NASA Facts Lightning and the Space Program 2006

8/6/2019 NASA Facts Lightning and the Space Program 2006

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FS-2005-10-031-KSC (Rev. 2006) 2 NASA Facts

Lightning protection systems

Kennedy Space Center operates extensive lightning protec-tion and detection systems in order to keep its employees,the 184-oot-high space shuttle, the launch pads, payloads andprocessing acilities rom harm. Te protection systems and thedetection systems incorporate equipment and personnel both

at Kennedy and Cape Canaveral Air Force Station (CCAFS),located southeast o Kennedy.

Predicting lightning before it reachesKennedy Space Center

Air Force 45th Weather Squadron Te rst line odeense or lightning saety is accurately predicting when andwhere thunderstorms will occur. Te Air Force 45th WeatherSquadron provides all weather support or Kennedy/CCAFSoperations, except space shuttle landings, which are supported

by the National Atmospheric and Oceanic Administration(NOAA) National Weather Service Space Flight MeteorologyGroup at Johnson Space Center.

Inormation provided by the 45th Weather Squadron in-cludes lightning advisories and warnings critical or day-to-dayshuttle and payload processing, as well as launch day weatherdata essential in helping NASA determine when it is sae orthe space shuttle to li o. Te 45th Weather Squadron has de-veloped several techniques to orecast lightning and has teamedwith many universities to improve thunderstorm prediction.

Te 45th Weather Squadron operates rom RangeWeather Operations at CCAFS, a center or the orecastingand detection o thunderstorms and other adverse weatherconditions. Housed there are the Meteorological InteractiveData Display System, which processes and displays data romthe National Centers or Environmental Prediction, weathersatellite imagery and local weather sensors to help orecastersprovide the most accurate, timely and tailored support possibleor Kennedy operations.

Among the local sources o weather inormation are twoweather radars that can identiy and track storms within a150-mile range o Cape Canaveral, and the Wind InormationDisplay System, a network o towers with wind, temperatureand moisture sensors. Wind measurements can reveal some othe conditions that can cause thunderstorm development.

Lightning Detection Systems Te Launch PadLightning Warning System, Lightning Detection and Rang-ing system and the Cloud to Ground Lightning SurveillanceSystem provide data directly to the R ange Weather Operations

on atmospheric electrical activity. Tese systems, along withweather radar, are the primary Air Force thunderstorm surveil-lance tools or evaluating weather conditions that lead to theissuance and termination o lightning warnings.

Te Launch Pad Lightning Warning System comprises 31electric eld mills uniormly distributed throughout Kennedy

and Cape Canaveral. Tey serve as an early warning systemor electrical charges building alo or approaching as part o astorm system. Tese instruments are ground-level electric eldstrength monitors. Inormation rom this warning system givesorecasters inormation on trends in electric eld potentialand the locations o highly charged clouds capable o support-ing natural or triggered lightning. Te data are valuable indetecting early storm electrication and the threat o triggeredlightning or launch vehicles.

Te Lightning Detection and Ranging system, developedby Kennedy, detects and locates lightning in three dimensions

using a time o arrival computation on signals received atseven antennas. Each part o the stepped leader o lightningsends out pulses, which the system receives at a requency o 66MHz (equal to V channel 3). By knowing the speed o lightand the locations o all the antennas, the position o individualsteps o a leader can be calculated to within 100-meter accuracyin three dimensions. Te system provides between one and1,500 points per ash.

One o the 31 electric eld mills that compose the Launch Pad

Lightning Warning System. They are called mills because they

have a rotating, our-bladed shield much like arms o a windmill.

The shield contained in the bottom o the round housing alter-

nately exposes and covers metal sensing plates, resulting in an

alternating current proportional to the atmospheric electric eld.


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For many years, this was the only system in operational me-teorology able to provide detailed inormation on the verticaland horizontal extent o a lightning ash, rather than just thelocation o its ground strike. Lightning Detection and Rangingdetects all lightning, including cloud-to-cloud and in-cloud aswell as cloud-to-ground.

