Natural and triggered lightning forecasts for space shuttle landings

Guidelines for operating the Space Shuttle are documented in the Shuttle Flight Rules. These flight rules contain weather constraints that are similar to the meteorological flight rules used in general aviation. Most individual components of the Shuttle's Orbiter are designed to withstand a 200 kA peak current from a lightning strike. However, the inability to predict the peak currents of lightning strokes and the uncertain effects of a lightning strike to the Orbiter as an entire system have resulted in Flight Rules that attempt to avoid natural or triggered lightning strikes to the vehicle. This paper describes the current Space Shuttle Flight Rules related to natural and triggered lightning. The tools and techniques used by the National Weather Service Spaceflight Meteorology Group (NWS SMG) at NASA Johnson Space Center to predict the lightning hazard are also discussed.

There are five weather conditions in the Flight Rules related to lightning avoidance: lightning, thunderstorms, precipitation, anvil clouds, and cumulus clouds produced by a fire. The Orbiter cannot pass within specified distances of these conditions. The distance may depend on the phase of the mission, the weather condition, and the age of the weather phenomena. These rules are written to avoid flying the orbiter in an environment with high electric fields and active lightning.

The primary tools used by SMG to evaluate the flight rules are lightning detection systems, weather radar, satellite, and weather reconnaissance. The National Lightning Detection Network (NLDN) provides coverage for all 3 primary US landing sites: Kennedy Space Center (KSC), Edwards Air Force Base, and White Sands Space Harbour. The KSC Lightning Detection and Range (LDAR) system, the USAF 45th Space Wing cloud-to-ground lightning sensor system, and a network of field mills also are used to evaluate weather conditions at Kennedy Space Center. The Sferics detection network operated by the United Kingdom's Meteorological Office and the Spanish National Institute of Meteorology cloud-to-ground lightning detection system provide coverage for Spain and northern Africa. These tools are fully integrated with conventional meteorological data using SMG software.

The use of these Flight Rules and forecasting tools will be demonstrated for two recent Space Shuttle launch attempts. The STS-93 launch attempt on 22 July 1999 was postponed due to thunderstorms and lightning occurring near the Shuttle Landing Facility. As an example of a triggered lightning concern, the STS-101 launch attempt in December 1999 was postponed due to low cloud ceilings and precipitation although no lightning was observed in the area. Interestingly, the field mill network at Cape Canaveral Air Force Station/KSC measured high electric fields and the pilot flying weather reconnaissance reported corona discharges on the aircraft.