This catastrophic risk disparity is what the NESC team was requested to evaluate. 
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Figure 6.2-1. MRM-2 

Figure 6.2-2. Location of MRM-2 on the ISS Russian Orbital Segment 
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Figure 6.2-3. MRM-2 on Orbit 

7.0 Data and Evaluation 

7.1 Risk Estimates 

The purpose of this NESC assessment was to quickly provide the ISS Program with an opinion 

regarding the Russian and NASA risk assessments performed for the MRM-2. Time constraints 

limited the NESC team to a high-level review of the methods used by RSC-E and NASA and to 

understand how both parties arrived at their results. Further study would result in an increased 

understanding of the results. 

The PNP and catastrophic risk values originally presented to the NESC team were those from the 

October 26, 2009 5R SORR (see Appendix A) and are shown in Table 7.1-1.

Table 7.1-1. Risk Comparison from October 2009 

Agency PNP R-factor Catastro

phic Risk

Odds Models

RSC-E 0.991 0.010 9.0x10-51 in 

11,111

ORDEM2000/1991 meteoroid

NASA 0.992 0.090 7.2x10-41 in 

1385

ORDEM2000/MEM
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As the understanding of the MRM-2 MMOD risk matured, the PNP and PNCF evolved from the 

values in contention at the beginning of the assessment. By December 4, 2009, reassessments 

showed that the difference between the RSC-E and NASA catastrophic risk had decreased from a 

roughly tenfold difference to a difference closer to a factor of two (see Table 7.1-2).

Table 7.1-2. Risk Comparison from December 2009 

Agency PNP R-factor Catastrophic 

Risk

Odds Models

RSC-E 0.9955 0.0512 2.3x10-41 in 

4347

2000 debris/1991 meteoroid

NASA 0.994 0.077 4.8x10-41 in 

2083

2000 debris/1991 meteoroid

One reason RSC-E’s PNP increased from 0.991 to 0.9955 is because earlier RSC-E PNP 

calculations used estimated stand-off distances between the outer shield (bumper) and the 

pressure wall that were smaller than the actual configuration. Also, the older PNP runs used 

1998 as the starting year for the debris and meteoroid models, instead of the actual year of 

MRM-2’s launch (2009). Similarly, an increased understanding of the MRM-2 configuration 

increased the NASA PNP from 0.992 to 0.994. 

7.2 Impact of Divergent PNP and R-Factors on Assessed Risk

A simple sensitivity study was performed to assess the relative impact PNP and R-factor values 

had on the assessed risk. Table 7.2-1 presents the most recent values of PNP, R-factor, assessed 

risk, and risk odds for the MRM-2 module, and includes the RSC-E-to-NASA risk ratio. As seen 

in this table, the assessed risk value calculated by the RSC-E is ~50 percent of the value 

calculated by NASA. 

Table 7.2-1. Risk Values Uses as Baseline for Sensitivity Study 

Agency PNP R-Factor Risk Odds RSC-E to 

NASA Risk 

Ratio

RSC-E 0.9955 0.0512 2.31E-04 1 in 4331 0.50

NASA 0.994 0.077 4.63E-04 1 in 2159 ----

This sensitivity study considered two scenarios. In Case I, the R-factor was held constant at the 

value equal to the average of the most recent RSC-E and NASA R-factors, while the PNP was 

varied between the most recent RSC-E and NASA values. In Case II, the PNP was held constant 

at the value equal to the average of the most recent RSC-E and NASA PNPs, while the R-factor 
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was varied between the most recent RSC-E and NASA values. Tables 7.2-2 and 7.2-3 present 

the results of this study. It is important to note that since PNP and R-factor are interrelated, 

treating them as independent variables will allow only qualitative conclusions regarding their 

relative effect on assessed risk values. 

Table 7.2-2. Case I: Effect of PNP Values on Risk Assuming a Constant R-factor 

Agency PNP Average 

R-Factor

Risk Odds RSC-E to 

NASA Risk 

Ratio

RSC-E 0.9955 0.0641 2.89E-04 1 in 3460 0.75

NASA 0.994 0.0641 3.86E-04 1 in 2593 ----

Table 7.2-3. Case II: Effect of R-factor Values on Risk Assuming a Constant PNP 

Agency Average 

PNP

R-Factor Risk Odds RSC-E to 

NASA Risk 

Ratio

RSC-E 0.99475 .0512 2.69E-04 1 in 3711 0.66

NASA 0.99475 0.077 4.04E-04 1 in 2468 ----

As illustrated in Tables 7.2-2 and 7.2-3, increasing the PNP from the NASA value of 0.994 to the 

RSC-E value of 0.9955, while assuming a constant average R-factor would result in an assessed 

risk by RSC-E that is 75 percent of what would be NASA’s assessed risk. Also, decreasing the 

R-factor from the NASA value of 0.077 to the RSC-E value of 0.0512, while assuming a 

constant average PNP would result in an assessed risk by RSC-E that is 66 percent of what 

would be NASA’s assessed risk. This shows that the assessed risk is more sensitive to PNP 

changes than changes to R. 

