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SKO 4 SLOVEN / Č I T E R 0 GLISH 2 ON MAV T N E N A / . Č I R T D RE ON MAV DR. ANT

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DR. A

ETIČ R V A M N TO N

avtor dr. J. D. Hunley

Prof. dr. Anton Mavretič je nadvse prijazen in ugleden inženir elektronike, ki je bil rojen v Sloveniji, vendar že večino svojega življenja živi in deluje v ZDA, kjer ga družina in prijatelji poznajo kot Tonyja. Prispeval je k načrtovanju in razvoju številnih instrumentov za vesoljska plovila, s pomočjo katerih smo pridobili ogromno podatkov o plazmi in Sončevem vetru. S plazmo se nanašamo na plin, ki ga sestavljajo pozitivno nabiti ioni (atomi, ki so izgubili enega ali več elektronov) in negativno nabiti elektroni (podatomski delci). Fizika plazme nas zanima, ker so iz plazme sestavljene zvezde in ker se ta uporablja v plazemskih televizorjih, med drugim pa je povezana tudi z delovanjem AM radiev ter z eksperimentalnimi fuzijskimi reaktorji. Sončev veter je tok plazme, ki z nadzvočno hitrostjo potuje od Sončeve korone (atmosfere) v medplanetarni prostor. Sončev veter je pomembno raziskovati, ker lahko med solarnim maksimumom, ko je oddajanje sončne energije največje, poškoduje satelite, ki krožijo okoli Zemlje, pa tudi druga vesoljska plovila. Brez zaščitnega dežnika, ki ga predstavljata Zemljino magnetno polje in Zemljina atmosfera, bi lahko Sončev veter resno poškodoval tudi Zemljo. Sončev veter namreč utegne vplivati na podnebne spremembe na Zemlji in lahko preobremeni električne napajalne vode. Čeprav znanstveniki Sončevega vetra še vedno ne razumejo v celoti, so instrumenti, kakršni so tisti, ki jih je pomagal zasnovati dr.

Mavretič, prispevali k temu, kar danes vemo o njem.

Anton Mavretič se je rodil 11. decembra 1934 na domači kmetiji očetu Antonu in mami Mariji, rojeni Črnugelj. Kot najstarejši otrok ima še dva brata, Jožeta in Iva-

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na, ter sestro Jožefo, poročeno Lorbek. Očetova kmetija stoji v vasi Boldraž na jugu Slovenije, štiri kilometre od Metlike. V tistem času so Boldraž osvetljevale samo kerozinske svetilke. Dr. Mavretič se je pozneje spominjal, da se je njegovo zanimanje za znanost in tehnologijo vnelo s »čarobno svetlobo«, ko so nemški ujetniki kmalu po 2. svetovni vojni v Boldražu pomagali napeljati elektriko, pri čemer je bila Mavretičeva kmetija prva, ki je dobila skrivnostni tok, in sicer ko je bilo Antonu 12 let. V srednji šoli je bodoči inženir postal spreten izdelovalec radiev. Šolanje je nadaljeval s študijem elektrotehnike na Univerzi v Ljubljani med letoma 1954 in 1957, kjer je zaključil dve leti dodiplomskega študija. Med srednjim in univerzitetnim šolanjem je živel pri babici Ani Črnugelj v Črnučah.

Takrat ga je njegov stric Mike Mavretič, ki je živel v Denverju v Koloradu na zahodu ZDA, spodbudil, da se je odpravil na pot s tovorno ladjo iz Reke v današnji Hrvaški do ZDA. To je bil cenovno ugoden način, kako priti v Ameriko, kjer je obiskoval Univerzo v Denverju in decembra 1959 diplomiral iz elektronike, junija 1961 pa z istega področja tudi magistriral. Potem se je vrnil v Jugoslavijo, katere del je bila takrat Slovenija, dokler se ni jeseni 1962 zopet vrnil v ZDA. Prispel je v mesto New York, od koder je šel na Univerzo Syracuse v severnem delu zvezne države New York. Tam je leto prej pridobil mesto asistenta in raziskovalca. Na Univerzi Syracuse je dokončal eno leto doktorskega študija vključno z opravljenim strokovnim izpitom in izpiti iz tujega jezika. Njegov potni list ni bil veljaven dovolj dolgo, da bi lahko dokončal doktorski študij na Univerzi Syracuse, zato je študij prekinil in se zaposlil na Centru za raziskave in razvoj Westinghouse v Pittsburghu v Pensilvaniji.

Podjetje Westinghouse ga je sponzoriralo, da je lahko pridobil zeleno karto, ki mu je omogočila delo v ZDA. Za Westinghouse je delal med letoma 1964 in 1965. 7. marca 1964 se je v slovenski cerkvi v Pittsburghu poročil z Darinko Šesek, ki jo je že prej spoznal v Sloveniji. Jeseni 1965 je nadaljeval z doktorskim študijem na državni univerzi v Pensilvaniji, kjer je decembra 1968 doktoriral, zopet s področja elektronike.

Istega meseca je postal postdoktorski sodelavec na Centru za raziskave vesolja na Tehnološkem inštitutu Massachusettsa (Massachusetts Institute of Technology – MIT), kjer je kot sodelavec in projektni inženir zasnoval najsodobnejše nizkošumne verige ojačevalcev signalov iz Faradayeve čaše (glej spodaj). Prav tako je vodil ekipo inženirjev in tehnikov pri zasnovi elektronike za raziskovanje plazme in Sončevega vetra. Njegova naloga je bila zasnovati instrumente tako, da bodo omogočili pridobitev podatkov, ki jih je iskala skupina znanstvenikov in glavni raziskovalec. Zasnove instrumentov je integriral v vesoljsko plovilo, ki jih je poneslo v vesolje, testiral njihovo pravilno delovanje in zanesljivost, nato pa jih usposobil za vesoljski polet. Med tem zahtevnim delom na MIT-ju do leta 1978 je bil dr. Mavretič tudi pomožni profesor na Northeastern University v Bostonu v Massachusettsu, ki stoji na drugem bregu reke Charles nasproti MIT-ja. Na Northeastern University je med letoma 1969 in 1980 vodil večerni pouk.

Njegov prvi projekt na MIT-ju je bil instrument za merjenje plazme za vesoljsko plovilo Explorer, ki ga je pozneje izstrelila Nacionalna zrakoplovna in vesoljska uprava (Nasa). Instrument je uporabljal Faradayevo čašo, napravo za merjenje električnega toka v žarku nabitih delcev, v tem primeru plazme. Naprava ima ime po Michaelu Faradayu (1791–1867), znanem po pionirskih eksperimentih na področju elektrike in magnetizma. Instrument, imenovan Faradayeva čaša za Sončevo plazmo (Solar Plasma Faraday Cup), je bil prvi tak instrument, ki so ga razvili za polet v Explorerju 47, imenovanem tudi Medplanetarna merilna platforma (IMP)-H (Interplanetary Measuring Platform), čigar namen je bil izmeriti smerno intenziteto elektronov in pozitivnih ionov v Sončevem vetru za energije do 7000 elektronvoltov. Instrument, pri katerem je bil dr. Mavretič projektni inženir, prof.

Herbert S. Bridge pa glavni raziskovalec na MIT-ju, je proučeval elektrone in ione v prehodnem območju ter v magnetnem repu, ki je raztegnjen podaljšek Zemljine magnetosfere na tisti strani, ki gleda stran od Sonca. Oblika magnetnega repa je odvisna od tlaka Sončevega vetra, ko veter potuje okoli magnetosfere, ki je posle-

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dično območje okoli Zemlje, v katerem se nabiti delci ujamejo zaradi svojega obnašanja, ki ga uravnava Zemljino magnetno polje. Instrument je imel dva senzorja.

