169 degrees, 38 minutes West longitude (30 km, or about 18 mi, southwest of Johnston Island). The actual weapon yield was very close to the design yield, which has been described by various sources at different values in the very narrow range of 1.4 to 1.45 megatons (6.0 PJ). The Thor missile carrying the Starfish Prime warhead actually reached an apogee (maximum height) of about 1100 km (just over 680 miles), and the warhead was detonated on its downward trajectory when it had fallen to the programmed altitude of . The nuclear warhead detonated at 13 minutes and 41 seconds after liftoff of the Thor missile. Starfish Prime caused an electromagnetic pulse (EMP) which was far larger than expected, so much larger that it drove much of the instrumentation off scale, causing great difficulty in getting accurate measurements. The Starfish Prime electromagnetic pulse also made those effects known to the public by causing electrical damage in Hawaii, about away from the detonation point, knocking out about 300 streetlights, setting off numerous burglar alarms and damaging a telephone company microwave link (the detonation time was nine seconds after 11 p.m. in Hawaii). A total of 27 sounding rockets were launched from Johnston Island to obtain experimental data from the shot, with the first of the support rockets being launched 2 hours and 45 minutes before the launch of the Thor missile carrying the nuclear warhead. Most of these smaller instrumentation rockets were launched just after the time of the launch of the main Thor missile carrying the warhead. In addition, a large number of rocket-borne instruments were launched from a firing area at Barking Sands, Kauai, in the Hawaiian Islands. A very large number of United States military ships and aircraft were operating in support of Starfish Prime in the Johnston Island area and across the nearby North Pacific region, including the primary instrumentation ship USAS American Mariner providing measurements conducted by personnel provided by RCA Service Company and Barnes Engineering Company. A few military ships and aircraft were also positioned in the southern conjugate region for the test, which was near the Samoan Islands. In addition, an uninvited observation ship from the Soviet Union was stationed near Johnston Island for the test and another Soviet scientific expeditionary ship was located in the southern conjugate region, permanent features of all future oceanic nuclear testing. After the Starfish Prime detonation, bright auroras