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Triggered-Lightning Avoidance for Space Launches

Shown is a photograph (below) of a lightning flash that was triggered 49 seconds after the 1987 launch of the Atlas/Centaur-67, resulting in total destruction of the vehicle and payload. Unfortunately, little quantitative data are available on the causes of triggered lightning. The most important physical parameter, the ambient electrostatic field aloft, cannot currently be measured from the ground. For these reasons, the triggered-lightning Launch-Commit Criteria, under which the national ranges now operate, comprise a set of purely meteorological rules. To decrease the high rate of false alarms due to these criteria, and to increase launch availability, the Air Force needs an airborne platform to measure the electric field aloft in support of launch operations, and a knowledge of the ambient-field conditions under which lightning can be triggered.

The Space Vehicles Directorate of the Air Force Research Laboratory at Hanscom Air Force Base, Massachusetts, developed a small sounding rocket to measure vertical profiles of the electric field immediately prior to lighting-triggering attempts using the well-developed “rocket-and-wire” technique. A field experiment was conducted in the summer of 1996, during which 15 pairs of sounding and triggering rockets were launched into thunderstorms, triggering nine lightning flashes. The development of the discharges was recorded both electromagnetically and optically, so that the phenomenology of the triggering process could be related to the available electrostatic energy.

Although data analysis from this experiment is still in progress, it is already clear that lightning can be triggered at fields aloft much smaller than previously estimated. First, the new results will be used to validate a French/Italian model of the triggering process. The model will then be applied to operationally relevant situations to determine safe bounds on the ambient fields for space launches. [1]

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"Triggered-Lightning Avoidance for Space Launches: Factsheet" Air Force Research Laboratory, Space Vehicles Directorate (1998).

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