Meril je elektrone med 17 in 7000 elektronvolti ter pozitivne ione med 50 in 7000 elektronvolti. Satelit Explorer 47 so izstrelili 22. septembra 1972 iz Cape Canaverala na Floridi z nosilno raketo Delta. Satelit so utirili na skoraj krožno orbito okrog Zemlje, in sicer na približno pol poti med Luno in Zemljo. Deloval je do 31. oktobra 1978, ko ga je Nasa izključila.

Dr. Mavretič je bil tudi projektni inženir za še en instrument, ki se je prav tako imenoval Faradayeva čaša za Sončevo plazmo (Solar Plasma Faraday Cup), čigar osnovni namen in glavna zasnova sta bila enaka. Glavni raziskovalec tega instrumenta je bil dr. Alan J. Lazarus, raziskovalni fizik na MIT-ju, ki je bil tudi soraziskovalec pri prvem instrumentu. Ta instrument je bil del satelita Explorer 50, znanega tudi kot IMP-J ali IMP-8. Izstreljen je bil 26. oktobra 1973 kot zadnji v seriji desetih vesoljskih plovil IMP. Tudi ta satelit je iz Cape Canaverala v vesolje ponesla nosilna raketa Delta. Satelit je deloval 33 let v skoraj krožni orbiti med Zemljo in Luno, rahlo bliže Zemlji kakor Explorer 47. Zadnji dostopni podatki s tega satelita se nanašajo na 7. oktober 2006. Zagotovil je osnove za vesoljski plovili Voyager 1 in 2, namenjeni v globoko vesolje. Dr. Mavretič je delal tudi kot projektni inženir za instrumente za merjenje plazme v Voyagerju 1 in 2, misijah, po katerih najbrž najbolj slovi. Leta 1975 je Nasin Center za vesoljske polete Goddard dr. Mavretiču podelil posebno priznanje za njegov izjemen doprinos pri misiji Explorer 50 (IMP-J).

Glavni raziskovalec za Voyagerjev instrument za merjenje plazme ali plazemski spektrometer (Plasma Spectrometer – PLS) je bil prof. Herbert Bridge. Dr. Mavretič je kot projektni inženir vodil in integriral delo štirih inženirjev in približno šestih tehnikov pri načrtovanju, testiranju in usposobitvi instrumenta, celoten projekt pa je bil pod vodstvom prof. Bridgea. Zlasti pomembno je bilo instrument zavarovati pred vibracijami med izstrelitvijo, pred mrazom v vesolju in pred radijskimi

frekvencami, ki so jih oddajali drugi instrumenti v obeh vesoljskih plovilih Voyager.

PLS je sestavljen iz štirih Faradayevih čaš, ki merijo ionske in elektronske tokove v energijskem območju od 10 do 5950 elektronvoltov. Tri čaše so usmerjene proti Sončevemu vetru, medtem ko je četrta obrnjena vstran glede na smer Sončevega vetra, tako da gleda na vsakega izmed planetov, ki so del itinerarja obeh Voyagerjev, in pri tem zaznava elektrone. Instrument analizira tokove, ki jih zaznajo trije detektorji, usmerjeni proti Sončevemu vetru. Ti določajo temperaturo, hitrost, tok, gostoto in dinamični tlak Sončevega vetra. Prav tako je zbral podatke o interakciji Sončevega vetra z zunanjimi planeti našega Sončevega sistema: z Jupitrom, s Saturnom, z Uranom in Neptunom. Podatki vključujejo informacije o virih, lastnostih in morfologiji (obliki, strukturi in spremembi oblike) plazme, ki prihaja iz magnetosfere omenjenih štirih planetov in njihovih lun. Med drugimi cilji PLS-a je tudi proučevanje ionov, ki izvirajo iz medzvezdnega prostora, in narave ter lokacije terminacijskega šoka – predela, kjer se hitrost Sončevega vetra upočasni do podzvočne hitrosti, ko se približuje heliopavzi – območju, kjer se srečata izstopajoč Sončev veter in vstopajoča plazma iz medzvezdnega prostora, kar predstavlja mejo našega Sončevega sistema. Končni cilj je prvič zaznati mejo heliopavze in plazme, ki prihaja iz območja izven Sončevega sistema.

Voyager 2 so izstrelili iz Cape Canaverala 20. avgusta 1977 z nosilno raketo Titan-Centaur. Vesoljsko plovilo je potovalo mimo Jupitra leta 1979, Saturna leta 1981, Urana leta 1986 in Neptuna leta 1989, med poletom pa zbiralo podatke.

Tudi Voyager 1 so izstrelili iz Cape Canaverala na nosilni raketi Titan-Centaur, in sicer 5. septembra 1977. Vesoljsko plovilo je Jupiter srečalo leta 1979, Saturn pa leta 1980. Eden izmed najpomembnejših dosežkov PLS-a je bil izpolnjen ob dveh srečanjih z Jupitrom leta 1979. Vrhunec tega srečanja so bile meritve torusa plazme, oblaka ionov in elektronov, ki pod vplivom Jupitrove lune Io v obliki obroča obdaja planet. Drugi vrhunec so bile meritve materiala, ki odletava s Titana, Saturnove največje lune. Voyager 1 je naknadno izmeril plazemske razmere v Sat-

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urnovi magnetosferi, in sicer novembra 1980, medtem ko je Voyager 2 to naredil avgusta 1981. Voyager 2 je opravil tudi meritve Urana januarja 1986 in Neptuna avgusta 1989. Obe vesoljski plovili sta trenutno (aprila 2013) namenjeni proti zunanji meji Sončevega sistema in iščeta heliopavzo, mejo, ki je ni doseglo še nobeno vesoljsko plovilo, kar jih je ustvaril človek. Voyager 1 je šel skozi terminacijski šok decembra 2004, Voyager 2 pa avgusta 2007, pri čemer je Voyager 2 poslal nazaj podatke, ki jih je izmeril PLS. Zatem sta Voyagerja vstopila v Sončevo ovojnico, zunanje območje heliosfere malo za terminacijskim šokom. Voyager 2 je zopet poslal nazaj podatke, ki jih je izmeril PLS. 20. marca 2013 so mediji poročali, da je Voyager 1 zapustil Sončev sistem, vendar so Voyagerjevi projektni znanstveniki na Kalifornijskem inštitutu za tehnologijo blizu Pasadene soglasno sklenili, da vesoljsko plovilo še ni zapustilo Sončevega sistema in prestopilo v medzvezdni prostor.

Decembra 2012 so Voyagerjevi znanstveniki izjavili, da je Voyager 1 znotraj »magnetne avtoceste«, novega območja, ki ga je odkril Voyager, kjer se energijski delci drastično spremenijo in za katerega so številni znanstveniki prepričani, da je zadnje območje, skozi katerega mora iti vesoljsko plovilo, preden vstopi v medzvezdni prostor. V to območje vstopajo nabiti delci iz notranjosti naše heliosfere, hkrati pa tudi delci višjih energij izven naše heliosfere. Toda spremembe smeri magnetnega polja, kar je zadnji odločilni pokazatelj vstopa v medzvezdni prostor, Voyagerjevi instrumenti vse do marca 2013 še niso izmerili. Obe Voyagerjevi vesoljski plovili naj bi imeli dovolj pogonskega goriva in električne energije za delovanje vsaj do leta 2020. Takrat bo Voyager 1 19,9 milijard kilometrov od našega Sonca, Voyager 2 pa 16,9 milijard kilometrov od našega Sonca. Podatki PLS-a znanstvenikom pomagajo razumeti plazmo in Sončev veter znotraj njegovega energijskega območja.

