One of the most powerful heroes in Astro City's history, though his time there wasn't very long.
The entity that became the super-human Atomicus was formed in the Atomic Research Lab at Fort Kanigher in April of 1961. His first act of heroism was to rescue Dr. Vasily Petrov, a Soviet defector. (Soviet agents were attempting to return Dr. Petrov to Russia.) The hero was dubbed Atomicus by Elliot Mills, a reporter for the Astro City Rocket, on April 7th, 1961.
There were many thoughts as to how he came into being; one thought was that he was a disembodied alien intelligence, who was trapped by the reactor energies, which drew him into human form. While others speculated that he was an air force pilot thought vaporized in an atomic test and reborn through atomic power. Some just thought he might be an ancient spirit brought back to life.
Atomicus' alter ego, Adam Peterson, started working for the staff of Astro City's mayor, Morton (first name unknown), shortly thereafter. Also working for the staff was Irene Merryweather, a mayoral aide. Atomicus and Merryweather were known to be a couple before Peterson's appearance. At the time, it was still not known that Peterson was Atomicus. Meriwether soon suspected the connection, however, and went about mercilessly attempting to prove that Peterson and Atomicus were one in the same. When she finally succeeded, the furious Atomicus left the Earth, never to be seen again.
In his Adam Peterson disguise, he wears the Atomicus costume underneath his clothes, but in a different form, flatter and simplified. He transforms it into the regular costume through some application of his powers.
It has been confirmed by several galaxy travelling heroes, including the First Family, that Atomicus is still alive, pursuing his life-path out in space. No further details are available, and this information is now over a decade old.
According to Kurt Busiek, Atomicus' storyline was inspired by an image Alex Ross drew of a Superman-esque character finally getting fed up with his love interest constantly trying to expose his secret identity.