 Doomsday left the planet after killing Bertron and Krypton's natives found the remains of Bertron's lab, thus obtaining the knowledge of cloning.In the newer continuity, Superman also became aware of his alien heritage only sometime after his debut as a superhero - initially assuming himself to be a human mutated in some manner and launched as part of an Earth space program - when a holographic program encoded into the craft which brought him to Earth uploaded the information into his brain (although Lex Luthor had earlier discovered his alien heritage when his attempts to create a clone of Superman were complicated by the unexpected x-factor of Superman's alien DNA).In "Action Comics" #600 (May 1988), Krypton was close enough to Earth that the radiation from its explosion (traveling only at light speed) was able to reach Earth.