Learning how to diagnose bad flying weather

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Lightning poses hazard to rockets, aircraft

One of a series of stories covering the quadrennial International Conference on Atmospheric Electricity, June 7-11, 1999, in Guntersville, Ala.

June 8, 1999: In the 1950s, a research scientist discovered that small rockets he launched to study lightning would sometimes trigger lightning. That heralded a new hazard for the space age, the problem of space launchers falling prey to lightning before they even reached space.

But no one worried much about the problem until Nov. 19, 1969, when Apollo 12 was struck twice by lightning in the 36.5 seconds after liftoff, when it was just 1.8 km (6,000 ft) high.

"That opened our eyes to the hazards that lightning could pose to a spacecraft traveling through an electrified cloud," said John Willett, a retired research scientist with the U.S. Air Force Research Laboratory outside Boston. The subject was reemphasized in 1987 when an Atlas Centaur was struck by lightning that damaged its electronics and caused an engine to steer the rocket off course.

Willett was an invited speaker Monday afternoon at the International Conference on Atmospheric Electricity being held this week in Guntersville.

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"What are the conditions in the atmosphere that are conducive to triggering lightning?" Willett asked. "If we know what they are, we have a better chance of avoiding them."

For now, the conditions are not fully known, so Willett and scientists at other institutions have developed a rocket program to find out. They use a small sounding rocket that uses surplus 2.75-inch-diameter Mighty Mouse motors designed for launch from attack aircraft. With a diagnostic payload designed for the rocket, the complete package is 6 feet tall.

Above: Lightning strikes the launch pad 36 seconds after Apollo 12 takes off for the moon. (NASA)

Willett said the rocket is launched into electrified clouds to measure conditions, principally how the electric potential or voltage changes with altitude. Then a small rocket, trailing a copper wire, is launched to trigger a lightning flash.

"When we do it we take great care to protect ourselves," Willett said. The launch team and their equipment are inside a trailer that is modified to serve as a Faraday cage, a sort of electrical isolation chamber that conducts any stray lightning around them.

The same cannot be done for rockets.

"I don't anticipate any way of preventing the lightning from striking the spacecraft," Willett said. "We'll have to take measurements in advance and say it is safe to launch, or it is not safe."

For the time being, launch sites like Cape Canaveral and Kennedy Space Center operate with lightning safety constraints that cost them a few launch opportunities, but minimize the chance of getting struck.

Web Links

Human Voltage (June 18, 1999) Scientists discuss biology, safety, and statistics of lightning strikes.
News shorts from Atmospheric Electricity Conference (June 16, 1999) Poster papers on hurricanes and tornadoes summarized.
Soaking in atmospheric electricity (June 15, 1999) 'Fair weather' measurements important to understanding thunderstorms.
Lightning position in storm may circle strongest updrafts (June 11, 1999) New finding could help in predicting hail, tornadoes
Lightning follows the Sun (June 10, 1999) Space imaging team discovers unexpected preferences
Spirits of another sort (June 10, 1999) Thunderstorms generate elusive and mysterious sprites.
Getting a solid view of lightning (June 9, 1999): New Mexico team develops system to depict lightning in three dimensions.
Learning how to diagnose bad flying weather (June 8, 1999): Scientists discuss what they know about lightning's effects on spacecraft and aircraft.
Three bolts from the blue (June 8, 1999): Fundamental questions about atmospheric electricity posed at conference this week.
Lightning Leaders Converge in Alabama (May 24, 1999): Preview of the 11th International Conference on Atmospheric Electricity.
What Comes Out of the Top of a Thunderstorm? (May 26, 1999): Gamma-rays (sometimes).
Lightning research at NASA/Marshall and the Global Hydrology and Climate Center.

"There's a very definite set of launch commit criteria imposed on national launch ranges," Willett said. "We are quite sure that it's safe before a rocket launches. But it leads to a reduction in launch opportunities."

Aircraft pose a different challenge. Most aircraft are well shielded against lightning because of the design of their metal airframes. Lightning is conducted around the skin and continues on its way in most cases. But as designers move to lightweight composites that don't conduct electricity, they face an increased risk of damage to the aircraft.

Willett noted that some strikes have splintered the plastic or rubberized radar domes on the nose of some aircraft. "You don't want that happening to wings or other major structures," he said. Further, the electronics that really control the aircraft will have to be shielded so they don't blank out for several seconds, as happened when Apollo 12 was struck.

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