IEEE Spectrum December, 2012 - 60

Pioneer Anomaly

At JPL , veteran navigators Neil
Mottinger and Jordan Ellis analyzed
the expanded Doppler observations. As
Continued from page 44
with the original orbital analysis, they
NASA's Goddard Flight Space Center, had to take into account all the informawho helped recover corrupted files and tion we had on anything that might affect
get all the Doppler data into usable the transmissions. They had to consider
condition. In the end, we were able to Earth's spin, the locations of the receiving
double the amount of available Doppler antennas, and both terrestrial and solar
measurements, increasing the span of weather. They also had to factor in the
orbital data to 23 years for Pioneer 10 changing communications strategies over
time-31
years for Pioneer 10
and toIEEE
more
than 10 Ad
years
for Pioneer
11. the
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60

NA * iEEE SpEctrum * DEcEmbEr 2012

and 17 for Pioneer 11-during which the
probes were capable of sending back signals that could be used for precision navigation. With their completed analysis
in hand, we found that the Doppler data
were still statistically consistent with a
constant deceleration but that a value
that decreased over time fitted the data
even better.
i f t h e e f f e c t d i m i n i s h e d slightly over
time, we reasoned, maybe the heat generated by the RTGs, which would have
slowly decayed, was responsible for the
anomaly after all.
To find out, we enlisted the help of
a team of engineers at JPL led by Gary
Kinsella, who had plenty of experience modeling the thermal behavior
of spacecraft, to construct a detailed
thermal model of Pioneer 10 and 11. In
the absence of computer-aided-design
files, which didn't exist 40 years ago,
Kinsella's team had to comb through old
blueprints and consult with the retired
TRW engineers who built the probes in
order to reconstruct their 3-D structure.
The team's eventual geometric model
divided the surface of each spacecraft
into 3300 patches and incorporated documented thermal properties of the materials
used to construct the probes. We used the
model to show how heat flowed across and
radiated off various parts, and with those
results, we estimated the magnitude of the
thermal recoil force at different times over
the course of the Pioneer missions. After
matching the model to the Pioneers' temperature and electrical readings, we found
that the spacecraft did experience a sizable
thermal recoil force, corresponding to an
excess of about 60 W even after 20 years in
deep space. The magnitude of the force was
still tiny by Earth standards-about the
same as the backward push your car experiences in reaction to the photons spit out by
its high-beam headlights. The team found
that a good half of the force came from heat
from the RTGs, which bounced off the back
of the spacecraft antenna. The other half
came from electrical heat from circuitry in
the heart of the spacecraft. Most of that heat
was radiated through louvers at the back of
the probes, which weren't as well insulated
as the rest of their bodies, further contributing to the deceleration.
We also calculated the thermal recoil
force on the spacecraft using just the
Doppler data, by computing the force
needed to match the probes' trajectories. When we compared this independent estimate with the one derived
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Table of Contents for the Digital Edition of IEEE Spectrum December, 2012