Whys and Wherefores of Lightning

The strike on Apollo 12 led to another study of lightning protection, this one focusing on the atmospheric conditions that might threaten a launch. At a meeting of the American Geophysical Union in December 1969, experts discussed the incident and offered NASA some observations. The scientists generally agreed that Apollo 12 triggered the lightning discharges. There were no other signs of lightning or thunder for six hours before and six hours after the launch. However, readings on electrical field meters in the Cape area indicated disturbed weather conditions. Apparently Apollo 12 had entered an electrical cloud and distorted the field sufficiently for breakdown to occur. The 110-meter space vehicle and its 500-meter ionized exhaust plume then formed an excellent conductor. The space vehicle had probably triggered a lightning stroke from an electrified cloud incapable of producing lightning on its own. Although the launch vehicle's design incorporated safeguards against electrical discharges, lightning could damage components in the spacecraft such as solid-state electronic devices. The Apollo 12 experience prompted NASA officials to reexamine the space vehicle and the weather criteria for a launch.19

The lightning investigation team opposed any modifications to the spacecraft. They recommended, instead, further launch restrictions to reduce the possibility of touching off another lightning strike. The new "severe weather restrictions" appeared in the launch rules for Apollo 13. The space vehicle would not be launched if the nominal flight path would carry the vehicle within 8 kilometers of a thunderstorm, through cold-front or squall-line clouds, or through cumulus clouds with tops at 3,050 meters or higher.20 The additional weather limitations would have a moderate effect on winter and spring launchings; in those seasons, high winds would more often cause delays. On a February afternoon, the probability of delay would increase from 10% to 18% with the new restrictions. In the summer, the probability of a scrub would jump from 3% to 18%. Despite the new rules, the odds for acceptable weather were still better than nine out of ten for most three-hour launch windows.21

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