Skylab and the Sun

8/8/2019 Skylab and the Sun

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N A T I O K I A I A F R O N A U T i r S A N D S P A C F A D M I N I ^ T R A T I O K I


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S K Y L A B A N D T H E S U N

J U L Y 1973

NATIONAL AERONAUTICS AND S P A C E ADMINISTRATI

O F F I C E O F M A N N E D S P A C E FLIGHT / OFFICE O F S P A C E S C IE N C E


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PREFACEThe purpose of this brochure is to familiarizethe reader with the Sun and what we expect togain by observing it. The articles in the brochurewere supplied by scientists, astronauts andengineers, individuals all closely associated withthe ATM Program and all deeply dedicated tom a k i n g it a success.Appended are several sources for information tofacilitate the exchange of ideas and results.

L. N. WernerSkylab Program O f f i c e

For sale by the Superintendent ol Documents, U.S. Government Printing Office, Washington, D.C. 20402


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F O R E W O R D

Since the earliest crude observations of the Sunby Galileo Galilei, man has taken giant strides inincreasing his knowledge of the Sun and the

;ftS .1 vfirVsW^Ff^'Adva0-068 m instrumentation and theability to' use a better location for observations,especially satellites above the disrupting effectsof the Earth's atmosphere, have been significantfactors in shaping the course of astronomicalresearch.Scientific instruments with which to make theseobservations from above the atmosphere haveevolved f r o m the early small rocket payloads tothe large high resolution man-operated instru-ments developed for the Apollo TelescopeM o u n t (ATM) which will significantly enhanceour capability to better study and understandthe activities of the Sun.T h e A T M carries five large experiment telescopepackages to permit simultaneous viewing of solaractivity in different wavelengths.The ATM experience will contribute to the evo-lution of future advanced orbiting observatorysystems and help define instruments which w i l lperform astronomical observations of a qualitywell beyond the capabilities of any ground-basedor space instrumentation known today.The ATMprogram will, for the first time, permitthe evaluation of man's utility and ability to op-erate complex scientific instruments in a spaceenvironment. T h e f i r s t A T M mission m ay wellbe the beginning of the modern era ofastronomy.Dixon L. ForsytheA T M Program ManagerNational Aeronautics & Space Administration

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CONTENTS

PRECEDING PAGE BLANK NOT FILMEDSection

PageForeword iiThe Skylab Program 1

Description 1Objectives 1Mission Profile 1K ey Mission Events 2

The Sun 3Solar Studies in Perspective 14

Skylab Solar Studies 17The ATM Solar Observatory 18The Scientific Instruments 21Crew Operations and Crew Training 28The Joint Observing Program 30

Skylab Associated Solar Programs , 35Coordinated Observing Program 36Guest Observer Program 37Skylab Ground Astronomy Program 38Solar X-ray Spectroscopy Program 40A T M Calibration Rocket. Program 41National Oceanic and Atmospheric Administration Solar Forecast Services . 42

AppendixAccess to Scientific Data A-lInformation for Teachers A-lInformation for Coordinated Observers A-lA T M Joint Observing Programs (JOP's) A-2A T M Principal Investigators Teams A-2A T M Guest Investigations A-4


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THE SKYLAB PROGRAMDESCRIPTIONSkylab is an experimental space station launchedin 1973 by the NASA. The station is a 100-toncomplex of highly versatile laboratories thathave the capability for multipurpose scientificinvestigation unmatched by any institution onEarth. The three-man Skylab crews performmore than 50 major research programs devel-oped by specialists f r o m universities, observa-tories, medical schools, hospitals, health institu-tions, and other public and private agencies inthe United States and abroad.M o v in g at a velocity of 5 miles/sec at an altitudeof 235 nautical miles, Skylab orbits the Earth inan easterly direction in an orbit canted 50 f romthe plane of the equator. During 8 months ofservice, Skylab covers an area 3450 miles wideon either side of the equator. Skylab will criss-cross every section of the United States and willbe visible for as long as 10 min at sunrise orsunset; the complex resembles a large starstreaking southeast or northeast. The stationcompletes one revolution every 93 minutes, itsmany delicate sensors finding and recordingnewinformation about the Sun and the M il k y Way,the atmospheric sheath, the remote sections andthe composition of the Earth, and about manh i m sel f .OBJECTIVEST he Skylab Program objectives are: Advancement of the sciences: T o increaseknowledge of medicine, astronomy, Earthmeteorology, physics, and other fields, in-cluding the effects of space and solar systemphenomena on the Earth environment Practical applications: T o perfect sensing

and data systems for use in agriculture, for-estry, oceanography, geography, geology,water and land management, communica-tions, ecology and pollution-control applica-tions, and to develop zero-g manufacturingtechniques Durability o f m a n a n d systems in space: T odetermine the ability of man, materials, andsystems to maintain their qualities and capa-bilities during a long period of weightlessness

Space-flight effectiveness an d economy: T oimprove space-flight technology to developlong-duration mission capability for futureprogramsMISSION PROFILEThe Skylab workshop was launched by a two-stage Saturn V vehicle from P ad A of launchcomplex 39 at the N A S A John F. KennedySpace Center (KSC) and was inserted into anearly circular orbit. In the first 7.5 hours offlight, Skylab jettisoned the payload shroud, ma-neuvered into a Sun-pointing mode, rotated theApollo telescope mount (ATM) solar observa-tory 90 to an operating position, and deployedall solar-cell power arrays. The ATM pointing1control system maintains the sun-pointing atti-tude, and the Skylab is pressurized to 5 psia [3.7psia oxygen (O2), 1.3 psia nitrogen (N2)] toprepare for docking with the manned commandand service module (CSM) and the astronauts'entry.A f t e r Skylab lift-off, a Saturn IB vehiclelaunches th e C S M an d three crewmen f r o m PadB at the KSC launch complex 39. First, the CSMis inserted into an interim elliptical orbit at analtitude of 93 to 138 miles above the Earth.U s in g the service propulsion system, the CSMclimbs to an altitude of 235 nautical miles, torendezvous with Skylab, and dock with the axialport of the multiple docking adapter ( M D A ) ,completing the cluster. The crewmen enter andactivate th e Skylab for habitation. T h e C S M i spowered down, allowing operation of only es -sential elements of the communication, instru-mentation, and thermal controls systems. Crew-m en conduct assigned experiments which, onFlight 2, emphasizes medical and solar researchan d the evaluation of the long-term habitabilityof the Skylab; the Earth resources experimentsalso are activated and their operation verified.O n the 27th day, the crewmen will prepareSkylab for storage in orbit; on the 28th day thecrewmen will board the CSM, undock, deorbit,an d land in the Pacific recovery area.Sixty days later, a second CSM with a three-mancrew will be launched from th e KSC. Orbitalinsertion, rendezvous, and docking will be thesame as for Flight 2, but Flight 3 will have more


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em phasis on solar as tronom y an d Earth resourcesexperim ents. Flight 3 is planned for up to 56days . A s s um ing nom inal m is s ion duration an ddeorbit, landing will be in the Pacific recoveryarea.T he third CSM and crew w ill be launched ap-proximately one month after the Flight 3 crewreturns to Earth. Flight 4 will com plete the ob-jectives of the plann ed experim ents.K EY M IS SIO N E VE N T SFlight 1Day 1:

Flight 2Day 1:

Days 2 to 25:

D ay 26:

T h e u n m a n n e d S ky la b i slaunched; inserted into opera-tional orbit; pressurized forentry of the c r e w m e n ; and theA T M is deployed.

T he first C S M i s launched, ren-d e z v o u s w i t h S k y l a b , an dd o c k s ; th e c r ew m e n e n te r an dpowe r up the orbiting cluster.T he astronauts m a n t h e labo-ratory to conduct sequencesof experim ents .O ne cre wm an exi ts , m ane uversouts ide the station to theA T M , a n d r e t r i e v e s a n dreplaces f ilm .

Days 27 and 28: T he crew deactivates experi-m e n t s an d prepares th e Skylabs y s t e m s for a 2 - m o n t h un -m ann ed s torage in orbit.Day 28:

FlightsDays 1 and 2:

T h e C S M is undocked, deor-bited, and returned to Earthwi th the c r e w m e n and thee xperim e nt f i l m , s pecim e ns ,an d records.

T he second crew a nd CSM a relaunched, rendezvous, dock,an d reactivate the d or m a n tSkylab.

Days 3 and 4: T he crew conducts miss ionexperim ents .D ay 5 : A n e x t r a v e h i c u l a r a c ti vi ty( E V A ) i s perform ed to loadfilm into th e A T M .Days 6 to 28: M ission experim ent operationsar e pe rf orm e d.Day 2 9 : A n E V A i s pe rf orm e d to re-trieve film from and to reloadth e A T M .Days 30 to 53: M is s ion experim ent operationsare continued.D ay 5 4: A n E V A i s pe rf orm e d for thepurposes of retrieving and re-p l a c i n g A T M film and in-s p e c t i n g d e g r a d a t i o n o f

therm al coatings.Days 55 and 56: Skylab and the experim entsare deactivated for a 1-monthperiod of un m an ned s torage.Day 56 :

Flight 4Days 1 and 2:

Day 3:Days 4 to 53:

Day 54 :

Th e CSM i s separated, deor-bited, and returned to Earth.

T he c r e w m e n and CSM arelaunched, and rendezvous anddock wi th th e Skylab; th eSkylab is activated.A n EVA i s pe rf orm e d to loadfilm in the A T M .M is s ion experim ent operationsar e pe rf orm e d.A n E V A i s pe rf orm e d to re-trieve A T M f i l m .

Days 55 and 56: S k y l a b s y s t e m s aredeactivated.Day 56 : T he crew and the CSM sepa-rate f rom Skylab, deorbit, andreturn to Earth.


