Castillo, Terri

Duncan, Gary

Gingras, Benjamin

Hartman, William

Haught, Megan

Koontz, Steve

Schmidl, William

Vera, Jerry

English

NASA Technical Reports Server

INTERNATIONAL SPACE STATION SPACECRAFT CHARGING 
HAZARDS: HAZARD IDENTIFICATION, MANAGEMENT, AND 
CONTROL METHODOLOGIES, WITH POSSIBLE 
APPLICATIONS TO HUMAN SPACEFLIGHT BEYOND LEO
Steve Koontz, Terri Castillo 
NASA Johnson Space Center
William Hartman, William (Danny) Schmidl
The Boeing Company
Megan Haught, Gary Duncan 
Barrios
Benjamin Gingras, Jerry Vera 
The Boeing Company
10th IAASS Conference
15-17 May 2019
https://ntrs.nasa.gov/search.jsp?R=20200001610 2020-03-28T19:10:23+00:00Z
Publically Available – NASA EDAA TBD
Background: ISS Spacecraft Charging Hazard
| 2
 This presentation will provide a review of how the International Space Station (ISS) safety engineering 
methodology for controlling extravehicular activity (EVA) crew electrical shock hazards has evolved over the 
past 25+ years, and will provide an overview of the present methodology 
 The specific EVA crew electrical shock hazard is due to the ISS floating potential (FP, the electrical potential 
difference between ISS conducting structure and the surrounding ionospheric plasma) which ranges from 
small positive voltages (~+10) generated at one truss tip to larger negative voltages (~-40 V) at the other end 
tip due to: 
 Current collected by the 160 V solar arrays
 Electrical power system operations
 Electric field induced by the geomagnetic field at high latitudes
 Large vehicle size (~ 100 yards between truss tips)
 Auroral electrons encountered at high latitude
External EMU Surfaces
Crew electrical shock hazard 
is present when electrically 
conducting EMU components 
make contact with ISS 
conducting structure, and 
charge passes across the 
EVA crew person’s bodyC) 
C) 
Ruled out as a possible contact point 
Possible direct contact point 
Ankle*, Arm*, BSC, OBS/DCM, Thigh*, Waist* or Wrist• contact ISS 
"" ~ Olf 
.~ , 
Publically Available – NASA EDAA TBD
ISS Spacecraft Charging Hazard Management Process
| 3
 ISS Program requires two-fault tolerant hazard controls for catastrophic hazards
 Replacing an ISS Program hazard control process requiring two-fault tolerant 
hazard controls with a detection and warning process is possible if and only if it 
can be demonstrated through a combination of in-flight measurements, ground 
based testing, and probabilistic analysis that the following criteria are true:
 (1) The probability of hazardous FP values (both positive and negative) on ISS 
is nominally low
 Depends on ionospheric electron density and temperature at ISS flight altitude, which depends on 
solar activity and the magnitude of the 11 year solar cycle
 Depends on ISS vehicle flight attitude and orientation with respect to the velocity vector, especially 
orientation of the active surface of the photovoltaic arrays and the 100 meter truss.  
 Depends on the specifics of ISS electrical power system operations 
 (2) Space weather events and/or vehicle configuration changes can be 
identified with sufficient lead-time to enable activation of controls or 
rescheduling of the EVA 
 Note: In addition, the frequency of occurrence of hazard control activation or 
rescheduling of EVAs must be small
 (3)The probability (P2) of completing the EVA crew-hazard shock-circuit during 
any EVA is low
 (4) The net probability of an EVA crew shock event, Ps, as a function of P1 and 
P2, is small enough for the ISS Program to accept the residual risk when EVA 
is conducted without any active hazard controls
Publically Available – NASA EDAA TBD
ISS Floating Potential Measurements
| 4
 Long-term measurements of ISS charging severity and frequency-of-occurrence have been 
made that provide support for the basis for the present electrical shock hazard methodology
 Early measurements were obtained with the ISS floating potential probe and ISS plasma 