Te Cloud to Ground Lightning Surveillance Systemdetects, locates and characterizes cloud-to-ground lightningwithin approximately 60 miles o the Range Weather Opera-tions. Electromagnetic radiation emitted rom lightning is rstdetected by the systems six direction nder and time-o-arrivalantennas, located in Orange and Brevard Counties. Lightningpositions are computed using triangulation and time-o-arrivalrom as many sensors as possible and relayed to a color displayvideo screen in the center. Once lightning-producing cells areidentied and located, the orecaster can more easily predictjust where the next lightning bolts will hit.

Te 45th Weather Squadron also has an extremely activeprogram to educate its customers and the general public onlightning saety.

Kennedy lightning policy

Kennedy pioneered a two-phase lightning warning policy.In Phase I, an advisory is issued that lightning is orecast withinve nautical miles o the designated site with a desired lead-timeo 30 minutes. Te 30-minute warning gives personnel in un-protected areas time to get to protective shelter and gives those

working on lightning-sensitive tasks time to secure operationsin a sae and orderly manner.

A Phase II advisory is issued when lightning is imminent oroccurring within ve miles o the designated site. All lightning-sensitive operations are terminated until the Phase II advisoryis lied.

Tis two-phase policy provides adequate lead-time orsensitive operations without shutting down less sensitive

operations until the hazard becomes immediate. Because it isessential that no lightning go unwarned, there is a alse-alarmrate o about 40 percent. Improved orecasting tools may enablethe alse-alarm rate to be reduced without compromising saety.

Te lightning policy is dened by the Kennedy LightningSaety Assessment Committee. Tis group is also responsibleor seeing that all structures at Kennedy, as well as the spaceshuttle, are adequately protected. Structures that particularlyneed protection against lightning include those containingignitable, explosive or ammable materials, and personnel.

Protection at the padSome Kennedy acilities that incorporate extensive light-

ning-shielding devices include the service structures at LaunchPads 39A and 39B, the Vehicle Assembly Building and theOrbiter Processing Facility.

An 80-oot-tall berglass mast on top o the xed servicestructure at each pad is the most visible means o protectingthe structure itsel, the shuttle while it is on the pad, and the en-closed launch equipment. Te mast supports a 1-inch stainlesssteel cable that runs over its top. Tis cable stretches 1,000 eet

in two directions, and each end is anchored and grounded. Itsappearance is similar to that o a suspension bridge tower and itssupporting cables.

A 4-oot-high lightning rod on top o the mast is con-nected to the cable. Te rods purpose is to prevent lightningcurrent rom passing directly through the space shuttle and thestructures on the pad. Any strikes in this area would be con-ducted by the cable, called a "catenary wire" because o its shape,to the grounded anchor points.

Other grounding systems in the Launch Complex 39

area include a network o buried, interconnected metal rodscalled the "counterpoise" that run under the launch pads andsurrounding support structures. All structures in the area aregrounded, including the Vehicle Assembly Building.

Additional protection devices at the pads include agrounded overhead shield cable to protect the crew emergencyegress slidewires, attached to the xed service structure.Grounding points on the pad surace connect the pedestalsthat support the mobile launcher platorm to the pad counter-

Attached to the wing o a Cessna Ciltation aircrat are cloud physics probes

that measure the size, shape and number o ice and water particles in clouds.

The plane is also equipped with eld mills, used to measure electric elds.

The plane was fown into anvil clouds in the KSC area as part o a study to

review and possibly modiy lightning launch-commit criteria.



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poise. Te platorm itsel has electrical connections in its twintail service masts that make contact with the space shuttle.Tese connections complete the system that conducts anylightning-related electrical discharges saely away rom thespace plane. Most other launch pads at CCAFS have similaroverhead wire lightning protection systems.

Overhead gridwire systems protect hypergolic uel and oxi-

dizer storage areas at the pads. Te huge 900,000-gallon liquidhydrogen and oxygen tanks at each pad are constructed ometal and do not need overhead protection since they providetheir own grounds.

Away rom the pad, the shuttle is well protected romboth inclement weather and lightning when it is in the VehicleAssembly Building. Tis 525-oot-high structure, one o thelargest in the world, has its own system o 11 lightning conduc-tor towers rising 25 eet high on its roo. When lightning hits

the system, wires conduct the charge to the towers, which thendirect the current down the sides o the building and into itsoundation pilings that are driven into bedrock.

