NASA launched Pioneer 10 and Pioneer 11 to explore the outer planets in 1972 and 1973, respectively. They were the first spacecraft to explore the asteroid belt and also the first to encounter Jupiter and Saturn. They paved the way for deep space missions continued by Voyager 1 and 2.
The Pioneers were designed with unique features that allowed them to be precisely tracked by Earth-based receiving stations. They were spin-stabilized and required very few course corrections that could perturb their motion.
The Beginning of an Anomaly
The Pioneers are regarded as the most precisely tracked and navigated space probes to date. However, the accuracy of measuring their acceleration has led to an unusual discovery. It was understood that Pioneer 10 and 11 would slow down because of the gravitational attraction from the Sun.
In 1980, a NASA's Jet Propulsion Laboratory researcher noticed that the probes were slowing down more than predicted data. This happened as the Pioneers had reached escape velocity from the Sun while preparing to leave the Solar System.
Both manifested a sunward acceleration of 8.74 ± 1.33 × 10-10 m/s2. The probes were still heading out of the Solar System, but a force acting on them pushed them toward the Sun and slowed their progress.
This figure seems like a very small difference in measured acceleration from predicted acceleration. However, it led the spacecraft to be thousands of kilometers closer to Earth per year than they should have been. Even such a slight sunward acceleration could suggest a possible anomaly against the principle of gravitation described by general relativity.
After several years, the mystery surrounding the Pioneer anomaly has deepened more. The effect first observed in 1990 persisted until NASA lost touch with Pioneer 11 in 1995. The findings were finally made public in 1998 by JPL senior research scientist John Anderson.
Since then, other space probes have demonstrated unexplained changes in speed. More significant than expected boosts in velocity were also observed when NASA's Galileo and NEAR spacecraft and ESA's Rosetta flew past Earth. The largest difference was recorded for NEAR, whose velocity changed 13 millimeters per second more than it should have. This excess is much larger than the expected errors in measurement.
Unveiling the Mystery
While this may sound trivial, the fact that both probes and other succeeding spacecraft went through this led some physicists to propose that something could be wrong with our current understanding of gravity, particularly Newton's gravitational inverse-square law. According to this law, the gravitational force between two objects is proportional to their masses and inversely proportional to the square distance between them. From this point of view, we should expect the influence of the Sun to weaken as we get further from it.
According to a study, "The Pioneer anomaly as acceleration of the clocks," by Antonio F. Rañada from Madrid, Spain, the acceleration is unrelated to any anomalous or unmodelled spacecraft motion. Instead, it is a manifestation of the universe's expansion, which increases the background potential. As a result, the increase accelerates the cosmological time and causes the anomaly.
Moreover, later probes did not encounter the same deceleration, indicating that another explanation is needed, which might likely have to do with the spacecraft themselves. After looking through the recovered Doppler data, a new study suggests that the anomalous acceleration of the Pioneer 10 and 11 probes is brought by the recoil force linked to an anisotropic emission of thermal radiation off the spacecraft. The study also concludes that once the thermal recoil force is properly accounted for, then no anomalous acceleration remains.
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