Discover Blogs | Cosmic Variance | Sean Carroll | 2010 Dec 15
The Pioneer Anomaly Resolved?Here’s an excellent article in Popular Science about the Pioneer anomaly. The Pioneer spacecraft, launched in the early 1970′s, have been moseying through the outer regions of the Solar System for quite some time now. But a careful analysis of tracking data indicated that the acceleration of the two spacecraft didn’t quite match what we’d expect from gravity; there appears to be an anomalous acceleration, nearly constant over time and pointing toward the Sun. Many new-physics explanations have been proposed, but it’s always been a difficult scenario to master; it’s very hard to imagine a new force that would account for the Pioneer data but not also show up in observations of the outer planets. (The Voyager spacecraft aren’t as useful for this purpose, as they are guided by tiny thrusters that overwhelm the signal, while the Pioneers float freely and are pointed using gyroscopes.)
The most likely explanation has always been that we didn’t completely understand the spacecraft, or the tracking system. Indeed, it’s been recognized for a while that a small imbalance in how the spacecraft radiated heat could account for the acceleration — but that imbalance didn’t seem to be supported by what we knew about the vessels. That may be changing, however. The Popular Science article is a little cagey, but it mentions a new and unprecedentedly thorough analysis by Viktor Toth and Slava Turshyev that should be coming out soon. Here is as much as they would let on:
- Five years have passed. Using the telemetry data, the two scientists created an extremely elaborate “finite element” 3-D computer model of each Pioneer spacecraft, in which the thermal properties of 100,000 positions on their surfaces are independently tracked for the duration of the 30-year mission. Everything there is to know about heat conduction across the spacecraft’s surfaces, as well as the way that heat flow and temperature declined over time as the power of the generators lessened, they know. The results of the telemetry analysis? “The heat recoil force accounts for part of the acceleration,” said Turyshev. They wouldn’t tell me how significant a part. (Turyshev: “We’d like to publish that in the scientific literature.”) But according to Toth, “You can take it to the bank that whatever remains of the anomaly after accounting for that thermal acceleration, it will at most be much less than the canonical value of 8.74 x 10-10 m/s2, and then, mind you, all those wonderful numerical coincidences people talk about are destroyed.”
Doesn’t look good for people who prefer to imagine that wild new physics is responsible. Not that they will go away — the power of wishful thinking is strong. You can already hear them staking out territory, even before the new report comes out:
- Other physicists are more combative. “Heat? That’s simply not the right explanation. They are wrong,” commented Johan Masreliez, an independent researcher in Washington who supports the expanding spacetime model of cosmology, for which it is crucial that the value of the Pioneer anomaly equals c times H. “But then I’m biased,” he added.
Even if the new analysis gives a very sensible and believable account of the Pioneer anomaly in terms of very ordinary physics, expect the true believers to hang on for years to come. The rest of us will move on — at least until the next exciting anomaly pops up.
Centauri Dreams | Notes & Queries | Paul Gilster | 2010 Dec 17
The Pioneer Anomaly - Slava G. Turyshev, Viktor T. Toth8.74 x 10-10 m/s2 isn’t much, but it’s an apparent Sun-ward acceleration sufficient to throw the paths of our Pioneer spacecraft off by a few hundred miles from where they ought to be each year. Noticing the effect in 1980, astronomer John Anderson, who was leading the analysis of Pioneer Doppler ranging data as part of a study of gravitational effects in the outer Solar System, came to the conclusion that outgassing from the spacecraft thrusters was responsible, but the effect persisted longer than it should and a whole range of alternate theories soon came into play after Michael Martin Nieto (Los Alamos National Laboratory) began to study the anomaly in terms of modified Newtonian dynamics (MOND).
The background is given in a fine article by Natalie Wolchover for PopSci. What accounted for Nieto’s fascination was the fact that the value of the anomaly almost exactly equaled the speed of light multiplied by the Hubble constant. At that point, with the possibility that the Pioneers were telling us something fundamental about physics, the Pioneer anomaly took on a life of its own. The discovery of dark energy in the same year that Anderson, Nieto and Slava Turyshev announced their findings about the Pioneer acceleration in Physical Review Letters only added to the interest, and hundreds of papers, many but hardly all on MOND notions, followed.