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such as TIROS and Mariner IV**. Solar devicescatch the Sun's rays to heat homes, swimmingpools and ultrahigh-temperature furnaces, and aportable solar stove is being developed for use infuel-poor countries.If in such a manner we could collect and effi-ciently use the sunlight falling on just the city ofLos Angles, it would supply more energy than isconsumed in all the homes on Earth. The Sunproduces this life-supporting energy so steadilythat astronomers cannot detect with certainty asmuch as one percent variation in the totaloutput.What makes the Sun shine so steadily? Primitiveman thought of the Sun as a ball of fire, butscientists determined long ago that the Sun wasnot merely burning like a great ball of coal. In-deed, if it were merely coal it would haveburned for only a few thousand years, andwould have turned to cold cinders billions ofyears ago. Some other explanation was required.STELLAR A T O M E N E R GYFinally, in 1925, Sir Arthur Eddington, a bril-liant British astronomer, proposed the answernow accepted as correct: It is atomic, or nuclear,energy that fires the stars. This energythe sameas that of the hydrogen bombcomes from theprocess we call nuclear f u s i o n , in which the nu-clei, or cores, of hydrogen atoms collide, unitingto f o r m helium nuclei and giving off bursts ofenergy.No other process we know of could possiblepour out such sustained quantities of energy.Moreover, we know that for the Sun to stabilizeat its present size, it must have a temperatureand pressure at its core s uf f i c i e n t to supportnuclear reactions.Thus, deep within the Sun, each second, 564million tons of hydrogen are converted to 560million tons of helium. The remaining four mil-lion tons each second radiate a wa y as heat andlight.If the Sun has been shining at its present bright-ness since the Earth was formed nearly five bil-

lion years ago, each pound of solar matter musthave yielded already at least 4,000,000 kilo-watt-hours of energy. At that rate, a pound ofthe Sun would keep a kitchen stove go ingwith allburners on for several hundred years.Fantastic as the Sun's outflow of energy mustappear, the nuclear fusion actually goes on at aslow pace, atomically speaking. The Sun may beconsidered as a very slow-burning hydrogenbomb, since it takes, on the average, about amillion years for two hydrogen nuclei to collideand fuse. These tiny particles, even in the Sun'sdense interior, are on the average almost as farapart, in proportion to their size, as the Earthand Venus. Moreover, they require a head-oncrash at extraordinarily high speeds in order tofuse.H U M A N "HOTTER" T H A N SUNOnly because the Sun is so large is its total pro-duction of energy so enormous. Pound forpound, the Sun actually produces less heat thanthe human body. If the m a s s of the Sun couldbe matched with live bodies and if the n o r m a lhuman metabolism of those bodies could con-tinue, they would generate more heat than theSun now radiates.H ow do we know this? It's a simple matter ofarithmetic: The Sun's output of radiant energy,divided by the Sun's mass, shows a daily pro-duction of two calories a pound. By contrast,the average human body generates somethinglike 10 calories a pound each day.M a n ki n d is now embarked on a great new adven-tureM;he exploration of space. M o r e than halfthe scientific satellites launched by agencies ofthe United States government are, in one way oranother, devoted to the study of the Sun's activ-ity and its close relationship to Earth'senvironment.!M y own agency, the O f f i c e of Naval Research,isdeeply involved in studying the Sun; we main-tain a series of satellites called S O L R A D in orbitat all times for solar radiation measurements.Until very recently, man's view of the heavensw as seriously hampered by a murky, shimmering

* * A n d of course, the Skylab. f T h i s was the case at the time this article was written.


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atmosphere, which distorts light beams and blotsout the Sun's atmospheric X-rays and much ofits ultraviolet and infrared radiation. HenryNorris Russell, the noted Princeton astronomer,once jested, "All good astronomers go to themoon when they die so that they may observethe universe without the intereference of a dirtyatmosphere."But beginning in 1946, rockets became availableto carry small telescopes and spectrographsabove the atmosphere, and for a few minutes ofeach rocket's flight the ultraviolet and X-rayemissions of the Sun can be studied. Within re-cent years balloons have lifted heavy telescopesand cameras above 99 percent of the atmo-sphere, to an area where the distortion of visiblelight is largely eliminated.And now$ satellites, such as the Orbiting SolarObservatories of the National Aeronautics andSpace Administration, provide stable platformsthat can point 80 pounds of instruments steadilyat the Sun, with f i n e accuracy. Dozens of solarultraviolet and X-ray pictures can be transmittedto Earth daily.

SU N MESSAGESRadio astronomy, which is only about a third ofa century old, provides another effect ive tool forstudying the higher levels of the solar atmo-sphere. During World War II, British radar engi-neers were puzzled when their instrumentstracked intense static signals descending into thewestern ocean, instead of N a z i bombers comingf r o m the east. They found that such ghost sig-nals rose and fell with the rising and setting ofthe Sun, which was sending out its own radiomessages.The Sun's emanations constantly flicker and pul-sate, with frequent violent outbursts. Astrono-mers tune in on these broadcasts with sensitiveantennas. Using huge radar transmitters, theycan bounce beams off the swollen outer atmo-sphere of the Sun and probe its structure andmovements.

The years 1964 and 1965 were designated theInternational Quiet Sun Years (IQSY). Observersin 43 countries kept a diary of the face of theSun at a time when it is relatively undisturbedby sunspots and solar storms.The IQSY is sequel to the InternationalGeophysical Year (1957-1958), when scientistsstudied the Sun and Earth under conditions ofmaximum solar activity. Changes since 1958have been substantial, since solar activity goesfrom active to quiet to active again in an averagecycle of about 11 years.In these coordinated international surveys, solartelescopes take regular pictures of the Sunthrough various filters; mountaintop observa-tories watch the Sun's outer atmosphere throughcoronagraphs; magnetographs make magneticmaps of the Sun's fa c e ; radio telescopes capturethe Sun's radio signals as w a v y lines and num-bers on paper tapes; and satellites, rockets, andballoons monitor the solar w i n d s and storms andthe Sun's output of high-energy radiations suchas X-rays.At the U. S. N a va l Research Laboratory's E. O.Hulburt Center for Space Research inWashington, D. C., we are especially interestedin rocket and satellite observation of X-rays,which can't be detected f rom the ground. Theytell us much about the most energetic processeson the Sun. In two decades at the laboratory, Isuppose I have instrumented more than 50rockets and a dozen satellites for this kind ofresearch.When astronomers examine the Sun with a solartelescope, its edge appears sharp as if it marked adefinite surface. This apparent surface is in facta transparent, though highly luminous, layer ofga s about 200 miles thick, called the photo-sphere. From the photosphere comes most ofthe light we get f r o m the Sun. At the bottom ofthe photosphere, the gas becomes so opaquethat no light f r o m the interior can escapethrough it directly.Thus the photosphere is a thin, bright shell thatsurrounds the main body of the Sun like an

f Refers to 1960, the next O r b i t i n g Solar Observatorywil l point about 250 Ibs steadily at the Sun with nearly100 times greater stability. Complete t hroug h 1965; m a n y more have s ince beeni n s t r u m e n t ed a n d f l o w n b y t h e H u lb u r t Cen ter .


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onion skin. Outside this layer lie two othersaregion of flamelike outbursts of gas called thechromosphere, and an almost endless outeratmosphere called the corona.As we look at the Sun f rom Earth, we can "see"only these three layers directlythrough visiblelight, infrared, and radio, observed by groundobservatories, and through ultraviolet and X-raysdetected by instruments in rockets and satellites.All that we know of the Sun's hidden interiormust, therefore, be deduced f r o m observation ofthese external features.The ancients held many strange notions aboutthe flaming ball that courses daily across thesky. Epicurus, the Greek philosopher, estimatedabout 300 B. C. that the sun "is just as great asit appears"^that is, about two feet in diameter.To Anaxagoras, another Greek philosopher, theSun was "a mass of red-hot metal" larger thanthe Peloponnesus. Even some Eskimos, until re-cently, believed that after the Sun set in thewestern ocean, "he" was paddled back in akayak through the night, to the eastern horizon.Modern astronomers can gauge the size and dis-tance of the Sun very accurately, using triangu-lation with other celestial objects. Its diameterof 864,000 miles compares with Earth's 8,000.The Sun's distance f ro m Earth averages93,000,000 miles, a length scientists use as theastronomical unit for measuring the solar sys-tem. Since Earth's orbit is slightly elliptical, theactual distance varies f r o m 91 to 94 millionmiles.When we compare Earth's size with that of theSun, we f i n d that the Sun would hold some1,300,000 Earths, and that it contains nearly330,000 times as much mass as the Earth. Sincegravitational pull depends directly on the mass,but decreases with the square of the distancef r o m the center of the body, a man on the Sunwould weigh some two tons."INTELLECTUALBORING"Having determined the Sun's mass and diameter,the astrophysicist can then deduce the tempera-ture, density, and pressure at all distances f romthe center to the surface, even though he is un-

able to see deeper than the 200 miles of theluminous photosphere. Sir Arthur Eddingtondescribed this deductive process as "intellectualboring."As a result of this boring, we have good reasonto believe that at the center of the Sun, close tohalf a million miles deep, pressure reaches 100billion atmospheres. (An "atmosphere" is 14.7pounds per square inch, the weight of the col-umn of air over a square inch of Earth's surfaceat sea level.)To produce such great pressure, we know thatgas must be heated to a temperature of about16,000,000 C. (Astronomers, like other scien-tists, use centigrade. To convert to Fahrenheit,multiply by 9/5 and add 32.)How hot is 16 million degrees? Sir James Jeans,in The Universe Around Us, calculates that apinhead of material at the temperature of theSun's core would emit enough heat to kill a mana hundred miles away.Although the density at the center of the Sunmust be about 11.4 times that of solid lead, theSun remains gaseous everywhere. That is, theatoms are free to move about, unlike those in asolid, which are fixed in a regular pattern. How-ever, the atoms in the core are not normal. Mostof their outer electrons have been sloughed offby collisions of atoms, and rush about as freeparticles.Normal atoms cannot get closer than about ahundred-millionth of an inch because their outerelectrons would touch. Electrons, which areneg-atively charged, repel one another. Thus theykeep atoms widely spacedwidely, that is, inatomic terms. But when electrons are strippedaway, the remaining nuclei can approach verymuch closer. That is why the Sun's inner corecan be so extremely dense. The bare nuclei aremore tightly squeezed.X - R A Y S T U R N VISIBLEIf we could go into the Sun's interior to makemeasurements, we would find that roughly 90percent of the energy that eventually floods outinto space is produced within a central corewhich reaches only one-quarter of the distanceto the surface.


