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Europe’s Rosetta spacecraft flew past Earth three times, but experienced the flyby effect only once. Nobody knows why. (ESA/C. Carreau)

The Force Is With Them

What changes the speed of spacecraft flying by Earth?

Good science is built on consistency, great science on anomaly. Albert Einstein devised his theory of relativistic mechanics partly to explain a tiny but stubborn irregularity in the orbit of Mercury. And now, some equally subtle glitches in the trajectories of deep-space probes have some scientists hoping to overthrow Einstein.

In 1990, NASA’s Galileo spacecraft swung toward Earth to use our gravity for a speed boost on its way to Jupiter. Problem was, it picked up more speed than relativity predicted. The unexpected increase amounted to just tenths of an inch per second, and could easily be corrected later in the flight. But it persisted. A second Galileo approach in 1992 produced another anomaly, except this time the spacecraft lost momentum. Three more times in the next decade, other probes swung past Earth and emerged with the “wrong” speed—which got some ambitious scientists thinking. “Sure, it’s in the back of people’s minds,” says Frank Jordan, “that there may be something incomplete about the Einstein business.”

Jordan and his colleagues at NASA’s Jet Propulsion Laboratory in Pasadena, California, have studied the flyby anomalies since 1990; the only consistency in the velocity change is that it happens abruptly. The team discovered in 2007 that a spacecraft approaching Earth at a shallow angle relative to the equator and departing at a steeper angle got a boost, while a probe that did the opposite experienced drag. The only spacecraft to approach and leave at symmetric angles, the Messenger mission to Mercury, showed no effect.

Just when a pattern seemed to emerge, two subsequent asymmetric flybys showed no anomaly. Those passes occurred tens of thousands of miles higher above Earth than earlier oddball flybys, which might be a contributing factor. But Jordan admits he and his colleagues have made no more progress in resolving the mystery.

Other scientists have trotted out exotic explanations for the anomalies: changes in the speed of light, gravity waves longer than the universe, a hidden fifth dimension. Stephen Adler of the Institute for Advanced Study in Princeton, New Jersey, floated the idea of the probes colliding with webs of dark matter—shadow particles that barely interact with regular atoms. Adler concluded that dark matter could theoretically cause strange flybys, but only if it has properties much different than most physicists believe. He suspects the anomalies arise instead from subtle, overlooked forces in conventional physics; with so many spinning bodies flying around, the equations can get hairy. Nevertheless, he puts the odds of discovering new physics at one in three.

Skeptics think those odds are too generous. Slava Turyshev, a researcher with JPL’s astrophysics and gravitation group, recently helped dismantle the unrelated but similar “Pioneer anomaly.” Scientists first noticed that effect in the 1970s, when Pioneers 10 and 11 passed by the outer planets, then drifted out of the solar system. The sun’s gravity still pulls on the probes, so they are slowing down, but more so than they were originally expected to. Turyshev and colleagues have explained some of the deceleration by taking into account (among other things) the warming effect of onboard power sources on the spacecraft structure. Turyshev plans to scrutinize the flyby data in the coming year, and he expects that similarly overlooked effects, thermal or otherwise, will emerge. “I will be the last to believe in new physics,” he laughs.

Sam Kean is the author of The Disappearing Spoon: And Other True Tales of Madness, Love, and the History of the World from the Periodic Table of the Elements (Little, Brown, 2010).

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