Pioneer Anomaly

The Pioneer anomaly or Pioneer effect is the observed deviation from predicted accelerations of the Pioneer 10 and Pioneer 11 spacecraft after they passed about 20 astronomical units (3×109 km; 2×109 mi) on their trajectories out of the Solar System. The apparent anomaly was a matter of tremendous interest for many years.

Both Pioneer spacecraft are escaping the Solar System, but are slowing under the influence of the Sun's gravity. Upon very close examination of navigational data, the spacecraft were found to be slowing slightly more than expected. The effect is an extremely small but unexplained acceleration towards the Sun, of 8.74±1.33×10−10 m/s2. The two spacecraft were launched in 1972 and 1973 and the anomalous acceleration was first noticed as early as 1980, but not seriously investigated until 1994. The last communication with either spacecraft was in 2003, but analysis of recorded data continues.

In 2011, papers reanalyzing the thermal radiation pressure forces inherent in the spacecraft showed that a careful accounting of this could account for the entire anomaly, and thus the cause was mundane and did not point to any new phenomena or need for a different physical paradigm. The most detailed analysis to date, by some of the original investigators, explicitly looks at two methods of estimating thermal forces, then states "We find no statistically significant difference between the two estimates and conclude that once the thermal recoil force is properly accounted for, no anomalous acceleration remains."