The Pioneer Anomaly, a 30-Year-Old Cosmic Mystery, May Be Resolved At Last

What is the mystery force slowing down the Pioneer spacecraft? Do we finally know the answer?

By Natalie Wolchover Posted 12.15.2010 at 12:00 pm 48 Comments

Gold Plaques Carried By Pioneer Spacecraft:

Slava Turyshev, who had just arrived at JPL from Moscow, soon joined Anderson and Nieto. Together with three others they launched a detailed investigation of all the available Pioneer Doppler data. At the same time they also checked out data from other missions, and found tentative evidence of anomalies in the trajectories of Ulysses and Galileo as well. [NOTE: Only the Pioneers, Ulysses and Galileo float freely. All other NASA spacecraft are three-axis stabilized: They exert gas thrusts in three directions to keep themselves on track, and these corrections erase any small divergences in motion. All four of the non-three-axis-stabilized craft seemed to the JPL scientists to be diverging.] In 1998 they announced their findings to the world in a can-of-worms-opening article in Physical Review Letters.

Mayhem followed. “1998 was a very interesting year, because right when we made the Pioneer anomaly known, there was the discovery of dark energy,” Turyshev explained. “So basically we realized that the universe accelerates because of dark energy, and people were ecstatic, saying, ‘look, we see something very exciting in the solar system, and maybe we need to modify gravity and all of that will go away, and Einstein and Newton will be, you know, dethroned.’”

Small anomalies in celestial motion have led to upheavals in physics before, after all. It was the famous “anomalous” precession of the perihelion of Mercury that helped prove Einstein's theory of general relativity in 1915. In 1998, comparisons to that historical precedent undoubtedly raised the profile of the Pioneer anomaly, as well as the profiles of many “outsider” theories that tried to account for it. Of these, one example is the aforementioned MOND, which posits that gravity does not follow an inverse square law at great distances from a massive body, but something slightly different. Hundreds of physics papers published since 1998 assert that MOND offers a viable explanation for the Pioneer anomaly.

Hundreds more hypothesize that the presence of huge quantities of as-yet-undetected “dark matter” pervading galaxies’ outskirts might be exerting a frictional drag force on the Pioneer spacecraft and slowing them down.

Yet another horde of physicists picked up on the coincidence Nieto had noticed four years earlier, when he almost fell out of his chair. In the context of the contemporaneous discovery of dark energy and the accelerated expansion of the universe, it seemed highly significant that the value of the Pioneer anomaly equaled that of the cosmic acceleration. Perhaps, scientists thought, NASA had been measuring the cosmic expansion of space for years!

More conservative physicists pointed out quite rightly that if dark matter or MOND were causing the anomalous acceleration of the spacecraft, then they ought to affect the motion of the outermost planets as well, yet nothing of the kind is observed. In their opinion the Pioneer anomaly was either nonexistent (i.e. the JPL scientists had misinterpreted the Doppler data) or it derived from plain old equipment malfunctions. The most likely culprit was deemed to be heat emission on board the spacecraft.

A Thermal Anomaly?

The plutonium inside the Pioneer’s generators gave off 2,500 joules of thermal energy per second at the height of its powers. Some of that heat got converted into electricity and ran the instrumentation. The rest simply radiated into space. If for whatever mechanical reason the heat radiated out from the generators unevenly, the extra heat radiating in one direction would exert an unbalanced recoil force, causing the spacecraft to accelerate. In fact, as the physics community was quick to point out, just five percent more heat radiating in one direction than the other would cause a recoil force large enough to account for the Pioneer anomaly.

Duly noting that point, the JPL team spent the next few years investigating all heat-related evidence. They came back with their verdict in 2002. Heat: not guilty. For one thing, they said, as the plutonium inside the generators decayed, the heat they gave off decreased, and so if heat were its cause then the anomalous acceleration of the spacecraft ought to have lessened with time as well. But it didn't – it seemed constant. Secondly, the generators were positioned quite far from the body of the spacecraft on the ends of long poles. From that remote distance, they calculated that very little heat would hit the spacecraft and exert a recoil force – an order of magnitude too little to cause the observed effect. Third and fourth, there was the tentative evidence offered by Galileo and Ulysses, both of which employed quite different power systems from that of the Pioneers.

Their arguments persuaded hordes of physicists, who began vying with great gusto for the thrones of Einstein and his non-relativistic assistant Newton. At conferences and meetings in Germany, Switzerland, the United States, and elsewhere around the world, they pronounced from the podiums such theories as: The anomaly really reflects a cosmic acceleration of time itself! The anomaly shows that Riemannian geometry, from which Einstein’s spacetime is built, is incomplete! We have discovered a new force! The solar system is expanding! The solar system is a hologram! String theory dimensions are tugging on the spacecraft! The lively joust of ideas—or as Viktor Toth describes it, “wild speculation”—has not died down since.

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