Four Pioneer Venus Probe spacecraft entered the Venusian atmosphere on December 9 1978, and descended to the surface while making in situ measurements of chemical composition, structure, and radiative balance. In the lower Venusian atmosphere, a number of anomalous events were noted in some of the scientific instruments and Probe engineering data. Many of these anomalies occurred on each of the Probes at approximately the same altitude. The anomalies are thought to be the result of some unexpected electrical interaction between the Probe and the Venusian atmosphere. This conclusion is based on analysis and testing of similar Probe hardware on the ground. However, the possibility that some anomalies were a result of latent design of manufacturing flaws cannot be ruled out

Polaski, L. J.

English

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NASA Technical Memorandum 84301
NASA-TM-84301 19830011397
AnomalousBehaviorof the
PioneerVenusEntry Probes
During Lower Descent
Louis J. Polaski
December1982
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LANGLEY RESEARCH CENTER
LI2P,ARY, NASA
HAMPTON, VIRG!N_A
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NationalAeronauticsand
Space Administration
https://ntrs.nasa.gov/search.jsp?R=19830011397 2020-03-21T03:51:53+00:00Z

NASA Technical Memorandum 84301
AnomalousBehaviorof the
PioneerVenusEntry Probes
During Lower Descent
Louis J. Polaski, Ames ResearchCenter, Moffett Field, California
NASA
NationalAeronautics and
Space Administration
Ames Research Center
Moffett Field. California94035
A/_3-. /q 6/o2_