7.3 Causes of PNCF Disparity: Finite Element Model (FEM) 

Both NASA and RSC-E use Bumper to perform MMOD risk assessments and to calculate the 

PNP. An FEM that describes the spacecraft geometry is created in Integrated Design and 

Engineering Analysis Software (IDEAS) and used by Bumper. The FEM is separated into 

different elements, each identified with a property identifier (PID). During the course of this 

activity, it was verified that the FEM used by NASA for their ISS Bumper risk assessment was 

different than the one used by RSC-E. The FEM was provided by RSC-E, but RSC-E was using 

an older version for their risk assessments. Figures 7.3-1 and 7.3-2 graphically illustrate the 

differences between the two FEMs in question. 
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Figure 7.3-1. FEM used by RSC-E in Bumper Assessment Showing PID Assignments 

Figure 7.3-2. FEM used by NASA in Bumper Assessment Showing PID Assignments 

7.4 Causes of PNCF Disparity: R-factor 

Another reason for differences in PNCF is disparity in R-factors chosen by RSC-E and NASA. 

The R-factor used by RSC-E in the PNCF estimate presented in the October 2009 NCR was 

0.010, resulting in a catastrophic risk of 9.0x10-5. However, NASA used an R-factor of 0.090 to 

arrive at a catastrophic risk of 7.2x10-4 (see Table 7.1-1). Like the PNPs, R-factors also changed 
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from October to December 2009. RSC-E’s reassessment of catastrophic risk changed the Rfactor from 0.010 to 0.0512, causing an increase in catastrophic risk. The NASA catastrophic 

risk actually decreased because the R-factor of 0.090 used in the October assessment was 

changed to 0.077 in the December assessment. 

Table 7.4-1 illustrates the individual risk factors and the assumptions that led to the respective Rfactors for NASA and RSC-E. In general, the differences can be summarized as: 

 NASA does not include docking unit failure as a risk,

 RSC-E considers the loss of crew due to hypoxia to be a higher risk factor than NASA, 

 NASA considers the risk due to fragmentation to be higher than RSC-E, and 

 RSC-E does not consider the risk due to the catastrophic destruction of internal 

pressurized tanks. 

Table 7.4-1. R-factor Comparison (N/C = Not Calculated in Analysis Provided) 

NASA

R

RSC-E

R

Risk Description Comments

0 N/C Critical crack (unzipping) 

causes loss of ISS

0 N/C External equipment 

penetration causes loss of ISS

0.063 N/C Internal systemic equipment 

penetration causes loss of ISS

NASA assumption is the presence of internal 

pressurized tanks.

0 0.02435 Docking unit failure

0.004 0.02519 Hypoxia causes loss of crew Depends on hole size and time it takes crew to 

egress ISS. RSC-E assumes time of 9.5 

minutes, while NASA uses a distribution 

relating to crew position and amount of time 

spent in different areas of the station.

0.010 0.00168 Fragmentation causes loss of 

crew

NASA probably assumed a higher occupancy 

that RSC-E.

0 N/C Thrust induced angular 

velocity causes loss of crew

To further examine the differences between the Russian and NASA risk assessments requires a 

closer examination of the Russian and NASA probability risk assessment (PRA) processes. The 

ISS MMOD PRA process is described in Appendix C, with details pertaining to RSC-E in 

Reference 3 and NASA in References 4 and 5. Due to the short timeline and high-level review, 
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many questions remained unanswered that, if addressed, would result in an increased 

understanding of the respective PRA processes and their differences. 

