published in EE Times
“The Voyager spacecraft has left the solar system,” read many headlines over the last few months. But its predecessor, the Pioneer spacecraft, should not be overshadowed. It left an important legacy as well, not for its planetary astronomy discoveries, but for its engineering lessons.

The pathfinding mission for the Voyager spacecraft’s tour of the outer planets was the Pioneer mission, with Pioneer 10 launched on March 2, 1972. Just 19 months later, Pioneer 10 passed within 132,000 km of Jupiter, returning more than 500 images taken with its single-pixel camera.

Its primary mission was to explore Jupiter. In fact, Pioneer’s discovery of the Jovian radiation belt, 1,000 times as intense as expected, came just in time to add radiation-hardened electronics to the still-under-construction Voyager craft — electronics that are credited with enabling its long life.

Artist’s impression of Pioneer 10 near Jupiter, courtesy of NASA.

Perhaps the part of Pioneer’s mission most important to the engineering community, however, began only after it left Jupiter’s influence.

The Pioneer 10 and 11 spacecrafts were the last crafts to have their long range radio dishes aimed back at earth by spin stabilization, rather than adjusted with thrusters or reaction gyro wheels. Between its roughly annual thrust corrections, the Pioneer 10 flew a ballistic trajectory, acting as a test mass to plot out the gravitational fields of the outer solar system.

A group at JPL, lead by John Anderson, had the idea of using the motion of Pioneer 10 as a high-precision probe of the gravitational environment of the solar system, searching for unknown planets and maybe even searching for low-frequency gravitational waves from small wobbles.

After all, this was how Neptune was discovered in 1846. Urbain Le Verrier, astronomer and mathematician, not willing to let go of the accuracy of the Newtonian Theory of Gravity, realized he could account for the well-documented anomalies in the motion of Uranus by postulating a hidden planet in a precise orbit. Neptune was found precisely where he predicted it to make Newtonian gravity work. As François Arago said, Le Verrier was the first astronomer to “find a planet with the point of his pen.”

He was not so lucky in applying the same analysis to the motion of Mercury. To account for the well-documented 43 arc-second-per-century anomalous shift in the perihelion of Mercury, Le Verrier proposed the Planet Vulcan, in orbit much closer to the Sun than Mercury. Vulcan was never found, but this anomaly with Mercury was one of the three observations that confirmed general relatively, 70 years later.

This was the context in which Anderson’s team set out to do precision celestial mechanics analysis of the motion of the Pioneer spacecraft.

The most dramatic-looking feature of the Pioneer spacecraft was a 2.74 m diameter parabolic dish used to stay in communications with earth. One of its tasks was to take a 2.1-GHz S-band carrier signal transmitted from earth and coherently up-convert it by a factor of 240/210 to about 2.29 GHz and retransmit it back.

Since the received frequency stability was as accurate as the earth-station’s frequency stability, small frequency shifts on the order of 1-mHz could be detected. This is a frequency shift of a part in 1 trillion over a few hours.

Precision measurements of frequency shifts received on earth could be interpreted as the Doppler shift in the signal. This enabled precision measurement of the spacecraft’s line of sight velocity, which could be translated to position and acceleration information.

By 1980, an anomaly had been detected in its motion. The unexplained frequency shift was only a few tens of milliHertz, but it was far above the noise. The spacecraft was slowing down more than expected. The deceleration was about 0.09 nano gees. This is about the same deceleration a car would have from the photon pressure from its high beams.

It could not be accounted for by any known influence, such as the gravitational effects of the sun or planets, the solar wind, cosmic ray fluxes, the precession of the poles of the earth, even residual fuel leakage from the spacecraft. This slight deceleration became known as the Pioneer Anomaly.

It appeared the acceleration was constant and not changing over time. This seemed to confirm the early estimate that the influence of thermal radiation from the radioisotope thermoelectric generators (RTG) was not contributing significantly to the deceleration. If it was a thermal radiation effect, the 88-year half-life of the Plutonium 238 should have contributed to a change in the deceleration as the thermal radiation decreased.

