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Projects: Pioneer Anomaly

Update from the Pioneer Anomaly Team

January 2006

As Pioneer 10 and 11 head toward the farthest reaches of our solar system, something strange is happening -- they are mysteriously slowing down. Scientists do not yet know why the spacecraft aren't acting as expected; however, The Planetary Society has stepped in to help fund the effort to analyze roughly 25 years of data in hopes of solving the mystery. We're happy to report that the Pioneer Anomaly Team has successfully retrieved much of the data and are preparing to begin analysis. Our detective work continues, and here, Pioneer Anomaly Team members Slava Turyshev and Viktor Toth report on their progress.

Update from the Pioneer Anomaly Team

The Pioneer 10 and 11 spacecraft, launched in 1972 and 1973, respectively, were the first man-made objects to leave the solar system. Unique characteristics of the spacecraft, including a thermally "clean" design (significant heat sources on board, the radioisotope thermoelectric generators, or RTGs, were situated at the end of long booms away from the main spacecraft body) and spin stabilization that reduced the need for attitude control thruster firings, made these spacecraft excellent for the purposes of studying small forces acting on the spacecraft. It was hoped that such precise navigation would make it possible to use the Pioneers in search for a hypothetical tenth planet or to place tighter limits on a low-frequency gravitational radiation arriving in the solar system from our stellar neighborhood.

Instead, we found a small, nearly constant acceleration in the approximate direction toward the center of the solar system, slowing the spacecraft as they are moving away from it.  The cause of this acceleration is as of yet undetermined. This signal has become known as the "Pioneer anomaly."

The Planetary Society, using support from its members, is currently sponsoring the recovery of Doppler and spacecraft status data, as well as analysis of that data.

New Data and Objectives for New Investigation

The original investigation used about 11.5 years worth of data for Pioneer 10, and slightly less than 4 years of data for Pioneer 11. We can now report on the recovery of a much larger data set: essentially all Doppler data for the entire 30-year time span of the Pioneer 10/11 missions will soon be available for analysis. This is not an easy process: 30 years is a very long time! The challenges are formidable, including having to deal with numerous obsolete data formats, no longer supported software packages, and last but not least, the fact that people with the right expertise are no longer around. Nevertheless, the formats are now understood and the media has been collected. We now have up to 60,000 data points (about 20 gigabytes) of data for Pioneer 10, and 50,000 data points (about 15 gigabytes) for Pioneer 11. The data recovery process in nearly complete and the data will be available for the analysis in early this year.

Since the previous analysis, models of the Earth improved to the extent that we now know the locations of Deep Space Network receiving stations with an accuracy of 1 centimeter. Planetary orbits are also known more accurately. With this and the extended Doppler data set, it will be possible to determine the motion of the Pioneer spacecraft for a longer time span, and with a better accuracy.

In addition to the recovery of Doppler data, in a separate effort, we successfully recovered almost all of the original Pioneer 10/11 telemetry. As the spacecraft traveled across the solar system, every bit of information they radioed to the Earth that was captured by the Deep Space Network's antennae was converted into a format called Master Data Records (MDRs) for further processing. The MDRs contained both telemetry readings about the health and status of the spacecraft, and the results of scientific measurements.

Normally, MDRs are further processed to extract science information on the one hand, which is sent to the experimenters, and engineering data on the other hand, which is then used by the spacecraft operations team to guide and command the spacecraft. Afterwards, the MDRs are no longer of much use; they were archived, but the retention schedule prescribed that they be destroyed after 7 years.

Fortunately, this is not what happened in the case of the Pioneer missions. Not only were the original magnetic tapes not destroyed but, in the early 1990s, before deterioration would have made the tapes unusable, they were copied to much more durable magneto-optical media. Since then, the files, about 40 gigabytes in total, have been copied to modern computers and we have now developed tools to extract information from them.

Why is this important? One of the most likely explanations of the Pioneer Anomaly is that it is due to an on board so-called "systematic" effect. The most likely culprit is heat radiation: the RTGs produce nearly 2.5 kilowatts of heat in total, and only a small portion of it may be enough to explain the anomaly if it is emitted in the right direction. Previous studies suggested that this mechanism is not sufficient to produce an acceleration of the required magnitude; however, with the MDRs at hand, we are now in a position to develop a better understanding of the "thermal inventory" on board the spacecraft, and predict any acceleration due to "systematics" to a much greater accuracy.

It should be emphasized that this has never been done before: never before was on-board telemetry incorporated into orbit determination computations. We are, therefore, in uncharted waters. Nevertheless, this is a win-win situation: at the very least, this analysis will help us better predict spacecraft orbits in the future. It is also possible that the analysis will confirm previous results that a "systematic" explanation is not sufficient to explain the anomaly, strengthening the case for new physical theories.

Our strategy for the upcoming analysis of the newly available data can be summed up as follows: First, studying the early mission data may help us unambiguously determine whether the acceleration points towards the Earth, the Sun, or some other direction. Second, we hope to find out how the anomaly begins or if it was present throughout the mission. Third, we hope to be able to determine how this anomalous acceleration changed over time. Fourth, we will compare data from the two Pioneers to see if we can discern any notable differences in their behavior. Lastly, we will use the MDRs to develop a better engineering model of the spacecraft, making use, for instance, of finite element analysis methods to understand its thermal behavior.

In March 2006, for the very last time the Earth will be in a favorable position to receive Pioneer 10's radio signal. It is possible that Pioneer 10 is still able to transmit, despite the age of its electronics, the extreme coldness of deep space, and the diminishing amount of electrical power on board. The possibility that the Deep Space Network will attempt to reacquire Pioneer 10's weak signal is currently being investigated, raising the hope, however faint, that we may yet have another data point to aid in our investigation of the Pioneers' enigmatic behavior.