NASA PRA Process – Hole Diameter and Crack Length Calculations 

The NASA PRA process is based on the application Manned Spacecraft and Crew Survivability 

(MSCSurv), which provides as an output the R-factor. In the version of MSCSurv [ref. 5] used 

for these assessments, hole diameter and crack length resulting from an MMOD impact are 

calculated using empirical equations that were developed for 14 common ISS wall configurations 

[ref. 6]. These equations are based on impact test data obtained at an impact velocity of ~6.5 

km/s. The effects of impact velocity are incorporated into the hole diameter and crack length 

predictions by NASA using a momentum scaling factor for hole diameter and an energy scaling 

factor for crack length [ref. 7].

RSC-E PRA Process – Hole Diameter and Crack Length Calculations 

With respect to the RSC-E PRA process, the equations for module wall hole diameter and 

maximum tip-to-tip crack length (Equations 7.4-1 and 7.4-2) presented in Section 2.2.2 of 

Reference 3 appear to be based on the empirical equations provided in Reference 6 that were 

developed in 1995 using data collected from high speed impact tests performed at impact 

velocities near 6.5 km/s. 

 

 

 

 Eq. 7.4-1 

 

 

 

 Eq. 7.4-2 

where: 

 = effective hole diameter (cm) 

 = maximum crack length (cm) 

 = diameter of a spherical particle (cm) 

 = incidence angle 

 = ballistic diameter limit for an impact velocity of 6.5 km/s and angle of incidence θ

A, B and C = factors that vary with hole diameter and crack length. For hole diameters 

in a typical Russian module shield: A = 4.323 cm, B = 0.416 cm, and C = 1.474 cm. For 

a pressurized hull crack length, A = 4.89 cm, B = 0.633 cm, and C = 1.44 cm. 

No velocity effects are evident in the RSC-E hole diameter and crack length equations. 

Therefore, these equations are strictly valid at a single impact velocity of 6.5 km/s. RSC-E is 

apparently aware of this restriction and their PRA process and catastrophic risk calculations 
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ignore velocity effects (see Appendix B). As such, the RSC-E risk and PNCF values are 

obtained without considering impact velocity effects. 

RSC-E presented a method for incorporating impact velocity effects for very large projectiles 

(dp/hb > 20) in Reference 3, pages 4-5 (shown in equations 7.4-3 and 7.4-4), but not for smaller 

projectiles. 

 

 Eq. 7.4-3

where: 

 = effective hole diameter (cm) 

 = diameter of a spherical particle (cm) 

h = barrier thickness (aluminum equivalent) 

V = impact velocity (km/s) 

 

 

 

 

 

 

 

 

 

 

 Eq. 7.4-4

where: 

 = particle mass (units not given) 

 = particle velocity before impact (km/s) 

 = particle velocity after impact (km/s) 

 = effective hole diameter (cm) 

 = shield thickness (cm) 

 = shield material density (units not given) 

Equations 7.4-3 and 7.4-4 for the larger projectiles are based on the conservation of momentum. 

The RSC-E PRA predictions might change if an energy balance were used (i.e., energy losses 

due to shock heating, etc. would be subtracted out). It would also be instructive to compare the 

predictions of the momentum-based approach (or a combined energy-and-momentum-based 

approach) against actual data, and against the predictions of empirical hole size and crack length 

equations for the particle size regime considered. 

Empirical Equation Coefficient Issues 

Inspection of the values of the coefficients A, B, and C in Equation 7.4-1 (from Reference 3) for 

hole diameter reveals that all three values match the values of A, B, and C in Reference 6 for the 

Research Module hole diameter equation. However, the values of B and C for crack length as 

given in Reference 3 (Equation 7.4-2) do not match the values of B and C in Reference 6 for the 
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Research Module crack length equation (Table 7.4-2, in blue). The value of A for the crack 

length equation in Reference 3 does match the value of A in Reference 6, indicating that the 

asymptotic value of crack length according to Reference 3 will eventually match that of 

Reference 6. 

= ( Eq. 7.4-2 

Coefficient Value for Hole 

Diameter from 

Ref. 3 (cm)

Value for Hole 

Diameter from 

Ref. 6 (cm)

Value for Crack 

Length from Ref. 

3 (cm)

Value for Crack

Length from Ref. 

