The Pioneer Anomaly
The Pioneer 10 & Pioneer 11 spacecrafts have managed to venture farther into space than most objects we’ve launched from Earth. Yet it seems that they have a lot of periods where there is unexplained deceleration when passing 20 astronomical units. This would get a spacecraft just outside of nearly outside of our solar system. This has caused many to propose some exotic possibilities, which include possible alien technology. It has been one of our most notable cosmic mysteries for a long time now, but perhaps not for long. Oddly, each Pioneer spacecraft were going to exit our solar system yet slow down under the influence of our Sun’s gravity. When slowing down more than expected, scientists knew something was odd.
In 1980, we initially saw an anomalous acceleration but this would change. Most of the proposals for deceleration have to do with the spacecraft while others reference gravitation. Neither held up entirely. One theory proposed did seem to have some acceptance for acceleration. Since each spacecraft is surrounded by an ultra-high vacuum powered by a radioisotopic thermoelectric generator or RTG, both can only shed heat via thermal radiation. Due to this, it is possible more heat is emitted in a particular direction causing radiative anisotropy. If this is the case, they’d accelerate slightly in the direction opposite of the excess emitted radiation due to the recoil of thermal photons. While this explains acceleration changes but it does not seem to fully explain the deceleration.