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Projects: Pioneer AnomalyFinding a needle in the haystack or proving that there may be none...A Pioneer Anomaly Update
October 5, 2008 marked the tenth anniversary of the first announcement of the Pioneer anomaly's discovery. On that date in 1998, The Physical Review Letters published a paper titled “Indication, from Pioneer 10/11, Galileo, and Ulysses Data, of an Apparent Anomalous, Weak, Long-Range Acceleration” with the initial results of a detailed study of the newly discovered effect. Now, over ten years after that announcement, the mystery of the anomalous acceleration of the Pioneer 10 and 11 spacecraft still remains, but hopefully not for long. In this report, we discuss our current efforts aimed at finding the nature of the Pioneer anomaly. The Pioneer anomaly is so small, about one ten billionth the magnitude of the gravitational acceleration here on the surface of the Earth, that detecting it can be rightfully likened to the near impossible task of finding a needle in the proverbial haystack. The earlier studies have shown that there may be a needle in the haystack, but its presence remained to be proven rigorously. As you may know, our preliminary thermal modeling indicates that anisotropic thermal radiation (that is, when the spacecraft emits more heat in one direction than the other) may account for some, but not necessarily for all of the anomalous acceleration of the Pioneers. Rather than treating this result as the “smoking gun”, we realized that our further study of the Pioneer anomaly requires a thorough and complete understanding of this thermal recoil force. This means that the most difficult part of our work only just began and our task just became harder. What if the Pioneer acceleration is entirely of a mundane origin? For instance, what if anisotropic thermal radiation emitted by the spacecraft accounts fully for the observed acceleration? In other words, what if there is no anomaly at all? Try proving to someone who saw a glimpse of something shiny that there is no needle in the haystack. Or at least, what if much of the acceleration has an explanation within the standard laws of physics, and only a smaller anomaly exists? These thoughts characterize our current efforts. Using the recently recovered Pioneer 10 ad 11 flight telemetry, including data on their thermal, electrical, power, propulsion, and communication subsystems, we continue to investigate the extent to which the anomalous acceleration seen in the Doppler data received from both Pioneers could be caused by thermal radiation. To do that, we have built an elaborate finite-element software model of the spacecraft that numerically solves equations of heat conduction and thermal radiation within the spacecraft and focuses on thermal radiation from the spacecraft to outer space. As boundary conditions, the model uses power and thermal information taken directly from the now available flight telemetry data. A built-in optimization algorithm allows us to find a solution that satisfies the set of various constraints that guide our search in this “treasure hunt”. The model is capable of calculating the anisotropy in the thermal radiation emitted by the spacecraft at different points in time, when they were at various heliocentric distances along the flight trajectory. Using the thermal analysis and the recently recovered on-board telemetry together, we can investigate the thermal, electrical, and dynamical behavior of the spacecraft and perform a detailed study of on-board systematics – a critical path in the study of the anomaly. This thermal model is developed at JPL by Gary Kinsella and colleagues with the help from a number of experts, including the thermal engineers who built the vehicles. For instance, we received advice from Jim Moses, a TRW retiree who nearly 40 years ago led the development of the thermal system of the two Pioneers. Jim still clearly remembers the design elements of the entire system, offering invaluable help in verifying our model. Last year we realized that at 25 astronomical units (AU) from the Sun, the Pioneer 10 thermal model indicates the presence of a sizable amount of anisotropically emitted heat. After examining the temperature map of the exterior surfaces of the spacecraft, we noticed that some of the louver blades of the thermal control louver system on the back of the spacecraft compartment were still partially open, letting out excess heat. This was a surprising revelation, indicating that even as far as 25 AU out from the Sun, heat from the Sun and all the instruments had a greater effect on Pioneer 10 than we expected. In order to verify and refine the model, we needed to examine its performance under conditions when the louver system was in a stable, firmly closed configuration. So, we decided to evaluate the thermal behavior of Pioneer 10 at greater distances, 50, 65 and 70 AU from the Sun. The subsequent analysis of the thermal model’s output for larger heliocentric distances suggested that even when all the blades of the louver system were closed, there was still a noticeable amount of anisotropically emitted heat. However, the results seemed to contradict the known properties of the Pioneer anomaly. Not only was the amount of heat smaller than what is needed to explain the anomaly, it also decreased with time. We will learn more about this behavior by running the model for all the heliocentric distances from 5 to 85 AU in increments of 10 AU. Once we complete this work (each case runs for nearly 2 weeks), we will be in a much better position to evaluate the role of the thermal radiation off the vehicles in the formation of the anomaly. Another important part of our ongoing investigation is the analysis of the newly recovered Doppler data. Yes, we finally assembled all the recently recovered data files and we have initiated the analysis of this much longer Doppler data set. We conduct this analysis of Doppler data together with the thermal analysis, in order to achieve a stronger, more convincing outcome. Initial results are very intriguing, leading us to believe that we will be able to address all the main objectives of the study of the Pioneer anomaly. These objectives are: i) analysis of the early data that could yield the direction of the anomaly, ii) analysis of planetary encounters, that should tell more about the onset of the anomaly, iii) analysis of the entire dataset, to improve our understanding of the anomaly's temporal behavior, and iv) comparative analysis of individual anomalous accelerations of the two Pioneers. We hope that this strategy will result in a solution that is accurate enough for an unambiguous determination of the origin of the Pioneer anomaly. Today, 10 years after the initial publication of a paper reporting the discovery of the Pioneer anomaly, we think that we see the light at the end of this long tunnel. By now, we have investigated nearly all branches of our “decision tree” and we have eliminated most of the possibilities. The search narrows and we expect to have solid results from our analysis in the very near future. Some may ask why we are making such an extraordinary effort to understand an anomalous acceleration this small. Especially if the result is a negative result, if all we prove in the end is that the laws of gravitation laid down by Newton and Einstein work with even greater precision than previously shown. What's the point? What is the benefit to human society? In the short run, knowing the gravitational constant to one more decimal digit of precision or placing even tighter limits on any deviation from Einstein's gravitational theory may seem like painfully nitpicking detail. Yet one must not lose sight of the "big picture". When researchers were measuring the properties of electricity with ever more refined instruments over two hundred years ago, they did not envision continent-spanning power grids, an information economy, or tiny electrical signals reaching us from the unfathomable depths of the outer solar system, sent by manmade machines. They just performed meticulous experiments laying down the laws connecting electricity to magnetism or the electromotive force to chemical reactions. Yet their work paved the way to our modern society. Similarly, we cannot envision today what research into gravitational science will bring tomorrow. Perhaps one day humankind will harness gravity. Perhaps one day a trip across the solar system using a yet to be devised gravity engine may not seem a bigger deal than crossing an ocean in a jetliner today. Perhaps one day human beings will travel to the stars in spacecraft that no longer need rockets. Who knows? But one thing we know for sure: none of that will happen unless we do a meticulous job today. Our work, whether it proves the existence of gravity beyond Einstein or just improves the navigation of spacecraft in deep space by accounting for a small thermal recoil force with precision, lays down the foundations that may, one day, lead to such dreams. Did you like this story? Send
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