6 (cm)

A 4.323 4.323 4.89 4.89

B 0.416 0.416 0.633 0.498

C 1.474 1.474 1.44 9.518

When RSC-E was queried regarding the source of the values of B and C for the crack length 

equation in Reference 3, their reply was that they are based on the “experimental results obtained 

in MSFC [where] the crack length = 1.5 x penetration hole.” From the form of equation 7.4-2, it 

is evident that coefficient B factors in the effects of impact obliquity, while coefficient C governs 

the rate at which the equation’s asymptotic value is reached. However, the values of B and C 

given for the crack length equation do not appear to cause the crack length values calculated 

using that equation to be 1.5 times the pressure wall hole diameter. If that were the desired end 

result, then it would appear that the value of coefficient A for the crack length equation would 

need to be 1.5 times the value of the coefficient A for the hole diameter equation. However, 

those two values as given in Reference 3 are within approximately 10 percent of each other 

(Table 7.4-2, in pink). Hence, the explanation provided by RSC-E regarding how the values of B 

and C for the crack length equation are obtained is not clear and should be revisited to more fully 

understand the role of these coefficients in affecting the outcomes of the RSC-E PRA process. A

review of the Research Module pressure wall hole diameter and maximum tip-to-tip crack length 

data obtained at impact test velocities near 6.5 km/s reveals that crack lengths are, on average,

approximately 1.33 times larger than corresponding hole diameters [ref. 8] over all of the impact 

obliquities and projectile diameters tested. Therefore, the RSC-E contention that the crack length 

= 1.5 x penetration hole diameter appears to be reasonable and should yield more conservative 

(i.e. higher) values of assessed risk if implemented in a PRA process. 

Figure 7.4-1 compares predictions of pressure wall tip-to-tip crack length for normal 6.5 km/s 

impacts on a Research Module wall target as given by Equation 7.4-2 using the values of the 

coefficients B and C given in Reference 3 (the RSC-E equation), and in Reference 6 (the NASATable 7.4-2. Coefficients for Equation 7.4-2 
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equivalent equation). Empirical crack length data for this type of impact on this wall system are 

also shown. As can be seen in Figure 7.4-1, crack-length predictions can vary significantly 

depending on the values of the coefficients B and C. Specifically, using the values of B and C 

identified by RSC-E results in predictions of crack length that are smaller than if the coefficients 

generated by the NASA-equivalent equation were used.

Figure 7.4-1. Comparison of RSC-E Empirical Predictions of Tip-to-Tip Crack Length for the 

Russian Research Module, Normal Impact at 6.5 km/s 
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8.0 Findings, Observations, and NESC Recommendations 

8.1 Findings 

The following findings were identified: 

F-1. Based on the risk assessment for either source, augmented MMOD shielding for MRM-2

is warranted for MRM-2.

F-2. According to the most recently produced values, the discrepancy between NASA and 

RSC-E risk is approximately a factor of 2, instead of the factor of 10 displayed at the 5R 

SORR.

F-3. The planned NASA and RSC-E tasks are appropriate to close the gap between the 

MMOD risk assessments.

F-4. Variations in PNCF between NASA and RSC-E may be caused by:

 RSC-E using older FEM than NASA,

 PNP calculations using different property identification mapping, and 

 Differing assumptions resulting in different R-factors. 

F-5. The components of the individual R-factors are different, and this may cause a larger 

variation in PNCF than either NASA or RSC-E are currently accounting for. Variations 

in R-factor between NASA and RSC-E may be caused by: 

 RSC-E assuming no risk due to pressurized tanks within the module while NASA 

does,

 NASA not accounting for docking mechanism failure, 

 RSC-E assuming higher risk of hypoxia, 

 RSC-E assuming lower risk from fragmentation (function of time spent in module), 

and

 Differences between how NASA and RSC-E calculate rear wall hole diameter and 

crack length, including an apparent lack of velocity effects in the RSC-E approach 

across the entire impact velocity spectrum (i.e., 1-16 km/s) and the use of empirical 

equation coefficients of unknown origin. 

F-6. With the information available, the two PNCFs cannot be adequately evaluated. 
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F-7. Different uncertainty assumptions may contribute to different risk assessments.

F-8. There is no NASA mechanism for incorporating changes in the R-factor due to model 

changes, shield changes, and operational changes. 

8.2 Observations 

O-1. It is unclear how NASA and RSC-E take into account the differences in construction 

between the actual MRM-2 and the Research Module wall system as tested in 1995 in 

their hole diameter and crack length equations. 

O-2. Some of the empirical coefficients used by RSC-E in its empirical crack length equation 

do not match corresponding coefficients in the original reference for the equation, and the 

explanation provided by RSC-E does not appear to fully explain the differences. 

O-3. Using the RSC-E equation for crack length could lead to values of assessed risk that are 

lower as compared to those that might be obtained using the NASA-equivalent equation, 

assuming no other variations between the RSC-E and NASA PRA processes. 

