Extreme Ultraviolet Explorer Long Look at the Next Window

8/7/2019 Extreme Ultraviolet Explorer Long Look at the Next Window

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(NASA-TM-lO8618) EXTREMEULTRAVIOLET EXPLORER. LONG LOOKTHE NEXT WINDOW (NASA) 26 p

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"'Produce ships and sails that can be used Inthe celestial atmosphere. Then you will alsofind men to man them, men not atraid otthe vast emptiness of space. '_

Johannes Kepler


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RJ HAL COl Teli Table of Contents

Unexplored Window 3Extreme Ultraviolet Explorer 3Unique Spacecraft 3Advanced Technology 4Cosmic Survey 4Astronomical Windows 4New Secrets 5

Extreme Ultraviolet Radiation 6Roadblocks to Extreme Ultraviolet Astronomy 7

Accepted Knowledge 7Technological Limitations 7Geocoronal Interference 7

Extreme Ultraviole t Astronomy Becomes Possible 8Thin Bubble 8Apollo-Soyuz 9White Dwarfs 9

Exploring Space: Rockets, Explorers, and Observatories 10Scorpius X- I 10Uhuru 10

Opening the Extreme Ultraviolet I IThe EUVE Mission 12

Orbital Checkout 12All-Sky Survey 12Deep Survey 12Survey Phase 13Spectroscopy Phase 13Better Than Voyager 13Retrieval 13

The EUVE Instruments 14Smooth Mirrors 14Spectrometer 15Sir I saac Newton 15Cutting Edge 15

The Explorer Platform 16Solar Arrays 16Toaster Oven 1610-Year Lifetime 17Command and Control 17Tracking Satellite 17Newton's Law 17

What Will EUVE Find? 18Mapping Galactic Gas 18Not Ordinary 18White Dwars 19Neutron Stars 19Red DwarLs 20B Stars 21Dwarf Novae 21Io Torus 22Quasars 2:2

EUVE Mission Management 24EUVE Team 25


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Unexplored WindowsA NASA satellite designed to scan theheavens in one of the last largely unex-plored windows of the electromagneticspectrum will be launched from CapeCanaveral, Florida in 1991. The ExtremeUltraviolet Explorer (EUVE) will be avital element in astronomers" efforts tounderstand the universe by discoveringwhat objects are present, and how theyform, change, and die. Scientists wantto learn what natural processes are atwork in the alien environment of space,where the everyday physical conditionsare often so unl ike those on Earth thatthey cannot be duplicated usefully inthe laboratory.

Extreme Ultraviolet ExplorerEUVE will map the entire sky to

determine the existence, direction, bright-ness, and temperature of thousands ofobjects that are sources of so-calledextreme ultraviolet (EUV) radiation. TheEUV spectral region is located betweenthe x-ray and ultraviolet regions of theelectromagnetic spectrum. From the skysurvey by EUVE, astronomers will deter-mine the nature of sources of EUV lightin our galaxy, and infer the distributionof interstellar gas for hundreds of lightyears around the solar system. It is fromthis gas and the accompanying dust inspace that new stars and solar systemsare born and to which evolving anddying stars return much of theirmaterial in an endless cosmic cycle ofbirth, death, and rebirth. Besidessurveying the sky, astronomers willmake detailed studies of selectedobjects with EUVE to determine theirphysical properties and chemicalcompositions. Also, they will learnabout the conditions that prevail andthe processes at work in stars, planets,

and other sources of EUV radiation,maybe even quasars.

EUV radiation, a principal emission ofmany types of celestial objects, is whollyblocked by the Earth's atmosphere.Until recently, it was believed to beblocked by the interstellar gas of ourMilky Way galaxy as well. As such, itwas sometimes known as "'theunobservable ultraviolet."Unique Spacecraft

The EUVE will be a spacecraft of anew type. Its scientific instruments willbe mounted in a single module on anorbiting platform, or spacecraft "bus,"like containerized cargo that is easilytransferred from an ocean freighter to arailway flatcar. The platform will provideelectrical power, communications,mechanical support and pointingcontrol for the EUVE module. AlthoughEUVE will be launched on anexpendable rocket, a new module,containing new scientific instruments,will be transported to it by the SpaceShuttle once the EUVE mission iscompleted. The new module will be

PRECEDING PAGE BL_NK NO) I'ILMLD

Three examples of interstel lar gas and dust.LEFT: The North American Nebula inCygn usTOP: Ring Nebula in LyraABOVE: Orion's Horsehead Nebula

RIGHT: Artist conception of the EUVEspacecraft in orbit


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swappedortheEUVEnitbythecrew,usingheShuttle'semoteManipulatorystem.hisdesignllowsreuseofunmannedpacecraftustastheShuttlellowseusefamannedvehicle.hisconceptlsoallowsorreadyservicingnspace,houldpace-craftsubsystemsrinstrumentsegrade.TodevelopheEUVE,ASAenlistedtheUniversityf CaliforniatBerkeleyandtheNASA/GoddardpacelightCenternGreenbelt,aryland.heBerkeleystronomersndphysicists,ledbyProfessortuartBowyer,avepioneeredheexplorationfspacentheextremeltravioletithinvesti-gationsysoundingocketsndmannedpacecraft.lsoBerkeleyasdevelopedhenecessaryechnologyandnstrumentsorEUVastronomy,andpioneeredhescientificinterpretationftheresultsfsuchexperiments.oddardasaproudhistorysthedeveloperf unmannedExplorer-classndobservatory-classsatellites,world-wideracking/communicationsetwork,ndassociated

groundontrolcenters.oddardlsopioneeredhedevelopmentndoperationfthefirstreusableunmannedpacecraftesignedpecificallyforrecoveryndservicingnspace.Advanced Technology

EUVE will carry a full complement oftelescopes, detectors, and a spec-trometer. The equipment incorporatesadvanced technology from the UnitedStates and Japan. The EUVE instrumen-tation will be mounted in a PayloadModule on NASA's new reusable ExplorerPlatform, designed and managed by theGoddard Space Flight Center (GSFC)and built by the Fairchild SpaceCompany. The Payload Module wasdesigned, built and tested at GSFC.Cosmic Survey

The scientific mission of EUVE willconsist of a six-month all-sky survey, inwhich the heavens are mapped in fourchannels of the extreme ultravioletspectrum while a narrow band in thesky is mapped at even greater sensitivityin a deep-sky survey. This will be

followed by a spectroscopy phase of atleast one year.

In the spectroscopy phase, individualtargets, whether discovered in the all-skyand deep-sky surveys or identified fromother information, will be analyzed indetail through individual observationsmade with an on-board EUV spectrometer .Typically, a spectrometer observationwill last from one to several days. TheEUVE surveys will be conducted by theBerkeley astronomers, while the spectro-scopic studies will be performed byGuest Observers who may be associatedwith scientific or educationalinstitutions. NASA Headquarters, inWashington, D.C., will select theseGuest Observers from throughout theUnited States and around the world, onthe basis of scientific merit.

