Britain tested its first atomic bomb in 1952 and exploded its first hydrogen bomb in 1957. Operation Grapple saw Valiant bombers testing the dropping of hydrogen bombs over Christmas Island. Advances in electronic countermeasures were also applied to the V bombers over the same period and the remaining V bombers came into service in the late 1950s. During the Cuban Missile Crisis of October 1962, Bomber Command aircraft maintained continuous strip alerts, ready to take off at a moment's notice, and the Thor missiles were maintained at advanced readiness. The Prime Minister did not disperse Bomber Command aircraft to satellite airfields, lest that be viewed as an aggressive step.