Ti podatki zajemajo hitrost, gostoto, temperaturo in tlak Sončevega vetra. Omeniti moramo, da je PLS v Voyagerju 1 prenehal delovati kmalu po srečanju s Saturnom, vendar je še vedno nudil omejene podatke do sredine leta 1986. Od takrat naprej je izključen – z izjemo obdobja po maju 2003 –, in do aprila 2013 še vedno ni bil priključen nazaj. Na dan 10. aprila 2013 je PLS v Voyagerju 2 še vedno deloval in

pošiljal podatke o Sončevem vetru nazaj na Zemljo. Medtem ko se približuje medplanetarnemu prostoru, prvenstveno pošilja podatke o kozmičnem šumu (sevanju, ki prihaja izven Zemljine atmosfere).

2. junija 1981 je Nasa dr. Mavretiču podelila skupinsko nagrado za dosežek pri »razvoju Voyagerjevega znanstvenega instrumenta«, zlasti za »znanstveni instrument za merjenje plazme«. V Nasini izjavi beremo: »V znak priznanja za izjemen dosežek pri zasnovi, razvoju in testiranju Voyagerjevih znanstvenih instrumentov, ki so priskrbeli osupljive znanstvene rezultate med raziskovanjem Jupitrovega in Saturnovega sistema ter medplanetarnega prostora.« Leta 1978 je dr. Mavretič zapustil MIT in postal višji sodelavec na Centru za astrofiziko Harvard-Smithsonian v Cambridgeu v Massachusettsu. Tam je ostal do leta 1980, istočasno pa je tudi nadaljeval z večernim poukom kot pomožni profesor na Northeastern University. Na Harvardu je delal v ekipi z devetimi drugimi inženirji in s fiziki, s katerimi je razvijal koronograf za spremljanje Sončeve korone in ultravijolične svetlobe v zunanjih območjih Sonca. Gre za območje, od koder izvira Sončev veter. Koronograf je bil zasnovan za polet na do tedaj še nedokončanem raketoplanu. Ko se je ekipi pridružil dr. Mavretič, je bil instrument že skoraj dokončan. Intenzivno se je ukvarjal s testiranjem naprave v raketnem izstrelišču White Sands v Novi Mehiki, kjer so instrument trikrat testirali.

Leta 1979 je dr. Mavretič začel poučevati na bostonski univerzi (BU), kjer je postal izredni profesor elektronike, računalništva in sistemskega inženirstva. V študijskem letu 1980/1981 ga je predstavniško telo študentov inženirstva izvolilo za »izjemnega profesorja leta« ter mu na svečani podelitvi diplom podelilo plaketo. Med zaposlitvijo na BU je prof. Mavretič obnovil sodelovanje z raziskovalci z MIT-ja pod vodstvom dr. Alana Lazarusa, da bi zasnovali instrumente Faradayeve čaše. V tem obdobju (pozna 80. in zgodnja 90. leta) so zasnovali instrument za

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merjenje plazme za Nasino vesoljsko plovilo Wind. V sklopu Windove misije je prof.

Mavretič nadzoroval sedem dodiplomskih študentov pri eksperimentu, ki se je takrat imenoval Eksperiment Sončevega vetra (Solar Wind Experiment – SWE). Šlo je za enega izmed več različnih instrumentov satelita Wind, medtem ko so druge instrumente zasnovali drugje. Tako kot pri IMP-ju in Voyagerjevih satelitih so tudi pri SWE-ju meritve Sončevega vetra opravljali detektorji ionov Faradayeve čaše. Za meritve so zopet uporabili dve Faradayevi čaši, vendar tokrat z boljšimi in manjšimi komponentami, kar je bila posledica vmesnega tehnološkega napredka v času med Voyagerjem in Windom. Kljub temu so dr. Mavretič in študenti za podlago pri delu uporabili Voyagerjevo zasnovo. Medtem ko je dr. Lazarus vodil delo na MIT-ju in izdelavo Faradayevih čaš, večinoma na BU-ju, je bil glavni raziskovalec celotnega projekta SWE dr. Keith W. Ogilvie na Nasinem Centru za vesoljske polete Goddard.

Znanstveni cilji SWE-ja so bili (in še vedno so, saj še vedno deluje) raziskati Sončev veter in njegove fluktuacije pa tudi interakcije z Zemljino magnetosfero. SWE naj bi meril tridimenzionalno porazdelitev hitrosti komponent ionov v Sončevem vetru, in sicer za energijsko območje od 200 do 8000 elektronvoltov.

Eden izmed ciljev je bil tudi izmeriti tridimenzionalno porazdelitev hitrosti tokov plazme vključno z elektroni v energijskem območju med 7 in 22.000 elektronvolti.

Tretji cilj meritev je bila visoka kotna ločljivost žarka elektronov v Sončevem vetru, in sicer tako v smeri medplanetarnega magnetnega polja kot tudi v njegovi nasprotni smeri, za energijsko območje od 5 do 5000 elektronvoltov.

Nosilna raketa Delta je satelit Wind iz Cape Canaverala izstrelila v vesolje 1. novembra 1994. Satelit je sprva z gravitacijskim manevrom sledil Lunini orbiti okoli Zemlje, ko je bil apogej (najbolj oddaljena točka orbite) nad sončno stranjo Zemlje.

Opravil je meritve magnetosfere. Pozneje med misijo so pogoni vesoljsko plovilo utirili v posebno orbito v zgornjem delu toka (v smeri proti Soncu) Sončevega vetra stran od Zemlje, ko je bilo plovilo na ugodni točki med Soncem in Zemljo.

Do leta 1996, ko je bilo vesoljsko plovilo še vedno v izhodiščni Lunini orbiti, je SWE že opravil podrobne kvantitativne meritve ionov in elektronov v območju pred udarnim valom (tj. v območju Sončevega vetra, kjer imajo najhitrejši elektroni nekolizijske trke; za ta območja je značilna visoka stopnja magnetne fluktuacije).

SWE je tudi v Lunini brazdi zaznal dva žarka ionov, ki sta imela različno hitrost. Skupaj z drugimi instrumenti je SWE že leta 1996 pomembno prispeval k dodatnemu razumevanju Sončevega vetra.

Nekaj več kot desetletje pozneje je skupina znanstvenikov na MIT-ju in Centru za vesoljske polete Goddard, med drugimi dr. Lazarus, dr. Ogilvie in dr. Justin Kaspar z MIT-ja, med proučevanjem SWE-jevih meritev hitrosti, gostote in temperature vodika ter helija v Sončevem vetru odkrila nekaj, kar je takrat poimenovala kot »dušilec« Sončevega vetra. Vodik je najpogostejši element Sonca in Sončevega vetra. Helij je sicer drugi najpogostejši element, vendar je v Sončevem vetru veliko redkejši kakor pa drugod v vesolju. Znanstveniki so odkrili, da se je količina helija povečala, ko je hitrost Sončevega vetra narasla, in sicer od praktično nič helija pri najnižji hitrosti do več kot štirih atomov helija na 100 vodikovih atomov pri hitrosti, višji od 500 kilometrov na sekundo. Znanstveniki so bili tako prepričani, da je helij nekako povzročil najnižjo hitrost. Sklenili so, da je majhno električno polje vodikovih atomov povleklo za sabo helij, kar je vodikove atome upočasnilo. »Pri najnižji hitrosti – hitrosti, pri kateri Sončev veter ne more več povleči ven helija –, tudi Sončev veter ne more več pobegniti,« pravi dr. Ogilvie. »Še vedno ni jasno, kako točno helij vzpostavi najnižjo hitrost, ki znaša okoli 260 kilometrov na sekundo, ali zakaj najdemo več helija v Sončevem vetru, ko se hitrost vetra poveča.