When the extreme ultraviolet astronomyresearch is complete, a Space Shuttlewill dock with the EUVE so that theShuttle crew can bring it onboard withthe Remote Manipulator System (robotarm). The extreme ultraviolet astronomypayload will be replaced with thepayload for a new scientific investi-gation, the X-ray Timing Explorer (XTE).Astronomical Windows

The EUVE studies come as a logicalcomplement to past Explorers that havescanned space in the infrared,ultraviolet, x-ray, and gamma-rayregions. As each of these so-called"windows" of the universe has beenopened to detailed study by Explorer-class instruments, wholly unexpectedobjects and phenomena have beendiscovered, and unusual physical

Milky Way Galaxy as a satellite streaks infront of it

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processes have been found to be atwork. Astronomers learned that theycan no more study our galaxy--or asingle star--in proper depth with onekind of telescope operating in a singlespectral region, than biomedicalresearchers can study the human bodyFinal inf lation stage of balloon carryingscientif ic instruments at sunrise nearPalestine, Texas.

with a single kind of microscope,chemical tool, or analytical procedure.Just as each kind of biochemicaltechnique involves different types oflaboratory apparatus or a different setof chemical reagents, each form ofelectromagnetic radiation or lightrequires distinct types of telescopes,associated instruments, and sensors forits study.New Secrets

Each new window brings unexpectedmarvels into the astronomers' view.Mapping the sky in infrared light, theInfrared Astronomical Satellite, a jointUnited States-United Kingdom--Netherlands spacecraf t, discoveredwhat may be planetary systems information, circling stars beyond theSun, and the eerie glow of dust trails oflong-vanished comets that once orbitedin our solar system.

Einstein, as the High EnergyAstrophysics Observatory-3 satellite waspopularly known, found andinvestigated binary stars in the MilkyWay and a neighbor galaxy in which

one member is almost surely a blackhole. A black hole is a condensed deadstar whose gravity is so powerful thateven light cannot escape it.

The International Ultraviolet Explorer(IUE), operated jointly by NASA, theEuropean Space Agency and the UnitedKingdom Science and EngineeringResearch Council, found a hot coronasurrounding our Milky Way galaxy. IUEalso discovered a glowing shell of gascaused by the collision of high- andlow-speed winds emitted by a star thatlater exploded as a supernova, and apreviously undetected form of sulfur ina comet that made a close swing pastthe Ear th.

Telescopes sensitive to gamma raysaboard balloons, rockets, and the SolarMaximum Mission satellite detected aspectral emission from the centralregion of the Milky Way that is causedby the mutual annihilation of matterand antimatter. The message is clear:open a new window on space and weare sure to find as-yet-unknown secretsof the universe.

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Extreme Ultraviolet RadiationEUV radiation is defined as the range

of the electromagnetic spectrum thatlies at shorter wavelengths (higherenergies) than ordinary ultraviolet lightand at longer wavelengths (lowerenergies) than x-rays. As such, itconsists of light with wavelengthsbetween about 100 Angstroms and1000 Angstroms. (One Angstrom, a unit oflength named for the Swedish physicistAnders Angstrom, equals one one-hundred-millionth of a centimeter, orabout four billionths of an inch.)

Both ordinary ultraviolet light andx-rays also are blocked by theatmosphere. They are known to reachthe Earth's vicinity from very greatdistances in space, and have beenextensively studied by satelliteobservatories above the atmosphere,developed by the United States andother technologically-advanced nations.For example, ultraviolet radiation fromstars, nebulae and galaxies was studiedby NASA's Orbiting AstronomicalObservatories and by the AstronomyNetherlands Satellite, and is underinvest igation by the InternationalUltraviolet Explorer.

X-rays from stars and galaxies havebeen explored by the High EnergyAstrophysics Observatories, the SmallAstronomy Satellite "Uhuru," and bysuch foreign craft as ESA's EXOSAT,Japan's TENMA and GINGA, Germany'sROSAT and the Soviet Union's MIR SpaceStation. Yet, despite almost two decadesof work, exploration of space in theextreme ultraviolet has been minimal.

Sagittarius star cloud in the direction of the center o/the Milky Way

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Naval Research Laboratory experiment aboard Apollo 16 pro_.idedthis image of our geocorona

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Roadblocks to Extreme Ultraviolet AstronomyThe study of the universe in the

extreme ultraviolet, still in its infancydespite space experiments flown since1971, has been hampered by threefactors that astronomers can nowsurmount with the EUVE: accepted"knowledge," geocoronal interference,and technological limitat ions.Accepted Knowledge

Conventional wisdom on our MilkyWay galaxy was painstakingly developedthrough extensive observations withradio and optical telescopes on theground during the 1950s. Theseobservations suggested that the disk ofour spiral-shaped galaxy is pervaded byan interstellar medium of hydrogen,helium, and less abundant gases. Bothhydrogen and helium absorb extremeultraviolet light. Calculations indicatedthat the hydrogen alone was enough tocut off the extreme ultraviolet light fromalmost any known object beyond ourown solar system, which is located inthe galactic disk. As early as 1959,experts asserted that it would beimpossible to observe objects muchbeyond the limits of the solar system inthe extreme ultraviolet. It was not until1975 that a crucial experiment inspace, carried out by Professor Bowyerand his associates on the Apollo-Soyuzmission, disproved this view.Technological LimitationsExtreme ultraviolet light cannot becollected and focused usefully withtelescopes of conventional design; ifordinary optical telescopes were used,most of the individual photons ofextreme ultraviolet light would beabsorbed in the coated surfaces of themirrors or scattered in directions awayfrom the optical axis.

The extreme ultraviolet can beobserved usefully only with telescopemirrors designed for operation ingrazing incidence mode, in which theincoming light strikes a mirror at a verysmall angle to its surface like a stoneskipping off the sea. Thus, in anextreme ultraviolet telescope, theprimary or light-collecting mirror doesnot face directly toward the target likethe mirror in a conventional telescopeor a searchl ight.

Instead, while the EUV telescopepoints at the target like a gun, its

primary mirror is positioned roughlylike the inner surface of the gun barrel,or like the inner surface of a coneopening toward the incoming light.

Even if appropriate grazing incidencetelescope mirrors are used, problemsremain. When the extreme ultravioletlight strikes the carefully shapedtelescope mirrors at angles of a fewdegrees to their surfaces, the slightestlocal irregularity interferes with theskipping light rays, scattering them inwrong directions like golf balls that hitslight irregularities on the green andveer away from the cup. Accordingly,the mirror surfaces must be made withexceptional smoothness, even by thedemanding standards of the optician.

There are other technologicallimitation as well. When EUVastronomy began in the 1960s, theavailable detectors--the devices thatactually sense the EUV radiationcollected by a telescope--wererelatively insensitive. Also, there wereno diffraction gratings--opticalcomponents that spread light into aspectrum--that were well-suited to usein the EUV. The EUV represented sucha new departure for astronomers'studies that there was no nationalstandard for calibrat ing laboratorymeasurements of the intensity of EUVlight.Geocoronal Interference

At the outer limits of the Earth'satmosphere, far above almost allorbiting satellites, the ambient gases areso thin that a gas atom can orbit all theway around the Earth without strikinganother atom. Under such conditions,the gases in this region readily escapethe Earth's gravity. They make up thegeocoronoa, a huge region that thinsout into interplanetary space.