Aer leaving the building, the space shuttle is vulnerableto lightning strikes as it is transported to the launch pad. Tistrip takes about six hours. Te primary method o reducing

lightning risk is by scheduling rollout during periods o verylow lightning probability typically in the late night and earlymorning hours.

Launch pad detection systems

A lightning measuring system is located at the launchpads so that any electrical activity in the immediate area can becontinually observed, recorded and assessed. Data gathered byits sensors and cameras is sent directly to the Launch ControlCenter so NASA personnel can determine when it is sae to

launch the shuttle.One o the monitors closest to the shuttle is the CatenaryWire Lightning Instrumentation System. Tis system senseslightning currents in the wire and evaluates them to see whatpotential they may have or causing damage to sensitive electri-cal equipment. Te current sensors are located at each end othe catenary wire, and they detect and record lightning strikesto provide potential damage assessment data or the lightninginstrumentation system.

Another launch pad monitoring system, the LightningInduced Voltage Instrumentation System, detects and records

any transient electrical impulses that might occur in spaceshuttle electronic systems or on the vehicles skin. Te systemis installed in the mobile launcher platorm and monitorsconditions while the shuttle is on the way to the launch pad viathe crawlerway and at the pad itsel. Voltages and currents maybe induced by nearby lightning even i the pad is not directlystruck.

Te electromagnetic elds rom the intense lightningcurrents are enough to cause currents to ow in nearbyconductors.

Data recorded by both systems are compiled and sent tothe Launch Control Center through the computers o theLightning and ransients Monitoring System.

A new Sonic Lightning Locator being tested at the shuttlelaunch pads precisely determines the strength and exact loca-tion, to within 5 meters, o any lightning strikes within theimmediate area o the pad. Tis helps shuttle engineers assessthe need to conduct additional tests on sensitive systems aernearby lightning events. Te sonic lightning locater uses an

Space Shuttle Endeavour passes the lightning mast and attached catenary

wire as it hurtles into space on mission STS-111 to the International Space

Station June 5, 2002. The mast and wire provide the primary lightning

protection system or the orbiter while it is on the pad. Mission STS-111 is

the 18th fight o Endeavour and the 110th fight overall in NASAs spaceshuttle program.



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electric eld detector and an array o acoust ic detectors tolocate the lightning contact point with great precision.

Visual detection o lightning activity is also essential.A network o video cameras positioned to observe the xedservice structures lightning mast and the top o the shuttlesexternal tank are linked to television monitors in the Launch

Control Center. Any lightning ashes can be seen on thescreen and recorded or later analysis. Tese data, along withthe launch pad protection systems, help veriy and calibrate thelightning detection systems.

Does it all work?

Te elaborate lightning detection and protection systemsat Kennedy have proven their worth t he hard way. Te lightningmasts at Launch Pads 39A and 39B are struck about ve timesper year, sometimes with a space shuttle on the pad. Tere has

been no damage to any equipment.In 1983, lightning struck the launch pad with the shuttleon the pad beore three o the our launches. o this date, noNASA-Kennedy employee has ever been injured by l ightning due in part to the lightning protection policy and educationprograms.

Tanks to the extensive weather and electric eld detec-tion systems, no space shuttle has ever been endangered bylightning during launch although several launches have beendelayed due to observed and orecast weather conditions.

Lightning research by NASA,other governmental agencies

Kennedy Space Center needs to protect space shuttles andother launch vehicles, payloads, associated ground processingequipment and acilities, and its personnel. Tereore, it hasperormed extensive research into lightning and its causes, andhow to detect and orecast it. Tis inormation is applied towardimproved lightning warning and protection systems.

For more than 20 years, Kennedy has hosted internationalprojects to study thunderstorms and atmospheric electric-ity. Te three largest programs have been the TunderstormResearch International Projectconducted in the mid-1970s, the Rocket riggered LightningProgram conducted rom the mid-1980s to 1992, and the Con-vection and Precipitation/Electrication program o 1991.

