An anonymous reader sends word that NASA scientists using the Relativistic Electron Proton Telescope (REPT) about the Van Allen Probes have discovered a third radiation belt surrounding Earth. Scientists have been aware of the Van Allen radiation belts since the 1950s, but it was thought that there were only two of them. The probes were sent up to simply map the belts in fine detail; the discovery of a third belt was a complete surprise. Deputy mission scientist Shri Kanekal said, "By the fifth day REPT was on, we could plot out our observations and watch the formation of a third radiation belt. We started wondering if there was something wrong with our instruments. We checked everything, but there was nothing wrong with them. The third belt persisted beautifully, day after day, week after week, for four weeks." Part of the reason they caught a glimpse of this belt was that they turned the REPT on early, so it would overlap with another probe that had reached end-of-life and was about to de-orbit. If they hadn't decided to do so, or if the REPT hadn't worked perfectly, we still might be in the dark about a third Van Allen belt.