A materials interaction experiment has been approved to study atomic oxygen interaction mechanisms and develop coatings for Space Station elements requiring long-lived operation in the LEO environment. A brief summary of this experiment is presented and the required exposure conditions are reviewed

Leger, L. J.

Visentine, J. T.

English

NASA Technical Reports Server

SPACE SHUTTLE NECHANISTIC STUDIES TO CHARACTERIZE 
ATOMIC OXYGEN INTERAGTIONS WITH SURFACES 
Lubert J. Leger and James T. Visentine 
NASA/Lyndon B. Johnson Space Center 
Abstract. 
study atomic oxygen i n t e r a c t i o n  mechanisms and develop coa t ings  f o r  
Space S ta t ion  elements requi r ing  long-lived operation i n  the LEO en- 
vironment. A b r ie f  summary of t h i s  experiment i s  presented and t h e  
required exposure conditions are reviewed. 
A materials i n t e r a c t i o n  experiment has been approved t o  
Introduction 
An understanding of t he  surface chemistry which gives rise t o  atom/ 
surf ace i n t e r a c t i o n s  within the  o r b i t a l  environment i s  c r u c i a l  t o  es- 
t a b l i s h i n g  a r e l i a b l e  materials i n t e r a c t i o n  da ta  base f o r  Space S ta t ion  
and ve r i fy ing  the  operational capab i l i t y  of ground-based neu t r a l  beam 
faci l i t ies  which simulate the space environment. One of the more im-  
po r t an t  e f f e c t s  of these  i n t e r a c t i o n s  i s  oxidation of material su r faces  
by atomic oxygen, a major cons t i t uen t  of the low-Earth o r b i t a l  environ- 
ment. Material i n t e r a c t i o n  s tud ie s  conducted during f l i g h t s  STS-5 and 
STS-8 have provided most of the c u r r e n t  information regarding the  re- 
a c t i v i t y  of spacecraf t  materials t o  atomic oxygen, and the  r e s u l t s  of 
these  s tud ie s  ind ica t e  many materials such as organic fi lms, polymers, 
and many composites react readi ly  with atomic oxygen and have reactivi- 
t i e s  i n  the range 2.5 x t o  3.0 x cubic centimeters per atom. 
The da ta  base provided by these,  as w e l l  as those t o  be obtained on 
LDEF, w i l l  be l imi ted  i n  i t s  appl ica t ion ,  however, because no informa- 
t i o n  w i l l  be ava i lab le  which adequately explains the  basic mechanisms 
responsible f o r  atom/surface in t e rac t ions .  Another more ser ious  l i m i t a -  
t i o n  t o  t h i s  data base i s  t h a t  the t o t a l  i n t eg ra t ed  atomic oxygen f l u x  
( f luence ) ,  derived f o r  these f l i g h t s  and used t o  determine m a t e r i a l  
i n t e r a c t i o n  rates, must be estimated using thermospheric models t o  
p r e d i c t  atomic oxygen number d e n s i t i e s  within the  o r b i t a l  environment. 
Typically, errors of f25% or  greater can be expected f o r  these  dens i ty  
es t imat ions ,  and since they are used t o  compute fluence, these  e r r o r s  
a l s o  appear i n  the  surface recession rates f o r  Space S ta t ion  materials. 
Experiment Description 
To reslove many of these  unce r t a in t i e s  i n  t h e  da ta  base, a f l i g h t  
experiment has been proposed f o r  the space s h u t t l e  t h a t  u t i l i z e s  an ion- 
neu t r a l  m a s s  spectrometer t o  obtain i n  s i t u  ambient dens i ty  measurements 
and i d e n t i f y  r eac t ion  products from modeled polymers exposed t o  the 
atomic oxygen environment. Using the  ambient dens i ty  measurements from 
the  mass spectrometer along w i t h  material recession measurements ob- 
ta ined  during the  same exposure, accurate r eac t ion  rate da ta  w i l l  be 
provided f o r  fu ture  spacecraf t  performance assessment and design. 
The mass spectrometer which w i l l  be provided by the  A i r  Force Geo- 
phys ica l  Laboratory (AFGL) and a mounting arrangement used t o  expose 
sur faces  t o  ambient impingement w i l l  be mounted i n  the payload bay on an 
appropriate s t ruc tu re  as shown i n  Figure 1. For the  required exposure 
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the Orbiter w i l l  be oriented with the -Z ax i s  pointed i n t o  the veloci ty  
vector (payload bay i n  ram). This or ientat ion w i l l  provide normal 
impingement on the exposed surfaces and, with the mass spectrometer 
entrance por t  aligned with the Z-axis, w i l l  provide fo r  optimal ambient 
density measurements. A t o t a l  exposure of 40 hours will be requested to  
provide s igni f icant  recession and insure assessment of bulk material 
reac t iv i ty .  Several ambient density measurements w i l l  be made during 
the  40-hour exposure. 
In  addition t o  the ambient density measurements, the mass spectrom- 
eter w i l l  a l so  be used t o  measure products of reaction from atmospheric 
interact ion with surfaces. For these measurements the m a s s  spectrometer 
w i l l  r o t a t e  90° r e l a t ive  t o  the Z-axis to  sample gases evolved from 
several surfaces being exposed on a carrousel arrangement as shown i n  
Figure 2. The carrousel w i l l  hold f ive  surfaces which w i l l  be used t o  
study the interact ion mechanism and w i l l  be selected on the basis of 
information provided i n  t h a t  regard. One of the f ive  surfaces may be a 
material which is  known t o  produce glow such a s  2-306 black paint.  
To aid i n  providing the best ambient density and reaction product 
evolution measurements, the AFGL mass spectrometer w i l l  be cal ibrated 
using an atomic beam currently being developed a t  the Los Alamos 
National Laboratories. This b e a m  generation concept i s  based on a laser 
heated plasma concept described i n  another paper of t h i s  meeting (see J. 
C r o s s ,  these proceedings) and should provide kinet ic  energies of up t o  5 
eV. Since the beam f a c i l i t y  vacuum chamber i s  large,  the f l i g h t  m a s s  
spectrometer can be cal ibrated i n  s i t u  and should allow a good under- 
standing of t rans ien t  processes i n s i d e  the ion source. Furthermore, the 
b e a m  f a c i l i t y  w i l l  be used t o  characterize reaction products generated 
on the specif ic  surfaces selected for  inclusion on the f l i g h t  experi- 
ment. This product d i s t r ibu t ion  w i l l  be comparable with reaction pro- 
ducts produced during f l i g h t  t o  ver i fy  simulation f i d e l i t y  and study 
reaction mechanisms. 
This lat ter data should be of i n t e r e s t  i n  glow mechanism studies,  i n  
t h a t  major const i tuents  produced by ambient impingement w i l l  be ident i -  
f i ed  and thereby provide a base of chemical species t o  consider fo r  
l i g h t  production mechanisms. 
l a t e  1986 or ear ly  1987 f l i g h t  date w i l l  be requested from the STS. 
Funding for  experiment development i s  being provided by Space Stat ion 
and the Office of Aeronautics and Space Technology/NASA Headquarters. 
Hardware development fo r  the f l i g h t  experiment i s  underway, and a 
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Space shuttle mechanistic studies to characterize atomic oxygen interactions with surfaces

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