Additionally, three programs using aircra with electriceld measurement capability have been conducted atKennedy. Te rst occurred around the time o the Apollo-Soyuz program to saely enhance launch availability or short-

launch-window docking missions.

Te second was an Airborne Field Mill Program in theearly 1990s that studied revising our lightning launch-commitcriteria, to saely relax them based on better understandingo the actual hazards. It was conducted by NASAs LangleyResearch Center, Marshall Space Flight Center and Kennedy,

Stanord Research International, and New Mexico echnologi-cal University.

Finally, the latest airborne eld mill program ew missionsin 2000 and 2001. Te data rom that program is still beinganalyzed, but has already led to modied lightning launch-com-mit criteria and improved launch saety.

Many investigators rom other governmental agencies,leading universities, utilities and international organizationsconducted ground-based and airborne lightning experimentssupporting Kennedy Space Center's Rocket riggered Light-ning Program. Te French government was a major participantand pioneered this type o research, along with the UnitedStates.

Te program investigated potential lightning strikes overland, water and in the air. Tree-oot-tall rockets trailing cop-per wires were launched into clouds rom a launch pad, roma ra in the lagoon, and rom a platorm suspended beneath aballoon.

Other NASA centers have also been heavily involved inlightning-related research. NASA-Langley scientists studiedaircra-triggered lightning by ying specially instrumented

and weather-hardened aircra directly through thunderstormsin Virginia and Oklahoma. Much o what we know about thisphenomenon was discovered through work with an F-106Bghter airplane.

During eight years o research, the airplane was struck bylightning more than 700 times. Nearly all o these strikes weretriggered by the aircras motion through the intense thunder-storm-electric eld, rather than as the result o intercepting anatural lightning bolt. Te Federal Aviation Administrationand the Air Force have conducted similar experiments to deter-

mine how to better protect aircra electronics.Marshall, in conjunction with Langley and Kennedy,measured electric elds alo in the early 1990s using airborneeld mills to assess what weather conditions pose a threat otriggered lightning during space vehicle launches.

Similar measurements were made in 2000 and 2001 by ateam led by Kennedy that included scientists rom Marshall,the University o North Dakota, the National Center orAtmospheric Research, NOAA and others.



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Scientists rom the University o Arizona, New Mexicoech and other universities are examining Kennedy/CCAFSground-based eld mill data or additional clues concerningwhat conditions are sae and which are hazardous. Tis will helpdesign launch rules providing maximum opportunity to launchwithout compromising saety.

Marshall has investigated thunderstorms by ying overthem with U-2 aircra, and is also investigating lightning viasatellite. Its Optical ransient Detector is able to detect andlocate lightning rom orbit over large regions.

Te detector is a highly compact combination o opticaland electronic elements that represents a major advance overprevious technology by gathering lightning data in daytime aswell as night. Some o its most important science results are therst-ever consistent lightning climatology covering most o theglobe, and contributing to the use o lightning data in severeweather orecasting.

Te detector, and its ollow-on, the Lightning Mapper,

enables more accurate estimates o the energy and current as-sociated with the global electrical circuit.

Lightning one of the mostviolent forces of nature

At any instant, there are more than 2,000 thunderstorms

taking place throughout the world. Tese storms combine toproduce about 100 lightning ashes per second, each one withan average o 300 million volts, currents ranging up to 20,000amps, and temperatures over 50,000 degrees Fahrenheit.Extreme lightning can reach a billion volts, over 200,000 amps,and over 54,000 degrees Fahrenheit.

A moderate-sized thunderstorm at its peak ca n generateseveral hundred megawatts o electrica l power, equivalent tothe output o a small nuclear power plant. With so much energybeing released, there is little wonder that lightning has consider-

able potential to cause damage.

Lightning is at once beautiul and earsome. Here, a cloud-to-ground strike

is caught in the act as it zaps a tree.