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In this nuclear furnace the fantastically hot,dense gas is almost pitch black, since nearly allits radiation is invisible X-rays produced by nu-clear reactions and the collisions of fast-racingnuclei and electrons.

Finally the Sun's energy reaches the surface andthere, in the photosphere, its form is againchanged, being largely converted once more toradiation that leaves the Sun to flood throughspace.The path of an X-ray as it escapes f r o m the coreof the Sun resembles the zigzagging track of thesteel ball in a pinball machine. Even though therays travel at the speed of light, 186,300 miles asecond, the devious trip to the surface takesabout 20,000 years!During that long period the X-rays graduallychange. Each time one is deflected, the fre-quency of its vibration is reduced slightly, andits wavelength is increased. In time, all theX-rays gradually turn into ultraviolet and visiblelight.To vnderstand this relationship, think of X-rays,ultraviolet, and visible light as all being cousins,or related forms of electromagnetic vibration, ona spectrum, or scale, like that of a piano. Just aseach note on the piano varies f r o m its neighborsby its frequency (the rate at which its stringvibrates), so do vibrations in the electromagneticspectrum. X-rays are comparable to high notes,ultraviolet represents notes with a somewhatlower frequency, and visible light waves fall stilllower, near the middle of the keyboard, so tospeak. Farther down the scale come infrared andthen radio waves, the "low notes." These, too,are electromagnetic vibrations, differ ing f r o mlight only in their rate of vibration.At three-quarters of the distance to the surface,the solar interior has cooled to about150,000 C, and the density has fallen to about atenth that of water. Up to this point, the Sun'senergy has been transferred in radiant form. Ra-diant energy travels directly by waves movingwith the speed of light, as when one feels theheat of a fire at a distance.Now, however, still more than 100,000 miles be-low the surface, the Sun's gas begins to convect,like boiling water, and energy seeths upward in aturbulent flow of hot gas. Convection occurswhen chaotic masses of gaseous atoms flow incurrents, each atom carrying its own parcel ofenergy all the way.

SUNSPOTSVISIBLEAristotle taught that the Sun was a globe of purefire without blemish. This belief persisted untilGalileo's time, when the newly invented tele-scope showed that dark spots come and goacross the face of the Sun.Normally, bright Sun blinds the naked eye, butwhen fog and haze reduce the glare we canreadily detect large sunspots, especially near sun-rise or sunset. Two hundred years ago, peoplethrought the spots were solid mountaintops pro-truding above an ocean of glowing lava, thephotosphere. They reasoned that the photo-sphere would have high and low tides. As thetide ebbed, the higher mountaintops wouldshow as dark bodies.In 1774, however, Alexander Wilson, a Scottishastronomer, observed that spots had inclinededges, like the slopes of a crater, leading to adark interior inside the brilliant shell. Sir WilliamHerschel, the British court astronomer, proposedabout 1800 that a spot reveals the surface of acold, solid crust. Above this surface, he thought,were two cloud layers, the outer being brilliantand hot, and the inner, a cool, protective shield,shading the crust. According to this notion, aspot would appear when the clouds parted toreveal the underlying cool crust.Herschel went so far as to suggest that the darkinterior of the Sun supported intelligent life. Sogreat was his authority that the idea of a coolsolar surface persisted through most of the firsthalf of the nineteenth century, even though it issimple to calculate that suc h a sun could notshine for more than a day or two.Actually, the surface of the Sun, the photo-sphere, appears granular at its base, as though itwere paved with cobblestones. A sunspot beginsto form as a dark pore in the midst of the finegranular pattern. Soon several pores coalescewith each other to f o r m a spot. Sometimes thespot lasts only a few hours, but occasionally onewill grow and persist for weeks or months.


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ROTATIONVARIESThe shape of the spot most often resembles afunnel 400 or 500 miles in depth. In the darkcentral area the temperature is only about4,200C. This is hotter than the hottest blastfurnace on Earth; yet compared to the 5,700C.temperature of the surrounding photosphere,the spot appears cool and dark.A relatively small spot measures only a fewthousand miles in diameter, roughly the size ofthe Earth. The largest spot group on record, in1947, expanded to more than seven billionsquare miles.Sunspots act as markers on the clear disk of theSun and show us that its globe rotates f r o m eastto west, but in a very peculiar way. Unlike thesolid Earth, the Sun does not rotate uniformlyat all latitudes. A spot close to the equator, forexample, completes a rotation in 25 days; one at30 latitude takes 26 days; the rotation of thepolar zone may take as long as 34 days.Thus the gaseous Sun twists on its axis so thatthe equatorial regions rotate faster than the po-lar caps. Most of the changing features observedon the surface of the Sun must be related insome way to this contortion.During quiet periods of the 11-year solar cycle,months may pass without visible spots. At peakperiods, as in 1957, spots may number as manyas 25 at one time.Some people have tried to link the number ofsunspots to the number of admissions of mentalcases to psychiatric hospitals, to the behavior ofthe stock market, to the pattern of annualgrowth rings in trees, or to the catch of Atlanticsalmon. None of these proves out. But sunspotsare clearly connected with radio communica-tions, magnetic storms, and the auroras, ornorthern and southern lights.Some of the oldest indirect evidence of thecyclic nature of solar activity is documented inrecords of auroras, which are a direct product ofsolar bombardment. Unlike magnetic phenom-ena, which are revealed only by delicate instru-ments, the strange lights of the auroras can beseen at times by all the world.

Ancient peoples were terrified and awe-struckby the flaming, pulsating, brilliant red and greenglows. Aristotle wrote about them as longago asthe fourth century B. C. In the Middle Ages,auroras were often described as fiery dragons,burning spears, beams of fire, or divine revela-tions. Superstitious folk interpreted the infre-quent and sporadic nature of the heavenlyspectacles as portents of the end of the world.From such auroral accounts, science historianshave traced the 11-year sunspot cycle back morethan 2,000 years. By use of the spectroscope, aninstrument which breaks white light into itsfamiliar rainbow spectrum, we measure the mag-netism of sunspots. The magnetic field strengthis enormouscomparable to the most intensefields produced in modern particle accelerators,such as the Brookhaven synchrotron. ButBrookhaven produces such a powerful field overonly a few thousand square feet. When we con-sider that the sunspot field often covers an areabig enough to blanket ten earths, we know thata m a jo r portion of the energy in the solar atmo-sphere is bound up in magnetic fields.LIGHT YIELDS STAR'S SECRETSA s every high-school science student learns, amagnetic field can be established in the laborato-ry by a steel horseshoe magnet, or by an electriccurrent flowing in a coil of wire. Now there arecertainly no solid-steel magnets in sunspots, sotheir magnetism must come f r o m tremendouselectric currents, carrying as much as 10 millionmillion amperes.Some scientists suggest that these huge currentsoriginate in the highly convective gas sur-rounding the inner nuclear fur n a c e . The streamsof hot gas carry burned nuclear fuel outward,and cooler gases carry fresh fuel toward the cen-ter. Because of the rotation of the Sun, thesecirculating streams may be twisted into whirlswhich detach like smoke rings, rising to breakthrough the photosphere and thus to fo r m pairsof spots.W hy are sunspots relatively cool? Possibly gaswithin a spot f l o w s out along lines of magneticforce and cools by expansion.In the 1830's, the French philosopher AugusteComte wrote that man must reconcile himself to

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eternal ignorance of the composition of thestars. How utterly wrong he was! By analyzingthe quality of sunlight arriving f r o m 93 millionmiles away, we can tell what the Sun is made ofjust as accurately as if a sample of the star hadbeen brought to our Earthly laboratory.Atoms radiate light with precise frequencies thatuniquely identify different elementssomewhatlike the tones and overtones that produce thespecific color or quality of a musical instrument.The ear can pick out the various frequency com-ponents of instrumental sounds with muchgreater discernment than the eye can resolve thelight waves in a color mixture. But one of theastronomer's key instruments, the optical spec-troscope, does what the eye cannot, and permitsus to isolate the different "tunes," or character-istic frequencies, of every known atom.Perhaps the most remarkable accomplishment ofastronomy is the spectroscopic discovery that allstars are made of the same atoms we find onEarth.In 1814, Joseph von Fraunhofer, a youngBavarian lens designer, stumbled upon a mostsurprising phonemenon. He was trying to isolatepure colors f ro m sunlight, to test the refrac-tionthe bending of light raysby his telescopelenses. When Fraunhofer looked at the rainbowspectrum of sunlight with his instrument, henoticed many fine dark lines interrupting thesmooth progression of color f r o m red to violet.At first he blamed his glass for imperfections,but soon he became convinced that the darklines were a true feature of sunlight.The solar Fraunhofer line spectrum can be usedas a "fingerprint" of the elements in the Sun, foreach element shows its own combination oflines. Hydrogen, for example, produces a simplespectrum with just a few dark lines; iron hasmore than 3,000. By means of Fraunhofer lines,about 70 of the 92 elements naturally occurringon Earth have been identified in the Sun.Furthermore, the character of the spectrall ineswhether they appear sharp or f u z z y , darkor only half-shaded, slightly shifted toward thered or toward the blue end of the color spec-t r u m o f f e r s the astrophysicist tremendousamounts of information: He can deduce temper-