O-4. Considering the magnitudes of the changes in PNP and R-factor values and the resulting 

corresponding changes in assessed risk values, it is evident that assessed risk value is 

more sensitive to changes in PNP value than it is to changes in R-factor value. 

8.3 NESC Recommendations 

The following NESC recommendations were identified and directed towards the ISS Program 

unless otherwise identified: 

R-1. Install additional MMOD shielding on the MRM-2 to reduce the PNP to the level 

specified in the requirements. (F-1 and F-3)

R-2. Continue NASA and RSC-E collaboration to narrow the gap between R-factors and 

PNCF for MRM-2. (F-2, F-3, F-5, F-6, and F-7)

R-3. Define uncertainties in PNCFs and the terms factored into their calculation. 

(F-5, F-6, and F-7) 

R-4. Proceed with current NASA plans to update R-factors in risk assessments. (F-7 and F-8)
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9.0 Alternate Viewpoints 

There were no alternate viewpoints identified during the course of this assessment by the NESC 

team or the NRB quorum. 

10.0 Other Deliverables 

No unique hardware, software, or data packages, outside those contained in this report, were 

disseminated to other parties outside this assessment. 

11.0 Lessons Learned 

No applicable lessons learned were identified for entry into the NASA Lessons Learned 

Information System (LLIS). 

12.0 Definition of Terms 

Corrective Actions Changes to design processes, work instructions, workmanship practices, 

training, inspections, tests, procedures, specifications, drawings, tools, 

equipment, facilities, resources, or material that result in preventing, 

minimizing, or limiting the potential for recurrence of a problem. 

Finding A conclusion based on facts established by the investigating authority. 

Lessons Learned Knowledge or understanding gained by experience. The experience may 

be positive, as in a successful test or mission, or negative, as in a mishap 

or failure. A lesson must be significant in that it has real or assumed 

impact on operations; valid in that it is factually and technically correct; 

and applicable in that it identifies a specific design, process, or decision 

that reduces or limits the potential for failures and mishaps, or reinforces a 

positive result. 

Observation A factor, event, or circumstance identified during the assessment that did 

not contribute to the problem, but if left uncorrected has the potential to 

cause a mishap, injury, or increase the severity should a mishap occur. 

Alternatively, an observation could be a positive acknowledgement of a 

Center/Program/Project/Organization’s operational structure, tools, and/or 

support provided. 

Problem The subject of the independent technical assessment. 

Proximate Cause The event(s) that occurred, including any condition(s) that existed 

immediately before the undesired outcome, directly resulted in its 
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occurrence and, if eliminated or modified, would have prevented the 

undesired outcome. 

Recommendation An action identified by the NESC to correct a root cause or deficiency 

identified during the investigation. The recommendations may be used by 

the responsible Center/Program/Project/Organization in the preparation of 

a corrective action plan. 

Root Cause One of multiple factors (events, conditions, or organizational factors) that 

contributed to or created the proximate cause and subsequent undesired 

outcome and, if eliminated or modified, would have prevented the 

undesired outcome. Typically, multiple root causes contribute to an 

undesired outcome. 

13.0 Acronyms List 

DC-2 Docking Compartment #2 

FEM Finite Element Model 

IDEAS Integrated Design and Engineering Analysis Software 

ISS International Space Station 

LaRC Langley Research Center 

MEM Meteoroid Engineering Model 

MEO Meteoroid Environment Office 

MMOD Micrometeoroid and Orbital Debris 

MOD Mission Operations Directorate 

MRM-2 Mini-Research Module 2 

MSFC Marshall Space Flight Center 

MTSO Management and Technical Support Office 

NCR Noncompliance Report 

NDC Notification of Document Change 

NESC NASA Engineering and Safety Center 

NRB NESC Review Board 

PID Property Identifier 

PNCF Probability of No Catastrophic Failure 

PNP Probability of no Penetration 

PRA Probabilistic Risk Assessment 

R Risk Factor 

RSC-E S.P. Korolev Rocket and Space Corporation – Energia 

SORR Stage Operations Readiness Review 
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14.0 References 

1. SSP-30425, revision B, Space Station Natural Environment Definition for Design, 

February 8, 1994. 

2. SSP-41163, Russian Segment Specification, Revision J, 28 November 2008. 

3. Anon., A Study of the Probability of Catastrophic MMOD Penetration of the Docking 

Compartment Pressurized Hull, Technical Report No. П-38332-311 / SS40950, Russian 

Space Agency, 2005. 