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Lightning on other planets

Tese giant electric sparks are not unique to Earth.Among the mystiying and gargantuan storms that ragethroughout Jupiters atmosphere, one amiliar phenomenon lightning was captured by cameras on NASAs Voyager Iplanetary explorer spacecra. Both Voyager I and II detected

electrical signals rom Jupiter characteristic o lightning. Tisdiscovery was the rst hard evidence that such violent electricaldischarges take place on other planets. Te Galileo spacecraalso photographed what appear to be visible lightning ashesin Jupiters atmosphere. Electrostatic discharge detection onSaturn and Uranus by Voyager 2, along with radio signals as-sociated with lightning picked up by the Pioneer Venus orbiterand Russian Venera probe, may indicate that lightning is com-monplace in our solar system.

Lightning-like electrostatic discharges in the dust storms

o Mars have been hypothesized.

Lightning helps maintain

atmospheric charge, aids plants

Lightning on other planets may be too ar out or somepeople. For others, the earsome ashes and explosions thataccompany a midsummer nights thunderstorm here on Earthoen seem too close to home.

During a power blackout rom a lightning strike, its hardto remember that some good does come rom the powerul

bursts o electrical energy. When lightning bolts discharge,they ionize the air and produce nitrogen oxide. According to re-cent studies, this process could generate more than 50 percento the usable nitrogen in the atmosphere and soil. Nitrogen is anessential plant ertilizer.

Lightning also plays a critical role in orests natural cyclesby helping generate new growth.

Areas that are burned by lightning-triggered res arecleared o dead trees so that seedlings have the space and soil totake root. Te global array o thunderstorms serves as a world-wide circuit o electrical generators.

Trough the activity o the lightning they produce, thesegenerators continually maintain and renew the atmospherespositive electrical charge.

Nature takes its toll, though

With so many bolts o lightning, its no wonder that peopleand structures are hit. Each year, about 100 people are killedand about 245 are injured in the U.S. by the number two storm-

related killer.

Lighting-generated res destroy more than 30,000 build-ings at a loss o hundreds o mill ions o dollars yearly. Te aver-age total economic impact o lightning is more than $5 billionin the U.S. each year.

Airplanes and spacecra are subject to the tremendouselectrical orces that can build up in the atmosphere. Accordingto the Federal Aviation Administration, commercial aircraare struck an average o once every 3,000 ight hours, or about

once a year. However, only one U.S. airliner was conrmed aslost to lightning, in 1963.

Because o an airplanes metal construction, lightningows along its uselage rather than penetrating it.

Almost all lightning strikes on aircra cause only super-cial damage, and passengers are protected rom injury. With theadvent o new composite materials or airrames and digital y-by-wire control systems, newer aircra may be more vulnerablethan statistics would suggest.

An artist's concept o the descent module o NASAs Galileo interplanetary

probe shows the probe plunging through Jupiters atmosphere. The probe

carried instruments designed to investigate lightning in the Jovian atmos-

phere. Launched rom KSC Oct. 18, 1989, Galileo arrived at the planet in

1995, and ater eight years o data gathering, disintegrated as it ell toward

the surace .



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Spacecra are more vulnerable than aircra. On March26, 1987, an Atlas Centaur rocket and its satellite were lostwhen the unmanned NASA vehicle was struck by lightningthat it triggered itsel.

wo earlier triggered strikes that temporarily disabled theelectrical systems on the Apollo 12 spacecra onboard a Saturn

V rocket on Nov. 14, 1969, prompted NASA to develop waysto protect its launch vehicles, and to create a better system topredict when and where lightning might strike.

Reducing lightning damage

NASA, the Department o Deense, NOAA, t he FAA,various research and industry groups, and several oreign gov-ernments continue to investigate the ways lightning develops,better ways to predict its occurrence, and the means to reducedamage when it does strike.

o attempt to predict where the next stri kes will occur, aNational Lightning Detection Network has been establishedacross the U.S. Te network plots the strike location o eachcloud-to-ground ash.

Te precise three-dimensional Lightning Detection andRanging system developed at Kennedy Space Center was com-mercialized under a Space Act agreement between NASA andGlobal Atmospherics, Inc (a Vaisala Inc. subsidiar y).

Tis system allows the orecaster to view the height andhorizontal extent o each lightning ash and not just the point-o-ground contact. Unlike the National Lightning DetectionNetwork, the system can also detect in-cloud and cloud-to-cloud ashes.