In the geocorona there are alsohelium ions, atoms of helium that haveeach lost one electron. These ions areprevented from escaping into space bythe Earth's magnetic field. The heliumions scatter, that is, reflect in alldirections, EUV light emitted by similarhelium ions in the Sun.

As a result, if space travelers on theMoon or Mars were to look down onthe Earth with eyes somehow sensitiveto extreme ultraviolet radiation, theywould see a huge luminous zone

around our planet, consisting of thegeocoronal helium ions glowing in theextreme ultraviolet light of the Sun.

Other constituents of the geocoronaalso scatter solar radiation. Locatedabove the Earth, but below or withinthe geocorona, satellite telescopes areseverely hampered by this geocoronalglow when they operate in the extremeultraviolet. At some wavelengths, theglow may be thousands of timesbrighter than a celestial target ofinterest when viewed with a givenextreme ultraviolet telescope. Thereappear to be only two ways tominimize this interference: locate thetelescope outside the geocorona oroperate it in a highly selective manner.

Positioning a spacecraft such as EUVEoutside the geocorona would make itimpossible to service it with the SpaceShuttle in the event of malfunction, andimpossible to revisit it with the Shuttleto replace the payload at the end of itsintended operating life. Therefore,EUVE will be operated in low-Earthorbit, below the geocorona, butgenerally will make observations atnight, when the geocorona causes theleast interference.

A remaining source of naturalinterference, called the very localinterstellar medium, hampers EUVobservations and cannot be overcomewith current technology. The very localinterstellar medium consists ofelectrically neutral atoms of interstellargas that sweep through the solar systemas the Sun and the planets movethrough the Milky Way. Helium atomsin this medium scatter EUV radiationfrom helium in the Sun, producing adim, interfering glow that EUVastronomers cannot avoid.


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Extreme Ultraviolet Astronomy Becomes PossibleRecent advances in space astronomy

and in technology make it possible forthe EUVE mission to explore space in awavelength window previously calledthe "unobservable ultraviolet." Theinterstellar gas of the Milky Way wasonce thought to be a smoothlydistributed, absorbing fog thatthoroughly blocks extreme ultravioletlight. On further inspection, it wasrevealed to be a complex array ofdense clouds embedded in a thinnerand hotter gas, honeycombed by verythin regions shaped like bubbles andtunnels, like an ant's nest or a rabbitwarren beneath the seemingly solidground.

Detectors, diffract ion grat ings,mirrors, filters, and calibration methodshave been developed and optimized foruse in the EUVE. Detectors, the crucialcomponents that sense the radiationcollected by the telescopes, provide agood example. With a telescope of agiven size, a more sensitive detectorallows the astronomer to discover andmeasure fainter sources. At first, thedetectors available for sensing EUVradiation were not very sensitive.

The Berkeley scientists developed so-called photon counting detectors forthe EUV, work recognized by theaward of patents and the publication ofmany technical papers. At first, thedetectors were simple photometers thatregistered only the intensity of EUVradiation, but now high-resolut ionimaging detectors can sense theposition of each incoming photon ofEUV light within the field of view. Theycan record also the exact moment atwhich the photon was received. Likethe improvements in the othertechnology areas, the improved EUVdetectors may prove beneficial inapplicat ions outside astronomy.Thin Bubble

Experimental EUV telescopes flownon manned spacecraft, unmanned

LEFT: Schematic of EUVE detectorRIGHT: One of the EUVE flight detectors

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ABOVE: Ameri(an Apo/lo spa_ecraflphotographed by So_iet cosmonauts in jointUS/USSR Apollo-Soyuz programLEFT."Artist's conception of the Copernicusastronomical observatory

sounding rockets, and interplanetaryprobes have detected about a dozenEUV-emitting stars in the Milk Way.They have revealed also that there arelines of sight along which we can seeto distances of many light years fromthe Earth (one light year equals about5.9 trillion miles, or 9.5 trillionkilometers).

Further, from studies by NASA'sCopernicus satellite (Orbit ingAstronomical Observatory-3) and theIUE, it appears that our solar system,including the Earth, is located in one ofthe thin bubbles of interstellar gas, anideal location from which to explore tosignificant distances in the extremeultraviolet. As a result of these findingsit appears that the EUVE, rather thanbeing blocked from observing beyondthe solar system by the interstellar gas,will be able to survey objects in ourgalaxy out to a distance of about 300


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lightyearsnmanydirections.UVEwillviewtoevengreateristancesnalimitednumberfdirections,erhapsincludingfewlinesof sightextendingwhollyoutofourgalaxy.Apollo-Soyuz

Evidence that EUV astronomy isfeasible came during the 1975 Apollo-Soyuz Test Project, a manned missionwith on-orbit rendezvous and dockingof space capsules from the UnitedStates and the Soviet Union (theApollo-Soyuz work followed a series ofinconclusive EUV astronomyexperiments by sounding rockets).

After the joint activities of theastronauts and cosmonauts wereconcluded and the Soviet Soyuz-19capsule separated from Apollo, theNASA crew repeatedly oriented theircommand and service module to pointa 14-Y2-inch EUV telescope designed atBerkeley at 30 preselected celestialtargets. Five of the targets weredetected, including one, the unusualhot star HZ 43 in the constellationComa Berenices. HZ 43 was such astrong source of extreme ultraviolet

light that it was recognized when rawdata telemetered from Apollo weretraced on a chart recorder at the NASAJohnson Space Center in Houston,Texas.White Dwarfs

The 1975 discovery of intenseextreme ultraviolet radiation from HZ43 was a triple milestone inastrophysics. It established the feasibilityof exploring the galaxy through theextreme ultraviolet window. It identifiedHZ 43 as the hottest and mostluminous white dwarf star then known.It proved also that white dwarf stars,although dim as seen in ordinary visiblelight, may be beacons in the heavenswhen viewed through the extremeultraviolet.

White dwarf stars are the finalevolutionary stage of stars such as ourSun. They burn hydrogen by nuclearreaction for billions of year, then blowup into helium-burning red giants,throw off their outer layers andeventually become hot, denseobjects--the white dwarfs--that cooland fade thereafter throughout eternity.

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Through three decades of the spaceage, astronomers and space engineershave developed a proven method forthe exploration of space through newwindows in the electromagneticspectrum. There are so manyunknowns when a particular spectralwindow on the universe is yet to beexplored, that it is difficult and eveninappropriate to plan a major spacefacility for that purpose.

Instead, simple experiments are f irstconducted in which relatively crudeand inexpensive measurements aremade from low-cost platforms to learn"'What's out there?" and see what kindof equipment functions well in space.

Unusually hot star HZ 43 detected bytelescope aboardApo/Io/Soyuz spacecraft

Scorpius X- 1Thus, the pioneering observations in

x-ray astronomy were made fromsounding rockets, with results thatincluded the discovery of the firstknown x-ray source beyond the solarsystem, the strange binary star, ScorpiusX-1. The results of these suborbitalstudies in x-ray astronomy showedscientists how to design appropriateinstruments for simple exploratorysatellites.Uhuru

Explorer 42, better known as SmallAstronomy Satellite-I, or Uhuru, wasable to catalog hundreds of x-raysources around the sky, and to recordimportant physical data on Scorpius X-Iand many of the others. From this andeven more sensitive observations by thelater Einstein satell ite, ast ronomerslearned that the sky is furl of intensesources of x-ray emission that bl ink onand off, occasionally flare to greatintensity, and constantly change likethe flashing colored lights on aChr istmas tree.

