'Oumuamua

The first interstellar visitor accelerated on its way out. Harvard's chair said: sail.

The first confirmed interstellar object (October 2017): no coma, tumbling, an extreme aspect ratio (10:1, unlike any known asteroid), and — the detail that broke ranks — non-gravitational acceleration on exit with no detected outgassing. Avi Loeb, chair of Harvard astronomy, published the lightsail hypothesis in a peer-reviewed letter and a bestselling book, at real professional cost. Conventional models (hydrogen ice, nitrogen shard) each require their own exotica. It connects the Fermi file: the first visitor arrived, said nothing, accelerated, and left — which is either rock physics or exactly what a probe looks like.