NASA probes seemed to defy gravity, until the mystery force turned out to be heat
From the 1980s, tracking data showed Pioneer 10 and 11 drifting slightly off their predicted paths beyond the outer planets, decelerating a touch more than gravity alone explained. The 'Pioneer anomaly' fueled two decades of speculation about new physics. In 2012, a detailed thermal model showed the probes' own waste heat was radiating slightly asymmetrically off their structure, producing just enough recoil thrust to account for the entire effect.