The ultimate stage in exploring aspectral window is to follow the surveysmade by Explorer satellites, once thedata have been carefufly studied, with apowerful, highly instrumented spacecraft.The future Advanced X-ray AstrophysicsFacility (AXAF) of the "'GreatObservatory" Class is one example.

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Opening the Extreme UltravioletThe initial explorations in EUV

astronomy were conducted bysounding rocket experiments and bythe experiment on the Apollo-SoyuzTest Project Astronomers at otherUnited States institutions and in Europealso contributed to EUV astronomy.

Additionally, the Voyager 1 and 2interplanetary space probes, whichwere sent to Jupiter and beyond,carried spectrometers Developed bythe University of Southern California,these probes are still obtaining valuablespectra in a portion of the EUV rangefor selected bright targets.

EUVE is likely to detect and studyclasses of EUV targets such as whitedwarf stars and flare stars, early type-B

stars with shock-wave-heated winds,exploding stars called dwarf novae, andothers These studies laid thefoundation for conducting an all-skysurvey in four EUV wavelengthchannels with EUVE, for making acomplementary survey in a portion ofthe EUV with the German-U.K.-U.S.cooperative mission, ROSAT, and forthe deep survey of a limited sky regionby EUVE, along with EUV spectroscopyon selected EUVE targets

Once the observations of the EUVEare completed and analyzed, investigatorswill determine whether the phenomenadetected justify further study with aneven more advanced observatory.

Voyager- 1 photo of Jupiter and two of itssatellites, Io (left) and Europa, aboveJupiter 's Great Red Spot

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Survey PhaseThe EUVE spacecraft will rotate

around its spin axis. The three surveytelescopes or "scanners" will point outin the equatorial plane of the EUVE, sothat each telescope scans a strip fivedegrees wide along a great circle in thesky. (Such a strip is about 10 times aswide as the full Moon.)

One end of the rotation axis of theEUVE will be kept pointed at the Sun.As the Earth moves halfway around itsorbit of the Sun during six months, andEUVE continues to revolve around theEarth, each great circle traced by anEUVE scanner will slowly sweep all theway around the sky, and each of thethree scanners will separately map thewhole sky. At the same time, the deepsurvey telescope will always point alongthe rotation axis in the directionopposite the Sun. Accordingly, it alwayswill point down the Earth's shadowcone and will trace out half of a greatcircle on the sky during the six-monthinterval of the all-sky survey.

LLFI: 7rif id Nel)uAi in Sa_itt ,triu_ABOVE: /_,tronaut ,it end of Space Shuttle'sremote manipulator arm approaching SolarMaximum Mission spacecraf t to make repairs

Spectroscopy PhaseFrom the many interesting objects

that will have been discovered duringthe EUVE all-sky survey and otherobjects already known from otherastronomical investigations, GuestObservers will choose targets fordetailed analysis. Using the EUVspectrometer that incorporatesadvanced diffraction gratings, the GuestObservers will study these targets.Better Than Voyager

Because EUV radiation arriving fromthe stars is generally so weak thatindividual photons of extremeultraviolet light must be separatelydetected and counted, the exposuretimes for these spectroscopic studiesare likely to range from one day toseveral days per observation. The resultwill be detailed spectra, which willextend over the full range of extremeultraviolet wavelengths, with 10 timesthe spectral resolution of the EUVspectrometers on the Voyager probes.(The Voyager instrumentation wasmuch less sensitive in the EUV than thespectrometer and it was possible toobserve only a limited number ofpreviously known objects; no surveywas conducted.)

As the deep survey telescope collectslight simultaneously for thespectrometer and for its deep surveydetectors, the fields of view around thespectroscopy targets will be surveyedtoo. The Spectroscopy Phase is expectedto last at least one year.

RetrievalAfter the EUVE mission is completed,

the Space Shuttle will rendezvous withEUVE so the spacecraft can be broughtonboard the Shuttle by a MissionSpecialist operating the RemoteManipulator System, a 15-meter(49-foot) mechanical arm equipped withgrapples. The EUVE Payload Modulewill be removed from the ExplorerPlatform on which it is mounted, andan X-ray Timing Explorer (XTE) PayloadModule will be installed in its place. Ifnecessary, platform subsystems can bereplaced.

The Explorer spacecraft, nowoperating as the X-ray Timing Explorer,will be released in orbit. The XTE willinvestigate rapidly varying phenomenain cosmic x-ray sources, such asoutbursts and oscillations in x-rayemissions. The intended operatinglifetime of the Explorer Platform,starting with the EUVE launch, is 10years, so that other experiments can bemounted later.

BEZOt! /: Schematic showing EUVE inrelat ionship to Earth 's shadow and our Sun

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The three scanner telescopes, and thedeep survey telescope/spectrometer ofthe Berkeley-developed EUVE payloadrepresent the state-of-the-art in extremeultraviolet astronomy. The instrumentsand their associated electronicspackages are mounted in the PayloadModule, which is installed as a unit onthe Explorer Platform. Each telescopeuses metal mirrors that reflect EUV lightat grazing angles, like the rising Sunseen mirrored in the sea. Thetelescopes are equipped with filtersmade from thin films of metals andother substances, layered to isolatedesired regions of the EUV spectrumfor observation. The reflecting telescopeconcepts are derived from thoseproposed in the 1950s by the Germanphysicist, Hans Wolter, who attemptedto design an x-ray microscope.

Each of the three EUVE scannertelescopes is about as large as a55-gallon oil drum and weighs about 188kilograms (about 260 pounds). The deepsurvey telescope/spectrometer weighsabout 323 kilograms (about 710 pounds).

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ABOVE: Technicians working onspectrometer in large thermal vacuumchamberSmooth Mirrors

Different mirror designs are used forscanner telescopes that observe thelonger and shorter wavelength EUVlight. This is necessary to efficientlycollect the light while discriminatingwhere necessary against x-rays thatwould otherwise contaminate themeasurements made through some ofthe filters. For the same reason, someof the mirrors are gold-coated toincrease EUV reflectivity, while othersare left without such coatings in orderto attenuate x-rays that strike theiramorphous (non-crystalline) nickelsurfaces. The metal mirrors were turnedon a computer-controlled diamondlathe at the Lawrence Livermore

National Laboratory. The mirrors arepolished to far greater smoothness thannormally attained by skilled opticians inorder to insure low scattering of EUVlight, especially at the shortestwavelengths. Specifically, the surfaceroughness has been reduced to lessthan 15 Angstroms, or about 60 billionthsof an inch. The mirrors will focus EUVlight from a star or other point sourceto an image of about 10 arcseconds,making a star seem like a spot that isabout 180 times smaller in diameterthan the full Moon. Although this islarger than the image of a star formedby a conventinal optical telescopeoperating in visible light, it sets a newstandard for EUV telescopes.


