Electrostatic charging of insulators is well known to pose potential threats to electronic systems under terrestrial systems. We were asked to evaluate potential hazards associated with replacement of an electronics board in the vacuum of space during the upcoming Hubble Service Repair mission in September 2008. A device called a "tribot" was built to simulate triboelectric charging of several insulators that are capable of contact charging against astronaut's gloves and suit materials. Materials were evaluated as the extent of not only the magnitude of electric fields generated by the frictional action, but also any resulting discharge which would be a source of EMI. Experimental results are quite surprising and unexpected when compared with tests performed under ambient conditions under dry and humid air

Buhler, Charles R.

Calle, Carlos I.

Ritz, Mindy L.

NASA Technical Reports Server

ESD challenges for NASA Astronauts in upcoming missions. 
C.R Buhier', C.I. Calle2 and M.L. Ritz'; Electrostatics and Surface Physics Laboratory, ASRC Aerospace, Kennedy 
Space Center, FL 32899, Charles.R.Buhler@nasa.gov; 2Electrostatics and Surface Physics Lab, NASA, Kennedy 
Space Center, FL 32899, Carlos.I.Callenasa.gov ; 3Electrostatics and Surface Physics Laboratory, ASRC Aero-
space, Kennedy Space Center, FL 32899, Mindy.L.Ritz@nasa.gov 
Introduction: As NASA begins to send humans 
back to the moon and onto Mars, there are numerous 
challenges for future astronauts concerning ESD issues. 
One such issue of concern is the replacement of ESD 
sensitive parts and components while in the vacuum of 
space. Astronauts working outside of the International 
Space Station or Space Shuttle do not have access to 
common ESD mitigating techniques such as air ioniza-
tion, electrical grounding, antistatic sprays, etc... 
Therefore, each ESD concern must be addressed prior 
to launch if possible in order for mission success. 
One recent example examined by the Electrostatics 
and Surface Physics Laboratory (ESPL) at the NASA 
Kennedy Space Center in Florida was to assess the 
ESD hazards associated with the changing out an elec-
tronics board for an upcoming Hubble Space Tele-
scope service repair mission. Since astronauts have 
never touched or exposed an electronic circuit board 
while in the vacuum of space, engineers were not sure 
what possible ESD concerns might exist. It was the role 
of the ESPL to identify those concerns and mitigate the 
risks if possible. 
Current models such as the Human Body Model, 
Charge Device Model and the Machine Model do not 
address the ESD effects of highly charged ungrounded, 
insulators in vacuum. Therefore, the first measurements 
were taken to quantify the large electric fields gener-
ated on highly resistive materials such as the space 
suits when triboelectrically charged against common 
materials found within the payload bay of the orbiters. 
These tests would guide the astronauts as to which ma-
terials they should and should not come in contact with 
prior to board replacement. A second set of measure-
ments were designed to assess whether discharges of 
any kind could result from contact with a ground probe 
with a highly charged insulator surface in vacuum. 
Such discharges, which are called brush discharges in 
air [1], would be sources of EMI which is well-known 
form of stress on sensitive components. 
Tests were performed using a modified triboelectric 
test standard used at Kennedy Space Center for over 40 
years [2]. This device, called a Tribot, shown in Figure 
1 is able to rub two materials together and measure the 
surface potentials of both materials afterwards. The 
first material is attached to a rotating wheel that ap-
proaches and rubs against a second material for 10 
seconds [Figure 1(a)]. The second material is held in 
place using a rod attached to the wheel assembly. After 
rubbing, both materials separate which allows the sec-
ond material housed inside a special holder to fall 
down [Figure l(b)]. The surface potentials of both ma-
terials are simultaneously measured for long periods 
using JCI (John Chubb Instruments, Inc.) fieldmeters 
[Figure 1 (c)]. Finally, a grounded brass probe of 3/4 
inches diameter approaches the charged surface in or-
der to extract any surface charge [Figure 1(d)]. This 
charge is monitored using a Keithley 6514 Electrome-
ter. In order to get the correct values of charge ex-
changed the probe must be housed inside a Faraday 
cage before and after contacts with the test material. 
,7f p.. I 4 L.	 0 14 
-	
lei 
 