Vendar pa vse to pomeni, da se nam izmika nekaj bistvenega pri razumevanju tega, kaj povzroča, da Sončev veter piha,« je izjavil dr. Kaspar. Znanstveniki so teoretizirali, da je razlog, zakaj se hitrost Sončevega vetra povečuje preko svoje najnižje

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hitrosti, izbruhana koronarna masa (coronar mass ejections – CME), ki ima pet- do desetkrat več helija, kot ga je v Sončevem vetru. Ko je Sončev veter dosegel svojo najnižjo hitrost, naj bi CME silovito sprostila plazmo iz Sončeve korone in količina helija se je povečala. Izbruhana koronarna masa je dodala helij, ki je povečal hitrost vetra, hkrati pa je tudi prekinila delovanje satelitov, sistemov za električno energijo in satelitov za radijsko komunikacijo, ki krožijo okoli Zemlje. Novi podatki bi tako lahko bili pomembni za preprečevanje tovrstnih prekinitev, vsaj pri novih satelitih.

Aprila 2013 je dr. Lazarus o prof. Mavretiču zapisal, da je »Tonyja pogosto / srečeval/ in da mu je bilo v veselje sodelovati z njim pri Windu in Voyagerju. Bistveno je prispeval k projektiranju obeh vesoljskih plovil. Na bostonski univerzi je bil mentor študentom magistrskega in doktorskega študija, ki so delali na vseh področjih v povezavi z Windom /…/ Brez njegovega dela ne bi mogli narediti tako uspešnih instrumentov.« Pomemben prispevek prof. Mavretiča pri delu na instrumentu je nagradil Tudi Nasin Center za vesoljske polete Goddard, ki mu je 7. oktobra 1993 podelil posebno skupinsko nagrado za »zasnovo, izdelavo in testiranje SWE-ja«.

Prof. Mavretič se je na bostonski univerzi poleg dela, povezanega z vesoljem, ukvarjal tudi z drugimi raziskavami v okviru mentorstva študentov, ki so s svojim delom prispevali tako k lastni izobrazbi kot tudi k tehnološkemu napredku. Ena takšnih raziskav je bila hitra izdelava prototipov analognih vezij vključno z zasnovo in s testiranjem. Projekt je v višini 148.000 dolarjev financirala Agencija za napredne obrambne analize preko Univerze Južna Florida, ki je sklenila podizvajalsko pogodbo z bostonsko univerzo. Prof. Mavretič in njegovi študenti so zasnovali strukturne enote na siliciju, ki so bile ali bipolarnega značaja ali pa so temeljile na tehnologiji komplementarnega kovinsko-oksidnega polprevodnika. Projekt so dokončali v enem letu. Podjetje Unitrode Corporation je financiralo še en projekt za zasnovo in analizo visokofrekvenčnih pretvornikov električne energije. Finanč-

na sredstva so znašala 35.000 dolarjev na eno študijsko leto, kar je ustrezalo financiranju dveh dodiplomskih študentov za dobo treh let. Podjetje Analog Devices Corporation je leta 1984 za dobo petih let dr. Mavretiču kot glavnemu profesorju podelilo subvencijo za področje analognih naprav. S sredstvi v višini 300.000 dolarjev, ki jih je podjetje zagotovilo za obdobje petih let, so letno financirali dva do tri dodiplomske študente pri naprednih raziskavah na področju analognih vezij z zelo visoko stopnjo integracije, in sicer vse do leta 1989, ko je bil projekt končan.

Prof. Mavretič je leta 1995 zaključil delo na bostonski univerzi, vendar se je tja spet vrnil leta 2005, saj je prof. Theodore Allan Fritz s tamkajšnjega Oddelka za astronomijo priskrbel donacijo v višini približno 4 milijonov dolarjev od Zračnih sil ZDA, da kot del satelita zasnujejo instrument, imenovan Prikazovalnik stožca izgub (Loss Cone Imager – LCI) za merjenje visokoenergijskih elektronov in protonov ter nizkoenergijske plazme v Van Allenovih pasovih. Te elektrone zadržuje Zemljino magnetno polje. Če hočejo ljudje potovati na druge planete, je pomembno razumeti to območje od okoli 6000 do 12.000 kilometrov nad Zemljo, saj bodo morali prav tam potovati skozi močno sevanje, s sevanjem pa se morajo spopasti tudi sateliti v tem območju. LCI, za katerega so načrtovali polet v eliptično orbito, ki je v območju med 6000 do 12.000 kilometri nad Zemljo, ima tri dostavljive komponente, ki so jih zasnovali profesorji in študenti na bostonski univerzi, in sicer fiksni senzor (Fixed Sensor Head – FSH), centralno elektronsko enoto (Central Electronics Unit – CEU) ter najpomembnejši del – visoko občutljivi teleskop (High Sensitivity Telescope – HST). FSH sestoji iz treh silicijevih detektorjev iz trdne snovi za merjenje energije delcev. HST ima posledično dva detektorja iz trdne snovi, ki merita tok energijskih elektronov, zlasti v območju od 20.000 do 500.000 elektronvoltov ter pod opazovalnim kotom sedem do deset stopinj glede na geomagnetno polje (območje se imenuje tudi stožec izgub, tj. območje izgub v obliki stožca).

Prof. Mavretič je na Centru za kozmološko fiziko na bostonski univerzi postal

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pridruženi član in projektni inženir za LCI, po tem, ko so z Zračnih sil ZDA na prvotno ponudbo prof. Fritza in njegovega postdoktorskega sodelavca odgovorili, da potrebujejo več inženirjev v podporo projektu. Prof. Fritz je nato prosil prof. Mavretiča, da se pridruži ekipi. Dr. Mavretič je vodil delo petih doktorskih kandidatov dr. Fritza, šestih magistrskih kandidatov in osmih dodiplomskih študentov višjih letnikov, ki so sodelovali pri projektu. Večina študentov je bila z Oddelka za elektroniko in računalništvo ter z Oddelka za vesoljsko tehnologijo in strojništvo (ki se je pozneje preimenoval v Oddelek za strojništvo), kjer prof. Fritz službuje poleg svojega primarnega mesta na Oddelku za astronomijo. Pri projektu sta sodelovala tudi dva študenta astronomije.

Prof. Fritz je sicer raziskovalno podkovan v fiziki plazme v vesolju in v magnetosferski fiziki, vendar njegovo strokovno inženirsko znanje ni pokrivalo nekaterih področij, ki jih je zahtevalo delo za LCI, zato je prof. Mavretič dopolnil njegovo znanje. Prof. Mavretič je v okviru projekta pisal poročila o napredku, organiziral razprave z agencijo, ki je dodelila sredstva (Raziskovalni laboratorij Zračnih sil ZDA vključno z Direktoratom vesoljskih plovil na Direktoratu za zračne sile v Kirtlandu v Novi Mehiki) ter se udeleževal znanstvenih in inženirskih sestankov, ki so bili povezani z vesoljskim plovilom. Svoje delo je, čeprav v manj intenzivni obliki, nadaljeval tudi v letu 2013. Satelit, na katerem naj bi LCI deloval, se imenuje Demonstracijski in znanstveni eksperimenti (Demonstration and Science Experiments – DSX), znan pa je tudi pod imenom Eksperiment vesoljske znanosti in tehnologije-4 (Space Science Technology Experiment-4 – SSTE-4). Izstrelili naj bi ga v srednjo Zemljino orbito, ki se nahaja med 2000 in 35.786 kilometri nad Zemljo. DSX naj bi prvotno izstrelili na isti veliki nosilni raketi, na kateri naj bi izstrelili tudi satelit Obrambni meteorološki satelitski program (Defense Meterological Satellite Program – DMSP). Vendar je Kongres ZDA predal nadzor nad vsemi meteorološkimi sateliti Nacionalni upravi za oceane in ozračje, da bi privarčeval denar. Posledično DSX ni bil izstreljen skupaj z vremenskim satelitom in se to do leta 2014 ali celo 2015

verjetno tudi še ne bo zgodilo. Medtem so instrument v letu 2013 v celoti testirali in dokončali.