contactor unit (PCU) emission currents (CY 2000 and 2001). These measurements 
demonstrated that the severity and duration of ISS charging events were far less than predicted 
early in the ISS Program
 More recently, Floating Potential Measurement Unit (FPMU) measurements have been made 
from CY 2006 to the present. These FPMU measurements include ISS FP, as well as 
ionospheric electron temperature (Te) and density (Ne). FPMU data has been validated against 
comparable ground-based and satellite measurements of ionospheric temperatures and 
densities at or near ISS operating altitudes
 These measurements have confirmed the earlier result that the severity and duration of ISS 
charging events are less than predicted early in the ISS Program, and are a key part of the EVA 
shock hazard detection and warning process
 Note: the observed lower-than-expected ISS charging measurements are largely the result of:
 1) Unusually low solar activity between 2008 and the present, corresponding unusually low 
ionospheric density and,
 2) ISS photovoltaic arrays are only rarely in a worst-case ram-oriented configuration at 
eclipse exit leading to less than worst-case electron collection in most cases
Publically Available – NASA EDAA TBD
Criteria 1: Probability (P1) Of Hazardous FP Values 
on ISS (1 of 2)
| 5
 Between CY 2006 and the present, thousands of FPMU measurements have been 
made. Those measurements have demonstrated that hazardous ISS charging 
environments occur infrequently (though, not infrequently enough to ignore)
 Note, low solar activity contributes to the lower charging environment. Increases in 
solar activity may affect these conditions, so a detection and warning process is 
required to monitor for possibly hazardous conditions
 Negative potential hazard exceedances are provided here
Total FPMU 
Measurement 
Time (sec)
Tip 
Exceedance 
(sec)
SARJ 
Exceedance 
(sec)
Center 
Exceedance 
(sec)
Year
2006 9.6E+03 0 0 0
2007 5.2E+04 0 0 0
2008 1.3E+06 111 38 18
2009 2.4E+06 221 26 1
2010 2.3E+06 37 24 23
2011 3.8E+06 49 36 34
2012 3.4E+06 50 44 44
2013 9.7E+06 136 130 130
2014 9.4E+06 115 112 108
2015 9.3E+06 162 107 105
2016 6.7E+06 184 117 103
2017 1.2E+07 708 371 229
2018 8.7E+06 609 296 149
Sum (sec) 6.9E+07 2383 1300 943
Fraction of exceedances: 1/ 1/ 1/
28818 52824 72792
Annual FPMU Data Review for 
Exceedances
Probabilities (Fraction of Exceedances) provided to the 
PRA team at pre-defined ISS locations:
-Centerline of the vehicle location:             1/72792
-Solar Array Rotary Joint (SARJ) location: 1/52824
-Truss Tip location:                                     1/28818
"" ~ Olf 
.~ , 
Publically Available – NASA EDAA TBD
Criteria 1: Probability (P1) Of Hazardous FP Values 
on ISS (2 of 2)
| 6
 In addition, negative voltage spacecraft charging events were reviewed
 ISS charging events can be grouped into two categories:
 1) PV array driven charging and motional EMF charging
 2) Shorter duration Rapid Charging Events (RCE) such as Eclipse Entry (EE) Charging, Auroral Charging 
at high latitude, Power on Reset (POR), and electrical power system Regulation Events (RE)
 Effect of potential drop across the ionospheric plasma sheath in contact with ISS external 
surfaces on the hazard was also investigated and compared to the potential drop across 
exposed ISS dielectric surfaces 
FPMU Data Review Potential Drop Across Anodized 
Aluminum and the Sheathoo ··············································································· RCE 
:: ···:-.-wi·=-•·;: ... iJ•····~··.·,11';r;;; ... ri-i·;i-1lllii-· • 40 xlO 
lj j::::::: ::::::~::: :::;::::;::: ;:::::Bi!-¾.t:::::::::;' ·::::: "''. :: 
Time (years) 
"" ~ Olf 
.~ , 
50mA 0 ~ .,. , 
current ends---4 ~,f,- ------------------
here ·10 
-20 
-30 
i 
-40 
0 -50 
~ 
::; -60 
-70 
-80 
-90 
/ 
IN across dielectric 
\ ,,,...,... AV across sheath is significant 
