The Pioneer Anomaly

Recently, I was offered a fantastic opportunity: to take part in a research effort to help determine the origin of the Pioneer Anomaly.

Many people wrote many things about the anomalous acceleration of the Pioneer 10 and 11 spacecraft, so there's no need to repeat the details here. Briefly, these spacecraft were launched in 1972 and 1973; they are flying out of the solar system; they are spin stabilized, meaning they need very few propulsive maneuvers to keep themselves pointed at the Earth; consequently, most of the time their motion is determined solely by the laws of celestial mechanics. The spacecraft have radio equipment on board that can respond to a signal from the Earth with a transmission whose frequency is precisely known. This is called "coherent" mode; in this mode, changes in frequency as small as a few milliHertz can be detected. In accordance with the Doppler effect, such changes in frequency can indicate a velocity change. After subtracting all known forces, a small, but consistent change in velocity remains that cannot be accounted for by recognized physics. Both Pioneers accelerate towards the Sun (i.e., they are slowing down) by ~8.74×10–10 m/s2. (This is a tiny value; about one ten billionth the gravitational acceleration on the surface of the Earth. But over time, it adds up to a lot; by now, the two Pioneers may be several hundred thousand kilometers from their predicted course, i.e., a distance comparable to the Earth-Moon distance.)

The reason for this is unknown. Some people suggested that it may be due to an engineering cause; the most likely culprit is heat, radiated away from the spacecraft preferentially in one direction, pushing the spacecraft in the opposite direction. This is a plausible explanation, as the spacecraft has ~2.5 kW worth of heat sources on board (waste heat from its radioisotope thermoelectric generators, plus all the electrically generated heat), and it takes only ~64 W radiated in a direction away from the Sun to produce the required acceleration.

Others, however, dismissed this possibility: they point out that the acceleration does not decay over time while the amount of heat available on board does.

So how did I get involved in this? Simple: I wrote a bunch of programs over the last couple of years, processing raw Pioneer telemetry files. My original purpose was to help preserve the legacy of these amazing spacecraft, perhaps one day build a Web site where their history can be examined, "replayed" if you wish. It so happens that the same raw telemetry I learned to read can also be used to determine the exact amount of heat on board and the extent to which it could (or couldn't) contribute to acceleration.

In November 2005, I participated in an international conference in Bern, Switzerland, where we discussed the current state of affairs. I had the opportunity to give a presentation on my work to date, describing the nature and significance of the data files we preserved, and also the tools that I developed to process them.

Since then, I co-authored a paper with Slava Turyshev and others, in which we report on the availability of the Pioneer telemetry data as well as additional Doppler radio data, and our plans to proceed with this data.

I am now making good progress building a geometric/thermal model of the spacecraft that, hopefully, will bring us one step closer to a resolution of this intriguing mystery. In another paper, we report on our progress as of early March, 2006.

Our work already had one tangible result: in the fall of 2005 it became clear that one final opportunity existed to contact Pioneer 10, in the early spring of 2006. This was somewhat of a long shot: the spacecraft was incredibly far away (~90 astronomical units), and its power source is decaying, no longer able to maintain a stable bus voltage required for transmitter operation. Sadly, the attempt was unsuccessful: no signal was heard.