In August 1985 the University of Iowa's Plasma Diagnostics Package was used in the Spacelab 2 mission to study the plasma environment near the shuttle orbiter. Measurements of the plasma density and the percentage density fluctuations yielded information about the structure of the orbiter's wake. These data appear to be in general agreement with previous shuttle results and with laboratory observations of plasma flow-body interactions

Dangelo, N.

Murphy, G.

Pickett, J.

Tribble, A.

English

NASA Technical Reports Server

I- 
Measurements of ‘Plasma Density and Turbulence 
Near the Shuttle Orbiter 
A. TRIBBLE, N. D’ANGELO, G. MURPHY, 
and J. PICKETT 
{NASA-CR-180102) l l E A S U B E H E N I S  OF PLBSHA N87- 166 14 
C E N S l l Y  ANC I U E E O I E N C E  N E A R  ‘ X F E  SHUTTLE, 
0 f iE1? lE; ,  (Icwa U r i v . )  12 F CSCL 201 
Unclas  
G3/75 43237 
Department of Physics and Astronomy 
The University of Iowa 
Iowa City, Iowa 52242  
January 1987 
https://ntrs.nasa.gov/search.jsp?R=19870007181 2020-03-20T12:10:59+00:00Z
2 
ABSTRACT 
In August 1985 the University of Iowa's Plasma Diagnostics 
Package was used in the Spacelab 2 mission to study the plasma 
environment near the shuttle orbiter. Measurements of the plasma 
density and the percentage density fluctuations yielded information 
about the structure of the orbiter's wake. These data appear to be in 
general agreement with previous shuttle results and with laboratory 
observations of plasma flow-body interactions. 
. .  
3 
The Spacelab 2 mission PDP (Plasma Diagnostics Package), 
designed and built at the Univers ty of Iowa to study the plasma an1 
electromagnetic environment of the shuttle orbiter, was a satellite 
53.3 cm in radius and 66 cm in height, which could be either 
positioned relative to the shuttle by the RMS (Remote Manipulator 
System) or ejected into free flight. One of the instruments on the 
PDP was a Langmuir probe, consisting of a 3 cm diameter gold-plated 
spherical sensor and associated electronics. The Langmuir probe was 
mounted on a moveable boom which was extended to a distance of about 
60 cm from the cylindrical axis of the PDP during wake studies. 
The Langmuir probe is a relatively simple instrument which 
has two operational modes, working either as an electron density/ 
temperature measurement tool, or as a diagnostic for AN/N fluctuations 
in electron density over the frequency range 1 - 40 Hz. The elec- 
tronics is alternated between the two modes by a timing signal 
generated by the PDP spacecraft encoder. The total cycle lasts for 
approximately 13 s and consists of a 12 s "lock" period during which 
the probe is held at +10 V relative to the PDP chassis, followed by a 
1 s, 120-sample sweep from +IO V to -5 V (1). 
During the lock period, the probe output current is sensed by a 
logarithmic sensor and sampled through three filters: 
1 - 6 Hz bandpass, and 6 - 40 Hz bandpass. The 1 Hz filter provides a 
signal proportional to the plasma density, whereas the 6 - 40 Hz band- 
pass filter output is proportional to the percentage density fluctua- 
tion, AN/N. 
1 Hz low pass, 
4 
For almost one hour during t h e  Spacelab 2 miss ion  t h e  PDP was 
pos i t ioned  by t h e  RMS a t  approximately 10 m d i r e c t l y  above t h e  
o r b i t e r  cargo bay (Fig.  1) .  
a t  t h e  r a t e  of 1 degree per  second, so t h a t  t he  PDP would pass  a l t e r -  
n a t e l y  from t h e  ram of t h e  plasma flow i n t o  t h e  o r b i t e r ' s  wake, n ine  
times during t h e  one-hour period. During t h i s  maneuver t h e  o r b i t e r ' s  
x-axis, as def ined  i n  Fig. 1, remained perpendicular  t o  t h e  plasma 
flow vector .  
The o r b i t e r  then  underwent a slow r o l l ,  
In  Figs .  2 ,  3, and 4 we p re sen t  samples of d a t a  obta ined  by t h e  
Langmuir probe du r ing  t h i s  period. Figs .  2a,  3a, and 4a show t h e  
plasma d e n s i t i e s  measured by t h e  Langmuir probe as a func t ion  of time, 
while  Figs. 2b, 3b, and 4b ind ica t e ,  on t h e  same t i m e  scale,  t h e  
behavior of t h e  percentage  dens i ty  f l u c t u a t i o n ,  ANIN, measured by t h e  
6 - 40 Hz bandpass f i l t e r .  The da ta  i n  Fig.  2 r e f e r  t o  daytime 