FP is the sum of these 2 curves 
This is referred to as a Rapid Charging 
Event 
-100 -------,.------------------------
0 10 15 20 25 
Time (seconds) 
Publically Available – NASA EDAA TBD
Criteria 2: Events Leading to Hazardous FP Values 
Can be Identified with Sufficient Lead Time
| 7
 The detection and warning approach was developed to support the EVA shock 
hazard control process
 For each EVA, a forecast is requested prior to the EVA. The FPMU provides 
ionospheric Ne and Te variability data as well as ISS FP variability data  and a 
conservative assessment is performed
 After the forecast is issued, the space weather is monitored for significant events that 
may affect the hazard environment 
 The ISS Program has accepted that a low likelihood does exist that the forecast 
result may be affected by space weather events (possibly 1 every 6 years that may 
require review)
Plasma Hazard Monitor 
and Notification Criteria 
and Process
EVA-2 weeks 
"Space Weather 
No: No issue: PCU Event• plasma 
off, no exceedance hazard assessment of 5mA or (-45.SV) 
with plasma 
properties with 
enhanced density 
(factor of six) 
Monitor for Se_ace 
rossible issue Weather Event 
1 Solar wind prediction> 700 km/s @EVA day? I 
l No ,vo: NoiSSUE 
I Earth directed CME? f-l Yes, Earth directed CME 
Yes : 1 It so, does it meet CME speed threshold? f-Issue exists 
with CME 
No: 
No issue 
' 
-Inboard of the SARJ: I No issue: Proceed with EV A r-PCUs placed in discharge i 
- Outboard of the SARJ: 
Delay EVA 
"" ~ Olf 
.~ , 
Publically Available – NASA EDAA TBD
Criteria 3: Probability (P2) of Completing the EVA 
Crew-Hazard-Shock-Circuit During Any EVA is Low (1 of 2)
| 8
 To determine the probability of completing an EVA crew-hazard shock-circuit 
during EVA, the ISS PRA team performed an assessment
 ISS specialists met to determine possible shock circuit pathways, and 
galvanic contact probabilities
 ISS PRA team obtained and reviewed video records of possible EVA galvanic 
contact
 Results from this assessment/survey provided the basis for the probability of 
completing the circuit
 PRA team performed a discrete event simulation to model the EVA plasma 
shock events
Discrete Event 
Simulation Logic
Outer Loop (batches) 
Sample a per-hour (per-crewmember) contact rate for each direct and indirect continuity path 
Inner Loop (reps) 
Sample the times and durations of all types of contacts (for one crewmember) 
• For example, in 8 hours. for each crewmember, MWS Tools are expected to contact ISS about 10 
times and to contact Wrist 2 or 3 times, the safety tether wire is expected to contact EMU once or 
not at all; each contact has its own sampled time and duration 
• Direct contacts are rare and, as with indirect contacts, must be coupled with contact between the 
safety tether wire and EMU 
Compare all the crewmember's contacts to each other for overlap 
• All contacts must occur during the total time period (e.g., 8 hours) 
• The second contact (and, if applicable. the third contact) must occur within the duration of the 
first contact 
• The gauntlet/TMG must be ,n a position to expose metal (4/13) 
• The portion of the ISS that is contacted must be either stainless (5%) or damaged (1%) 
End inner loop 
The mean of the batch is the average number of overlapping contacts that occurred 
End outer loop 
"" ~ Olf 
.~ , 
Publically Available – NASA EDAA TBD
Criteria 3: Probability (P2) of Completing the EVA 
Crew-Hazard-Shock-Circuit During Any EVA is Low (2 of 2)
| 9
 Results were determined for the mean (1/290), 5th percentile, and 95th
percentile
Probability of Completing 
the Hazard 
Circuit/Contact (P2)
The probabilities were 
determined by the PRA team 
using a discrete event simulation 
that modeled the system as 
discrete events occurring at 
particular points in time.
The figure shows results for the 
mean (1/290), 5th percentile 