Te Lightning Detection and Ranging system has con-tributed much to our understanding o lightning, including thedistribution o lightning strike distances, and the use o light-ning in severe weather orecasting. Soon, satellites that observethe whole planet will supplement ground detectors to increasecoverage o thunderstorm activity.

Meteorologists can use this data to alert people in poten-tial strike areas. Te more accurate the prediction o where and

when lightning will occur, the better chance o reducing oreliminating the damage it causes. Kennedy and CCAFS use atwo-phase lightning policy (described on page 3).

Ground equipment needsmost protection

Since lightning tends to strike the highest local point,special care must be taken to protect tall structures rom direct

strikes. Tese structures are oen power lines, microwave relaytowers used in telephone communication, buildings lled withsensitive electrical equipment, or even launch pads.

Without protection, a lightning strike can cause powerline surges and arcing, electrical res and electrical or structuraldamage. Te lightning does not have to hit a acility directly to

cause damage. Voltages and currents induced by nearby strikescan burn out or damage components o modern electroniccircuits.

Te National Fire Code standards or lightning protection(NFPA-780) or structures call or a pathway, or conductor,that will saely redirect a lightning bolts electrical energy to theground. Circuit breakers, uses and electrical surge arrestorsprovide additional protection.

Sometimes even this equipment is not sufcient to preventdamage. Studies, including results rom the Rocket riggeredLightning Program, have shown that lightning strikes result inrapid current surges (reaching an initial peak within a milliontho a second) with such high peak current (over 20,000 ampereson average) that conventional protection methods are unable tosave complex electronic systems rom da mage.

Utilities and high-technology industries, among others, areinvestigating ways to better protect vital electrical equipment.

Better protection begins withbetter knowledge of lightning

Although lightning has been known to be a discharge oelectrical energy since Ben Franklins kite-ying days, the wayelectrical charges build up and discharge in clouds is still notully understood, even now in the 21st century. Researchers atKennedy and others throughout the world attempt to answerthese questions so improved means to detect and measure thecharges can be developed.

A lightning bolt is the transer o an electrical chargebetween regions inside a cloud, between clouds, rom cloud toair, rom cloud to the ground, or (more rarely) rom the groundto air.

For such a transer to take place, the two types o chargesmust be separated so the cloud is electried. Exactly how thecharges become separated and where in the cloud they arelocated are still not completely clear.

Is a thundercloud like a generator?

However the details may turn out, it is well understoodthat thunderstorms separate electrical charges. Usually, a posi-



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tive charge is pumped alo while a negative charge accumulatesnear the lower-middle part o the storm. A small amount opositive charge may collect near the base o the storm cloud. Ittakes energy to separate the charge, and this energy comes romthe rapidly rising air currents in the storm. Tus, like a genera-tor, a thunderstorm converts mechanical energy to electricalenergy.

Convection and thunderstorms

A thunderstorm is a natural heat engine. On a typical sum-mer day over Florida, the air is loaded with moisture and theland surace is hot. As the land heats the air near the surace, itexpands, becomes less dense (lighter) and begins to rise.

As it rises, the air expands urther, this time due to thelower pressure higher in the atmosphere, rather than due toheating. In act, as the air expands in the lower pressure, it cools

because its internal energy is spread out over a larger volume.When moist air cools enough, it can no longer hold all thewater it contained when it was warm. I it were on the ground,dew and og might orm. Alo, the excess water condenses outas a patch o og in the sky, which we call a cloud.

When water condenses, it releases heat to its surroundings,just as when it evaporates, it absorbs heat (which is why a wettowel cools you on a hot day). Te heat released when a cloudorms makes the air rise even more vigorously until a cloud isthousands o eet high. Te cloud can continue to grow as longas it has a good source o warm, moist air at its base. As it grows,

it eventually becomes tall enough or the air in the cloud to coolbelow the reezing point (0 C).

Surprisingly, water in the parts o the cloud cooler than 0does not actual ly reeze until it gets considerably colder: -10 Cto about -20 C. Liquid water colder than 0 is cal led super-cooled water. At temperatures below -10 to -20, water vaporcondenses directly to ice (subliming rather than condens-ing). As we will see, it is the mixture o ice and super-cooledwater that probably accounts or most thunderstorm electrica-tion.