ANOMALOUS BEHAVIOR OF THE PIONEER VENUS ENTRY PROBES DURING LOWER DESCENT
Louis J. Polaski
Ames Research Center
SUMMARY
i
Four Pioneer Venus Probe spacecraft entered the Venusian atmosphere on
December 9, 1978, and descended to the surface while making in situ measurements of
chemical composition, structure, and radiative balance. In the lower Venusian
atmosphere, a number of anomalous events were noted in some of the scientific
instruments and Probe engineering data. Many of these anomalies occurred on each
of the Probes at approximately the same altitude.
The anomalies are thought to be the result of some unexpected electrical
interaction between the Probes and the Venusian atmosphere. This conclusion is
based on analysis and testing of similar Probe hardware on the ground. However,
the possibility that some anomalies were a result of latent design or manufacturing
flaws cannot be ruled out.
INTRODUCTION
Four Pioneer Venus Probe spacecraft entered the Venusian atmosphere on
December 9, 1978, and descended to the surface while making in situ measurements of
chemical composition, structure, and radiative balance. In the lower Venusian
atmosphere, a number of anomalous events were noted in some of the scientific
instruments and Probe engineering data. Many of these anomalies occurred on each
of the Probes at approximately the same altitude.
The first indication of anomalous behavior came when the Atmosphere Structure
Experiment (LAS/SAS) temperature sensors on each of the four Probes suddenly failed
at approximately 640 K, which corresponds to an altitude of about 12.5 km (ref. i).
It was then observed that the Net Flux Radiometer's (SNFR) external sensors on the
North, Day, and Night Probes suddenly failed at about the same altitude (ref. 2).
A close examination of the data received from the other scientific instruments and
Probe housekeeping measurements uncovered additional anomalies that occurred prior
to, during, and following the aforementioned sensor failures.
ANOMALIES
The following anomalies were common to the North, Day, and Night Probes. There
was an apparent failure of the SNFR fluxplate and SAS temperature sensors. There
were also abrupt changes and spikes in the SAS pressure and sensor data and in the
SNFR maximum and minimum flux data. The SAS and SNFR boom deployment status
changed from deployed to stowed. Other anomalies included erratic data from the
two thermocouples embedded in the heat shield (at the stagnation point and frustum)
and from the thermistor measuring the junction temperature of the heat-shield
thermocouples. Finally, there were slight variations in the power bus current and
voltage levels and slight offsets or jumps in the values for Probe internal pressure
and forward and aft shelf temperatures.
On the Sounder Probe, the following events were observed. There was an
apparent failure of the Atmosphere Structure (LAS) temperature sensors along with
abrupt changes and spikes in the LAS pressure sensor data. Abrupt changes occurred
in the Cloud Particle Size Spectrometer (LCPS) laser alignment monitor as well, and
the LCPS return laser beam intensity decreased. On the other hand, there was a
steady increase in the Infrared Radiometer (LIR) flux readings and the LIR flux data
were "Noisy." Other events include spikes in the Mass Spectrometer (LNMS) analyzer
ion pump current monitor and in the receiver automatic gain control (AGC); an abrupt 4
decrease in the power bus current; and jumps in the receiver (transponder) static
phase error. Finally, values appeared for the heat-shield thermocouple measurements
whose leads were severed when the heat shield was separated from the pressure vessel.
DISCUSSION
It is important to note that on each Probe several seemingly unrelated events
occurred simultaneously. For example, on the North, Day, and Night Probes, the
apparent failures of the SAS and SNFR temperature sensors occurred within a few
minutes of each other. The failed sensors were each different, the SAS being a
platinum resistance device and the SNFR being a thermistor. The electronics for
these sensors were electrically isolated from each other; however, they both were
moun[ed on booms that extended from opposite sides of the Probe. On the Sounder
Probe, jumps occurred in the LAS pressure data when the heat-shield thermocouple
values changed or when the LNMS analyzer ion pump indicated current spikes, yet
the LAS sensor, heat-shield thermocouple, and the LNMS electronics were electrically
isolated from each other. During this time the thermocouple electronics were
attached to two severed (nonconnected) former thermocouple leads.
It was first suspected that the SAS and SNFR temperature sensor failures were
the result of a partial or catastrophic failure of the external harness or pressure
vessel feed-through connectors. Extensive analysis and environmental testing of the
harness materials and connectors had been conducted during the Probe development and
qualification test program. Additional tests were conducted after the flight to
determine the combined effects of sulfuric acid, pressure, and temperature on these
components. None of these revealed any material or design deficiencies.
A post-flight analysis was also conducted to reexamine the implications of an
uncovered Probe/Bus Interface Disconnect (IFD) containing the power data and command
lines between the Probe and the Bus spacecraft on each of the Probes and the
potential effects of the cable-cutter severance of the Probe/heat-shield IFD harness
carrying the power and signal lines between the heat shield and the pressure vessel
on the Sounder Probe. This analysis considered the effects of shorts (caused by
the presence of sulfuric acid or conductive debris) across the IFD socket contacts
or the severed IFD harness wires. The analysis revealed that even if such shorts
occurred, no damage or adverse performance would result. The possibility of elec-
trical shorts or arcing across the pins of the pressure vessel feed-through
connectors carrying the power and signal lines between the externally mounted sensors
and window heaters and the inside of the Probe was also considered. Shcr_s aad
arcing could occur only if the feed-through and its mating external harness connector
were not fully engaged, an unlikely situation to occur on all four Probes. The
likelihood of conductive debris or sulfuric acid entering the gap between partially
mated connectors is remote, because of their mechanical configuration. The low
voltage on the pins (28 VDC maximum) and the high atmospheric pressure rule out the
possibility of arcing. Thus, there is no possibility of a fe_d-through connector
failure causing the observed events. However, there is evidence that several Probe
failures did occur. The first was observed on the Sounder Probe shortly after the
LAS temperature sensor failure, when the Probe bus current dropped 2.1 A (from
15.4 A). Simultaneously, the LIR flux data indicated a change. It was verified
that the current drop exactly matched the current normally supplied to the LIR
window heater. The construction of this heater is different from the other window