(1/820) and the 95th percentile 
(1/130)
Probability 
of Contact 
1.0E-04 
2 Crew, 8 Hours 
1 in 820 
r-----
1.0E-03 
l in 290 l3,4E-3) 
1 in 130 
---; 
1.0E-02 
Left1)0i t: S percentile 
Break-1)0u1t: Me-an 
R[gbt-polnl: 95'1' percendle 
-I 
1.0E-01 
""~Olf 
.~ , 
Publically Available – NASA EDAA TBD
Criteria (4): Net Probability of an EVA Crew Shock 
Event, and the Detection and Warning Product (1 of 2)
| 10
 The ISS Program plasma hazard control process is based on a detection and warning process
 The detection and warning product is based on an analysis of the hazardous scenarios and 
their probabilities
 Scenarios vary depending on shock hazard (negative and positive), EVA location, PCU 
operational state, and conductive area. Example results are provided for two positive potential 
scenarios are shown here
 Chart on the left is for the Truss Tip location, and the chart on the right for the SARJ location
 The 5mA current level is indicated with the red arrow 
 Similar to the previous chart, results are provided for the mean, 5th, and 95th percentiles (for 
the Tip, mean= 1/290,000, and for the centerline, mean= 1/11,000,000)
Recommended 
cu"entlev• 
>25mA 
>lOmA 
>SmA 
>2mA 
>lmA 
>O.SmA 
>O.lmA 
l.OE-09 
Current exceedances at the Tip (PCU OFF) 
147.2m off centerline) 
I In 230,000,000 (4.!E-91 
l in 290,000 (3.SE-61 
Left.point sth percentlt 
Brtilk·poirt Mtan 
RietJt-point: 9S~ pem:otile 
l.OE-08 l.OE-07 l.OE-06 l.OE-05 l.OE-04 l .OE-03 l.OE-02 
Recommended 
current level• 
"" ~ Olf 
.~ , 
Current exceedances at 35m from CL (PCU OFF) 
(35m off centerline) 
>25mA l l in 300,000,000 (3,lE-91 
>lOmA 
>SmA 
>2mA 
>lmA 
>0.SmA 
I In 56,000,000 (UE-81 
I in ll,OOO,OOO(B.9E-8) 
I in 850,000 (1.2E·6) 
I In 96,000 (I.OE-SJ 
I in 19,000 (S.2E-SJ 
Leh-point s.• percentile 
Bre.Jk•poait Mean 
Richt·po01t 9S" pertentie 
....... -----·--'·i , ..... 
---+----+--------r----+---, I 
l .OE-09 l OE-08 l.OE-07 l .OE-06 l.OE-05 1.0E-04 l.OE-03 l.OE-02 . 
Publically Available – NASA EDAA TBD
Criteria (4): Net Probability of an EVA Crew Shock 
Event, and the Detection and Warning Product (2 of 2)
| 11
 The negative potential hazard results are shown below. Note: the negative 
potential hazard probability included all negative FP limit violations (for example, 
all RCE events were included)
 Similar to the previous chart, results are provided for the mean, 5th, and 95th
percentiles (for the mean, the negative shock hazard is 1/13,000,000)
N1egative Shodk 
"" ~ Olf 
.~ , 
Publically Available – NASA EDAA TBD
Validation and Programmatic Approval of the 
Detection and Warning Approach
| 12
 For high exceedances, driven by space weather events, the ISS Program may 
utilize PCUs (for EVA inboard of the SARJ). For EVAs outboard of the SARJ, the 
ISS Program maintains the option to defer the EVA until the event passes
 The detection and warning approach has been approved by the ISS Program 
safety review process
 As part of the process, a back-up methodology was requested, developed, and 
approved/implemented. The methodology, IRI Real-Time Assimilative Mapping 
(IRTAM) provides near real-time ionospheric environment measurements based 
on global ground based digisonde measurements 
 IRTAM has been developed and is provided by the University of 
Massachusetts at Lowell (UML) Space Science Lab
 IRTAM uses real time measurements from ~70 digisonde instruments
 In the event of an FPMU failure, IRTAM would be utilized to determine the 
environment variability for the plasma hazard forecast.
 In addition, IRTAM provides data daily, while the FPMU has to be activated, 
so IRTAM can be used to monitor for a changing environment at any time
Publically Available – NASA EDAA TBD
Applicability of Probabilistic Spacecraft Charging 
Hazard Control Methods to Human Spaceflight Beyond LEO
| 13
 ISS Space Environments Team has developed an EVA Shock Hazard Detection 