Cloud droplets are too small to all as rain, but turbulencein the cloud causes droplets and ice crystals to collide. Dropletsmay coalesce, and when a super-cooled droplet collides with anice crystal, it will reeze to the crystal, thus enlarging it. Soonthese larger ice crystals begin to all through the super-cooledwater and collect it, growing as they go. When they have allenenough or the temperature to rise above 0, they melt, becom-ing raindrops.

Sometimes a small ice pellet will become coated with waterand then be blown back up higher by a sudden updra. Laterit can all again and gather even more water. Tis can happenseveral times i the updras and turbulence are strong enough.Ten some really large ice particles can orm and they may notmelt beore hitting the ground. Tese large ice particles arecalled hail.

The lightning event begins when lightning strikes the rocket, launched rom

the Rocket Triggered Lightning launch pad at Kennedy Space Center, and

the 3,000-oot copper wire being trailed rom the rocket. The wire is then

vaporized as it ollows the path to a lightning rod attached to the instrumented

launcher. As the wire burn dissipates, it creates an eect called rosary bead

lightning. This can be the prelude to natural lightning restrikes. The initiationo a wire burn can also induce natural intercloud lightning during the event, as

seen in this sequence.



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Precipitation charging theory

Te most widely accepted explanation o how thunder-storms separate the charge is based on laboratory experimentsand atmospheric observations with aircra and radar. Te testsshow that when ice crysta ls and super-cooled water dropletscollide, i they dont coalesce, the pieces that are scattered aer

the collision are charged. Which pieces get which kind ocharge, positive or negative, depends on the temperature. But attemperatures typical o the electrically active part o thunder-storms, the smaller pieces usually get the positive charges. Tesesmaller, lighter ragments will be carried alo by the updraswhile the negatively charged, larger, heavier remnants all. Tisresults in charge separation and an upward transport o thepositive charge.

Mechanics of a lightning strike

It is a act o nature that positive and negative electricalcharges attract each other. Te strength o this attraction iscalled the electric eld. When enough charge has been sepa-rated, the orce o attraction overcomes the electrical resistanceo the air and a giant spark (lightning!) can occur.

Most lightning occurs within or between clouds. Tedestructive cloud-to-ground lightning bolt occurs much lessrequently and can carry either a positive or a negative charge.O the two, negative lightning is the most common type (about94 percent). Te process involved in generating a lightning

stroke explains why lightning usually seeks out and strikes thehighest point on the surace.

First, a long series o negatively charged branches about 50yards long, called stepped leaders, emerges rom the cloud andapproaches the ground. During the approach, the stepped lead-er causes electric elds on the ground to increase in strength.Positive ions gather around pointed objects as small as pineneedles and grass blades, then ow upward toward the steppedleader as several 50-yard sparks, called upward streamers.

When the stepped leader and upward streamer touch, thecloud-ground circuit closes, and a huge, rapid surge o current

ows up the ionized stepped leader channel rom ground tocloud. Te grounded object serves as the ocal point o thepositive ion ow. Tat object, such as a tree or a goler with anupraised club, is considered struck by lightning.

Te huge upward surge o current is called the returnstroke. It heats the channel to over 50,000 degrees Fahrenheitalmost instantly. Tis lights up the channel, which we see aslightning, and generates a large pulse o sound as the super-heated air rapidly expands, which we call thunder. Usually the

return stroke doesnt neutralize all the charge in that region othe cloud, and a dart leader races down the lightning channel tothe ground, initiating another return stroke.

Tere are usually three to our return strokes per lightningash, separated by about a tenth o a second. Tis is near thelimit o human perception and explains why lightning appears

to icker. Lightning with as ew as y return strokes has beenobserved. Te entire event is called a lightning ash.

Positive lightning carries a positive charge to the ground.It makes up less than 4 percent o a storms lightning strikes andtypical ly takes place at the end o a storm. However, the positivelightning strike has potential to cause more damage.

It generates current levels up to twice as high and o longerduration than those produced by a negative bolt. Its the long-duration, or continuing current components, o lightningthat causes heating and burning, and metal punctures. For thatreason, scientists are especially interested in developing ways todetect the areas o a thunderstorm that develop positive bolts.

