The ‘Pioneer anomaly’ that threatened to upend physics

Why tiny discrepancies have the power to unleash huge scientific change

NASA Ames Research Center

In 1972 and 1973, two unmanned spacecraft — called Pioneer 10 and 11 — were launched on missions to the outer reaches of the solar system. The probes sent beautiful images of Jupiter and Saturn back to Earth, along with exciting new data about their makeup. And then the scientists monitoring the crafts sat back and relaxed as the ships began their long, lonely journeys into deep space.

Until something weird turned up in the data.

Around 1980, John Anderson, a physicist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., noticed that the spacecraft were not as far away from Earth as they should have been. After careful measurements, he and his colleagues determined that both probes were slowing down, as though some tiny force were pushing them toward the sun. NASA had extremely detailed models of the solar system and of its deep space probes. And yet none of these could account for the deviation. The discrepancy came to be known as the Pioneer anomaly.


The deceleration Anderson observed was tiny, less than a nanometer per second squared. But it was extremely nettlesome to physics, a field that relies on being able to generate very accurate predictions about the universe. Over the ensuing decades, physicists have proposed any number of explanations for the slowdown, and even radical potential revisions to basic cosmology—including, in some cases, rejecting Einstein’s theory of gravitation—in attempts to explain this minute disagreement between theory and observation.

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It may seem strange that such a small discrepancy could prompt physicists to reconsider a century of established science. But in that sense, the Pioneer anomaly offers an unusually clear view of how big changes in science often work.

This summer marks the 50th anniversary of the publication of “The Structure of Scientific Revolutions,” in which the groundbreaking science historian Thomas Kuhn famously introduced the notion of a “paradigm shift” to describe the dramatic change in worldview that he claimed accompanied revolutionary periods in the history of science.

Before Kuhn, most people who wrote on such things took for granted that the history of science is essentially progressive, a steady march toward knowledge. Kuhn turned this picture on its head. He argued that actually the history of science is discontinuous, marked by periods when well established theories are rejected entirely, replaced by alternatives bearing so little resemblance to what came before that in some cases it would be impossible to even translate between new theories and old.

Central to Kuhn’s account of scientific change are tiny anomalies much like the Pioneer problem. Minor anomalies might normally be dismissed as irrelevant to big theories, but over time, as these anomalies accumulate, some scientists will come to see them as important. This can lead to a period of crisis, during which previously sacrosanct assumptions are questioned and revised. The result could be revolution.


One such anomaly was recognized in 1859, when a French physicist noticed that the motion of the planet Mercury deviated very slightly from theoretical predictions. The difference went unexplained until 1915, when Albert Einstein introduced his new theory of gravity, which exploded our previous understanding of the cosmos but accounted perfectly for Mercury’s deviation. Another such anomaly concerned how the temperature of physical systems known as “black bodies” affects the radiation they emit. In 1900, the German physicist Max Planck noted a slight discrepancy between his new mathematical model of the situation and the classical prediction of what should happen. Planck himself thought the discrepancy was of little significance, but over the next 25 years, others came to see that it called for a drastic revision of classical physics. And so quantum theory was born.

Physics has been waiting to see if the Pioneer anomaly might augur a similar revolution, and this summer the wait appears to have come to an end—perhaps a disappointing one, to those hoping for something radical. A team of Anderson’s colleagues at the Jet Propulsion Laboratory published a paper in Physical Review Letters in which they convincingly demonstrated that some electronic components on the ships were producing small amounts of heat, and the force imparted to the ships by this heat was just enough to produce the observed deceleration. “I think it is solved for good,” says physicist Slava Turyshev, lead author on the report.

If they’re right, it’s time to put the Pioneer anomaly to bed. Sometimes, perhaps most of the time, small deviations that seem insignificant really are insignificant, even if it takes years, or perhaps decades, to understand why. But physics still has plenty of intriguing discrepancies to worry about. For instance, according to theory, certain subatomic particles known as neutrinos ought to weigh nothing. But experiments over the last decade indicate that they may have tiny masses after all. Meanwhile, it seems that satellites experience a slight unexplained acceleration when they pass near Earth. And there are others. So perhaps a crisis is impending after all—or perhaps it has already arrived, and we won’t know until revolution is upon us.

James Owen Weatherall is an assistant professor of logic and philosophy of science at the University of California, Irvine. His book, “The Physics of Wall Street: A Brief History of Predicting the Unpredictable,” will be published in January 2013 by Houghton Mifflin Harcourt.

Correction: Because of a reporting error, an earlier version of this article misidentified the satellites whose slowing velocity long posed a problem for physicists. They are Pioneer 10 and 11.