PIONEER ANOMALY

One of the remarkable things about long-distance spaceflights is how accurately they can be modelled on a computer. Once clear of Earth, the main force acting on craft is gravity, which behaves in a highly predictable way. It came as a surprise when scientists found Pioneer 10 and 11 were slowing down more quickly than expected from the Sun’s gravitational pull. Some thought this indicated a problem with our understanding of gravity, but NASA traced the ‘Pioneer anomaly’ to an asymmetry in the way they radiated waste heat.