Projekt LCI je prinesel štiri doktorske disertacije študentov, ki so delali na njem, ter veliko število magistrskih del. Prof. Mavretič je bil član njihovih izpitnih komisij in mnogo teh študentov je pozneje zasedlo pomembna delovna mesta. Prof. Fritz je dejal, da je bil »Tony absolutno ključnega pomena za projekt« in da »brez njega ne bi uspeli dokončati umerjenega LCI-ja z novo tehnologijo za Zračne sile ZDA«.

Prof. Mavretič je razen pri projektu LCI sodeloval tudi z drugimi študenti.

Medtem je leta 1995 začel delati v industriji, primarno na področju polprevodnikov. Tistega leta je postal podpredsednik in glavni tehnolog v podjetju Radio Frequency Power Products, Inc., ki je snovalo in izdelovalo napredne računalniško vodene radiofrekvenčne generatorje moči od 40 do 30.000 vatov in frekvenc od 500.000 hertzev do 160 megahertzev (milijonov hertzev). Kot glavni tehnolog je predlagal nove ideje in pristope k reševanju problemov z generatorji in z njimi povezanimi omrežji. Produkti se uporabljajo za poganjanje plazemskih komor, ki proizvajajo radiofrekvenčno energijo z ekstremno natančnostjo in zanesljivostjo, kakršna je potrebna tudi pri zasnovi instrumentov, ki so namenjeni uporabi v vesolju.

To pomeni, da morajo delovati štiriindvajset ur na dan, sedem dni v tednu. Znotraj plazemskih komor so običajno polprevodne ploščice za računalnike z dragimi integriranimi vezji. Dr. Mavretič je za to podjetje delal v Voorheesu v New Jerseyju, ki se nahaja na drugi strani reke Delaware nasproti Filadelfije.

Leta 1998 je podjetje Advanced Energy Industries (AEI) prevzelo podjetje RF Power Products kot hčerinsko družbo v stoodstotni lasti. Dr. Mavretič je še naprej delal v New Jerseyju, čeprav je živel v 486 kilometrov oddaljenem Bostonu. S komercialnim letalom se je na stroške podjetja vsak ponedeljek vozil v Filadelfijo, ob petkih pa se je vračal nazaj v Boston. Med tednom je bival blizu podjetja,

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kjer je imel stanovanje. Podjetje mu je dalo službeni avto, ki ga je lahko uporabljal med delovnim tednom. Po tem, ko je AEI prevzelo hčerinsko družbo, je dr.

Mavretič postal podpredsednik in tehnološki vodja v matičnem podjetju. Med delom v tem podjetju je imel veliko seminarjev, napisal pa je tudi številne interne dokumente (v glavnem je šlo za izobraževalne seminarje) o tehničnih težavah in radijskih frekvencah. Predstavljal jih je raziskovalni in razvojni skupini v podjetju ter inženirjem tehničnega osebja. Neposredno je vodil deset do dvanajst inženirjev.

Včasih je sodeloval pri arbitražah zaradi kršitev patentov nasprotujočih podjetij.

Med sodelovanjem s podjetjem je izumil in patentiral štiri nove tehnike nadzora postopka v elektroniki, ki so še vedno v uporabi. Ko ga je leta 2003 podjetje prosilo, da se preseli v Fort Collings v Colorado, je dr. Mavretič selitev zavrnil in zapustil AEI ter se vrnil domov v Boston.

Od leta 2003 do leta 2004 je bil svetovalec podjetja Princeton Power Systems v Princetonu v New Jerseyju. Tam je analiziral kompleksne sisteme krmilnih zank pretvornika energije za uporabo Vojne mornarice ZDA. Takšni pretvorniki energije delujejo na patentiranem principu, pri katerem se enosmerni električni tok pretvori v izmenični tok (AC), da nadzoruje hitrost standardnih AC-motorjev. Sistem bi lahko odpravil težke kovinske menjalnike med pogonskim sistemom in propelerjem vojne ladje. S svetovalnim delom se je ukvarjal tudi sicer, medtem ko je od 1985 do danes direktor v svojem svetovalnem podjetju SIAT of Boston, LLC (d.o.o.) v Naticku v Massachusettsu, kjer z ženo Darinko trenutno živita.

Dr. Mavretič je v svoji karieri napisal in bil soavtor številnih strokovnih člankov ter poročil. Je doživljenjski član Inštituta inženirjev elektrotehnike in elektronike (IEEE), od julija 2007 pa je tudi dopisni član Slovenske akademije znanosti in umetnosti, mesta, ki se ga podeli samo z izvolitvijo, kar priča o ugledu, ki ga Mavretič uživa v svoji domovini. SAZU ima sicer najstarejše korenine katere koli znanstvene institucije v Sloveniji, saj začetki njene predhodnice segajo v leto 1693.

DR. A

ETIČ R V A M N TO N

by J. D. Hunley, Ph.D. Prof. Dr. Anton Mavretič an affable and distinguished electrical engineer born in Slovenia but living and working in the United States of America most of his adult life and known to his family and friends as Tony has contributed to the design and development of numerous instruments on spacecraft that have provided a great deal of data about plasma and the solar wind. The plasma referred to here is a gas consisting of positively charged ions (atoms that have lost one or more electrons) and negatively charged electrons (sub-atomic particles). Plasma physics is of interest because stars consist of plasma and it is used in plasma TVs and involved in the functioning of A.M. radios as well as experimental fusion reactors, among other places. The solar wind is a supersonic flow of plasma from the Sun’s corona (atmosphere) into interplanetary space. It is important to study the solar wind because it can cause damage to satellites circling the Earth and to other spacecraft during the solar maximum, when the Sun’s energy output is greatest. Without the protective umbrella of the Earth’s magnetic field and the Earth’s atmosphere, the solar wind could also seriously damage the Earth. As it is, the solar wind may affect climate change on Earth and can overload power lines. Although scientists still do not yet fully understand the solar wind, instruments such as the ones Dr. Mavretič has helped to design have contributed to what we now know.

Anton Mavretič was born on December 11, 1934, at home on a family farm to father Anton and mother Marija (maiden name Črnugelj). Mavretič was the eldest child among his three siblings: two brothers (Jože and Ivan) and one

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sister (Jožefa, married Lorbek). His father’s farm was located in Boldraž, a village in southern Slovenia about four kilometers from Metlika, Slovenia. At that time, Boldraž was lighted only by kerosene lamps. Dr. Mavretič later remembered that his interest in science and technology was kindled by “the magic of light” when German prisoners shortly after World War II helped to install electricity in Boldraž, with the Mavretič farm being the first to receive the mysterious current when Anton was 12. In secondary school, the future engineer became a skilled builder of radios. He went on to study electrical engineering at the University of Ljubljana from 1954 to 1957, completing two years toward a degree. During his high school and university studies he lived with his grandmother Ana Črnugelj in Črnuče, Ljubljana.

