Pioneer Anomaly Solved!

With the latest piece of the puzzle just published in a scientific journal, a solar system mystery that has perplexed people for more than 20 years has been solved, truly thanks to the support of Planetary Society members. That mystery is the “Pioneer Anomaly,” an anomalous acceleration that affected the two Pioneer spacecraft as they left the solar system.

Pioneers 10 and 11 were launched in the early 1970’s. As they traveled away from the Sun, they slowed down. Most of this slowing was expected, a result of the gravitational pull of the Sun and other massive objects in the solar system. But even when everything in the solar system whose mass could have any effect on the Pioneers was accounted for, both spacecraft were found to be slowing more than expected. The excess slowing was very tiny, but measurable.

Over the course of the past two decades, all sorts of solutions have been proposed, some of which invoked exotic “new” physics. In the end, recovery of more data and years of painstaking work have shown that no such exotic solution is necessary, but rather that anisotropic (big word for not-symmetric-in-all-directions) thermal radiation (big words for heat) can explain the mystery. This solution had already been suggested, and examination of the problem over time had made it seem increasingly likely, but only careful analyses could check whether anisitropic radiation could explain the anomaly. The latest piece of this analysis appears in a new scientific article by Slava Turyshev and colleagues.

The only way to solve this mystery was to look at much more data than had previously been available. That is where Planetary Society members stepped in. A few years ago, Slava Turyshev and his colleagues at the Jet Propulsion Laboratory were in desperate need of funding to help them do just that. With funds supplied by our members, Turyshev successfully chased down data from a number of sources. This was not an easy (or quick) task. These missions lasted for more than 30 years. Imagine all the people, computing formats, and hardcopy and electronic storage devices involved over that period, and you’ll start to get an idea of the problem.

As Slava mentioned to us before, “We recovered more data than we dared dream possible. 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. The recovery of Doppler and telemetry data and the entire effort in thermal analysis would not have happened without the Planetary Society.”

After they recovered the data, they had decades of Doppler data to analyze. That told them the true nature of the acceleration – the anomaly -- and how it acted over time including that it varied, rather than staying constant. See this discussion for more details on their Doppler results as well as more background on the Pioneer Anomaly.

To really nail down whether a thermal explanation alone could explain the anomaly required detailed thermal modeling of the spacecraft, and that is the focus of the new paper. This also could not have been accomplished without the saving of data facilitated by Planetary Society members. That process turned up spacecraft schematics used to construct the model, but also turned up data on spacecraft temperatures during the mission. The latter allowed comparison of real temperatures in a few locations with the model.

Why was the thermal emission from the spacecraft anisotropic and slowing the spacecraft down? First of all, because the Pioneer spacecraft were spin-stabilized and almost always pointed their big dishes towards Earth. Second of all, because two sources of thermal radiation (heat) were then on the leading side of the spacecraft. The nuclear power sources, more formally Radioisotope Thermoelectric Generators (RTG), emitted heat towards the back side of the dishes. When the dishes reflected or re-radiated this heat, it went in the direction of travel of the spacecraft. Also, the warm electronics box for the spacecraft was on the leading side of the spacecraft, causing more heat to spill that direction. Photon pressure, the same type of thing used in solar sailing, then preferentially pushed against the direction of travel, causing a tiny, but measurable, deceleration of the spacecraft – the Pioneer Anomaly.

These results once emphasize three things to me: the power of more data, the power of careful scientific analysis, and the power of Planetary Society members getting involved at critical times. In addition to providing the solution to a long-term solar system mystery, the technical results of these studies will be useful for planning the details of future spacecraft navigation.

You can find the latest scientific paper, being published in Physical Review Letters, here: "Support for the thermal origin of the Pioneer anomaly" by Slava G. Turyshev, Viktor T. Toth, Gary Kinsella, Siu-Chun Lee, Shing M. Lok, and Jordan Ellis.