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

Pioneer 10 Flies By Jupiter NASA

Thirty years ago, NASA scientists noticed that two of their spacecraft, Pioneer 10 and Pioneer 11, were veering off course slightly, as if subject to a mysterious, unknown force. In 1998, the wider scientific community got wind of that veering—termed the Pioneer anomaly—and took aim at it with incessant, mind-blowingly detailed scrutiny that has since raised it to the physics equivalent of cult status. Now, though, after spawning close to 1000 academic papers, numerous international conferences, and many entire scientific careers, this beloved cosmic mystery may be on its way out.

Slava Turyshev, a scientist at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and Viktor Toth, a Canada-based software developer, plan to publish the results of their strikingly comprehensive new analysis of the Pioneer anomaly in the next few months. Their work is likely to bring a conclusion to one of the longest and most tumultuous detective stories of modern astrophysics.

NASA launched Pioneer 10 in the spring of 1972 and Pioneer 11 one year later. The spacecraft's joint mission was to gather information about the asteroid belt, Jupiter, Saturn (in the case of Pioneer 11), and their moons. As they hurtled past those various celestial objects, the probes measured previously unknown properties of their atmospheres and surfaces; they also photographed Jupiter’s Red Spot and Saturn’s rings up close for the first time. Then, after completing their "flyby" missions in the mid-1970s, the Pioneers kept going. Carrying identical plaques depicting a man and a woman, the atomic transition of hydrogen, and the location of our planet within the galaxy—a message to aliens—the probes became the first manmade objects ever to plunge beyond the solar system into the inconceivable cold and dark of interstellar space.

Photograph of Saturn Taken by Pioneer 11:

JPL scientists continued Doppler tracking the Pioneers far into deep space. They sent and received a continuous stream of radio transmissions to and from both Pioneers, logging the velocity of each everywhere along its trajectory. An astronomer named John Anderson led the analysis of the Doppler ranging data. He and his team intended to use the data to study subtle gravitational effects in the outer solar system, far from the overwhelming influences of the sun and larger planets. It was thought, for example, that the Pioneers might oscillate in tune with low-frequency gravity waves.

Of course, in order to detect such curiosities in the motion of the spacecraft, the scientists needed to know exactly what to expect in the first place; this required the construction of an algorithm of truly staggering complexity. Contributing factors to the predicted Doppler shift included: the deceleration experienced by the Pioneers as they struggled against the gravitational pull of the sun, planets, moons, asteroid belts, and comet clouds, the positions and thus gravitational fields of which move constantly; the tiny push on the spacecraft by the sun's radiation, which weakened with time as the spacecraft moved progressively farther away, and also changed as the angle of the spacecraft changed; the increase in the delay time between the bounce of a radio wave and its reception back at Earth as the spacecraft grew more distant; the gravitational drag on the radio waves from the sun; the additional frequency shift in the radio transmissions caused by the rotation of the Earth… and the list goes on. Anderson synthesized that headache's worth of cosmic influences into a single algorithm. But unfortunately it didn't seem to work.

In 1980, he noticed a small discrepancy between the Doppler shifts he expected to receive based on his algorithm and the actual, measured shifts of the radio signals coming from the spacecraft. Their expected and actual motions weren't quite matching up. As they moved outward against the gravitational pull of the sun and planets, the spacecraft were, of course, slowing down. But the problem was they were slowing down too much. Each year, both of the spacecraft were a few hundred miles farther behind where they should have been on their respective paths, according to the algorithm. That isn't much in the context of space travel, to be sure, but it isn't trivial either. The constant, extra acceleration amounted to 8.74 x 10-10 m/s2 directed toward the sun– a factor ten billion times smaller than the acceleration due to gravity, but still, undeniably, there.

Anderson's first reaction was to think his algorithm must have been missing something. Some tiny influence on the motion of the spacecraft must not have made it into the mathematical mix. A few years of thinking and discussing led him and his immediate team to the conclusion that the anomalous acceleration must have been caused by “outgasing” - fuel dripping from the thrusters, exerting a recoil force against the spacecraft as it dripped. Since by that point the craft were cruising through interstellar space without propulsion, the scientists thought the fuel drops would soon finish dripping and the effect would go away. But perplexingly, it didn't: Over the next decade, the spacecraft racked up billions of frequent flier miles--but thousands less than they should have.

In 1994, Anderson received an email out of the blue from Michael Martin Nieto, a cosmologist at Los Alamos National Laboratory near Santa Fe, NM. Nieto had lately become interested in alternatives to Newton’s inverse square law for gravity, including a new theory called MOND (modified Newtonian dynamics), and so he contacted Anderson to find out how sure NASA was about the strength of gravity based on their observations of the motions of spacecraft. Anderson replied that, as a matter of fact, gravity didn't seem to be working right for the Pioneers.

When Nieto read the exact value of the small, anomalous acceleration experienced by Pioneers 10 and 11, he almost fell off his chair. (In typical physics-speak: “My office had a hard floor and my computer chair had wheels, so when I arched my back in a "wha?" reaction the chair started rolling.”) There was a profound cosmic coincidence afoot: As Nieto immediately noticed, the value of the Pioneer anomaly almost exactly equaled the so-called “cosmic acceleration”—the speed of light ‘c’ multiplied by the Hubble constant ‘H'—suggesting the anomaly's cause lay within the foundation of physics.

Right then and there, Nieto signed on to work with Anderson at JPL, got a major investigation of the Pioneer anomaly off the ground, and has spent most of his energy studying it ever since. Why? “The Pioneer anomaly could be the first evidence that gravity deviates from an inverse-square dependence,” he said recently. “It could be huge.”

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