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.”
The researchers, using data recovered from recently discovered Pioneer records and funded by sources outside of NASA, have figured out part of the problem but the rest remains a mystery.