NASA/JPL · Public domain

Did real planetary science — the greenhouse warming of Venus, the seasonal changes on Mars, organic chemistry in the atmosphere of Titan — and then did something rarer, which was to explain it to an audience of hundreds of millions without condescension. Cosmos remains the most watched science series ever made. He co-authored the first serious model of nuclear winter, designed the plaques bolted to the Pioneer probes, and argued for planetary humility from the vantage point of a photograph of Earth as a single pale dot.