Then, at the urging of his uncle Mike Mavretič who lived in Denver, Colorado, in the western United States, he took passage on a cargo ship from Rijeka in what is now Croatia to the U.S. This was an inexpensive way to get to America, where he attended the University of Denver, earning his Bachelor of Science in electrical engineering in December 1959 and his Master of Science in the same discipline in June 1961. At that point, he returned to Yugoslavia, of which Slovenia was then a constituent republic, before coming back to the U.S. in the fall of 1962. He arrived in New York City, from which he went to Syracuse University in upper New York State. There he had been awarded a graduate research assistantship a year earlier. At Syracuse, he finished one year of studies toward the doctorate including passing the qualifying exam and foreign language exams. His passport was not valid for long enough to complete his doctoral degree at Syracuse, so he discontinued his studies and went to work for Westinghouse Research and Development Center in Pittsburgh, Pennsylvania.

Westinghouse sponsored him in obtaining a green card allowing him to work in the U.S. He worked for Westinghouse for two years during 1964 and 1965. On March 7, 1964, in a Slovenian church in Pittsburgh, he married Darinka Šesek,

whom he had met in Slovenia. In the fall of 1965, he resumed work on his Ph.D. at Pennsylvania State University, earning that degree in December 1968, again in the field of electrical engineering.

The same month, he became a post-doctoral fellow at the Center for Space Research at the Massachusetts Institute of Technology (MIT), becoming a staff member and project engineer, a position in which he designed the state-ofthe-art low-noise amplifier chains for Faraday cup sensors (see below). He also oversaw a team of engineers and technicians designing electronics for the study of plasma and the solar wind. His job was to ensure that the designs provided the data sought by the group scientists and the principal investigator.

He integrated the design into the spacecraft that would carry it into space, tested it to ensure that it worked correctly and was reliable, and then qualified it for spaceflight. While doing this demanding work at MIT until 1978, Dr. MavretiÄ? also taught as an adjunct professor at Northeastern University in Boston, Massachusetts, across the Charles River from Cambridge, where MIT is located. He taught evening classes at Northeastern from 1969 to 1980.

His first project at MIT was a plasma instrument for an Explorer spacecraft to be launched by the National Aeronautics and Space Administration (NASA). This instrument used a Faraday cup, a device for measuring the current in a beam of charged particles, in this case, plasma. The device is named after Michael Faraday (1791-1867), known for his pioneering experiments in electricity and magnetism. Called the Solar Plasma Faraday Cup, the first such instrument was designed to fly on Explorer 47, also called Interplanetary Measuring Platform (IMP)-H, whose purpose was to measure the directional intensity of electrons and positive ions in the solar wind for energies up to 7,000 electron Volts. The instrument, for which Dr. MavretiÄ? was the project engineer and Professor Herbert S. Bridge was the principal investigator at MIT, studied these electrons and

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ions in a transitional region, and in the magnetotail, an elongated extension of the Earth’s magnetosphere on the side facing away from the Sun. The magnetotail is shaped by the pressure of the solar wind as it streams around the magnetosphere, which in turn is a region surrounding the Earth in which charged particles are trapped with their behavior dominated by the planet’s magnetic field.

The instrument had two sensors. It measured electrons between 17 and 7,000 electron Volts and positive ions between 50 and 7,000 electron Volts. Launched by a Delta space-launch vehicle from Cape Canaveral, Florida, on September 22, 1972, the Explorer 47 satellite was placed in a nearly circular orbit around the Earth, roughly halfway between the Moon and the Earth. It operated until October 31, 1978, when NASA turned it off.

Dr. Mavretič was also the project engineer for another instrument, also called a Solar Plasma Faraday Cup, with the same essential purpose and basic design. The principal investigator for this instrument was Dr. Alan J. Lazarus, a research physicist at MIT, who had been a co-investigator on the first instrument. This instrument was part of Explorer 50, also known as IMP-J or IMP-8.

It launched on October 26, 1973, and was the last of ten IMP spacecraft. It also was lifted into space on a Delta launch vehicle from Cape Canaveral. This satellite operated for 33 years in its near-circular orbit between the Earth and the Moon, slightly closer to the Earth than Explorer 47. Its last available data were for October 7, 2006. It provided a basis for the deep-space Voyager 1 and 2 spacecraft. Dr. Mavretič also served as project engineer for plasma instruments on those two spacecraft, the missions for which he is probably most famous. In 1975, NASA Goddard Space Flight Center awarded Dr. Mavretič special recognition for his outstanding contribution to Explorer 50 (IMP-J).

For the Voyager plasma science instrument or Plasma Spectrometer (PLS) Prof. Herbert Bridge was the principal investigator. As project engineer, Dr.

MavretiÄ? oversaw and integrated the work of four engineers and about six technicians in designing, testing, and qualifying the instrument under the overall direction of Prof. Bridge. Of particular concern were protecting the instrument from vibrations during launch, from cold in space, and from radio frequencies coming from other instruments on the two Voyager spacecraft.

The PLS consists of four Faraday cup detectors, which measure ion and electron currents in the energy range of 10 to 5,950 electron Volts. Three of the cups are directed at the solar wind, while the fourth is turned sideways in relation to the solar wind direction to look at each of the planets on the two Voyagers’ itineraries and to detect electrons. The instrument analyzes the currents obtained by the three detectors pointed at the solar wind to determine the temperature, speed, flux, density, and dynamic pressure of the solar wind.

It also has gathered data on the interaction of the solar wind with the outer planets of our solar system: Jupiter, Saturn, Uranus, and Neptune. These data include information about the sources, properties, and morphology (form and structure as well as transformation) of the magnetospheric plasma from those four planets and their moons. Among other objectives of PLS are the study of ions of interstellar origin, and also of the nature and location of termination shock, where the solar wind slows to subsonic velocity as it approaches the heliopause (the area in which the outgoing solar wind and incoming plasma from interstellar space meet, forming the boundary of our solar system). A final objective is to make the first detection of the boundary of the heliopause and of plasma coming from outside the solar system.

Voyager 2 launched from Cape Canaveral on August 20, 1977, aboard a Titan-Centaur space-launch vehicle. The spacecraft flew past Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989, gathering data as it flew. Voyager 1 also launched aboard a Titan-Centaur from Cape Canaveral.

Launched on September 5, 1977, it encountered Jupiter in 1979 and Saturn in 1980. One of the most important successes of PLS came from the two encoun-

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ters with Jupiter in 1979. A highlight of this encounter was the observation of the Io plasma torus, a ring-shaped cloud of ions and electrons that surrounds the planet Jupiter. Another highlight was the observation of material flowing off of Titan, Saturn’s largest moon. Additionally, Voyager 1 measured plasma conditions in Saturn’s magnetosphere in November 1980, while Voyager 2 did so in August 1981. Voyager 2 also took measurements at the planets Uranus in January 1986 and Neptune in August 1989. Both spacecraft are currently (April 2013) headed toward the outer boundary of the solar system in search of the heliopause, which had never yet been reached by any human spacecraft.

Voyager 1 passed the termination shock in December 2004 and Voyager 2 did so in August 2007, with Voyager 2 sending back observations by the PLS. After that, the Voyagers entered the heliosheath, the outer region of the heliosphere just beyond the termination shock. Again, Voyager 2 sent back observations from PLS. On March 20, 2013, media reports stated that Voyager 1 had left the solar system, but Voyager project scientists at the California Institute of Technology near Pasadena, California, reached a consensus that the spacecraft had yet to leave the solar system and pass into interstellar space. In December 2012 Voyager scientists stated that Voyager 1 was within a “magnetic highway,” a new region it had discovered where energetic particles change significantly and which many scientists believe is the final area the spacecraft has to cross in order to reach interstellar space. Into this area, charged particles from inside our heliosphere enter and higher-energy particles from outside our heliosphere also flow in. But a change in the direction of the magnetic field, the final critical indicator that interstellar space had been reached, had not yet been observed by Voyager instruments as late as March 2013. Both Voyager spacecraft are believed to have enough thruster fuel and electrical power to operate until at least 2020. At that time, Voyager 1 will be 12.4 billion miles (19.9 billion kilometers) from our Sun and Voyager 2 will be 10.5 billion miles (16.9 billion kilometers) from our Sun. Data from the PLS have contributed to

scientists’ understanding of plasma and the solar wind within its energy range.