condi t ions ,  those  i n  Fig. 3 are f o r  daytime cond i t ions  u n t i l  
0201:31 UT and f o r  n igh t t ime  condi t ions  a f t e r  0201:31 UT, whi le  those  
i n  Fig. 4 are f o r  n igh t t ime  condi t ions.  During t h e  one-hour r o l l  
per iod some of t h e  t h r u s t e r s  on t h e  o r b i t e r  had t o  be f i r e d  so t h a t  
t h e  proper a t t i t u d e  and r o l l  r a t e  were maintained. The times of 
t h r u s t e r  f i r i n g  are i n d i c a t e d  in  Figs .  2 ,  3,  and 4 by v e r t i c a l  arrows. 
Evident ly ,  t hese  t h r u s t e r  f i r i n g s  are r e spons ib l e  f o r  some of t h e  
b u r s t s  seen i n  t h e  d e n s i t y  f l u c t u a t i o n  da ta .  The times a t  which t h e  
PDP is i n  t h e  middle of t h e  s h u t t l e  wake are ind ica t ed  by a s t e r i s k s .  
5 
These times are spaced one from the next by approximately 6 min, 
corresponding to a roll rate of 1 degree per second. 
An examination of the wake transits indicates that the electron 
density in the wake of the orbiter is, typically, more than 2 orders 
of magnitude smaller than the density measured in the ram of the 
plasma flow, in general agreement with the results reported, e-g., by 
Murphy et al. (1). 
Langmuir probe is located within the wake produced by the wrist joint 
Density depressions are also observed when the 
of the RMS, these times being labeled by triangles (2). The arm wakes 
are, of course, of smaller depth and duration than the orbiter wakes. 
As far as plasma density fluctuations are concerned, one 
notices that, in the frequency range 6 - 40 Hz, AN/N is, typically, on 
the order of 1-3% all around the shuttle orbiter. However, a special 
feature of this noise stands out, namely a substantial decrease of the 
noise within the shuttle wake, with enhancements occurring at its 
edges, when the Langmuir probe moves either into or out of the wake. 
This phenomenon has been reported and discussed previously by Murphy 
et al. (1). It appears also to be in line with recent laboratory 
observations by Merlin0 and D'Angelo (3) even though the laboratory 
scale size was much smaller. In studies of plasma flow-body inter- 
actions performed in a so-called DP (Double Plasma) device, 
an interesting complex noise structure in the wake of a metallic 
object was uncovered. The positions at which the noise amplitude is 
large,st form an x-pattern downstream of the object. A scan of this 
noise pattern at just one fixed distance from the object would, of 
6 
course,  have produced a r e s u l t  s ,ai ,ar t o  the  one obtained by the 
Langmuir probe on the PDP as  i t  swept the ram-wake t rans i t ion  around 
the orbi ter .  
7 
REFERENCES AND NOTES 
1. G. Murphy, J. Pickett, N. D'Angelo, W. S. Kurth, Planet. Space 
2, Sci Val. 3 4 ,  NO. 10, pp. 993-1004, 1986. 
2. Due to the rotation of the orbiter, it was necessary to rotate the 
PDP on the RMS in the opposite direction so that the orientation 
of the PDP, relative to the plasma flow, remained unchanged. A s  a 
consequence, the PDP periodically passed through the wake of the 
RMS . 
3. R. L. Merlino, N. D'Angelo, J. Plasma Phys., in press. 
4 .  We thank the crew of the Challenger for doing an outstanding job 
in performing the orbiter maneuvers and positioning the PDP on the 
RMS; the personnel at Johnson Space Center involved with the 
design and testing of RMS procedures, in particular Ann Austin; 
and the mission planners and Payload Operations Control Center 
cadre at Marshall Space Flight Center. 
grant NAG3-449 from the NASA Lewis Research Center and contract 
NAS8-32807 from the Marshall Space Flight Center. 
This work was supported by 
8 
FIGURE CAPTIONS 
Fig. 1. Location of the PDP on the RMS, relative to the shuttle 
orbiter, during plasma wake studies. 
Fig. 2. Plasma density (a) and density fluctuation (b) data, 
0152-0158 UT, day 212, 1985. 
Fig. 3. Plasma density (a) and density fluctuation (b) data, 
0158-0204 UT, day 212, 1985. 
Fig. 4. Plasma density (a) and density fluctuation (b) data, 
0204-0210 UT, day 212, 1985. 
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Measurements of plasma density and turbulence near the shuttle orbiter

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