Figure 1. (a) The top view of the 1 nbot within the 
vacuum chamber. (b) The front view. (c) The side 
view. (d) The discharge electrode shown without the 
Faraday housing. 
The materials tested included RTV (room tempera-
ture vulcanized) silicone used for the astronaut gloves; 
Ortho fabric (Kevlar®, Gortex® and Nomex®) and 
PTFE or Teflon® fabric, the space suit outermost ma-
terials; and a variety of payload bay materials including 
insulating materials such as floroethylenepolypropyl-
ene (FEP), as well as grounded conductors such as 
Chromate. Tests performed at vacuum conditions (10
https://ntrs.nasa.gov/search.jsp?R=20130012039 2019-08-31T00:32:40+00:00Z
toff) are compared with those in low humidity ambient 
conditions. 
Results and Conclusions: An excellent representa-
tion of the results is shown below in Figure 2. The 
highest recorded surface potential reading on space suit 
and glove material is presented against the materials 
they are expected to come in contact with. The top 
graphic shows the results for low humidity testing in air 
while the bottom graphic shows the high vacuum re-
sults. We note that the highest charge levels recorded 
were +20 kV for RTV rubbed with FEP and -22.6 kV 
Teflon® fabric rubbed against the grid side of Ortho. 
In both cases the charge dissipation or charge decay 
times were quite long and high surface potentials could 
easily last for several hours. None of the materials 
tested exhibited short decay times except for a well-
grounded Ortho grid. Thus it should be assumed that 
all insulators will hold their charge for long periods of 
time. 
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Figure 2. (top) Summary of test results for low humid-
ity conditions at atmospheric pressure. (bottom) Sum-
mary of high vacuum test results. 
Like materials were not able to charge each other 
under high vacuum conditions. This is completely op-
posite of what happens in air. Although not shown, 
data taken on materials in air show that like materials 
will electrostatic charge upon contact and separation.
For example VectranTM was able to charge up to -13.8 
kV when rubbed against another sample of Vectran 
which is typical of insulator-insulator contact charging 
in air. Teflon®, Ortho, Nomex® and RTV all per-
formed the same way by not self-charging under high 
vacuum conditions. Clearly the transfer of ions which 
dominates insulator-insulator contact charging in air is 
not present under high vacuum conditions. 
Similarly, grounded metals such as Chromate which 
can deposit large amounts of charge on insulator sur-
faces in air, were unable to charge Teflon®, Nomex® 
and Ortho in high vacuum. However, Chromate was 
able to charge RTV and FEP. We do not know the 
amount of charge deposited onto the metals since they 
are grounded conductors and cannot be read using the 
fleidmeter from the backside 10 cm away. 
Finally, for all materials tested, there were no re-
corded brush discharges under high vacuum conditions 
using the 3/4" diameter grounded metal probe. All 
charge measurements were below the noise level of 10 
12 pC. Unlike discharge testing in air which can be as 
high as several nanocoulombs, charge insulators in 
vacuum conditions did not discharge to a grounded 
metal. Thus the brushlike discharges that occur in air 
must be solely due to air breakdown and not emission 
from surface ions or electrons. This is opposite from 
highly charged metals in vacuum which can discharge 
due to field emission as electrons overcome the work 
function. Insulators without the benefit of free electrons 
cannot discharge and thus it may be impossible to form 
an EMI event caused by triboelectrically charged insu-
lators in vacuum. 
The results can now be used to allow engineers to 
choose between materials that should be used in con-
junction with the space suit materials and which ones 
should not. All measured charge decay times are quite 
long and thus waiting for charge to dissipate is not an 
adequate mitigation strategy. Thus sensitive compo-
nents must be shielded from large DC electric fields 
but they may not require EM! shielding which is com-
monly caused by the discharge of insulators to 
grounded surfaces. 
References: 
I. Britton, L.G., Avoiding Static Ignition Hazards in 
Chemical Operation. 1999, New York, NY: American 
Institute of Chemical Engineers. 
2. Finchum, A., Standard Test Method for Evaluating 
Triboelectric Charge Generation and Decay. 1998, 
Kennedy Space Center Test Standard MMA-1985-98 
(Revision 3).
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ESD Challenges for NASA Astronauts in Upcoming Missions NAS10-03006 
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6. AUTHOR(S) 5d. PROJECT NUMBER 
Dr. Charles Buhler 
Di. Carlos Calle 5e. TASK NUMBER 
M. L. Ritz
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12. DISTRIBUTION/AVAILABILITY STATEMENT 
13. SUPPLEMENTARY NOTES 
14. ABSTRACT 
Electrostatic charging of insulators is well known to pose potential threats to electronic systems under terrestrial systems. We were 
asked to evaluate potential hazards associated with replacement of an electronics board in the vacuum of space during the upcoming 
Hubble Service Repair mission in September 2008. A device called a "tribot" was built to simulate triboelectric charging of several 
insulators that are capable of contact charging against astronaut's gloves and suit materials. Materials were evaluated as the extent of 
not only the magnitude of electric fields generated by the frictional action, but also any resulting discharge which would be a source of  
EMI. Experimental results are quite surprising and unexpected when compared with tests performed under ambient conditions under 
dry and humid air. 
15. SUBJECT TERMS 
brush discharges, high vacuum, Hubble Space Telescope, triboelectric charging. 
I C SECURITY CLAC* I F I ('ATION OF 17. LIMITATION OF 18. NUMBER 19b. NAME OF RESPONSIBLE PERSON . ABSTRACT OF 
PAGES Charles Buhier a. REPORT b. ABSTRACT c. THIS PAGE
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