May 17 2008
Beyond the edge of the solar system, something has gradually dragged two of America's oldest space probes -- Pioneer 10 and Pioneer 11 -- a quarter-million miles off course. Astrophysicists have struggled 15 years in vain to identify the infinitesimal force at play. The Pioneer anomaly, as it is called, throws a monkey wrench into celestial mechanics.
Slava Turyshev may have found the answer in NASA's trash. Reconstructing decades of discarded spacecraft data, the Russian-born astrophysicist and the private space enthusiasts helping him say they believe they are on the verge of solving a mystery of time and gravity that has perplexed a generation of physicists and might have confounded Newton and Einstein.
The anomaly officially materialized in 1988, 16 years after NASA launched Pioneer 10 toward the outer planets. The 568-pound spacecraft had been designed to stay in radio contact with Earth just 21 months, time enough for it to become the first spacecraft to pass through the asteroid belt, the first to fly past Jupiter and the first to visit the outer solar system. The plutonium-powered probe, however, transmitted data 31 years until 2003.
As it sped through space, a specialist in radio-wave physics named John Anderson at NASA's Jet Propulsion Laboratory noticed an odd thing. The spacecraft was drifting off course. The discrepancy was less than a few hundred-millionths of an inch per second for every second of spaceflight, accumulating year after year across billions of miles. Then Pioneer 11, an identical probe escaping the solar system in the opposite direction, also started to veer off course at the same rate.
Ordinarily, such small deviations might be overlooked, but not by Dr. Anderson. He monitored the trajectories six years before calling attention to the matter. "I'm a little like an accountant," Dr. Anderson said. "We have Newton's theory and Einstein's theory, and when you apply them to something like this -- and it doesn't add up -- it bothers me."
Not everything in the solar system adds up, of course. The moon's actual orbit is off its calculated course by about six millimeters a year. No one knows why. The standard yardstick for length on an interplanetary scale, the Astronomical Unit, grows by about seven centimeters a year. Scientists have yet to agree on an explanation. At least four recent planetary probes experienced such unaccountable changes in velocity as they passed Earth, Dr. Anderson and his colleagues reported this past March in Physical Review Letters.
None prompted the scrutiny given the Pioneer anomaly. In hundreds of technical papers, Dr. Turyshev and scores of other space scientists considered and eliminated most mundane explanations, including fuel leaks, software bugs, mechanical flaws, navigation errors, fading plutonium power, planetary influences, the solar wind, even the effect of ocean tides and local plate tectonics on the placement of ground antennas. Others proposed more far-fetched scenarios: the tug of dark matter, the accelerating expansion of the universe or a breakdown of gravity's most fundamental laws.
Indeed, Dr. Turyshev at the Jet Propulsion Laboratory and his colleagues around the world regard the Pioneer probes as the largest test of Newton's law of gravity ever conducted. By that axiom, refined by Einstein, any two objects in the universe exert gravitational attraction on each other proportional to their mass and affected predictably by the distance between them.
"We would expect the two spacecraft to follow Newton's law of gravity," Dr. Turyshev said, "but they in fact fail to confirm Newton's law. If Newton is wrong, Einstein is wrong too."
For 14 years, Dr. Turyshev sought a simpler answer. He finally wondered whether heat radiating unevenly from the probe might be the cause but lacked enough information.
Then, at JPL in 2002, he discovered 400 computer tapes of Pioneer data gathering dust under a stairwell. In 2005, he intercepted 70 filing cabinets of Pioneer engineering data on their way to the junk heap at the NASA Ames Research Center, at Moffett Field, Calif. The computer files held all of the Pioneer mission data, but they were unreadable.
With no formal NASA funding, almost 6,000 members of The Planetary Society, a space-exploration advocacy group based in Pasadena, Calif., donated $220,000 to translate the antiquated data into a digital format that a modern computer can read. "This is not something that should be brushed away just because it is old data," said society Executive Director Louis Friedman. Victor Toth, a noted Canadian computer expert, donated his time.
After six years of work, the researchers expect to finish restoring the last data files next month. Based on a partial analysis, Dr. Turyshev reported in April at a meeting of the American Physical Society in St. Louis that at least 30% of the force can be attributed to heat radiating from the probe. "The rest is unknown," he said.
In the year ahead, Dr. Turyshev and his colleagues plan to use the vintage data to create a computer flight simulation of the two Pioneer missions with a precision never before possible. That may finally lay it to rest.
"There is some hope that this would show a new physics," Dr. Turyshev said. "With the Pioneers, we are exploring uncharted territory."
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