ature, pressure, density, and composition; thestrength of gravity, density of radiation, electricforce, magnetic force, degree of turbulence, andconvective movements in the region of the Sunwhere the spectrum line is produced.The Fraunhofer spectrum tells us that the Sunconsists principally of hydrogen. Hydrogenatoms are roughly 10 times as abundant there ashelium, the next most abundant element, and1,000 times as abundant as carbon, nitrogen, oroxygen, which are so common on Earth. Exceptfor the overabundance of hydrogen and helium,the chemical composition of the solar atom-sphere is much the same as that of Earth's crust.Like the other close-in planets of the solar sys-temMercury, Venus, and M arsthe Earth haslost most of its hydrogen and helium. But theJovian planetsJupiter, Saturn, Uranus, andNeptunebecause they are cold and very heavy,retain a great deal of the original hydrogen andhelium and thus more closely resemble the Sun.SUN'S EXPRESSIONC H A N G E SIn white light we see m a in l y the lower levels ofthe photosphere with its granules and sunspots.The Fraunhofer lines originate in the higher por-tions of the photosphere where it is coolerthedarker the line, the higher and cooler its regionof origin.In 1889, George Ellery Hale, father of theworld's largest optical telescope, the 200-incheron California's Palomar Mountain, invented amost useful variation of the spectroscope calledthe spectroheliograph. In essence it is a highlyselective filter than enables astronomers to nar-row down their view of the Sun to that of asingle line of the color spectrum. Thus as thespectroheliograph scans the face of the Sun, itsees only one color, such as the red line ofhydrogen or the violet line of ionized calcium.Each line is produced in the Sun at a level wherethe temperature is just right. Thus the spectro-heliograph can probe deeper and deeper into theSun's atmosphere, photographing the entire faceof the Sun at each level. And at each layer theface of the Sun takes on a remarkably differentcomplexion, and the expression is constantlychanging.


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Hovering near sunspots, self-luminous clouds re-semble f luffs of wool in white light. They arecalled faculae, Latin for "little torches." Thesurface of the Sun, when photographed with aspectroheliograph in the violet light of ionizedcalcium, takes on a mottled appearance, like theskin of an orange. Near the sunspots the mottlesconcentrate into bright patches called plages,French for "beaches."ECLIPSE ERROR FATALA few thousand miles above the photosphere,the solar atmosphere is so thinned out that itbecomes virtually invisible in the glare of thephotosphere. But when an eclipse masks the faceof the Sun, we see a very interesting profile.If the eclipse occurs at sunspot maximum, thecorona assumes a symmetrical shape with petal-like streamers resembling a large dahlia with theblack moon at the center. At sunspot minimum,great equatorial streamers stretch millions ofmiles, distorting the symmetry.Most of what we know of the Sun's outer atmo-sphere comes f r o m studies of the solar eclipse,perhaps the most dramatic of all nature'sspectacles.The earliest historical record of an eclipse datesback more than 4,000 years to October 22,2137 B. C., and is documented in the Chineseclassic Shu Ching. This book contains regula-t ions of the emperor regarding his royalastronomers and their eclipse predictions:"Being before the time, the astronomers are tobe killed without respite; and being behind thetime, they are to be slain without reprieve."Although eclipsemanship is no longer a matterof life or death, astronomers have often riskedgreat personal danger in eclipse expeditions. OneEnglish astronomer traveled 75,000 miles to sixeclipses, but because of clouds or rain saw onlyone. A French astronomer, Pierre Janssen, wasso intent on photographing the eclipse of 1870,during the Franco-Prussian War, that he riskedGerman rifle fire to escape f ro m the siege ofParis in a balloon. Unhappily, when he reachedthe eclipse path over the A f r i c a n coast, rain hidthe event.

In 1842, astronomers in southern Europe werethe first to take careful note of the very faint,extended outer atmosphere of the Sun. As themoon blocked out the brilliant disk, a pearlywhite corona with delicate streamers and curvedarches stood revealed. Close to the black edge ofthe moon, a reddish ring encircled the Sun,giving rise to the name "chromosphere." Fromthis ring, luminous red clouds and streamers ofga s called prominences looped high into thecorona.Each century sees about 237 solar eclipses. Ap-proximately one-fourth are total, and on theaverage two total eclipses occur every threeyears. Among other institutions, the NationalGeographic Society has been active in eclipse ob-servations, with nearly a dozen expeditions sincethe early '30's.But in spite of the most persistent efforts, morethan a century of eclipse studies has given us lessthan a hundred minutes' worth of observation!We still know relatively little about the truestructure of the chromosphere and corona.

T E M P E R A T U R E P A R A D O XPresumably the temperature of the Sun's atmo-sphere should get progressively cooler the far-ther one measures out f r o m the Sun's surface.Recall that the temperature deep in the thermo-nuclear furnace is about 16 million degrees, anddrops steadily to about 5,700 degrees at the sur-face. In the solar atmosphere we would expecteven cooler gas.But the spectrum of the chromosphere and coro-na reveals a very interesting paradox: The tem-perature there begins to rise again, shooting upto above 100,000 degrees in less than 10,000miles, and eventually climbing to several milliondegrees.How can the chromosphere and corona derivetheir high temperatures through a much coolerphotosphere?Astrophysicists believe that the seething, bub-bling granules at the Sun's surface break likeocean waves and create a tremendous rumblingroar of sound. As these waves of sound rush

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upward into more rarefied gas, they accelerateuntil supersonic shocks occur, which heat thegas to its high temperatures.Pictures of the rim of the Sun show a fountain-like structure. Thousands of tongues of gas,called spicules, spring as jets above the burstinggranules. They surge up f r o m the base of thechromosphere and fall back again in five to tenminutes, rising with speeds of 10 to 15 miles asecond to heights as great as 6,000 miles. Someof the spicules seem to vanish into the corona.At any instant as many as 100,000 spiculescover the face of the Sun, and for this reason thechromosphere has been called "the spray of thephotosphere."C O R O N A NOT STATICWith the coronagraph telescope, which artifi-cially eclipses the Sun's disk, we can see hugestreamers of bright gas often looping as high as ahundred thousand miles into the corona, anddipping back to the photosphere as much as halfa million miles away. These prominences, whenphotographed in time-lapse motion pictures,show continuous changes in their over-all shapesand complicated internal streaming.Prominences usually appear to spring f r o m sun-spot groups. Their arched structure indicatesstrong magnetic f ieldsjust as iron filings formcurved lines on a sheet of paper when a magnetis placed under them. Where the streamers areanchored to the photosphere, violent convectiontwists and shifts the lines about, causing thearches to react in spectacular whipping,streaming, and eruptive patterns high into thecorona.The corona is not a static atmosphere that blan-kets the Sun the way our own atmosphere hugsthe Earth. Because the corona is so hot, it con-tinually expands into spacerelatively slowly atfirst, perhaps a thousand feet per second. Butthe rising coronal gas accelerates rapidly, be-cause there is almost no interplanetary gas pres-sure to resist the expansion. Ultimately it mayreach 500 miles per second. This "solar wind" ofhydrogen steadily blows out through space andraces toward the Earth and other planets.

The wind that reaches Earth today left the solarsurface about 10 days ago. Actually it never pen-etrates the atmosphere since it is deflected byEarth's magnetic shield, which bulges outthousands of miles from the surface.How far does the wind reach? We are not sure,but calculating f r o m its speed and strength, itmust travel at least to Neptune, 30 times fartherthan Earth from the Sun, and possibly to Pluto,40 times farther than Earth.Modern eclipse expeditions took on a new lookin 1958, when scientists first attempted to userocket astronomy to determine which layers ofthe solar atmosphere emit X-rays and ultraviolet.The expedition was a joint venture of ground-based astronomers, under the leadership of Dr.John W. E v a n s of the Sacramento Peak Observa-tory in New Mexico, and a team of rocketspecialists f r o m the U. S. N a va l ResearchLaboratory, under my direction.The eclipse began at sunrise on the equator nearN ew Guinea and raced across the Pacif ic Oceanfor about 8,500 miles to the coast of Chile nearValparaiso, where it left the Earth at sunset. Inits long path^iever more than 150 miles widethe eclipse missed all the large South Pacif ic is-lands, and could be observed on land f r o m onlya few coral atolls.ROCKETS TAKE SUN'S P UL S EThe astronomers chose the atoll Puka Puka inthe Danger Islandsabout 2,300 miles south ofHo n o l ul u o n which to set up their spectro-graphs. To support the rocket part of the expe-dition, the N a v y provided a floating hotel,machine shop, and laboratorya landing shipcalled the Point Defiance.Our six solid-fuel rockets, 1,500-pound combi-nations of Nike-booster first stages and Asp sec-ond stages, pointed like arrows f r o m the deck.The Asp second stages would enter the eclipseshadow about 100 miles up, reach a peak of 150miles, and splash into the sea 60 miles asternabout six minutes after firing.Eclipse day dawned gray and overcast where thePoint Defiance lay to, 30 miles off Puka Puka.At 8:38 we fired the first rocket, 10 minutes

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before totality. Two more were fired during thebrief interval of totality. Sixteen minutes laterNo. 4 flashed into the sky. Rocket No. 5 balked,but the sixth rocket took off almost onschedule.When the smoke had cleared, our thoughtsshifted to our colleagues on Puka Puka. The sadstory that we picked up shortly after on theradio told of rain and clouds that completelyruined their observations. A year's preparationbefore embarking, months of effort on PukaPukaall had come to naught.When we scanned our radio telementry recordsf ro m the rockets, the signals clearly showed thatX-rays are produced high in the corona. Evenwith the Sun's disk covered, 13 percent of theX-rays remained unobscured. In contrast, the ul-traviolet rays were completely eclipsed at totali-ty, indicating that they originate at the fringe ofthe photosphere. Furthermore, as the M o o nblotted out individual sunspot areas, the X-rayf low diminished abruptly, proving that sunspotgroups emit concentrated X-rays.Sunspots, plages, prominences^these dramaticactivities of a quiet Sun pale into insignificancecompared to the explosive phenomenon knownas a solar flare. A large flare can erupt with theforce of a billion hydrogen bombs within anhour's time, releasing enough energy, if it couldall reach Earth, to melt the north and southpolar ice.This tremendous power is released by a brilliantburst of light and all other electromagneticwavelengths, f rom X-rays and ultraviolet to in-frared and radio waves; by protons and electronsaccelerated to more than half the speed of light;and by clouds of ionized, or electrified, gas thatsweep through space at hundreds of m iles persecond. It was such a flare that disrupted earthlycommunications s o strikingly in N o v e m b e r ,1960.CLOUD SPAWNSSOLAR SYSTEMSometimes a large flare can be seen in whitelight; in fact, the earliest record of a flare isprobably an 1859 account by an English astron-omer named Richard Carrington, who thoughthe had witnessed the splash of a gigantic meteor-ite.