In 1994, out of the blue, Anderson got an email from Michael Nieto, a Los Alamos cosmologist. He was interested in MOND, MOdified Newtonian Dynamics, theories of gravity. Coincidentally, Nieto thought he could use precision measurements of the Pioneer satellite as a test for non-Newtonian gravity effects. He expected anomalous accelerations on the order of the expansion rate slow down of the universe, about 0.07 nano Gees. Here was one possible explanation. Wow! Could this agreement just be a coincidence?

At around the same time, Slava Turyshev joined Anderson to work on the Pioneer Anomaly reviewing all possible spacecraft-related causes for the anomaly.

In 1998, what was known of the Pioneer Anomaly was published to stimulate the scientific community to come up with other explanations.

Almost 1,000 papers were published with explanations for the Pioneer Anomaly, including dark energy, dark matter, string theory, violation of general relativity, extra dimensions, and even variations in the speed of light. The chart below shows the publication rate in papers just posted on arXiv over the years.

Publications explaining the Pioneer Anomaly posted on arXiv.

But Anderson’s team was not willing to let go of possible conventional explanations. They managed to find more historical Doppler data, some on old magnetic tape media, stuffed in moldy cardboard boxes under a staircase at JPL. With the new data, they increased the accuracy and time span of Pioneer 10 measurements to more than 23 years. In the new data, they saw a change in the deceleration. It was decreasing over time.

This important signature in the data hinted that maybe the deceleration was tied to the decrease in the thermal emissions from the radioisotope thermoelectric generators. Their generated electrical energy was only about 100 watts, but they radiated 2.5 kwatts of waste heat. If only 60 watts of the IR radiation were to hit the front and back of the spacecraft asymmetrically, the radiation pressure could account for the Anomaly.

But accurate analysis of the thermal radiation effects was incredibly complicated. Turyshev, joined by Viktor Toth, a software engineer from Toronto, Canada, turned old blueprint data into a 3D finite element surface model. Using the thermal properties of each surface element and temperature readings buried in the historical “housekeeping” data, they calculated the heat flow and thermal radiation emissions from the known electronics and the reflectivity and angle of reflections of the IR emissions from the two RTGs on 2.8 m long booms.

After 10 years of detailed modeling and computer simulations, in 2012, the Anomaly could be explained. The new model’s predictions matched the measured deceleration and its change over time to 20 percent, within the error of the measurement and analysis. About half the deceleration force was from IR emissions from heat leaked from the baffles of the electronics package pointing out the leading surface of the spacecraft. The remaining deceleration was due to excess reflected IR photon pressure from the back surface of the parabolic radio dish from the extended RTGs.

“Three decades after its discovery, we can now say there is no exotic cause for the Pioneer anomaly. The puzzling deceleration was produced by the asymmetrical radiation of waste heat created onboard the spacecraft,” Slava Turyshev and Viktor Toth reported in their 2012 paper in IEEE Spectrum.

This is not the only recent case of an observed anomaly, announced to the scientific community, turning out to have a conventional explanation. In September 2011, a CERN team working on the OPERA experiment, announced the speed of neutrinos to be faster than the speed of light. After careful detective work, a year later, this anomaly was traced to a loose fiber optic connector and a slower than expected precision clock crystal.

Engineers should never forget these examples. Good science and engineering is not easy. It takes tenacity, free and open brainstorming, and smart people applying careful modeling and analysis to “put in the numbers” in order to debug and confirm the root cause of any problem. Most importantly, before thinking about overthrowing a time-proven physics principle, it’s important to thoroughly investigate conventional explanations.

The last received signal from Pioneer 10 was on Jan 23, 2003, but its mission is not over. In 2 million years, Pioneer 10 will reach the Aldebaran star system, 68 light years away. It is carrying a gold-plated aluminum plaque, designed by Carl Sagan and Frank Drake, with a picture of a man and woman, and the location of our solar system referenced to pulsars in our local space. Who knows what will find it, and the consequences.

Pioneer plaque with directions to its origin, us.