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SpectrometerThe EUVE spectrometer represents a

novel design, providing maximumenergy throughput and spectralresolution in the extreme ultraviolet.The more throughput the spectrometerhas, the fainter the sources it candetect in a given exposure time with agiven telescope. As the spectralresolution of a spectrometer isincreased, it becomes possible toseparate independent spectral lines,arising in atoms and ions of differenttypes, which are blended togetherwhen observed at lower resolution andare thus difficult or impossible to study.Sir Isaac Newton

The key component of aspectrometer is the dispersing element,meaning the optical device that spreadslight out into a spectrum. The mostfamiliar dispersing element is the glassprism made famous by Sir IsaacNewton, who observed sunlight thatpassed through his prism to be spreadout into the colors of the rainbow.Modern spectrometers usually areprovided with dispersing elementscalled diffraction gratings. A diffractiongrating is a plate of glass or similarmaterial that is provided with a thin,smooth metal coating in whichthousands of very narrow, parallelgrooves are ruled at preciselycontrolled spacings. When light wavesreflect from the grooves, they bendslightly at the edges of the grooves (aphenomenon called diffraction) andthus change direction. The amount ofbending depends on the color orwavelength. Therefore, the differentwavelengths are bent by differentamounts and the incoming light, uponreflecting from the grating, is diffractedinto a spectrum.Cutting Edge

The key feature of the EUVEspectrometer is the use of threediffraction gratings of a new type in aconverging light beam and theexistence of only three reflectingsurfaces in each of the threewavelength channels. By using the newkind of diffraction gratings, in whichthe spacing between grooves or "lines"is continuously varied from one end of

a grating to the other, and by placingthe gratings in the converging beam ofthe telescope rather than in a parallellight beam, the Berkeley designers haveachieved high spectral resolution withonly three reflections per channel. Eachreflection of a light beam, especially anEUV light beam, results in some loss oflight. Therefore, by minimizing thenumber of reflections, the energythroughput is maximized

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The EUVE spacecraft consists of aPayload Module that contains the EUVEscient ific inst ruments and associatedelectronics packages. Also, it includesan Explorer Platform that providesstructural support, two-way communi-cations with the ground, electricalpower, stabilization, and pointingcontrol. This spacecraft design, developedat the Goddard Space Flight Center(GSFC), represents a natural evolutionof the earlier Multi-Mission ModularSpacecraft design, as first used by GSFCin the Solar Maximum Mission spacecraft.

The Solar Maximum Mission (SMM)spacecraft, launched in 1980, wasretrieved by the Space ShuttleChallenger, repaired onboard, andreleased back into orbit during a single,manned flight in 1984. The SolarMaximum Repair Mission demonstratedfor the first time that a spacecraftdesigned for on-orbit replacement ofindividual subsystems could be repairedsafely in space. The Explorer Platformdeveloped for EUVE and subsequentmissions carries this design philosophyfurther by providing a generallyadaptable Platform Equipment Deck,which can accept science payloads ofmany kinds, including some that maynot have been envisioned when theplatform was actually built. The key tothis new design is the concept of thePayload Module, which may containany scientific instruments that fit withinthe limits of available space and power.Rather than mounting individualscientific instruments to the spacecraft,as was done in the Solar MaximumMission, the entire complement ofEUVE instruments, contained in asingle, replaceable Payload Module, aremounted to the Platform EquipmentDeck of the Explorer Platform as asingle unit. By the same token, thePayload Module can be removed at afuture date--a process that will beperformed in space--and replaced by anew module that mounts on the deckin the same way, but which contains awholly different set of scientificinstruments.Solar Arrays

All power on the EUVE spacecraftcomes from the Sun. Solar energy isconverted to electricity in photovoltaiccells contained in the spacecraft's solararrays and is stored in batteries.

The Solar Maximum Mission (SMM)was designed so that the instrumentsand therefore a fixed surface of thespacecraft, would always point at theSun. On the other hand, EUVE willpoint telescopes in directions awayfrom the Sun, and future payloads onthe same Explorer Platform may needto point in still other directions.Accordingly, while the solar panels onSMM were fixed in orientation withrespect to the spacecraft structure, thesolar panels on the Explorer Platformmust be capable of articulation, that is,for pointing at the Sun as the spacecraftpoints instrumen_ i_" first one directionand then another..dso, the EUVE

arrays are larger, incorporate moreefficient photovoltaic cells, and producemore power than the SMM solar arrays.Toaster Oven

At the beginning of the EUVEmission, the solar arrays will providemore than 1,000 watts of power,

ABOVE: Ar tist 's concept ion of NASA'sTracking and Data Relay Satel li teLEFT: Support module for the EUVEscientific instrumentsBELOW: Cutaway drawing of EUVEspacecraft configuration


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averaged over a typical orbit. Of this, amaximum of 300 watts will be allocatedto the Payload Module. That allocationis less than one-fourth of the powerneeded to operate a typical toasteroven, but is ample to run the house-keeping subsystems and scient ificinstruments on EUVE, all of which useadvanced solid-state components. Forexample, the three EUVE scannertelescopes together use only about24 watts.lO-Year Lifetime

Solar arrays are known to degrade inlow-Earth orbit, where they are struckby high-speed oxygen atoms thatinduce defects in the cells. Therefore,to insure proper operation of theExplorer Platform over its intended10-year lifetime, the arrays are designedfor easy removal and replacement inspace during servicing missions of theSpace Shuttle.

Electricity generated by the solararrays is stored in a Modular PowerSystem (MPS), consisting of the storagebatteries, power regulators, and powercontrollers. Night and day occur about16 times each per day, as the EUVEoperates in low-Earth orbit. Powergenerated during orbit day is stored inthe batteries so that energy is availableto operate the spacecraft during orbitnight. Ground controllers carefullyregulate the constant cycle of batterycharging and discharging, so thatbattery lifetime is maximized consistentwith the requirements of on-orbitspacecraft operations. When thebatteries fail or deteriorate to anunacceptable extent, as all batterieseventually will, the MPS can bereplaced in orbit by a Shuttle astronaut.Command and Control

EUVE will be operated from aPayload Operations Control Center(POCC) at the Goddard Space FlightCenter. Commands will flow to thespacecraft from the POCC, and dataobtained by the spacecraft will berouted through to the POCC and thento the Science Operations Center at theCenter for EUV Astrophysics at Berkeley.Tracking Satellite

Normal communications to and fromEUVE will be via the Tracking and DataRelay Satellite (TDRS) System. A high-gain antenna, part of the Modular

Antenna Pointing System (MAPS), ismounted on a gimbal at the end of anextendable mast (deployed afterseparation from the launch vehicle) onthe EUVE Explorer Platform. (A high-gain antenna is one with a relativelynarrow beam, so that it can receiveand transmit radio communications in ahighly directional manner.) The high-gain antenna on EUVE can be steeredby the MAPS to establish communi-cations with a TDRS satellite locatedhigh above in geostationary orbit. Fromthe tracking satellite, data areforwarded to a TDRS ground terminalat White Sands, New Mexico, whichroutes them by land line to the POCC.

Telemetry is received, processed, andforwarded onboard the ExplorerPlatform by a replaceableCommunications and Data HandlingModule. This subsystem stores commandssent from the ground for execution atspecified times in both the platform andscience instrument systems. It alsoexecutes real-time commands from thePOCC to operate onboard equipment.It processes the housekeeping telemetrythat indicates the state of flight systemsand also the science data generated bythe EUVE telescopes and thespectrometer. All data are stored inmagnetic tape recorders, for playbackvia the TDRS System to the ground atcommanded times.

