The ‘Pioneer anomaly’ that threatened to upend physics

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Why tiny discrepancies have the power to unleash huge scientific change
By James Owen Weatherall
In 1972 and 1973, two unmanned spacecraft—called Pioneer I and II—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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Written by physicsgg

September 2, 2012 at 9:53 am


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