Some in the United States during the early 1960s pointed out that although all of the individual components of nuclear missiles had been tested separately (warheads, navigation systems, rockets), it was infeasible to test them all combined. Critics charged that it was not really known how a warhead would react to the gravity forces and temperature differences encountered in the upper atmosphere and outer space, and Kennedy was unwilling to run a test of an ICBM with a live warhead. The closest event to an actual test was 1962's Operation Frigate Bird, in which the submarine launched a Polaris A2 missile over 1,000 miles to the nuclear test site at Christmas Island. It was challenged by, among others, Curtis LeMay, who put missile accuracy into doubt to encourage the development of new bombers. Other critics pointed out that it was a single test which could be an anomaly; that it was a lower-altitude SLBM and therefore was subject to different conditions than an ICBM; and that significant modifications had been made to its warhead before testing.