Only a generation ago, most astronomers be-lieved that the solar system originated in a nearcollision between the Sun and another star, andthat the material of the planets was torn loosefrom the Sun by the tremendous gravitationalpull of the passing star.Today's view, however, holds that the Sun andthe planets condensed f r o m an enormous turbu-lent cloud of gas and dust. The Sun grew stead-ily warmer because of its immense gravitationalenergy. In time the protostar began to glowbrightly, and its core temperature rose millionsof degrees.Hydrogen nuclei, impelled by the tremendousheat, collided with such violence that thermo-nuclear fusion could occur, and nuclear energy,rather than gravitational energy, began to keepthe star hot.Some theoretical calculations indicate that theproportion of hydrogen in the core of the Sunhas decreased f ro m about two-thirds to aboutone-third in the past five billion years. Tempera-tures have risen somewhat, and the sun hasg r own about five percent larger in diameter andabout 25 percent more luminous. The great ma-jority of stars follow this gradual trend ofevolution.The Sun today is a very ordinary stara yellowdwarf midway between the largest and thesmallest, and between the hottest blue stars andthe coolest red stars. To Earth-based observers, itis a hundred billion times brighter than any oth-er star, though it would appear puny if it werematched at the same distance against the morebrilliant stars. Rigel, for example, is 15,000times more luminous, and 36 million suns couldbe fitted into Antares, a red supergiant.What of the future? Will the Sun burn out? Intime the core will deplete its hydrogen. With thecore spent, the thermonuclear reactions willspread to outer portions where unused hydrogenstill exists.A s the reaction zone moves closer to the surfaceof the Sun, the tremendous nuclear heat at itscore will also move outward, forcing the Sun toexpand, and the total amount of radiated heatand light will increase. The Sun will then be-come a giant red star like Antares: It will blow

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up to a monstrous ball of extremely rarefied,red-hot gas large enough to engulf Mercury,Venus, the Earth, and M a rs , the four nearestplanets.When will the sun reach this stage? We have nocause for immediate concern-^it may takeanother five billion years!SUN'S END IS D W A R F STARFinally, when all its hydrogen has been con-verted to helium, the Sun will cool and shrink,ultimately becoming a white dwarf no biggerthan the Earth but weighing several tons percubic inch.Not all stars reach this peaceful demise. Starsmuch more massive than the Sun end their evo-lution in a catastrophic explosion which fills vastregions of space with debris. Eventually this ma-terial recondenses into new stars. Our Sun issuch a second-generation star, and man on Earthis made of secondhand atoms left over f r o m astar that exploded before the Sun was born.

We know this because the Sun contains an ex-cess of heavy elements, such as iron, that couldnot have been produced by the simple nuclearburning of hydrogen, the primeval material ofthe universe.M ean w h i le , the Sun is our bridge to the stars. Itis the only star whose surface and atmospherew e can study in f i n e detail, and it typifies thegreat m a jo ri t y of stars in the M ilky Way. In itsspectacular flare outbursts, we can observe theinteraction of hot gases, intense magnetic fields,an d shock waves under conditions man cannotsimulate in his laboratories.But we stand today on the threshold of existingn ew knowledge. Rockets and satellites willprobe ever deeper toward the zones of intensesolar activity. With such m a g n if i c en t new toolsto observe the Sun, the c o m i n g years shouldbring a revolution in our understanding ofEarth's bright and awesome companion in theheavensand the myriad greater and lesser starsbeyond.

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SOLAR STUDIES IN PERSPECTIVEG. K. OertelN A S A HeadquartersS. D. JordanGoddard Space Flight CenterTHE SUN AND LIFE ONEARTHThe Sun provides virtually all of the heat, light,and general energy we use.For example, the Sun provides the energy thatdrives the atmosphere and is responsible for theweather. Variations in local solar input give riseto seasons. "Solar energy" lifts water vapor f ro mthe oceans and lets it rain down on land and inreservoirs for hydroelectric power plants. Oiland coal contain solar energy stored by lifeforms millions of years ago. The only exceptionin practice is nuclear energy.Interesting, but so what? After all, the Sun hasbeen pretty much the same and is believed to beincapable of changing its average output ap-preciably in less than hundreds of thousands ofyears. Ice ages? Maybe, but not for us or ourchildren. (Unless we manage to change ouratmosphere enough to alter the net solar energyabsorbed). So, why not leave the Sun alone andjust keep using its energy?Solar energy is free and comes in at a tremen-dous rate. Solar power farms are now understudy, such as one planned by astronomers AdenMeinel and his w i f e M a r jo r i e. We study the Sunto unlock its secrets: the Sun probably producesall of its energy by burning hydrogen to heliumin various ways. If we could do the same onEarth, we would have enough fuel in our oceansfor millions of years at current consumptionrates. The research ef f o r t in this field suffersf r o m the same types of problems as our at-tempts to explain solar flares and other plasma-magnetic field phenomena on the Sun.We also study the Sun because of its vagaries:the f lo w of solar energy is not all steady. A high-way carries mostly cars and trucks of moderatesize and speed, but small numbers of oversizedand overloaded trucks or speeding cars can causehazards and damage of many kinds.Similarly, some solar emissions are highly ener-getic or otherwise singular and can cause impor-

tant effects on Earth. M a n y of these effects arementioned or discussed in Dr. Friedman's 1965article in the National Geographic Magazinewhich is reprinted in this brochure: scrambledtelegram messages, interruption of shortwaveradio communications, and others.The magnetic storms which are caused by thearrival of solar plasma hours to days after solarflares can cause some curious effects, not onlyon a compass. Oilmen making electro-magneticmeasurements deep in wells f i n d their highlysensitive electrical instrumentation inoperabledue to interference f ro m magnetic storms. Com-mercial power systems have been blacked out byhigh voltage surges during magnetic storms. The1971 crash of an airliner in Alaska caused morethan 100 fatalities and occurred during a mag-netic storm due to an as yet unexplained electro-nic communications failure. It is still underinvestigation.Another area of investigation concerns theweather. One look at the energy involved in astorm system shows that the solar radiationswhich give rise to aurorae, communicationsblackouts, and magnetic storms are quite weakby comparison. Yet, we know that a single care-less skier, or the sound f ro m a shot fired nearby,can set off a deadly avalanche in the mountains,with effects which are incredibly greater thanthe cause. We also know that weather predictionis far f r o m perfect for some reason.Is there, then, any evidence that solar activityaffects the weather or the climate? M a n y expertsthink so. Others oppose the idea vehemently. Inthe avalanche, solar energy has been "used" tobring water f r o m the sea and store it as snow onthe side of a mountain until the mass is barelystable. Could solar energy be similarly stored inthe atmosphere to the point of near instabilitywhere a small effect such as solar high-energyradiation can trigger the formation of a stormsystem? This would not necessarily violate anylaws of physics, but the mechanism is notknown. Some possibilities have been proposed,but none are generally accepted. Investigationcontinues because there is really a good deal ofevidence.Douglass at the University of Arizona claimedearly in this century that the growth of trees

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evidence in the annual tree r ingsshows cyclicvariations with the 11-year solar activity cycle.He claimed that he could trace back the solarcycle for many centuries working simply f ro mtree rings.Just a few years ago Russians claimed that theyfound cyclic variations of deposits in variouslakes and that they could trace back the solarcycle for millions of years f rom these variations.A Russian weather forecaster in Siberia used so-lar measurements so successfully that he trig-gered a conference of Russian astronomers andmeteorologists in 1972 to discuss his approachand to improve weather forecasting throughoutthe Soviet Union.The National Center of Atmospheric Research( N C A R ) at Boulder, Colo., and scientists atmany academic institutions have maintained orrecently acquired a strong interest in solar activ-ity effects on the weather and climate. N C A Rand N A S A are jointly sponsoring a workshop onthis subject to be held at the Goddard SpaceFlight Center late in 1973, with the goal to iden-t i fy what is necessary to understand the phe-nomenon and to perhaps use it for the benefit ofall who are affected by the weatherand who isnot?Solar studies with Skylab and other observa-tories will help understand the way in which so-lar high-energy radiation is generated, how ittravels to Earth, and how much arrives here.Meteorological studies will pinpoint the reactionof the atmospherepredict the effects on cli-mate and weather. Perhaps, if we will learnenough, we will some day see the Sun and itsrelevant features on the TV screen and be giventhe weather forecast for a week or even a monthor be told to plant more of one or another cropnext year. Wedon't know the limitations to thissort of forecasting, but there is enough promiseto investigate.The study of the Sun is part of astronomy andas such a branch of basic research, done for thesake of expanding human knowledge about theuniverse in which we live.We don't always expect fringe benefits f rombasic research, but we often get them. When

