The Pioneer 10 and 11 spacecraft have experienced an unexplained drag that has caused them to travel slower than predicted, a phenomenon known as the Pioneer anomaly. Attempts to explain it using normal physics have failed, which leads people to speculate that it will require brand-new physical theories to explain. I personally hope that this is like the discovery of radioactivity by Roentgen in 1895 — a first initial glimpse into a new world.