Were the Pioneer probes measuring the cosmic expansion whose effects are now thought to be intertwined with dark energy? It was one amongst a sea of possibilities. Wolchover’s article describes how Viktor Toth, running an independent analysis using home computers, became skeptical of the earlier work of Anderson and Turyshev and later, with Turyshev, became instrumental in saving the 30-plus years of Pioneer Doppler data and logbooks. With the help of a retired Pioneer mission control engineer named Larry Kellogg, the two acquired Pioneer telemetry data and went back to work from scratch on a much more thorough analysis.
That was five years ago. The result:
- Using the telemetry data, the two scientists created an extremely elaborate “finite element” 3-D computer model of each Pioneer spacecraft, in which the thermal properties of 100,000 positions on their surfaces are independently tracked for the duration of the 30-year mission. Everything there is to know about heat conduction across the spacecraft’s surfaces, as well as the way that heat flow and temperature declined over time as the power of the generators lessened, they know. The results of the telemetry analysis? “The heat recoil force accounts for part of the acceleration,” said Turyshev. They wouldn’t tell me how significant a part. (Turyshev: “We’d like to publish that in the scientific literature.”) But according to Toth, “You can take it to the bank that whatever remains of the anomaly after accounting for that thermal acceleration, it will at most be much less than the canonical value of 8.74 x 10-10 m/s2, and then, mind you, all those wonderful numerical coincidences people talk about are destroyed.”
If the Pioneer acceleration declines with time, it’s obviously not the constant force it was originally thought to be. The question of decay in that acceleration, and whether the acceleration is indeed in the direction of the Sun or elsewhere, is still open, and Toth and Turyshev are planning to supplement their recently published review of the phenomenon with a forthcoming publication that recounts their latest work. Wolchover thinks the duo have become convinced that thermal effects aboard the spacecraft themselves are the cause of the anomaly, but Turyshev won’t reveal the results, telling the writer only, “Physics as we know it worked well.”
- arXiv.org > gr-qc > arXiv:1001.3686 > 20 Jan 2010 (v1), 19 Aug 2010 (v2)
Living Reviews in Relativity 13 (2010) 4 (online)
Abstract: Radio-metric Doppler tracking data received from the Pioneer 10 and 11 spacecraft from heliocentric distances of 20-70 AU has consistently indicated the presence of a small, anomalous, blue-shifted frequency drift uniformly changing with a rate of ~6 x 10^{-9} Hz/s. Ultimately, the drift was interpreted as a constant sunward deceleration of each particular spacecraft at the level of a_P = (8.74 +/- 1.33) x 10^{-10} m/s^2. This apparent violation of the Newton's gravitational inverse-square law has become known as the Pioneer anomaly; the nature of this anomaly remains unexplained. In this review, we summarize the current knowledge of the physical properties of the anomaly and the conditions that led to its detection and characterization. We review various mechanisms proposed to explain the anomaly and discuss the current state of efforts to determine its nature. A comprehensive new investigation of the anomalous behavior of the two Pioneers has begun recently. The new efforts rely on the much-extended set of radio-metric Doppler data for both spacecraft in conjunction with the newly available complete record of their telemetry files and a large archive of original project documentation. As the new study is yet to report its findings, this review provides the necessary background for the new results to appear in the near future. In particular, we provide a significant amount of information on the design, operations and behavior of the two Pioneers during their entire missions, including descriptions of various data formats and techniques used for their navigation and radio-science data analysis. As most of this information was recovered relatively recently, it was not used in the previous studies of the Pioneer anomaly, but it is critical for the new investigation.
Popular Science | Natalie Wolchover | 2010 Dec 15
What is the mystery force slowing down the Pioneer spacecraft? Do we finally know the answer?