Backup communications modes areprovided via an omnidirectional antennaon the Explorer Platform, which cantransmit directly to ground stations locatedin sight of the spacecraft, and viaNASA's Deep Space Network, operatedby the Jet Propulsion Laboratory inPasadena, California.Newton's Law

The Explorer Platform providesreaction wheels, gyros, and magnetictorquers to stabilize and point theEUVE spacecraft. To steer thespacecraft, the reaction wheels areaccelerated by small electric motors, asinstructed by commands from theground. Then, by Newton's Third Lawof Motion (every action has an equaland opposite reaction), the spacecraftturns in the opposite direction.

The magnetic torquers, which containelectromagnets that can be energizedby ground command, are used toreduce, or "dump" momentum byreacting against the Earth's magneticfield. Otherwise, the reaction wheelswould eventually spin too fast. Gyrosare used to keep track of exactly wherethe spacecraft is pointing. The ModularAt titude Control System that incorporatesthese components receives input dataon orientation from star- and Sun-sensors.

Astronaut in shut tle bay _,.ith Solar Maximum Mission spacecraftin backg,round

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What Will EUVE Find?

18

As with every Explorer spacecraft, themost remarkable objects that EUVEfinds are likely to represent phenomenathat scientists have not predicted yet.Nevertheless, from the properties ofpresently known celestial objects,astronomers predict that among theastronomical bodies that EUVE is likelyto detect are red and white dwarf stars,neutron stars, and B-type stars withextended atmospheres heated to hightemperatures by shock waves. OtherEUVE objects likely to be investigatedinclude binary stars of the RS CanumVenaticorum type, the hot outeratmospheres (coronae) of stars similarto the Sun, and so-callod dwarf novaeor cataclysmic variable stars.

Additionally, EUVE will investigatephysical processes in our solarsystem--notably in the auroras ofJupiter and possibly of Saturn, and inthe Io torus, a doughnut-shapedformation of electrified gas atoms thatcircles Jupiter at the position of theorbit of its volcanic moon, Io.Mapping Galactic Gas

Detecting members of each of theclasses of stars in our Milky Waygalaxy, EUVE will obtain the necessarydata to estimate the amount ofinterstellar gas along the line of sight tothe star, and perhaps the relativeamounts of the two predominantconstituents of the gas, hydrogen andhelium, in that direction. Thisinformation will allow astronomers todevelop basic information on thedistribution of interstellar gas in thevicinity of the solar system, and out todistances of hundreds of light-years. Ineffect, they map not only the distri-bution of detectable EUV-emittingobjects in the sky, but also the distri-bution of invisible interstellar gas. Themap is likely to reveal the existence ofsome tunnels or lines of sight in whichthere is very little gas. In thesedirections, it may even be possible to

observe quasars that are located atdistances of many millions oflight-years.Not Ordinary

EUV radiation is generated underconditions so different from those thatwe commonly experience that itnecessarily follows that most of thesources that EUVE will detect areremarkable by ordinary standards.

EUV light can arise from intenselyincandescent surfaces, like those foundin the thick atmospheres of very hotstars, and it can be produced in thinnergases at high temperatures. Theradiation can be continuous withwavelength, like the glow from anincandescent light bulb, or it can becharacterized by an emission linespectrum, like the light from a neonsign. The glow from a light bulb occursat wavelengths of visible and infraredlight, while many of the likely sourcesof EUV radiation emit their energypredominantly in that spectral region.

EUV spectral emission lines can begenerated also when high-speedelectrified, subatomic particles smashinto atoms or molecules of familiargases such as oxygen.

"IH( )_.'t.: Z()ta/ _(_/,Ir_'(lips('

BELOW: White dwarf in planetary nebula

,. ............ k) I U,j ",,l{ rt


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White DwarfsHot, white dwarf stars, with surface

temperatures of tens of thousands ofkelvins (the Sun's surface temperature isabout 6,000 kelvins, or about 11,000degrees Fahrenheit), are likely to bedetected in great numbers by EUVE.Some white dwarfs in our galaxy areeven likely to be detected at distancesof 3,000 light-years.

EUVE all-sky survey data on thesehot, white dwarf stars, combined withdata from existing optical andultraviolet telescopes, will indicate theirtemperatures and the relative contentof hydrogen and helium in theiratmospheres. Also, the data will revealhow much interstellar gas is locatedbetween each white dwarf star and theEarth. When white dwarfs areexamined with the EUVE spectrometer,spectral lines of other elements may bedetected, which can be interpreted toyield more information on chemicalcomposition. Theories for the structureof the atmospheric layers in these starscan be tested, and mechanisms bywhich various elements may diffuse

through, or settle in the atmospherescan be elucidated.

Small and dense, white dwarf starsconsist largely of so-called electrondegenerate matter, which is not foundon Earth. A typical white dwarf hasabout 65 percent of the mass of theSun, or about 650 times the mass ofthe giant planet, Jupiter, yet all of thismaterial is compressed into a tiny starabout the same size as the Earth. Asingle teaspoonful of white dwarfmatter would weigh tons on Earth.

White dwarfs are thought to representthe final evolutionary state of stars likeour Sun and other stars, rangingupward at the times of their formationto perhaps as much as eight times thesolar mass.

During their earlier evolution, thesestars must shed much or most of theirmaterial in order to eventually becomea white dwarf. According to a theoryfirst proposed by the Nobel Prize-winning astrophysicist, SubrahmanyanChandrasekhar of the University ofChicago, no white dwarf can evercontain more than about 1.44 times the

mass of the Sun. In contrast, normalstars made of hot, electrified gases arebelieved to be capable of forming withmasses up to about 120 times the solarmass. So far as is known, a white dwarfstar, if isolated from neighbors in space,will slowly cool forever, graduallychanging from a source of EUVradiation to just a visible-light source,and eventually dimming intoindetectable obscurity. By studyingwhite dwarf stars with EUVE,astronomers will learn about matterexistent in strange conditions and aboutthe future, final life stage of our Sun.Neutron Stars

Neutron stars are even smaller,denser and more massive objects thanwhite dwarfs. A single teaspoonful ofneutron star material, if it could beconfined, would weigh trillions of tonson Earth. Neutron stars form at highertemperatures than white dwarfs, yetrapidly cool. They are believed to bethe remnant cores of supergiant starsthat exploded as supernovae, like thegreat supernova that was seen in thesouthern sky in February 1987. Some

Voyager-2 photograph of some detai l anddifference in Saturn's complex system ofrings


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are known to have powerful magneticfields and to be rotating at high speed;they are detectable as pulsars, sourcesof rapidly repeating bursts of radiowaves that may be produced as a beamof radio emission turns with theneutron star, like the beam of a rotatingradar scanner at an airport. As theneutron star beam rotates past theEarth, astronomers detect a radio burstor "pulse." According to presentinformation, a neutron star that is atypical pulsar gradually spins slowerand slower, its magnetic field weakens,and its radio emission cuts off.

