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With the latest piece of the puzzle just published in a scientific journal, a solar system mystery that has perplexed people for more than 20 years has been solved, truly thanks to the support of Planetary Society members. That mystery is the "Pioneer Anomaly," an anomalous acceleration that affected the two Pioneer spacecraft as they left the solar system.
Second of all, because two sources of thermal radiation (heat) were then on the leading side of the spacecraft. The nuclear power sources, more formally Radioisotope Thermoelectric Generators (RTG), emitted heat towards the back side of the dishes. When the dishes reflected or re-radiated this heat, it went in the direction of travel of the spacecraft.
Also, the warm electronics box for the spacecraft was on the leading side of the spacecraft, causing more heat to spill that direction.