These data have included the speed, density, temperature, and pressure of the solar wind. It should be noted that the PLS on Voyager 1 failed shortly after the Saturn encounter but was still able to offer limited data until mid-1986. Thereafter it was turned off except for a period after May 2003 and was still not turned back on in April 2013. As of April 10, 2013, PLS on Voyager 2 was still operating and sending data on the solar wind back to Earth. As it approaches interplanetary space, it provides data primarily about cosmic noise (radiation coming from outside the Earth’s atmosphere).

On June 2, 1981, NASA bestowed a group achievement award on Dr. Mavretič for “Voyager Science Instrument Development,” specifically the “Plasma Science Instrument.” Its citation reads, “In recognition of outstanding accomplishment in the design, development and test of the Voyager science instruments which provided spectacular science results in the exploration of the Jupiter and Saturn systems and interplanetary space.”

In 1978, Dr. Mavretič left MIT and became a senior staff member at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

He remained there until 1980 while continuing his teaching of evening classes at Northeastern University as an adjunct professor. While at Harvard, he worked with a team of nine other engineers and physicists developing a coronograph to monitor the Corona and the ultraviolet light in outer regions of the Sun. This is the region where the solar wind originates. The Coronograph was designed for flight on the as yet uncompleted Space Shuttle. The instrument was nearly completed when Dr. Mavretič joined the team. He was heavily involved in testing the device three times on rockets at White Sands Missile Range in New Mexico.

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In 1979, Dr. Mavretič began teaching at Boston University (BU), becoming an Associate Professor of Electrical, Computer, and Systems Engineering. In the academic year 1980/1981 he was voted by the engineering student body the “Outstanding professor of the year” and awarded a plaque at the graduation ceremony. While at BU, Prof. Mavretič again collaborated with MIT researchers, headed by Dr. Alan Lazarus, to design Faraday cup instruments. At this time (the late 1980s and early 1990s) a plasma instrument was designed for NASA’s Wind spacecraft. For the Wind mission, Prof. Mavretič supervised seven graduate students for what was called the Solar Wind Experiment (SWE), one of a number of different instruments on the Wind satellite, these other instruments being designed elsewhere. As on the IMP and Voyager satellites, the measurement of the solar wind on SWE would be done by Faraday cup ion detectors, two of them again, but with better and smaller components as a result of technological developments in the interim between Voyager and Wind. Nevertheless, Dr. Mavretič and the students used the Voyager design as a basis for their work. While Dr. Lazarus headed the effort at MIT and the construction of the Faraday cups, mostly at BU, the overall principal investigator for SWE was Dr. Keith W. Ogilvie at NASA Goddard Space Flight Center.

The scientific objectives of SWE were (and are, since it is still operating) to study the solar wind and its fluctuations as well as its interactions with the Earth’s magnetosphere. SWE was to measure the three-dimensional velocity distributions of the solar wind’s ion components for ions with energies ranging from 200 to 8,000 electron Volts. It also had the objective of measuring the three-dimensional velocity distributions of plasma flows, including electrons with energies in the 7 to 22,000 electron-Volt range. A third measurement objective was the high-angular resolution of the beam of electrons in the solar wind in both the direction of the interplanetary magnetic field and in the opposite direction, for energies ranging from 5 to 5,000 electron Volts.

A Delta space-launch vehicle propelled the Wind satellite from Cape Canaveral into space on November 1, 1994. Initially, the satellite followed a lunar swingby orbit around the Earth with an apogee (high point of the orbit) over the portion of the Earth that was in daylight. It made magnetospheric observations. Later in the mission, thrusters inserted the spacecraft into a special orbit in the solar wind “upstream” (toward the Sun) from the Earth at a vantage point between the Sun and the Earth.

As of 1996, when the spacecraft was still in the initial lunar swingby orbit, SWE had already made detailed quantitative measurements of the ion and electron foreshocks (in the area in the solar wind where the fastest electrons have collisionless shocks; these regions exhibit considerable levels of magnetic fluctuation). In the lunar wake, SWE also detected two ion beams of different velocities. Together with other instruments, SWE was already able by 1996 to contribute significantly to the further understanding of the solar wind.

Over a decade later, studying SWE’s measurement of the speed, density, and temperature of hydrogen and helium in the solar wind, scientists at MIT and Goddard Space Flight Center, including Dr. Lazarus, Dr. Ogilvie, and MIT’s Dr. Justin Kaspar discovered what they called the “throttle” for solar wind. Hydrogen is the most common element in the Sun and the solar wind. Helium, while the second most prevalent element, is much rarer in the solar wind than elsewhere in the universe. The team of scientists found that the amount of helium increased as the solar wind speeded up, from practically no helium at the minimum speed to more than four helium atoms per 100 hydrogen atoms at speeds higher than 310 miles (500 kilometers) per second. Thus, the scientists believed that helium somehow established the minimum speed. They concluded that hydrogen atoms’ small electric field dragged the helium with it, slowing down the hydrogen atoms.

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“At the minimum speed the speed where the solar wind is no longer able to drag out helium the solar wind can’t escape either,” according to Dr. Ogilvie. “It’s still not clear exactly how the helium sets the minimum speed at its particular value of around 260 kilometers per second, or why more helium is found as the solar wind speed increases, but it’s a clue that we are missing something fundamental about what makes the solar wind blow,” Dr. Kaspar stated. The scientists speculated that what speeded up the solar wind beyond its minimum velocity were coronal mass ejections (CMEs), which have five to ten times as much helium as the solar wind. As the solar wind hit its slowest speed, plasma apparently was explosively released by the CMEs from the Sun’s corona as the helium increased. These CMEs added the helium that speeded up the wind. But they also caused disruptions in the operations of satellites, power systems, and radio communication satellites circling the Earth. So the new data could be valuable in countering these disruptions, at least in new satellites.

In April 2013, Dr. Lazarus wrote about Prof. Mavretič that he saw “Tony often and had the pleasure of being a colleague on Wind and Voyager. He contributed substantially to the engineering design for both spacecraft. He supervised Ph.D. and Masters thesis students at BU who worked on all aspects of Wind. /.../ We could not have made such successful instruments without his work.” NASA’s Goddard Space Flight Center also recognized the value of Prof.

Mavretič’s work on the instrument by granting him a Special Act Group Award on October 7, 1993 for the “design, fabrication, and testing of the SWE.” Besides his space-related work at BU, Prof. Mavretič did other research involving his supervision of students as their work contributed to their education as well as to technological development. One involved rapid prototyping of analog circuits, including their design and testing. The Defense Advanced Research Projects Agency funded this project for $148,000 through the Uni-

versity of South Florida, with a subcontract to BU. Prof. Mavretič and his students designed building blocks on silicon, either bipolar in character or using complementary metal-oxide semiconductor technology. They finished the project in one year. Unitrode Corporation funded another project for design and analysis of a high-frequency power converters. The amount of funding was $35,000 per academic year, which supported two graduate students for three years. Analog Devices Corporation awarded Dr. Mavretič an Analog Devices Professorship Chair in 1984 for five years. The $300,000 it provided over the five years supported two to three graduate students per year to advance research in very large-scale integrated analog circuitry through 1989, when the project ended.