There may be millions of deadpulsars (those whose radio emissionshave ceased) in our galaxy that areundetectable by present techniques. YetEUVE may find one or more such deadpulsars, if theoretical speculations panout. According to these ideas,interstellar gas attracted by thepowerful gravity of the dead pulsarsmay accrete onto their surfaces,reheating them to temperatures of afew hundred thousand degrees. If theyget that hot, then although a neutron

star is typically no larger than a majormetropolitan area (although containinghalf again as much matter as the Sunand the rest of our whole solar system),it may glow fiercely in extremeultraviolet light.

It appears that the EUVE offers thebest-known possibility of detectingthese hypothesized accreting deadpulsars. The detection of even one suchobject would be a major advance inastronomy. The possibility exists that adead pulsar lurks within 10 light-yearsof the Sun, yet has escaped detectionby present means. If so, EUVE may find it.Red Dwarfs

Red dwarf stars, like ProximaCentauri, the nearest known starbeyond the Sun, have intense magneticactivity that heats their outeratmospheres or coronae. In thesecoronae, there often are greatexplosions called stellar flares, whichperhaps mimic the solar flares thatoccur on our Sun, but which are vastlymore powerful .

Similar activity occurs in binary starsystems of the RS Canum Venaticorum

type (named for the first known systemof this type). In this class of binarysystem, the two member stars are closeto each other and are locked insynchronous rotation, so that as theyorbit around the center of mass of thesystem, the same hemisphere of eachstar always faces toward the other. Incontrast, as the Sun turns once every27 days, we on Earth see first onehemisphere and then the other.

Studies with x-ray telescopes and theInternational Ultraviolet Explorer (IUE)

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reveal that disturbed regions in theatmospheres of these binary stars arethe sites of intense magnetic activity,like giant analogs of the sunspots onour own star. Flares also occur in theatmospheres above these regions.

EUVE is expected to glean important,new information about the coronae andflares of red dwarf stars and RS CanumVenaticorum systems. EUVE also shoulddetect interesting EUV spectra from thehot, outer layers of other classes ofnearby cool stars.

Observations by the Einstein andEXOSAT satellites showed that manytypes of cool stars, including "mainsequence" stars like the Sun, familiarbright stars such as Capella andProcyon, and red dwarf stars, emit softx-rays. The radiation presumably comesfrom the upper atmospheres of these

LEFT: Crab Nebula containing a neutron starat i ts centerBELOW: Proxima Centauri, a Red Dwarf starappearing as a very tiny point of light adjacentto Alpha Centauri (lower left quadrant)

stars, which are much hotter than theirsurface layers.

Soft x-rays are those with relativelylow energies and relatively long wave-lengths; they constitute the region ofthe electromagnetic spectrum that adjoinsthe EUV on its short-wavelength-side.The Einstein and EXOSAT findingshave led astrophysicists to infer thatmany of these cool stars also must bestrong EUV emitters, even if they donot undergo major flares or othereruptions. Calculations suggest severalthousand cool stars should be detectedand studied in the EUVE all sky-survey.B Stars

Early type-B stars like the familarSpica, brightest star in the constellationVirgo, have unexpectedly proved topossess extended coronae. In thesehuge, outer atmospheres of the B stars,the gases probably are heated by shockwaves generated when a faster-movingwind from the star crashes into slower-moving gas that left the lower Tayers ofthe star at an earlier time. Or, fast andslow gas currents may collide in someas-yet-unspecified way. In any case, justas a shock wave or sonic boomprecedes the supersonic transport,Concorde, as it flies through the nearlystationary gases of the Earth's loweratmosphere, shock waves occur in aB-star corona when the fast movingstreams move through coronal regionsat speeds greater than the local speedof sound. Much energy is released,making the corona so hot that it glowsbrightly in the EUV. Observations bythe EUVE spectrometer are expected todetect spectral lines in the 80- to120-Angstrom spectral range that canbe analyzed to determine temperatures,densities, and other physical conditionsin these unusual stellar atmospheres.

The data should allow EUVE observersto test and perhaps distinguish betweenthe different theories of these shock-heated winds that were recentlyproposed by Joseph Cassinelli of theUniversity of Wisconsin and Roll-PeterKudritzki of the Max Planck Institute ofAstronomy and Astrophysics in Munich,Federal Republic of Germany.Dwarf Novae

Dwarf novae are a fascinating subjectfor study by the EUVE. These so-calledcataclysmic variable stars are actuallybinary star systems in which anordinary main sequence star, notremarkably different from our Sun, isclosely orbiting a common center ofgravity with a white dwarf starcompanion.

Because of the powerful gravity ofthe white dwarf, gas streams out fromthe atmosphere of the main sequencestar and fails down on the white dwarf,spiralling inward toward it through astructure known as an accretion disk.As the gas in the disk spirals closer andcloser toward the white dwarf star, itmoves into a smaller and smaller

Early-type B _star_,n Plemdes

(,: L l " :, )"t,_i:Z:\ ; '.':';"{


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volume. Therefore, the density of thefalling gas inexorably increases. Ineffect, the gas is being compressed bythe gravity of the white dwarf star.Because a compressed gas becomeshotter (and when it expands it cools,the principle on which homerefrigerators and air conditioners work),the gas in the accretion disk of thedwarf nova system get hotter and hotter,and accordingly it glows in the EUV.

At irregular intervals of days, weeks,or months, eruptions occur in theaccretion disk. During these eruptions,one of which was actually observed bythe EUV telescope during the Apollo-Soyuz Test Project in 1975, there occurgreat outbursts of extreme ultravioletradiation. Studies of dwarf novae withthe EUV will yield basic physicalinformation about these strange starsystems and how they explode.Io Torus

EUVE also will yield basic newinformation about the planet Jupiterand its strange planet-girdling gaseousbelt, the "1o torus," which glows ininvisible forms of radiation, including

the extreme ult raviolet .Io is one of the four large moons of

Jupiter discovered by Galileo in theearly 17th century. The great Italianastronomer could hardly have imaginedthat almost four centuries later, theVoyager-1 spaceprobe would pass nearthe moons that he first glimpsed andwould discover frequently- orconstantly-erupting volcanoes on Io,which release huge amounts of sulfurand oxygen in various forms into trans-Jovian space. Some of these substancesbecome individual ions (electrified

22

TOP: Center of Andromeda Galaxy, one ofthe nearest to our own Milky Way

atoms) that orbit Jupiter at roughly Io'sdistance from the planet, spreading outto fill a doughnut-shaped plasma cloud,the to torus.

Other charged particles stream downthe lines of magnetic force that loop farout from Jupiter and pass through theregion of Io. Striking the upperatmosphere of the giant planet near thepolar regions, the charged particlesstimulate atoms and molecules, makingthem glow in an ultraviolet aurora thatis reminiscent of the aurora borealisand australis (the northern andsouthern lights) sometimes seen onEarth.

The EUVE spectrometer will gatherbasic new data on the spectralproperties of the Io torus and theJovian aurorae and may be capablealso of gathering such data on auroraeon the planet Saturn or even on distantUranus. From this information,aeronomers--scientists who specializein the physical study of light emissionsand physical processes in planetaryatmospheres--will learn about theparticles that stimulate the observedglows, the atoms and moleculesinvolved in the glows themselves, andthe natural processes at work.Quasars

Quasars are as a class, the mostdistant and powerful sources of energyin the known universe. Some quasarsblaze fqrth so brightly that presenttelescopes can glimpse them atdistances far beyond almost allidentif ied galaxies.

