PHILADELPHIA -- On the day that the United States stunned the world by dropping an atomic bomb on Hiroshima, a young Black man from the Main Line area of Philadelphia died in obscurity in a remote Oregon forest, the end of a brief military career that became enmeshed in some of the most significant currents of U.S. and world history. The story of paratrooper Malvin L. Brown was a saga of racial prejudice, secrecy, a cosmic discovery in the annals of atmospheric science, tragedy, and, ultimately, a 70-year mystery. Brown has just received a high honor posthumously: The government officially has...