heaters in that its outer sheath is made from tantalum (the others are Kovar). One
suggested failure mechanism is that the tantalum reacted with the sulfuric acid and
. the atmospheric CO2, creating holes in the outer sheath. These holes then permitted
the insulation to become contaminated, thus forming conductive paths between the
heating element and ground (sheath). This short caused the heater fuse to blow.
The change in the LIR flux data was most likely a response to the rapid change in
the diamond window temperature when the heater failed.
A second failure was noted on each of the small Probes when the SAS and SNFR
sensor boom status changed from deployed to stowed at about the same time the SAS
and SNFR temperature sensors failed. Unfortunately, this status was only sampled
once every 512 sec. Thus, precise knowledge of when the status changed is impossi-
ble. Since it was mechanically impossible for the booms to re-stow themselves,
failure of the boom status switch was suspected. Post-flight testing of identical
switches indicates that the switches most likely failed as a result of the atmo-
spheric temperature and pressure.
The anomalies in the Probe housekeeping data are more difficult to explain,
since no spare hardware was available for testing. The most interesting anomalies
are those associated with data from each Probe's heat-shield thermocouple and
thermistor. Each Probe heat shield was instrumented with two thermocouples: one in
the nose cap (stagnation region) and the other in the aft end of the forebody
(frustum region) (ref. 3). These thermocouples were installed to obtain heat-shield
performance data during entry. Each set of thermocouple leads was routed to a
terminal board located on the inside of the heat-shield support structure. Here
the thermocouple wires were terminated into copper Probe harness wires that were then
routed into the pressure vessels through feed-through connectorsL A thermistor
was mounted on each terminal board to monitor the temperature of the thermocouple
lead/copper wire junction. J
On the North, Day, and Night Probes, during the period immediately following I
the failures of the SAS and SNFR temperature sensors, the outputs of the terminal
board thermistors dropped from their saturated value of 305 K. These sensors then
returned to their normal saturated value after about i0 min. During this same
interval, the thermocouple produced anomalous readings. One suggested cause of
these anomalies is that the external harnesses or connectors failed, buttas
previously noted, this possibility is ruled out. The data could also be explained
by an atmospheric temperature inversion that the Probe passed through; however,
there is no evidence to support this assumption. The only remaining suggested
possibility is thah the Probe was covered with a plasma. Just how a Probe could
pick up a plasma sheath is unknown.
On the Sounder Probe, the leads to the thermocouples and the terminal board
thermistors, along with other wires, were severed by an anvil-type cable cutter just
before the separation of the pressure vessel from the heat shield. About 2 min
before the loss of the LAS temperature sensors, the readings for the thermocouples
3
changed from an "open" value to a value equivalent to a 0.2-mV output between the
severed thermocouple leads. The readings from these leads occasionally changed (up
and down) during the remainder of the descent. Again, a possible explanation for
this behavior is a failure of the external harness or feed-through connectors. A
tantalizing possibility, however, is that the open leads were acting like a Langmuir
Probe in a plasma. The other anomalies observed were power variations, jump
increases in each of the Probe's internal pressure and temperature readings, and
changes in the Sounder Probe's transponder static phase error (SPE) and receiver
automatic gain control (AGC). Although not the prime candidates for cause, each of
these anomalies could have resulted from the effects of static discharge within or
outside of the Probe.
CONCLUSIONS
An analysis of the Probe anomalies suggests that they are the result of an
unexpected electrical interaction between the Probes and the Venusian atmosphere.
The source of the electrical energy for such an interaction is unknown. Further
analysis of the instrument anomalies and the scientific data obtained in lower
altitudes may offer more clues. However, the possibility that some anomalies were
a result of latent design or manufacturing flaws cannot be ruled out.
REFERENCES
i. Seiff, A.; Kirk, D. B.; Sommer, S. C.; Young, R. E.; Blanchard, R. C.; Juergens,
D. W.; Lepetich, J. E.; Intrieri, P. F.; Findlay, J. T.; and Derr, J. S.:
Structure of the Atmosphere of Venus up to ii0 Kilometers: Preliminary
Results From the Four Pioneer Venus Entry Probes. Science, vol. 203, 1979,
p. 787.
2. Suomi, V. E.; Sromovsky, L. A.; and Revercomb, H. E.: Preliminary Results of
the Pioneer Venus Small Probe Net Flux Radiometer Experiment. Science,
vol. 205, 1979, p. 82.
3. Pitts, W.: Performance of Entry Heat Shields on Pioneer Venus Entry Probes.
Journal of Geophysical Research, vol. 85, 1980, p. 8333.
1. Report No. 2. GovernmentAccessionNo. 3. Recipient'sCatalog No.
NASA TM-84301
4. Title and Subtitle 5. Report Date
ANOMALOUS BEHAVIOR OF THE PIONEER VENUS ENTRY January 1983
PROBES DURING LOWER DESCENT 6. PerformingOrganizationCode
7. Author(s) 8, Performing Organization Report No.
Louis J. Polaski A-9119
i'0. Work Unit No. •
9. Performing Organization Name and Address T- 7378
NASA Ames Research Center 11.Contractor GrantNo.
Moffett Field, Calif. 94035
'1"3. Type of Report and Period Covered
12. Sponsoring Agency Name and Address Technical Memorandum
National Aeronautics and Space Administration 14 SponsoringAgencyCode
Washington, D. C. 20546
15. Supplementary Notes
Point of Contact: Louis J. Polaski, Ames Research Center, MS 213-1
Moffett Field, Calif. (415) 965-5483 or FTS 448-5483
16. Abstract
Four Pioneer Venus Probe spacecraft entered the Venusian atmosphere on
December 9, 1978, and descended to the surface while making in situ measure-
ments of chemical composition, structure, and radiative balance. In the
lower Venusian atmosphere, a number of anomalous events were noted in some
of the scientific instruments and Probe engineering data. Many of these
anomalies occurred on each of the Probes at approximately the same altitude.
The anomalies are thought to be the result of some unexpected electrical
interaction between the Probes and the Venusian atmosphere. This conclusion
is based on analysis and testing of similar Probe hardware on the ground.
However, the possibility that some anomalies were a result of latent design
or manufacturing flaws cannot be ruled out.
17. Key Words (Sugg_ted by Author(s)) 18. Distribution Statement
Pioneer Venus Probe anomalies Unlimited
Subject Category: 91
19. S_urity Cla_if. (of this re_rt) 20. Security Classif. (of this _) 21. No. of Pa_s 22. Price"
Unclassified Unclassified 8 A02
"For sale by the National Technical Information Service, Springfield, Virginia 22161




Anomalous behavior of the Pioneer Venus entry probes during lower descent

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