IEEE Spectrum December, 2012 - 42

Magnetometer

Ultraviolet
photometer
Radioisotope
thermoelectric
generators (RTGs)

Attitude thrusters
Imaging
photopolarimeter

Spin/despin
thrusters

Geiger tube telescope

Low-gain antenna
Separation ring

Plasma analyzer

Asteroid/
meteoroid
detector sensor

Meteoroid detector
sensor panel
Trapped-radiation
detector

Thermal
control louvers

High-gain
antenna reflector
High-gain antenna
feed assembly

Stellar reference
assembly
light shield

Medium-gain
antenna
RTG power
cable

Cosmic ray telescope
Infrared radiometer
Charged-particle instrument

RTGs
Sun sensor

iLLuStRAtion: noemotion; SouRce: nASA

Attitude thrusters

pair of rabbit ears. These generators converted heat from the
radioactive decay of plutonium-238 pellets directly into electricity using bimetallic thermocouples. The RTGs were ultrareliable: They contained no moving parts, and the 88-year
half-life of the fuel meant the spacecraft would have adequate
power for decades.
To communicate with Earth, each spacecraft was
equipped with a 2.74-meter-wide parabolic antenna. It was
decided early on that the probes would be stabilized by spinning them along the axis of their antennas. This kept each
spacecraft's antenna pointed in a consistent direction, cutting down on the need for frequent attitude corrections. If an
antenna were to fall out of alignment with Earth, incoming
signals would hit the antenna off-axis. Special circuits would
then pick up on the modulation of the signal that would arise
as it wobbled back and forth across the antenna while the
spacecraft rotated. These circuits would then fire hydrazine
thrusters to correct the spacecraft's orientation.
Armed with this basic technology, Pioneer 10 and 11
set off where no man or probe had gone before, on trajectories that would eventually take them to nearly opposite
ends of the solar system. After its launch on 2 March 1972,
Pioneer 10 traversed the asteroid belt and swung past
Jupiter in December 1973, its off-axis single-pixel photodetector spinning slowly to construct pictures of the giant
42

BIG DISH: the largest single
component on both pioneer 10
and pioneer 11 was the high-gain
antenna, a 2.74-meter-wide
reflector that was designed to
be able to communicate with
earth from well beyond the orbit
of mars. most of each probe's
control and science instruments
were located on the back of
this dish. specialized circuits
could open sets of thermal
louvers to release excess
heat. each spacecraft carried
a set of radiothermal power
sources, which were held away
from the spacecraft body
on 2.5-meter-long booms to
protect against radiation. A third,
even longer boom was used
to house a magnetometer.

NA * iEEE SpEctrum * DEcEmbEr 2012

planet. Pioneer 11, which launched on 6 April 1973, performed an even more ambitious mission: flying past Jupiter
a year after Pioneer 10, and then using the gas giant's gravity to change course-performing one of the first of the
now- common "slingshot maneuvers." The spacecraft flew
beneath Saturn's rings some five years later.
The probes, of course, continued hurtling through space
after their planetary flybys. As Earth receded into the distance, our planet's apparent motion around the sun diminished as well. Consequently, the thrusters needed to fire less
and less often every year to adjust the orientation of Pioneer 10
and 11's homeward-directed antennas. That turned the spacecraft into largely ballistic objects, and as such, nearly ideal
probes of the effect of external forces like gravity.
At this point, a group of researchers led by astronomer
John D. Anderson at NASA's Jet Propulsion Laboratory
(JPL) set out to use navigational data on these mostly passive
probes to make high-precision measurements of the gravitational environment of the outer solar system. The team, which
included one of us (Turyshev), was specifically looking for
subtle changes in trajectory that might be caused by the tug
of as-yet-undiscovered planets or else very-long-wavelength
gravitational waves produced during the big bang. Neither
signal turned up. Instead, the investigation uncovered something completely unexpected and far harder to interpret.
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Table of Contents for the Digital Edition of IEEE Spectrum December, 2012