Lightning and Launches
Excitement mounts as the Space Shuttle's launch day grows closer. Astronauts prepare for liftoff, and the ground crew anticipates its duties. Then, a frustrating announcement: the launch is delayed because of bad weather.
Image to right: Thunder and lightning can cause delays in launch schedules
A hurricane delay is easy to understand, but why would a few clouds or some light rain stop a launch into space? Is poor visibility the reason? Is NASA overly cautious? Weather is the single greatest cause for launch delays and scrubs, and about 30 percent of weather delays are related to lightning avoidance rules.
If the Shuttle were to be struck by lightning, the most likely consequence would be short circuiting of the electronics, including guidance and navigation systems, says John Madura, manager of Kennedy Space Center's Weather Office. The vehicle could veer out of control; if the self-destruct mechanisms were also disabled, the Shuttle could crash into populated areas. Even if the Shuttle itself were unharmed by the lightning strike, the payload electronics -- equipment and scientific experiments -- could be adversely affected.
Postponing a launch because of a thunderstorm makes sense. What about a merely cloudy day, or a light sprinkling of rain? Even when there's no thunder, rain, or lightning present, the risk of lightning still exists, but it's a different type of lightning than meteorologists worry about.
A launch vehicle and its plume ascending through clouds can trigger lightning at lower electrical fields than required for natural lightning. That's because the vehicle and the plume act as conductors and decrease the electrical field strength necessary to create a lightning flash. "That can create what we call triggered lightning," Madura says. "The lightning is triggered by the vehicle passing through the moderate electrical field in clouds of sufficient thickness near the freezing level."
Image to left: Lightning strikes the launchpad of Space Shuttle Challenger prior to the launch of Mission STS-8
Credit: Sam Walton
The phenomenon of triggered lightning was experienced in 1969 when Apollo 12 launched through clouds that produced a weak electrical field. The spacecraft triggered two lightning strikes, even though no natural lightning was present. Adequate backup systems allowed the flight to proceed without disaster, but that mission brought increased attention to the problems of natural and triggered lightning.
In 1987, an Atlas-Centaur rocket and its payload were destroyed when the launch of the vehicle triggered lightning. That mishap led to the formation of the Lightning Advisory Panel. The panel then developed rules, used today by the U.S. space program, that determines when a launch must be scrubbed because of weather conditions.
Because it's so difficult to directly measure the charge in clouds during a launch countdown, meteorologists rely on weather data collected from radar, aircraft and meteorological balloons, Madura says. These instruments measure cloud thickness, reflectivity, and temperatures.
Weather experts pay particular attention to the height and thickness of clouds located near the freezing level. A significant portion of electrical charge is contained in that area where water droplets in clouds turn to ice. Electrical charge may be present in clouds greater than 4,500 feet thick and with temperatures between 0 and -20 degrees Celsius (32 and -4 degrees Fahrenheit).
To supplement the gathered data, a network of field mills was installed around the launch area. The mills measure electrical fields near the surface of the Earth. If any of the mills within 5 nautical miles (NM) of the launch pad registers 1,000 volts per meter or greater, a launch is postponed.
Besides thick clouds and the potential for triggered lightning, thunderstorms in and of themselves are monitored and must be an acceptable distance from the launch site.
"Thunderstorms have natural lightning present," says Madura. "The risk is obvious, and those highly charged clouds take a while to dissipate. When a thunderstorm explodes, the air rises until it becomes stable, and then it spreads out. The top part of this storm cloud is called the anvil. Our rules say we can never launch within 10 NM of an anvil because it may be heavily charged."
Once a thunderstorm has dissipated, there are debris clouds that also require attention, Madura says. A debris cloud is one that has become detached from the main cloud or thunderstorm and, thus, may still contain electrical charge. Spacecraft cannot launch into or near newly formed debris clouds.
Precipitation is also dangerous to the Space Shuttle. The white thermal protection blankets that cover much of the outer surface of the Orbiter are a cloth-like material that absorbs moisture. If the blanket gets wet and stays wet in space, the moisture could freeze at high altitudes. That could cause the insulation tiles to buckle, and when the Shuttle re-enters the Earth's atmosphere, the crew and spacecraft would not be fully protected from the intense heat.
Airplanes often trigger lightning when in flight, Madura notes. Airplanes are designed to give lightning a conductive path away from sensitive electronics, and there is no long exhaust plume extending behind the craft. The plume significantly magnifies the electrical charges.
Postponing a launch costs about $300,000, and can also delay other launches that are dependent on the launch. NASA meteorologists are studying weather patterns and cloud characteristics hoping to better understand the physics behind lightning. Because so little is known about triggered lightning, launch delays are now made very conservatively. As weather scientists gain more understanding, they will be better able to predict which conditions rule out a launch without unnecessarily delaying others.
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Courtesy of NASA's Aerospace Technology Enterprise
Published by NASAexplores: November 27, 2001
Marshall Space Flight Center