Prof. Mavretič retired from Boston University in 1995, but he returned to BU in 2005 because Prof. Theodore Allan Fritz in the Department of Astronomy there had secured a grant of about $4 million from the U. S. Air Force to design an instrument designated the Loss Cone Imager (LCI) to measure highly energetic electrons, protons, and low-energy plasma in the Van Allen belts as part of a satellite. These electrons are held by the Earth’s magnetic field. It is important to understand this region from some 3,720 to 7,440 miles (6,000 to 12,000 kilometers) above the Earth because if humans want to travel to other planets, they will have to pass through the heavy radiation there, and satellites in this region also have to contend with the radiation. The LCI, which was planned to fly in an elliptical orbit from 3,720 to 7,440 miles above the Earth, has three deliverable packages the Boston University professors and students designed, a Fixed Sensor Head (FSH), a Central Electronics Unit (CEU), and most importantly, a High Sensitivity Telescope (HST). The FSH consists of three silicon solid-state detectors for measuring particle energies. The HST, in turn, has two solid-state detectors to measure fluxes of energetic electrons, primarily in the range of 20,000 to 500,000 electron Volts within a seven-

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to ten-degree cutoff angle with respect to the geomagnetic field (also called lossed cone).

At the Center for Space Physics at BU, Prof. Mavretič became an associated member and the project engineer for LCI after the Air Force responded to the original proposal submitted by Prof. Fritz and a postdoctoral fellow, saying that it needed more engineering support. Prof. Fritz then asked Prof. Mavretič to join the team. He oversaw the work of five of Dr. Fritz’s Ph. D. candidates, six M. S. candidates, and eight upper-class undergraduates, who worked on the project. These students were mostly in the Department of Electrical and Computer Engineering and the Department of Aerospace and Mechanical Engineering (later renamed Department of Mechanical Engineering), where Prof.

Fritz holds joint appointments, together with his primary appointment in the Astronomy Department. Also participating in the effort were two Astronomy students.

Prof. Fritz has research interests in space plasma physics and magnetospheric physics, but his engineering expertise did not extend to some of the components needed for the LCI, so Prof. Mavretič supplemented Prof. Fritz’s skills. For the project, Prof. Mavretič wrote progress reports; held discussions with the granting agency (the Air Force Research Laboratory, including the Space Vehicles Directorate at Kirtland Air Force Base, New Mexico); and went to scientific and engineering meetings having to do with the spacecraft. His work continued, though with less effort, into 2013. The satellite on which it would operate, designated the Demonstration and Science Experiments (DSX; also known as Space Science Technology Experiment-4 or SSTE-4) was expected to be launched into medium Earth orbit, defined as the region above the Earth between 1,245 miles (2,000 kilometers) and 22,235 miles (35,786 kilometers). DSX was initially expected to be launched on the same large space-

launch vehicle as a Defense Meterological Satellite Program (DMSP) satellite.

But Congress had given oversight of all meteorological satellites to the National Oceanic and Atmospheric Administration, hoping to save money. As a result, DSX was not launched with a weather satellite and probably won’t be launched until 2014 or even 2015. Meanwhile, in the course of 2013, the instrument has been fully tested and delivered.

The LCI effort resulted in four Ph. D. theses by students who worked on it and a large number of Masters theses. Prof. Mavretič served on their examining committees, and many of the students have gone on to important jobs. Prof. Fritz stated that “Tony” was “absolutely critical to the project, which would not have succeeded in delivering a calibrated LCI with new technology to the Air Force without his contribution.” Prof. Mavretič has also worked with other students outside of the LCI project.

Meanwhile, in 1995 Dr. Mavretič began working in industry, primarily in the semiconductor field. That year, he became Vice President and Chief Technical Officer at Radio Frequency Power Products, Inc., which was designing and producing sophisticated computer-driven radio-frequency generators at power levels from 40 to 30,000 Watts and at frequencies from 500,000 Hertz to 160 MegaHertz (millions of Hertz). As chief technologist, he suggested new ideas and approaches to solving problems with these generators and associated networks. The products are used to drive plasma chambers to deliver radio-frequency power with extreme precision and reliability equal to that needed in the instruments designed for use in space. That is, they have to work twenty-four hours per day, seven days a week. Inside the plasma chambers there typically are wafers for computers with expensive integrated-circuit chip designs. For this firm, Dr. Mavretič worked in Voorhees, New Jersey, across the Delaware River from Philadelphia.

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In 1998, another firm, Advanced Energy Industries (AEI), acquired RF Power Products as a wholly owned subsidiary. Dr. Mavretič continued working in Voorhees while living in Boston. He was commuting to Philadelphia by commercial airline on Mondays and back to Boston on Fridays, all at company expense. He had an apartment close to his work site where he lived during the week. The company supplied him with a car to use during the work week. After AEI acquired the subsidiary, Dr. Mavretič became a Vice President and Technical Fellow at the parent firm. While at the company, he held many seminars and also wrote numerous internal documents (essentially training lectures) about technical issues in radio frequency. He presented them to the research and development group in the firm and to the technical staff engineers. He directly supervised ten to twelve engineers. At times, he participated in the arbitration of patent infringements by an opposing company. He invented and patented four new electronics process-control techniques while with the company that are still in use. When the firm asked him to move to Fort Collins, Colorado, in 2003, Dr. Mavretič declined and left AEI, returning home to Boston.

From 2003 to 2004, he served as a consultant to Princeton Power Systems in Princeton, New Jersey. There he analyzed the control-loop complexities of a power converter for U. S. Navy applications. Such power converters work on a patented principle in which direct-current power is converted to alternating current (AC) to control the speed of standard AC motors. This system could eliminate heavy metallic gear boxes between the power train and the propeller of a naval vessel. He has done other consulting work, and from 1985 to the present, he has been president of his own consulting firm, Siat of Boston, LLC (Limited Liability Company), now in Natick, Massachusetts, where he and Darinka currently live.

Over the course of his career, Dr. Mavretič has written or co-authored a large number of professional papers and reports. He is a life member of the

Institute of Electrical and Electronics Engineers (IEEE) and since July 2007 he has been a corresponding member of the Slovenian Academy of Sciences and Arts, an election-only position, suggesting the esteem in which he is held in his native Slovenia. Known by the acronym SAZU, it has the oldest roots of any scientific institution in the country, with its predecessors dating back to 1693.

Viri: biografija temelji na gradivu, ki mi ga je poslal prof. Mavretič, na njegovih dodatkih in popravkih mojih osnutkov, ter na številnih zanimivih spletnih straneh, ki jih je preveč, da bi jih naštel, poleg vseh drugih virov. Dr. Lazarus, prof. Fritz in prof. James Sullivan, ki so vsi sodelavci prof. Mavretiča, so pomagali z informacijami in prijazno prebrali osnutek te biografije ter predlagali pomembne popravke. Brez njihove pomoči in pomoči ostalih to besedilo ne bi bilo takšno, kakršno je.

Sources: This account is based on materials Prof. Mavretič sent me, his own additions and corrections to my drafts, and many internet sites too numerous to list, among other sources. Dr. Lazarus, Prof. Fritz, and Prof. James Sullivan, all colleagues of Prof. Mavretič, provided information and kindly read a draft of this biography, suggesting important changes. Without their help and that of others, this document would not have been possible in its present form.

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CIP - Kataložni zapis o publikaciji Narodna in univerzitetna knjižnica, Ljubljana 929Mavretič A. 520.64 HUNLEY, J. D., 1941Voyager, dr. Mavretič / J. D. Hunley ; [prevod Andreja Nastasja Terbos]. Ljubljana : KSEVT, 2013 ISBN 978-961-92999-4-4 269239808

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