I have read a book claiming that the speed of the Voyager probes do not fit neither the Newton's laws of motion nor Einstein's relativity theory. Is it a recognized scientific fact? Where can I find the relevant information?
Wikipedia article "Pioneer anomaly" seems to address this. The accepted answer to this slowing down of the spacecraft slightly more than predicted is that the deceleration was caused by "an anisotropic radiation pressure caused by the spacecraft's heat loss."
So the anomaly was caused by radiation pressure caused by heat being emitted from the spacecraft.
The is also an article here at Nature Physics