Triggered lightning a bolt from thegray?

Te phrase a bolt rom the blue originated rom observa-tions o a seemingly inexplicable phenomenon a ash o light-ning on a day without a storm cloud nearby. Tis event wouldbe startling under any circumstances, but imagine the shock oseeing such a bolt strike the 363-oot-high Apollo 12/Saturn Vrocket while it was more than a mile above Kennedy (Nov. 14,1969). Perhaps being in an airliner as it was zapped by light-ning at 20,000 eet would be more o a scare, though. While notreally bolts rom the blue, because they occur inside o clouds,they occur in clouds that otherwise do not contain lightning.

Why are rockets and airplanes struck in these circum-stances? It was rst thought that they just got in the way o alightning bolt jumping rom a positive- to a negative-chargedarea o a thundercloud. Later research provided evidence thatthe buildup o strong electric elds at certain points o theaircra were the culprit.

Such concentrated elds o electrical energy can developbeore lightning occurs. When an aircra or a rocket enterssuch a high electric eld, electrical elds are compressed, andthey concentrate around the sharp edges and protuberances othe vehicle.

I the electrical elds around the airplanes sharp and pro-truding parts build up to where there is an electrical breakdowno the air, lightning leaders orm at two or more locations onthe airplane. Te aircra also contributes to the conducting



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Further lightning safety information is available from the National

Weather Service lightning safety Web site:

http://www.lightningsafety.noaa.gov.

path between a positive and a negative electrical eld, triggeringthe resultant lightning bolt.

In the case o Atlas Centaur-67, a lightning strike changedsome data in the rockets computer, which caused it to steer therocket sideways and begin breaking up in ight. Range Saetythen destroyed the out-o-control rocket, March 26, 1986.

Lightning Safety

Lightning is the second leading cause o storm-causeddeaths in the U.S., killing more people than tornadoes or hur-ricanes. Only oods kill more than lightning. Lightning alsoinicts lielong debilitating injuries on more than it kills.

Public education is the key to prevention. Lightning saetyis best taught as a multi-level process o decreasing levels oprotection. No place outside is sae when thunderstorms arewithin several miles.

Te rst and best level o lightning saety, Level-1, is toavoid the threat. Use the weather orecast and know your localweather patterns to plan your outdoor activities to avoid thelightning.

Level-2 is to use the 30-30 Rule while outdoors. I thereare 30 seconds or less between lightning and its thunder, goinside. Wait 30 minutes or more aer the last thunder beoregoing outside. Te saest, mostaccessible place to avoid lightning is a large, ully enclosed build-ing with wiring and plumbing, such as a typical house. Whileindoors, avoid using corded telephones, electrica l appliancesand wiring, and plumbing. I a solid building is not available,a vehicle with a solid metal roo and metal body oers someprotection.

Level-3 o lightning saety is getting into dangerous terri-tory. I you must be outside and thunderstorms are near, avoidthe most at-risk locations or activities. Avoid high elevationsor open areas. Do not go under trees to keep dry. Avoid tall,isolated objects. Avoid swimming, boating and shing. Avoidopen-cockpit arm or construction equipment.

Level-4 should be used only as a desperate last resort. Iyouve made several bad decisions and nd yoursel outside, inan at-risk location, and thunderstorms are threatening, someprocedures can reduce, but not eliminate the threat.

Level-5 is rst aid. All lightning deaths result rom cardiacarrest or stopped breathing rom the cardiac arrest. CPR orrescue breathing is the recommended rst aid, respectively.

The future of lightningprediction, detection andresearch

As society becomes more dependent on computers andother electronic devices, more eective ways must be developedto protect this equipment against high-voltage shock. Futureaircra made o nonconductive composite materials, that y-by-wire or by computer command instead o manual hydraulicsystems, will need advanced protection systems. As the globalpopulation expands, the increase o people and property callsor improved lightning prediction and detection throughadvanced weather equipment and methods.

As one o the more lightning-sensitive residents o thelightning capital o the United States, Kennedy will continueto apply its technical expertise to support these eorts.


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Documents

NASA Facts Lightning and the Space Program 2006