Scientists working with recovered data from the Pioneer 10 and 11 missions are closing in on a solution to the famous Pioneer Anomaly. Their just-published results show that the mysterious effect on the two spacecraft is not constant over time, probably indicating that no outside force is acting on the Pioneers, but rather, something inside the spacecraft is to blame.

Slava Turyshev and a team of researchers are publishing an upcoming issue of Physical Review Letters their analysis of radio transmission data from the spacecraft. (The article is available on-line.) Their work strengthens the case that the source of the anomaly lies in the spacecraft themselves, not in any mysterious outside force acting on them. The most likely cause is heat generated by spacecraft systems, producing a recoil force.

The Pioneer Anomaly was defined as "anomalous acceleration in the direction of the Sun" or, as seen from Earth, the spacecraft appeared to be slowing down. It was first detected in 1980 by John D. Anderson of the Jet Propulsion Laboratory through his analysis of the Doppler shift in the radio signal from the Pioneers 10 and 11 on their way out of the solar system, after becoming the first spacecraft to fly by Jupiter and Saturn.

Since its discovery, suggested solutions to the Pioneer Anomaly have ranged from such things as the gravity of as-yet undetected bodies in the solar system, dark matter or dark energy, the cosmic expansion, to some sort of New Physics, such as modifications to the theory of gravity. For over 20 years, scientists around the world have been seeking an explanation.

Planetary Society members have been eager to help solve this mystery with their support to the research team and their enthusiasm for pushing the boundaries of what is known. With Society members' backing, Turyshev and his colleagues were able to recover spacecraft data preserved in obsolete formats or once thought lost. This vastly increased the "data arc" for the spacecraft's trajectories, enabling the team to analyze the Pioneers' behavior over many more years than earlier researchers were able to do.

"We recovered more data than we dared dream possible," Turyshev said, "Without the rescue of the Doppler data, we would have been blind, never able to claim the quantitative data we need to solve the anomaly."

Doppler measurements
Over 25 years ago John Anderson noticed that the Pioneers' radio signals drifted slightly from what was expected according to well-understood laws of physics. There was a small "blue shift" toward radio receivers on Earth, the opposite of the famous "red shift" of galaxies that appear to be moving away from our location on Earth.

The early analysis indicated that the anomaly was constant over time. This suggested some force external of the spacecraft was "putting on the brakes," not an on-board system that would slowly be running out of power. With the newly recovered data, Turyshev and colleagues were able to discern that the anomaly was, in fact, decreasing over time, consistent with decaying thermal power on-board the spacecraft.

The most likely suspect behind the acceleration is indeed heat radiating from the Pioneers' power source and leaking from the spacecraft's instruments. Turyshev and colleagues are completing their meticulous study of thermal forces and will soon publish their peer-reviewed final results.

Thermal modeling
Through the radioactive decay of plutonium, radioisotope thermoelectric generators (RTGs) provide the energy spacecraft needed to operate its instruments and communicate with Earth. To communicate from the outer solar system, a spacecraft needs a big antenna, hence the big, parabolic antennas that dominate the profiles of all really-deep-spacecraft.

The analysis of recovered telemetry data indicates that heat from the RTGs reflected off the backside of the big high-gain antenna and helped "push" the spacecraft back toward the sun.

There was another thermal effect at work: heat radiating from the instruments housed inside the spacecraft "bus" was vented through louvers that could be opened or closed as necessary. With the spacecraft "rebuilt" as a computer model for analysis, the team was able to determine that the escaping heat was pointed in the direction of the Pioneer Anomaly, thus slowing the spacecraft on their trajectories away from the sun.

"The recovery of Doppler and telemetry data and the entire effort in thermal analysis would not have happened without the Planetary Society," said Turyshev. "The members provided the money we needed to get started and demonstrated to NASA that the public was definitely interested in solving the mystery. Their interest and strong support made possible our work to solve the Pioneer Anomaly."

Benefits of a Mystery Solved
While solving the Pioneer Anomaly will not lead to a new physics, the work will have benefits in the endeavor of space exploration. Understanding how a spacecraft's flight can be affected by such tiny forces as the recoil force due to heat radiating from power sources and scientific instruments can lead to more accurate spacecraft navigation. This will have a tremendous impact on future mission aiming to search for gravitational waves, testing the fundamental laws of physics, and studying the universe.

The mystery contributed in other ways to modern science by stimulating work on the fundamental principles that govern the physics of objects in space, and in testing how gravity - as described by both Newton's Law of Gravity and Einstein's Theory of General Relativity - behaves in our solar system.

Bill Nye, Executive Director of the Planetary Society, commented: "What a remarkable business. Slava and his colleagues are analyzing the motions of spacecraft that have flown fantastically far away from us, but not quite as far as we once figured they should fly. So far, it's not new physics; just the surprising subtle push of photons. This kind of diligent analysis would be impossible without the good data that we recovered. I'm looking forward to the next level of this analysis. It will affect spacecraft operations for years to come."

The paper discussed above is titled "Support for temporally varying behavior of the Pioneer anomaly from the extended Pioneer 10 and 11 data sets," soon to be published in Physical Review Letters, and now available on-line at http://arxiv.org/abs/1107.2886. The authors are Slava G. Turyshev of the Jet Propulsion Laboratory; Viktor T. Toth of Ottawa, Ontario; Jordon Ellis of JPL; and Craig B. Markwardt of NASA Goddard Spaceflight Center.