NASA's Voyager spacecraft is not the only important deep-space mission to remember. The Pioneer spacecraft and the Pioneer Anomaly are part of a classic engineering story about good design.
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.
Related posts and resources:
- Voyager: The mathematics of interstellar space travel
- Voyager: Electronics for the long haul
- Turyshev, Slava G. "The Pioneer Anomaly," Jet Propulsion Laboratory, California Institute of Technology, from arxiv.org
- Pioneer Anomaly, 3.2 Acquisition of radiometric Doppler data
- Toth, Viktor T. and Slava G. Turyshev. "Finding the Source of the Pioneer Anomaly" IEEE Spectrum, posted Nov 30, 2012.
- "Plutonium Is Hot Suspect in Pioneer Spacecraft Mystery," Wired UK.
- NASA Pioneer Missions page
- Wolchover, Natalie. "The Mystery of the Pioneer Anomaly Solved at Last," Popsci, posted July 22, 2011.