4. J.E. Williamsen, Vulnerability of Manned Spacecraft to Crew Loss from Orbital Debris 

Penetration, NASA–TM–108452, 1994. 

5. Evans, H., Blacklock, K., and Williamsen, J.; Manned Spacecraft & Crew Survivability 

(MSCSurv) Version 4.0 User’s Guide; Sverdrup Technology, Inc., NASA-MSFC; Report 

no. 651-001-97-006; September, 1997. 

6. W.P. Schonberg, Pressure Wall Hole Size and Maximum Tip-to-Tip Crack Length 

Following Orbital Debris Penetration, Final Report, NASA/ASEE Summer Faculty 

Fellowship Program, Marshall Space Flight Center, Alabama, 1995. 

7. J.E. Williamsen, D.J. Grosch, and W.P. Schonberg, “Empirical Prediction Models for 

Hole and Crack Size in Space Station Shielding from 6 to 12 km/s,” in Proc. SPIE 

Symposium on Orbital Debris, Impact Modeling, and Penetration Effects, Paper No. 

2813-20, Denver, Colorado, 1996. 

8. Schonberg, W. and Williamsen, J. Empirical Hole Size and Crack Length Prediction 

Models for Dual Wall Systems Under Hypervelocity Impact, International Journal of 

Impact Engineering, Vol. 20, pp 711, 1997. 

Volume II: Appendices 

Appendix A. 5R Stage Operations Readiness Review 

Appendix B. Questions to RSC-E with Replies 

Appendix C. ISS Probabilistic Risk Assessment: MMOD Assessment 

Appendix D. MRM-2 Ballistic Limit Equation Inputs 

Appendix E. ISS Noncompliance Report NCR-RS-MRM2-01

Appendix F. Stakeholder Briefing for Independent Review of US and Russian PRAs for 

MRM2 MMOD Risk 
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Appendix A. 5R Stage Operations Readiness Review

The documents presented in Appendix A, while pre-decisional 

when created, are now considered to be post-decisional 

due to events that have transpired since the date of the presentation.
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Appendix B. Questions to RSC-E with Replies 
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Appendix C. ISS Probabilistic Risk Assessment: MMOD Assessment 
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REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

Independent Review of U.S. and Russian Probabilistic Risk 

Assessments for the International Space Station Mini Research Module #2 

Micrometeoroid and Orbital Debris Risk

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Squire, Michael D.

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 8. PERFORMING ORGANIZATION

 REPORT NUMBER

L-20002 NESC-RP-09-00592

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category 16 - Space Transportation and Safety

Availability: NASA CASI (443) 757-5802

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

The Mini-Research Module-2 (MRM-2), a Russian module on the International Space Station, does not meet its requirements 

for micrometeoroid and orbital debris probability of no penetration (PNP). To document this condition, the primary Russian 

Federal Space Agency ISS contractor, S.P. Korolev Rocket and Space Corporation-Energia (RSC-E), submitted an ISS 

non-compliance report (NCR) which was presented at the 5R Stage Operations Readiness Review (SORR) in October 2009. In 

the NCR, RSC-E argued for waiving the PNP requirement based on several factors, one of which was the risk of catastrophic 

failure was acceptably low at 1 in 11,100. However, NASA independently performed an assessment of the catastrophic risk 

resulting in a value of 1 in 1380 and believed that the risk at that level was unacceptable. The NASA Engineering and Safety 

Center was requested to evaluate the two competing catastrophic risk values and determine which was more accurate. This 

document contains the outcome of the assessment.

15. SUBJECT TERMS

International Space Station; Micrometeoroid and orbital debris; Probability of no penetration; NASA Engineering and Safety 

Center; Mini-Research Module-2

18. NUMBER

 OF 

 PAGES

96

19b. TELEPHONE NUMBER (Include area code)

(443) 757-5802

a. REPORT

U

c. THIS PAGE

U

b. ABSTRACT

U

17. LIMITATION OF 

 ABSTRACT

UU

Prescribed by ANSI Std. Z39.18

Standard Form 298 (Rev. 8-98)

3. DATES COVERED (From - To)

November 2009 - February 2011

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

869021.05.07.03.12

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

NASA/TM-2011-217070

16. SECURITY CLASSIFICATION OF:

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gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

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PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

01- 03 - 2011