and Warning Process that meets the criteria for acceptance by the ISS Program
 The general approach has applicability to future human spaceflight missions, as 
risk trades will need to be performed to support mission success
 For future missions, the spacecraft charging risks will be different (cis-lunar 
environment, missions that pass through the Van Allen belt, GEO, and GEO tail 
environments, etc…)
 Designing spacecraft specifically for the more severe charging environments 
beyond LEO will be the best approach. However, when the material selection 
does not support that approach, spacecraft charging assessments and hazard 
analysis will be required. 
 The frequency of occurrence and severity of expected charging environments 
will need to be quantified to determine whether or not a detection and warning 
approach to managing spacecraft charging hazards will be acceptable to support 
future space flight missions in environments beyond LEO
Publically Available – NASA EDAA TBD
Summary
| 14
 This presentation provided a summary of how the process utilized by the ISS 
Program for managing the EVA crew electrical shock hazard evolved over the years
 Successful implementation of a detection and warning approach depends on: (1) 
the probability of hazardous FP values (both positive and negative) on ISS 
remaining nominally low, and (2) space weather events and/or vehicle 
configuration changes being identified with sufficient lead-time, (3)the probability 
of completing the EVA crew-hazard shock-circuit during any EVA remaining low, 
and (4) the net probability of an EVA crew shock event remaining small enough 
for the ISS Program to accept the residual risk
 In addition, an overview of the present methodology was provided
 Detection of a potentially hazardous environment before the EVA with time to 
implement hazard controls has been approved by the ISS Program safety review 
process and accepted by the ISS Program
 In addition, a back-up methodology, IRTAM, was approved/implemented
 Finally, the applicability of the probabilistic spacecraft charging hazard control 
methods to human spaceflight beyond LEO was discussed. Applicability for those 
missions will depend on the frequency of occurrence and severity of the expected 
charging environments
Publically Available – NASA EDAA TBD
Back-up
| 15
Publically Available – NASA EDAA TBD
Back-up: ISS Vehicle Charging
| 16
Magnetic Field (Bz Component)
Example to demonstrate the possible potential 
difference between ISS and Plasma for a worst 
case environment and solar array position
Example ISS spacecraft charging prediction shows that depending on the ISS solar array 
operations, the environment, and the EVA location, there may be negative potential 
exceedances
ISS spacecraft charging prediction depends on the 
location and time of the EVA (and the corresponding 
location with respect to the Earth’s magnetic field)
-> -4.21e+ 07 
-  -4.48e+07 
_ , -4.74e+01 
-> -5.0le+0l 
_ , -5.27e+01 
-,-S.54e+01 
-> -5.81e+Ol 
_ , -6.34e+01 
-> -6.60e+01 
-  -6.87e+01 
_ , - 7.13e+01 
- >-7.40e-t07 
-  -7.67e+01 
_ , - 7.93e+01 
- >-8.20c+01 
+XW, PCU Off, Eclipse Exit: Southern Hemisphere -Summer Season, Negative (Port) Case 
Forecast Solar Storm Assessment for EVA on GMT 088 (Fri Mar. 29, 2019) Hours 0-24 
Voltage at outermost EVA location (P4,P5,P6) during ISS RAM (Worst Case) 
3 4 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 
"" ~ Olf 
-~ , 
GMT 088 
No exceedance'S 
- PO~TTIP <lel..,,lod, 
- ISSPORTS 


International Space Station Spacecraft Charging Hazards: Hazard Identification, Management, and Control Methodologies, with Possible Applications to Human Spaceflight Beyond LEO

International Space Station Spacecraft Charging Hazards: Hazard Identification, Management, and Control Methodologies, with Possible Applications to Human Spaceflight Beyond LEO

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