Groves told James Chadwick, the British liaison to Manhattan Project in Washington, D.C., that Penney was too important to the project to be released. On 27 April 1945 Penney went to Washington for a target selection meeting. He gave advice regarding the height of the detonation which would ensure optimum destructive effects, whilst ensuring the fireball did not touch the earth, thereby avoiding permanent radiation contamination on the ground. The committee selected four cities from a list of seventeen. Penney attempted to forecast casualties and damage effects, but this was difficult because the exact energy of the bombs was not known. This was answered by the Trinity test detonation on 16 July 1945. Penney was assigned as an observer on an aircraft, but the flight was cancelled due to bad weather, and he did not witness the test. Five days later, Penney gave a presentation on the results of the test, during which he predicted that the bomb would level a city of three or four hundred thousand people. The following month Penney went to Tinian Island as part of Project Alberta, the group of scientists and military personnel that assembled the atomic bombs. Along with Royal Air Force (RAF) Group Captain Leonard Cheshire from the British Joint Staff Mission in Washington, he represented the United Kingdom. The American authorities stopped them observing the bombing of Hiroshima but, after an appeal to Chadwick, they were permitted to accompany the second mission. On 9 August 1945 Penney witnessed the bombing of Nagasaki, flying with Cheshire in the B-29 observation plane Big Stink. Big Stink missed its rendezvous with the bomber Bockscar, so they witnessed the flash of the Nagasaki detonation from the air at too great a distance to photograph the fireball and the target was obscured by clouds. As the leading expert on the effects of explosions, Penney was a member of the team of scientists and military analysts who entered Hiroshima and Nagasaki following the Japanese surrender on 15 August 1945 to assess the effects of nuclear weapons. Penney returned to the United Kingdom on a civilian flight from the United States in September 1945. He brought artefacts from Hiroshima and Nagasaki with him, and was charged for excess baggage. He married Joan Quennell on 3 November 1945. Penney returned to Imperial College, where he wrote a report on Hiroshima and Nagasaki. He estimated that the bomb dropped on