Yet a quasar is apparently an objectno larger than our solar system, and atypical galaxy contains hundreds ofbillions of stars like the Sun.

The mystery of why a tiny quasar canoutshine a huge galaxy remainsunsolved, despite almost 30 years ofstudy by hundreds of astronomers sincethe first quasars were found. Theleading idea postulates that at the heartof the quasar there is a supermassiveblack hole, a region of space in whichmatter amounting to hundreds ofmillions of solar masses is packed intoa radius so small that the gravitationalforce exerted is too powerful to allowthe escape of a ray of light.

ABOVE: Io, Jupi ter's volcanic Moon, f romVoyager- I C ! _ ", :


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Around the supermassive black hole,some experts postulate that there is a"reprocessing region," whereelectromagnetic radiation is reduced inenergy (increased in wavelength) as itpasses through a hot gas. Whether ornot this is true, many investigatorsconsider that the putative black hole issurrounded by an accretion disk. Thisstructure must be much larger than theaccretion disk in the dwarf novasystem, but presumably it forms in thesame way, through the infall or mattertoward the black hole.

Recent findings suggest that the EUVEcan help to explore the nature of eitheror both of these two hypotheticalstructures, the reprocessing region andthe accretion disk.

Specifically, x-ray observations madefrom space show that many quasarshave an unexpected emission in thelongest wavelength x-rays, whichborder on the EUV region of thespectrum. Extrapolation suggests thatthis same emission might be very strongin the EUV and theorists propose thatthe radiation may be coming from thequasar accretion disk and perhaps fromthe reprocessing region, if one exists.

At the same time, if a number ofrelatively gas-free lines of sight exist, itmight be possible to view extragalacticspace through them and perhaps detectthe EUV radiation from a quasar in thatdirection. This may be a long shot, butthe scientific results, should this difficultquasar observation succeed, couldinclude fundamental new informationon one of the greatest puzzles inmodern astronomy--the nature andenergy source of quasars.

Quasar 3C273, a very distant object withextremely high energy, observed for the firsttime in x-ray by HEAO-2 spacecraf t

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The EUVE Program is a component ofthe Astrophysics Program conducted bythe Office of Space Science andApplications at NASA Headquarters inWashington, D.C. Direct managementresponsibility for developing andoperating EUVE, including the ExplorerPlatform and Payload Module, is vestedin the Goddard Space Flight Center,Greenbelt, Maryland.

The Center has contracted with theUniversity of California at Berkeley forthe design, fabr ication, integration,calibration and test of the scientificinstruments, the conduct of the all-skysurvey, and the development of thenecessary computer software for theprocessing and scientific analysis ofdata from the EUVE telescopes andspectrometer.

A Flight Operations Team will staffthe EUVE Payload Operations ControlCenter (POCC) at Goddard to controlmission operations and conduct short-term and long-range mission planning.

To perform their assigneddevelopment tasks, the Berkeleyastronomers have built a state-of -the-art,

low-contamination EUV instrumentcalibration facility. It incorporates avacuum chamber with a capacity of3x5 meters (about 10x16 feet) whichis entered after passing through a seriesof clean rooms, to controlcontamination by airborne dust andother substances that can degradesensitive optical and sensorcomponents.

To coordinate science operationsplanning and the control, performancemonitoring, and data collection fromthe scientific instruments, the Berkeleyscientists have established an EUVEScience Center at the University. Thisincludes a Science Operations Center(SOC), a Science Data Storage Facility,and a Science Data Analysis Facility, allequipped with appropriately-networkedcomputer workstations. The EUVE ScienceCenter will manage the Guest ObserverProgram during the Spectroscopy Phaseof the EUVE Mission, when GuestObservers selected by NASAHeadquarters will study data obtainedwith the EUVE.

The SOC will work closely with themission planners at Goddard tocoordinate the acquisition of scientificmeasurements by EUVE and the propercommanding of the scientificinstruments on board. The SOC willsupport both the all-sky survey planningand the development of spectroscopyobservation plans for Guest Observers.The Science Data Analysis Facility willprovide an archive for the raw sciencetelemetry from EUVE and for theprocessed data. Members of theBerkeley science team will use thefacility to produce the all-sky surveycatalog and sky map, and to study datafrom the EUVE deep survey. GuestObservers will study these data and thespectroscopy observations from EUVE,using computer tools at the facility, orby analyzing magnetic tapes orelectronically transmitted data files attheir home institutions.

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In.strument C-alibration Facility


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EUVE TeamKey NASA PersonnelNASA HeadquartersEUVE Program ManagerEUVE Discipline ChiefEUVE Program ScientistGoddard Space Flight CenterDeputy Project Manager, Explorers and

Attached Payloads ProjectsEUVE Project ScientistProject Manager, Satellite Servicing

ProjectSpacecraft Manager , Satellite Servicing

Project

Mr. John LintottDr. Edward J. WeilerDr. Robert V. Stachnik

Mr. Donald U MargoliesDr. Yoji KondoMr. Frank J. Cepollina

Mr. Robert H. Spiess

EUVE Instrument Systems Manager Mr. Dale F. SchulzEUVE Payload Module Manager Mr. Richard M. DayEUVE Mission Operations Manager Mr. Hector ZayasKey University of Cali fornia at Berkeley PersonnelEUVE Science Principal InvestigatorEUVE Instrument Principal InvestigatorEUVE Science Payload Project ManagerEUVE Project AstronomerEUVE Guest Observer Project ScientistEUVE Instrument Scient istOther Principal Members of the EUVE TeamGoddard Space Flight CenterMr. Walter AncarrowMr. James BarrowmanMr. George Daelemans, Jr.Mr. David DoudsMr. James JewMr. Donald KirkpatrickMr. Kelly McEntireMr. Anthony MillerMr. William MocarskyMr. W. Lee NiemeyerMr. Peter O'NeillMr. John RobinsonMr. Robert ShelleyMs. Bonnie TetiMr. Clyde WoodallPhotographic CreditsBall Aerospace GroupCalifornia Institute of Technology and

Carnegie Institution of WashingtonFairchild Space CompanyGeoff Chester, Albert Einstein PlanetariumNASA, Goddard Space Flight CenterLick Observatory, University of CaliforniaMcDonnel Douglas Astronautics CompanyNaval Research LaboratoryNASA HeadquartersUniversity of California, BerkeleyU.S. Naval ObservatoryAuthor of the BrochureDr. Stephen P. MaranLaboratory for Astronomy and Solar PhysicsNASA Goddard Space Flight Center

Prof. Stuart BowyerDr. Roger MalinaMr. Steven J. BattelDr. Herman MarshallDr. Carol Christ ianDr. Patrick Jelinsky

University of California, BerkeleyDr. Supriya ChakrabatiMr. Carl DobsonMs. Trish DobsonDr. David FinleyDr. Isabel HawkinsMr. Henry HeetderksDr. Michael LamptonDr. Koji MukaiDr. Oswald SiegmundMr. lames TomDr. John VallergaDr. Peter VedderDr. Barry Welsh

Researched and edited bylira Lynch and Associates


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Documents

Extreme Ultraviolet Explorer Long Look at the Next Window