isolationist and forbade space exploration and communication with other worlds. The young scientist Jor-El was born into this world. By his adult years, a mysterious "Green Plague" was killing Kryptonians by the thousands, and upon researching the matter, Jor-El discovered that its cause was growing radiation produced by Krypton's increasingly unstable core. This process was going to cause the planet to explode. Unable to convince his associates to abandon tradition and consider escape, and reasoning that modern Kryptonian society had grown cold, unfeeling and sterile, Jor-El removed the Eradicator's planetary binding genes from his unborn son Kal-El's genetic pattern, took Kal-El's birthing matrix and attached a prototype interstellar propulsion system to the vessel. Just as the planet began to shake apart and massive, exploding streams of green energy erupted through the surface of Krypton, Jor-El launched the matrix towards Earth, where it would open and give birth to the infant upon landing (the Post-Crisis Superman therefore was considered to be technically "born" on Earth). Jor-El was not only determined that his son would survive the death of his birthworld, but that he would grow up on a world that vibrantly embraced living, as his forebears once did. The Last Son of Krypton A central theme of this version of the Superman mythos was that the character was to remain the last surviving remnant of Krypton. Thus, Silver Age elements such as Supergirl, Krypto, Beppo, and Kandor had never existed in this version (though Post-Crisis versions of these elements were eventually reintroduced). The supervillain Doomsday was revealed in the 1990s as a being genetically engineered by Bertron, an alien scientist, on an ancient Krypton. 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). Revisiting Krypton In Action Comics #600 (May 1988), Krypton was close enough to Earth that the