Faraday studied electricity, its use was not fore-seen for any practical purpose, and Faraday issaid to have received support for this "uselesstomfoolery" only by claiming that some daysomebody would f ind a way to tax it.It is not easy to see how the Sun could ever betaxed. But it is a basic research effort with ex-ceptional promise for practical applications.The Sun is the center of the solar system. Itsradiations and emissions affect everything withinthat system, f r o m planets like Earth and M a r s tocomets and spacecraft.Its atmosphere expands out into space in asteady stream called solar wind and engulfs theentire solar system. In a real sense, we are livingon a cool island within the hot outermost layerof the Sun, and our spaceship Earth in its orbitaround the center of the Sun moves through thesolar wind trailing a large wake.The Sun is a giant laboratory in space in whichphysical processes can be studied on a scalew h i c h cannot be produced in the laboratory.M a n y discoveries were made in studying theSun, including the light element helium. Thehigh-energy processes of modern astrophysicshave counterparts on the Sun where they can bestudied in sufficient detail to arrive at an under-standing of the mechanisms involved. The Sunserves as a testbed of the theory of gravitationit did so centuries ago for Kepler and Newtonw ho discovered and explained the laws of plane-tary motion. (The fall ing apple that reputedlystruck Newton on the head did, of course, makeits own unique contribution). Einstein's theoryof gravitation (general relativity) is testedthrough solar and related studies, because theSun has the strongest gravitational pull of anybody close enough for detailed study.The Sun is a. star-Hhe only on e close enough foraccurate and prolonged study. It has served asthe dictionary for our reading of the universe'sbillions of other stars. The theories of stars mustpass their most rigorous testthe explanation ofthe Sun. The Sun's mass loss, through the solarwind, radiations, and perhaps even dust; itsspin-down through solar wind drag; its mysteri-ous oblateness which indicates a rapidly spinning

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inner core; its failure to produce the nuclear re-action products called "neutrinos" which couldconfirm or disprove the theory of nuclear energyproduction in its interior; the curious tempera-ture structure which a "cool" (10,000F) inneratmosphere supporting a hot (millions of de-grees) outer layer; the mysterious structures in

its atmosphere including sunspots, polar caps,streamers, and m a n y others; all are among themany riddles which the Sun is presenting, andwhich will require years of intensive study, f r o mspace, from the ground, and with pencil andpaper, before the answers will be in hand.Skylab will be a giant step for war d.

Skylab orbits Earth 270 miles in space. A suited astronaut changes f i lm in the Apollo Telescope Mount's instruments.The Apollo Telescope Mount is the space laboratory's solar observatory. Elements of the orbiting cluster are, from left:the Apollo spacecraft which carries the Skylab crew between Earth and the space station, the Multiple DockingAdapter, Airlock Module, and the Orbital Workshop which contains crew quarters and laboratory faculties. The118-foot-long Skylab contains about the same volume as a moderate two-bedroom house.

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SKYLAB SOLAR STUDIESEdward G. Gibson*N A S A Scientist-AstronautThe solar Apollo Telescope Mount (ATM) mis-sions represent the first step taken by our coun-try's space program toward a large and sophisti-cated manned space observatory. Thisopportunity to obtain data of unmatched qual-ity will advance appreciably our knowledge ofthe Sun as well as demonstrate the value andfeasibility of future manned astronomicalobservatories now in the planning stages.The success of the ATM missions depends to alarge extent on the scientific knowledge,training, and decision - making capabilities ofboth the astronauts and the ground supportteam.The opportunity to exercise scientific judgmentduring flight and to enhance significantly thevalue of the data returned, arises directly f r o mthe nature of solar observations.We are close enough to the Sun to see muchdetailed structure in its atmosphere. Because ofthe complexity of this structure and the widerange that has been observed in its characteristictime for change ( f r o m many years down to sec-onds), a wide variety of observations is possible.Thus, decisions must be made which determinethe amount of new and significant informationin the returned data.The role of the onboard observer can be simplystated. He is presented with television picturesof the Sun at several wavelengths in the electro-magnetic spectrum, as well as with other indica-tors of the state of solar activity. Instrumentswhich are capable of high data-acquisition ratesand which can be operated to observe only a*Dr. Gi bs on i s the Scientist A s t r o n a u t on the ThirdSk yla b M i s s i on.

small portion of the Sun are available for use bythe observer. However, he is constrained bylimited quantities of photographic film in all butone of the instruments. Hence, the scientificvalue of the returned data is dependent upon theability of the onboard observer to make judi-cious decisions concerning when, at what rate,and from where on the Sun to take data witheach instrument.By operating above the Earth's atmosphere, theA T M gains several advantages over ground-basedobservatories. On the ground, only the solar ra-diation that falls within several relatively narrowwindows in the optical, infrared, and radio re-gions can be measured. The remaining radiationis absorbed by the atmosphere. However, oncein orbit, instruments can measure directly thetotal spectrum, including the ultraviolet andX-ray portions, which promise new informationon the higher energy processes taking place onthe Sun.When the corona (the Sun's very tenuous outeratmosphere) is observed out to several solar radiiby occulting the disk of the Sun, only the regionclose to the s u rf a ce can be seen because of therelatively bright background light of the daylightsky. The bright background is caused by lightf r o m the Sun's disk which has been scattered byour atmosphere and is small, relative to the faintcorona, only during times of total eclipse. Be-cause a total eclipse lasts only a few minutes,observations from above the atmosphere are re-quired to study both the three-dimensionastructure of the corona as it rotates with the Sunand the full history of coronal processes.Finally as sunlight passes through our turbulentatmosphere, refraction limits the resolution withwhich the Sun can be viewed. However, on aspace observatory like the ATM, the resolutionof an instrument is limited only by the diffrac-tion limit of its optics and its pointing stability

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THE ATM SOLAR OBSERVATORYOwen K. Garriott*N A S A Manned Spacecraft CenterDixon L. ForsytheN A S A HeadquartersEugene H. CagleN A S A Marshall Space Flight CenterT he Apollo Telescope Mount (ATM) ha s beendesigned and developed to house and supportm a n n e d telescopes for studying the Sun. Inm a n y respects it achieves the operational capa-bility an d flexibility of some of our more-advanced instruments in ground-based observa-tories, but extends their range to wavelengthsvisible only above the Earth's atmosphere. It re-flects the strong desire of many leading astron-omers to conduct experiments f rom space,above Earth's obscuring atmosphere, atwavelengths below about 3000 A.T he 22,000-lb A T M contains an octagonally-shaped structure supporting most of the subsys-tem components and elements. This structuralf r a m e surrounds a large cylindrical canister, thehousing for the scientific instruments. T he cyl-inder itself measures approximately 7 ft in diam-eter and 11 ft long. An internal cruciform struc-ture divides the cylinder into quadrants to housean d support the 2200 Ib of experiment instru-mentation. It serves as an optical bench to pro-vide the necessary stability so critical toinstrument pointing and acquisition ofhigh-quality data.To help maintain this stability, a loop within theskin of the cylinder circulates liquid coolant.This thermal-control system is self-containedwithin the canister. The walls are composed ofcold plates that absorb the heat dissipated in theexperiment package. The water/methanolcooling fluid transfers heat absorbed from th ecold plates to radiators on the exterior side ofthe experiment canister, where it radiates intospace. This active coolant system maintains anaverage temperature within the experimentpackage of approximately 53 F. Each experi-ment, moreover, has its own thermal-controlheaters, designed to maintain its temperaturewithin 1 throughout th e length an d width of*Dr. Garriott is the Scientist Astronaut on the SecondSkylab Mission.

th e instruments. M a n y precautionary meas-ures have been taken to avoid an y fluid leakagewhich could contaminate the optical elements ofthe instruments. All fluid lines and componentsare located on the outside to avoid leakage intothe canister.Besides these thermal controls for the experi-m e n ts , a passive system regulates t h e A T M sup-porting rack structure and the componentsm o u n te d on it. And a thermal shield attached tothe "Sun" end of the canister an d rack mini-mizes solar heating of these system componentswhile t h e A T M points directly at the Sun fordata acquisition.T he Attitude an d Pointing Control System( AP CS ) consists of two separate but interrelatedcontrol subsystems. Its primary portion providesattitude control an d stabilization for the entireSkylab assembly. T he other, th e ExperimentPointing Control (EPC), stabilizes an d fine-points t h e A T M experiment package.

T he primary system consists mainly of compu-ters, sensors, cold-gas thrusters, an d three Con-trol Moment Gyros ( CM Gs ) . It controls th eSkylab attitude in the presence of disturbingtorques of both internal an d external orgin. In -ternal disturbances include brief transients pro-duced by crew motion. M o r e important will beth e small, but steady, venting torques, only afew tenths of a foot-pound in magnitude. Everyprecaution has been taken to keep these ventingtorques low. External disturbances consist pri-m a ri ly of gravity-gradient an d aerodynamictorques, which reach a magnitude of about 7f t - l b , but are largely cyclic an d thereforecontribute less to an accumulation ofm o m e n t u m . ->E ac h C M G i s mounted within double gimbals,and any two of the three c an provide the neces-sary spacecraft stability. Attitude control withCM Gs of fer s m ajor advantages for orbital opera-tions such as Sky lab's which require long dura-tion an d minimum contamination. Nominally,th e CMGs will n ot require assistance from eithercold-gas thrusters or small rocket motors thatare ordinarily used as control devices; thesecould contaminate th e optical surfaces of manyof the scientific experiments.

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The constant-speed CMGwheels are oriented asa group in the proper direction to absorb themomentum required to hold the entire Skylabattitude within 3 arc min of the desired direc-tion at all times. Venting torques and non-cyclic components of the external torques willgradually cause more and more momentum tobe stored in the CMG configuration. To avoid"saturation," the ATM digital computer ob-serves the way momentum is accumulated eachorbit and commands a small (


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P E R F O R M A N C E REQUIREMENTS FOR SINGLECMCDynamic Performance:Rotor Angular Momentum, \ft-lb-secRotor Speed, rpmRotor Acceleration Time, hrMaximum Torque, ft-lbThreshold Torque, ft-lbPhysical:Weight, Ib

Volume, cu ftMountingPerformance life, hr

23009100141220.1641816.74-point CG10,000

A great deal of attention has been given to theproblem of minimizing contamination of theenvironment around Skylab. Since water vapor,other gases, and particulate matter can have as ignif icant ef f ec t on the collection of undis-torted data, all materials used in Skylab werecarefully selected for low outgassing character-istics. All sources of venting from the Skylabassembly have been thoroughly reviewed withrespect to the nature of the vented gases andmaterials; and, where necessary, correctivemeasures have been taken.

