Scientists have finally cracked a decades-old spaceflight riddle, figuring out why NASA’s Pioneer 10 and 11 probes began to slow mysteriously as they sped far from the sun. The cause of the so-called “Pioneer Anomaly,” it turns out, is heat coming from the electrical current flowing through the probes’ instrument and power systems. This heat pushed back on the spacecraft, causing them to decelerate slightly, according to a new study.

Pioneer 10 and Pioneer 11 launched in 1972 and 1973, respectively. They were the first spacecraft to fly through the main asteroid belt, and the first to study Jupiter up-close. The probes kept on cruising after their Jupiter encounters, speeding toward Saturn and beyond.