An April Space.com posting that was cited as the PHYSICS TODAY Online News Pick 'Thermal radiation may account for Pioneer anomaly' reported on a space physics mystery 'seen on two NASA probes that has caused scientists to question the laws of physics.' But just as neutrinos don't really exceed the speed of light, it turns out that well-understood science explains the minute slowing detected and studied over decades in the solar-system-transiting Pioneer 10 and 11 spacecraft. And as with those not-so-speedy neutrinos, the New York Times has sought to explain to the public what physicists wondered and how they reached resolution.
At the summary level, so has USA Today, where the 17 July article 'Pioneer anomaly confirmed as excess heat effect' reported that a 'new Physical Review Letters journal report led by Slava Turyshev of NASA's Jet Propulsion Laboratory finds [that a] slight pushback of thermal radiation, unaccounted for in initial analyses, explains the .00001 foot per hour slowing of the spacecraft, noted since 1980.'
USA Today also explained, 'Pioneer 10, launched in 1972, is now about 105 times farther from the sun than the Earth, and Pioneer 11, launched in 1973, is about 85 times as far. Both are headed out of the solar system.'
Now the Times, in its 24 July Science section, has offered Dennis Overbye's more comprehensive explanatory article 'Mystery Tug on Spacecraft Is Einstein's 'I Told You So'' It begins:
It's been a bad year to bet against Albert Einstein.
In the spring physicists had to withdraw a sensational report that the subatomic particles known as neutrinos were going faster than light, Einstein's cosmic speed limit; they discovered they had plugged in a cable wrong.
Now scientists from NASA's Jet Propulsion Laboratory have reported that they have explained one of the great mysteries of the space age, one that loomed for 30 years as a threat to the credibility of Einsteinian gravity.
Overbye asks rhetorically about the minute slowdown, 'Was there an unknown planet or asteroid out there tugging on the spacecraft? Was it drag from interplanetary gas or dust? Something weird about the spacecraft? Or was something wrong in our calculation of gravity out there in the dark?' He notes that the 'last explanation would have been big news indeed,' and that the story allowed 'the news media to ask their favorite science question: Was Einstein wrong?'
Here's the abstract of the now-popularized PRL article 'Support for the Thermal Origin of the Pioneer Anomaly' by Turyshev and his colleagues Viktor T. Toth, Gary Kinsella, Siu-Chun Lee, Shing M. Lok and Jordan Ellis:
We investigate the possibility that the anomalous acceleration of the Pioneer 10 and 11 spacecraft is due to the recoil force associated with an anisotropic emission of thermal radiation off the vehicles. To this end, relying on the project and spacecraft design documentation, we constructed a comprehensive finite-element thermal model of the two spacecraft. Then, we numerically solve thermal conduction and radiation equations using the actual flight telemetry as boundary conditions. We use the results of this model to evaluate the effect of the thermal recoil force on the Pioneer 10 spacecraft at various heliocentric distances. We found that the magnitude, temporal behavior, and direction of the resulting thermal acceleration are all similar to the properties of the observed anomaly. As a novel element of our investigation, we develop a parametrized model for the thermal recoil force and estimate the coefficients of this model independently from navigational Doppler data. We find no statistically significant difference between the two estimates and conclude that, once the thermal recoil force is properly accounted for, no anomalous acceleration remains.
Overbye explains how the investigators reconstructed what happened via a process that Turyshev calls 'space archaeology':
[Turyshev] and his colleagues had to scour NASA labs for old punch cards and magnetic tapes and for vintage devices that could read the data stored on them — then reformat all that data to a single modern standard.
Among other things, that meant ascertaining the positions of every antenna in NASA's Deep Space Network to an accuracy of one centimeter over all that time.
It took much longer than Dr. Turyshev had imagined, and he had to depend on money from the Planetary Society as well as from NASA to keep the project going, all the time aware that nobody was ever likely to retrace his footsteps. So he had better get it right.
'The more we learned, the less optimistic we became about new physics,' he said. It became apparent that the fault with the Pioneers' travels turned out to lie not in the stars or the shape of space-time but in the spacecraft themselves.
Near the end, Overbye answers that 'favorite science question' of the news media: 'Gravity did not need to be fixed. Einstein was right again.'
Steven T. Corneliussen, a media analyst for the American Institute of Physics, monitors three national newspapers, the weeklies Nature and Science, and occasionally other publications. He has published op-eds in the Washington Post and other newspapers, has written for NASA's history program, and is a science writer at a particle-accelerator laboratory.