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tion of several onboard visual displays. These in-clude not only the two Ha images mentionedearlier, but also an XUV image in a broad spec-tral band f r o m 170 to 630 A, a so f t X-ray imagein the region 2 to 10 A, a white-light coronalimage, an X-ray history chart record, a 6 cmradio monitor and a 0 to 8 A X-ray scintillationdetector.The two Ha telescopes have zoom lenses per-mitting the. astronaut to select a field of viewbetween 4.4 and 35 arc min. Spatial resolutionof the Ha video display is about 2 arc sec. Hawavelength discrimination is provided by solidetalon Fabry-Perot filters with 0.7 A half width.The Ha telescopes have movable crosshairs,which will be aligned to the entrance aperturesof the Harvard and NRL ultraviolet spectro-meters; pointing for the other instruments is lesscritical since they have wide fields of view.The broadband XUV monitor (provided byN R L ) has spatial resolution of only 20 arc sec,but it will reveal features in the transition layerand low corona such as "holes," bright coronalknots, or coronal flaring regions that are not evi-dent in the Ha display. Images f r o m the XUVmonitor will be transmitted to the ground on adaily basis to assist in-flight support activities atthe Mission Control Center, Houston.The X-ray monitor (part of the ASE instrument)will provide the astronaut with X-ray imageshaving a spatial resolution of about 1 arc minand will be used to monitor the X-ray emissionsof active regions.

of 30 arc sec and sensitivity of 1CT1 of the solardisk intensity; it should reveal the presence ofcoronal streamers, although its main purpose isto check alignment of the coronagraph.The ATM is generally operated f r o m the controlconsole by the astronaut who directly initiatesins trumen t observing sequences. However,during unattended periods, when astronauts areon board Skylab but not present at the console,limited operation of several of the instruments ispossible by ground command. Furthermore, theHarvard spectrometer, the ASE X-ray telescope,and the HAO coronagraph will also be operatedfor 8 to 12 hrs per day during the twounmanned intervals between the three-mannedvisits.W HI T E LIGHT CORONAGRAPH(HAO)The white-light coronagraph experiment willphotographically monitor the coronal brightnessand polarization from 1.5 to 6.0 solar radii overa wavelength band extending f r o m 3500 to 7000A . The instrument consists of an externallyocculated coronagraph designed to reduce theinstrumentally-scattered light to levels on the or-der of 10"10 B0, where B0 is the mean solarradiance. A removable camera contains a 750-ft.roll (8,025 exposures) of Kodak special fi lm 026- 02, a Panatomic-X-type emulsion with im-proved reciprocity characteristics. The camera isdetachable and will be replaced with additionalfilm-loads during astronaut extravehicularactivity.

These displays may be used, for example, to de-termine the location of a flare when the 0 to 8 Ascintillation detector, an automatic device in theASE instrument, indicates the onset of an X-rayburst with intensity above a predeterminedthreshold. Other indicators of flare onset includethe 6-cm radio burst detector and the X-ray his-tory event record (provided by Marshall SpaceFlight Center (MSFC) which will indicatewhether the general level of soft X-ray activityhas been rising or falling over the precedinghours.The white-light coronal display (derived f r o mthe HAO coronagraph) has a spatial resolution

The net angular resolution of the coronagraphfilm combination has been measured to be 8.2arc sec, corresponding to a distance of about6000 km in the corona. Because this resolutioncorresponds to a system response of about 3%for an input contrast ratio of 1.6:1, a somewhathigher spatial resolution should be achieved inthe actual coronal photographs.Because of the vignetting function caused by thepresence of the external disk assembly, the ef-fective coronal radiance is "flattened" over thefield of view. The net mean coronal brightness atthe fi lm plane varies by a factor of only 5 f r o m1.5 to 6 R, because of the vignetting action.

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Photograph of the solar corona made on June 5, 1973by the High Altitude Observatory White LightCoronagraph on Skylab, which views the corona f r o m1.5 to 6 solar radii from Sun center in the 3500 - 7000angstrom band. The photograph shows coronal formscaused by the interaction of electrons and magneticfields in the outer solar atmosphere. The bright disk ofthe Sun is occulted by the instrument. The black shadowat the bottom is caused by the support for the occulter,an d the white figure on the occulter is used forcalibration. (Courtesy of R. MacQueen High AltitudeObservatory)A step wedge, illuminated by sunlight, and cali-brated relative to the intensity of the mean solardisk over the range of 10~8 Ee to 10"1 B0 isimaged on each picture f ra m e by a supplemen-tary optical system. In addition, a television dis-play at the ATM console provides means for im-proving the quality of the data through directastronaut observation of the coronal image andthe pointing and internal alignment.Four picture-taking modes are available to thecoronagraph experiment. Two modes cycle threelinear polaroids through the field of view toallow determination of line-of-sight electrondensities in the corona. Two additional modesprovide rapid film-taking sequences for fol-lowing transient phenomena in the corona.X - R A Y( A S E ) SPECTROGRAPHIC TELESCOPE

The ASE X-ray spectrographic telescope has aprimary optical system consisting of a nestedpair of coaxial and confocal grazing-incidencemirrors of paraboloid-hyperboloid design. These

mirrors provide a geometrical collecting area of42-cm2 and form a soft X-ray image of the Sun1.92-cm in diameter. The field of view is 48 arcmin and the on-axis resolution is 2 arc sec. Theimage is recorded photographically on 70-mmKodak SO-212 film, a Panatomic-X-type emul-sion without an overcoating.A filter wheel with five filters and a blankopening is in the optical path and provides broadband X-ray filtergrams in the 3.5 to 60 A range.A n X-ray transmission grating with 1440 lines/mm can be inserted into the optical path to pro-vide spectrally dispersed images. The spectralresolution is 0.15 A and the grating will workbest for bright, small features such as flares.Several operating modes are possible. In thesingle mode, one sequence of exposures, each afactor of four longer than the last from 1/64 secto 256 sec, through a single filter is obtained.This mode provides s u f f i c i en t dynamic range toencompass virtually all expected coronal X-rayphenomena. To observe rapidly varying features,such as flares, the instrument can be switchedinto high rate mode wherein an abbreviated se-quence (e.g. 1/64 sec to 1 sec.) is repeated with

X-ray photograph of the solar corona obtained May 281973 by the American Science and Engineering X-rayTelescope on Skylab. The solar corona is the very thinouter portion of the Sun's atmosphere. Structures withtemperatures higher than one million degrees can beobserved. The loops, arches, and other features seen inthe photograph are produced by the interaction of theSun's magnetic field and the ionized gas of the corona(Courtesy of G. V a i a n a , Solar Physics Group, AmericanScience and Engineering, Inc., CambridgeMassachusetts)23


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an interval between exposures of 0.2 sec. Lessrapid time variations can be observed in the lowrate mode in which the interval between ex-posures is 12 sec. Aprogrammed mode is avail-able in which the instrument is operated in thehigh rate for f our m in and t he low rate for 9min. In the flare auto mode, operation is initi-ated automatically by the X-ray scintillationdetector.In addition to the above primary system, theinstrument contains an uncollimated X-ray scin-tillation detector and an X-ray "finder" tele-scope. The scintillation detector, which moni-tors the 0 to 8 A X-ray flux, provides a visiblean d an audible alarm to alert the astronaut tothe onset of a flare, controls automatic opera-tion of the film camera, and provides eight-channel pulse-height spectra in the range 10 to80 keV. The 0 to 8 A flux is displayed on theastronaut's display panel and is up-dated everysecond. By logarithmic compression a dynamicrange in flux of five decades is possible. T hepulse-height data are telemetered to ground anda complete spectrum is generated every eightsec. The "X-ray finder" telescope provides theastronaut with a TV image of the Sun, one arcm in in resolution, in the wavelength band 2 to10 A. The "finder" is aligned with the main tele-scope so that the astronaut can use it to point tobright features such as flares.The instrument can be operated during manned,unat t ended, or unmanned periods of the Skylab.However, during the latter two periods, the ca-pability of varying the experimental modes islimited.

X - R A Y TELESCOPE (MSFC)T he Marshall Space Flight Center (MSFC )-Aerospace instrument employs a glancing-incidence telescope to produce an image of theSun on SO-212 film. The telescope has two opti-cal elements: an internally-reflecting parabo-loidal primary element and a hyperboloidal ele-ment, one focus of which is coincident with thefocus of the paraboloid. It has an effective focallength of 190.5 cm and a collecting area of 14.8cm2, giving an effective photographic f-ratio off / 44. The resolving power of the instrument on-axis is limited by the film to approximately 3

arc sec. Off-axis the resolution is slightly de-graded by coma an d curvature of field. Thesealterations, together with vignetting, limit theuseful field of view to approximately 38 arcm i n .T he telescope operates at all X-ray and EUVwavelengths above about 5 A, but the responseis limited to certain wavelength bands of interestdefined by thin metal foils. T he filters arecarried on a wheel immediately in front of thefilm plane.Up to 7000 f ra mes of solar X-ray photographsm ay be taken with on e film cassette, containing1000 ft of SO-212 f i lm. Four such cassettes willbe used during the full Skylab mission. T he in-strument m a y b e operated in several modes: apatrol mode for observations of the quiet Sunand in the presence of moderate activity, an dactive an d flare modes employing shorter ex -posure tim es for the observation of activeregions and flares.In addition to the telescope, the M S F C instru-ment contains tw o proportional counters whichmonitor th e s of t X-ray flux from th e whole Sun.O n e o f these has an aluminum window (1.71m g / c m 2 ) and is sensitive in the wavelength re-gion 8 to 20 A, while th e other has a berylliumwindow (45.3 mg/cm2) and is sensitive in theregion of 2 to 8 A. The pulses f rom each counterar e sorted electronically into amplitude bands toperform coarse spectral analysis of the solarX-radiation. T he output from either counter c anbe displayed on the history plotter on the astro-naut's console, to give a guide to the rate ofchange of the solar X-ray flux, and hence to thelevel of solar activity.

