Donald F. Hornig was a top science adviser to three Commanders-in-Chief — Dwight Eisenhower, John F. Kennedy and Lyndon Johnson — and taught at Princeton and Harvard, in addition to serving a six-year stint as president of Brown University. But he achieved his greatest measure of fame as a young chemist a year out of graduate school, when he joined the Manhattan Project, the U.S. government’s top-secret effort during World War II to develop an atomic bomb.
Hornig, who died on Jan. 21 at age 92  in Providence, R.I., was such a gifted young scientist that, despite his inexperience, he rose to the level of team leader, supervising a group that developed the “X unit,” the mechanism that actually triggered the nuclear bomb. Here are five fascinating facts about the scientist who helped to end World War II and launch the nuclear age.
- In a 1995 Christian Science Monitor article , Hornig recalled that he was working as a physical chemist at the Woods Hole Oceanographic Institute in Massachusetts when he got a call from his old Harvard professor George Kistiakowsky, who said he needed Hornig’s help at a lab where he was working. When he agreed, Kistiakowsky told him to pack his bag immediately, and said no more. It wasn’t until Hornig got to Los Alamos, N.M., in May 1944, that he was told that he would be working on the effort to build an atomic bomb.
- Hornig made his most important contribution to the bomb after just a month on the job. In June 1944, J. Robert Oppenheimer, the project’s chief scientist, held a meeting to discuss a critical technical problem: how to trigger an implosion of the outer shell of the bomb’s plutonium, which would drive the fissionable material together and create the critical mass that would lead to a nuclear blast. Using explosives would destroy the firing mechanism, which would make testing difficult. Hornig suggested setting off electrical spark gaps, which would act as very precise switches that would trigger the process in a fraction of a microsecond. Oppenheimer was sufficiently impressed that he told Hornig to get some people together to work on the idea, and by October, the decision was made to use Hornig’s gadget.
- The first full-scale test of the bomb — scheduled for Monday, July 16, 1945, the day before President Truman’s crucial meeting with Winston Churchill and Josef Stalin at Potsdam, Germany — had high stakes, but it was no sure thing. “Many involved in the project doubted that the complicated implosion-bomb system would work effectively and with a large enough explosive yield,” Hornig recalled. The scientists’ nerves were rattled all the more when heavy clouds over the desert test area created a powerful electrical surge that might have set off the bomb prototype, had its trigger been set.
- On the evening before the test, Oppenheimer, who was worried about sabotage, assigned Hornig to go up in the bomb tower and keep watch over the nuclear device. Hornig climbed a 100-foot ladder to the top of the tower, with a paperback book to keep him occupied. It proved to be a hair-raising experience. The tower was battered by heavy rains from a thunderstorm, and Hornig anxiously counted the seconds between the thunder claps and lightning flashes. He recalled that he “tried not to think of what might happen if the tower got a direct hit and the gadget went off.” As a consolation, it occurred to him that if the bomb went off, “at least I would never know about it.”
- Just before daybreak, Hornig sat in a concrete bunker about five and a half miles away from the bomb tower, watching control panels. Just before 5:30 a.m., he saw a brilliant flash, and then — ignoring the possible risks — ran outside to get a better view. He saw “a great fireball rapidly rising, with peach, green, and rosy red colors, gradually transforming into a mushroom cloud.”
- Hornig, who was exhausted from going without sleep for 48 hours, had a subdued reaction to the event. “Boy, am I tired,” was his first thought, he later recalled. It took a moment for the significance to sink in. “Nobody knew where this would lead,” he later recalled. “But I had no regrets. If it ended the war without the tremendous casualties an invasion of Japan would cause, it was worth it.”
Here’s declassified film footage shot by the government of the historic bomb test: 
 died on Jan. 21 at age 92: http://news.providencejournal.com/breaking-news/2013/01/donald-f-hornig-dies-was-president-of-brown-university.html