To not bury the lead: His team studied data the government made available from military missile tracking systems, which conveniently also false-positives on asteroids. There was one in 2014 which slammed into the Pacific Ocean at a speed at least twice that of the speed stars around us move relative to the sun, which makes it likely to be extra-solar. Based on that speed & how much burned upon entry, they concluded its material must be tougher than iron. They're planning an expedition to the oceanic area around Papa New Guinea, mostly funded through private donations, to recover the object.