U L T R A V I O L E T S P E C T R O M E T E R -S P E C T R O H E L I O M E T E R (HCO)T he Harvard experiment is designed to performsolar observations in the extreme ultraviolet( E U V ) wavelength range from approximately280 to 1350 A with a spatial resolution of 5 arcsec. A n off-axis parabolic mirror images the Sunon the entrance slit of the 0.5m concave-gratingspectrometer. Small rotations of the mirror per-mit the instrument either to (a) build up two-dim ensional rasters of a 5 arc m in region of the

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Sun in 5 min of time: or (b) to scan a singleraster line of 5 arc min length every 5 sec oftime, in order to study more rapidly evolvingphenomena. The iridium-coated f/12 mirror hasa 2.3-m focal length, producing a solar image21.4 mm in diamter.Light from a 5-arc-sec square portion of the so-lar image enters an EUVconcave-grating spectro-meter containing an original gold grating, ruledat 1800 mm"1, and the spectrum is imaged at afocal surface which contains seven independent,open-channel, electron-multiplier detectionsystems. In the reference grating position, theintensity f ro m the selected portion of the Sun isrecorded simultaneously at seven importantwavelengths . These wavelengths contain spectrallines which span th e temperature range from104 to 2 x 106 K, covering the chromosphere,transition region, and corona. A slight rotationof th e grating places Lya, Ly/3, Ly-y , and theL y m a n continuum in position f or simultaneousrecording. M a n y other polychromatic positionsof th e grating produce chance coincidences o finteresting groups of lines. T he grating c an alsobe positioned to select for study any desired sin-gle wavelength in the range 280 < 7 < 1350 A.A f t e r th e grating position is selected, squarerasters, single-line rasters, or continuous moni-toring o f a n y desired point ( 4 0 - m s time resolu-tion) m a y b e performed. In an important alter-native mode, the instrument with stationary mir-ror is positioned at a selected solar feature andthe grating is scanned continuously, thus ob-taining a complete spectrum of the feature with1.6 A resolution in 3.8 min.This instrument is capable of operation in them a n n e d , unattended, o r u n m a n n e d modes. How-ever, in the latter two cases the capability forprecision pointing at selected fine-scale structureis much reduced.XU V SPECTROHELIOGRAPH (NRL)The NRL extreme ultraviolet spectroheliographis a slitless objective grating spectrograph oper-ating over the wavelength range 150 to 630 A.Sunlight entering the instrument is both disper-sed and focused by a single concave grating (f-ocal length 200 cm, 3600 lines/mm) which isrotated between two positions to select eitherthe short wavelength range 150 to 335 A or the

Coronal loop prominence at the solar limb in the 417Angstrom emission line of Fe XV, recordedphotoelectrically by the Harvard College ObservatoryScanning UV Spectrcheliometer on Skylab, June, 1973.To obtain this picture, the instrument scanned over af i v e - b y - f i v e - a r c - m i n u t e area, recording the 417 Angstromemission from discrete five-by-five-arc-second segmentsof the solar atmosphere. (Courtesy of E. Reeves, HarvardCollege Observatory).longer range 321 to 630 A. The only otheractive optical element is a thin (0.1 micron) alu-m i n u m filter in front of the individual Kodak1 0 4 (formerly SWR) f i lm strips (35 x 258 mm),w h i c h acts to exclude stray light of wavelengthlonger than 835 A.The resultant solar spectrum appears as a seriesof superimposed monochromatic images of theSun, one for each emission line in the wave-length range. Some images are overlapped, espe-cially those below approximately 230 A thatarise f ro m highly stripped iron (Fe VIII - XVI),but other images such as He I 584 A, He II 304A , M g I X 3 68 A F e X V 2 8 4 A , a n d F e X V I 3 3 5A ar e fairly well separated.Because small, intense features are well separa-ted, spectroheliograms of flares and active re-gions can be obtained in several hundred emis-s ion lines~.T he field of view of the instrument is approxi-mately 60 arc min and the dispersion is 1.29A /mm. Spectral an d spatial resolutions are inter-dependent. The spatial resolution is 2 to 10 arc

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Section of a spectroheliogram obtained by the Naval Research Laboratory XUV Spectroheliograph operatingunattended on Skylab just prior to the beginning of manned operation. The principal image in this section is from theHe II 304 Angstrom line, in which coronal holes, the chromospheric network, active regions and limb features can all beobserved. The complete spectroheliogram from which this image was selected contains a series of images, dispersed inwavelength, from the various ultraviolet line emissions. (Courtesy of R. Tousey, Naval Research Laboratory)

sec, depending on the wavelength, and is best atthe central portion of each range and degradedat the ends of the range. The spectral resolutionis approximately 0.13 A f o r a well defined fea-ture 10 arc sec in extent. Time resolution de-

pends on the exposure time, w h i c h varies fromthe shortest exposure of 2.5 sec to prolongedm a n u a l exposures of up to 48 min. On the threem a n n e d missions there are respectively 200,400, and 200 film strips available.26


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UV SPECTROGRAPH (NRL)The NRL ultraviolet spectrograph is a double-dispersion, high-resolution spectrograph withspatial and spectral fields defined by an entranceslit. A primary mirror ( focal length 100-cm)f o r m s a solar image on a fixed slit. Light fromthe slit is diffracted by either of two pre-dis-perser gratings, which select the wavelengthband (970 to 1970 A, or 1940 to 3940 A), forf inal dispersion by the main concave grating(radius of curvature 200 cm; 600 lines/mm),w h i c h focuses the slit spectrum on the photo-graphic f i l m .T he pre-disperser gratings are ruled in 10 stripsof differ ing dispersion, approximating a contin-uously changing dispersion. This technique in-creases the speed of the instrument by reducingthe residual astigmatism to approximately 1 arcm i n ; it does not produce a spectrum havingspatial resolution along the slit. Eight spectra arerecorded on each Eastman Kodak-type 104 filmstrip. The entrance slit defines the spatial resolu-tion of 2 x 60 arc sec, and the two values ofwavelength resolution, 0.04 A a nd 0.08A , i n t h eshort and long wavelength ranges respectively,result from the slit width and dispersions of 4.2

DISKLIMB

A / m m and 8.3 A/mm in the two wavelengthranges. Time resolution varies from the shortestexposure of 0.15-sec to l o n g manual exposuresup to 48 minutes.The instrument contains a white-light, slit-jaw,video camera system using an image dissectortube which presents the astronaut with a displayfor pointing the instrument very near the solarlimb. This is also used to secure coalignment be-tween the NRL spectrograph, the HCO spectro-meter, and the Ha video display, thus making itpossible to observe the same solar features witht he U V an d EU V instruments an d also have aphotographic record in Ha that establishes theidentity of the feature. A third use of the white-light video system is to operate a servo-systemthat controls the primary mirror so that spectraacross the limb can be made at automaticallyselected positions that are held stable to 7!". Inthe three missions there are, respectively, 200,400, and 200 filmstrips available, each capableof eight exposures.S U M M A R Y O FCHARACTERISTICS I N S T R U M E N TT he principal characteristics o f th e s i x A T M i n -struments are summarized in the fo l l o w i n g table.

2160 2200 2240 2280 2320 2360 2400 2440WAVELENGTHSpectra in the 2170 to 2450 Angstrom band horn the Naval Research Laboratory ultraviolet spectrograph on Skylab.The illustration compares the photospheric Fraunhofer absorption spectra, photographed with the instrument on thesolar disk, and the chromospheric and coronal emission spectra photographed with the instrument aimed at the edge(limb) of the disk. (Courtesy of R. Tousey, Naval Research Laboratory)

Summary of ATM Instrument Characteristics

N a m eWhite-l ightcor on ag r ap hEU V spectrometer-spectroheliometerX-ray spectro-graphic tele-scopeX-ray telescopeXU V spectro-heliographUV spectrograph

InstitutionH A D

HC OA SE

M S F CN R LN R L

WavelengthR a nge3700-7000 A

280-1350 A3.5-60 A

3-53 A150-630 A970-3970 A

WavelengthR e s o l ut i o n

1.6 A

(see text)

(see text)0.13(10 arcsec)0.04-0.08

SpatialField1.5 - 6.0 R

5x5 arc min5 arc min x 5 arc sec5 arc min x 5 arc sec48 arc min

38 arc min60 arc min48 arc min

SpatialR e s o l ut i o n8.2 arc sec

5 arc sec

2 arc sec

2 arc sec2-10 arc sec2x60 arc sec

TemporalR e s o l ut i o n>40.5 s

5 min5 s4 0 m s>2.5 s

>3.5 s>2.5 s>0.15 s

U n m a n n e d /U n a t t e n d e dOperationY es

Y esY es

N oN oN o

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CREW OPERATIONS AND CREW TRAININGOwen K. GarriettJohnson Space CenterDixon L. ForsytheN A S A HeadquartersEugene H. CagleM arshall Space Right CenterCrew participation in the solar observationstudies should greatly increase the scientificreturn.For instance, many solar features will be re-solved for the first time at EUV and X-ray wave-lengthse.g., narrow filaments, supergranules,flare centers, prominence structure, and coronalstructure.A crewman on board the spacecraft will be ableto assure the proper target identification andtracking. With such high resolution (1 arc-seccorresponds to about 700 km on the solar disc),even solar rotation contributes as much as 9 arc-sec per hour of target motion and becomessignificant at longer exposure times.O f particular importance, the ATM instrumentswill see a variety of transient p h en o m en a a c t i v eprominences, perhaps coronal fluctuations con-current with radio noise bursts, filament oscilla-tion, and, of course, flares. These transientstypically may las