Oral History Transcript — George W. Clark
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Interview with George W. Clark
George W. Clark ; July 15, 1976
Abstract:This telephone interview covers Clark's involvement in the founding of the discipline of non-solar x-ray astronomy. He discusses the factors — philosophical, theoretical and experimental — that led to the discovery of the first non-solar x-ray source in 1962. He relates his relationships with colleagues at the Massachusetts Institute of Technology and American Science and Engineering, Inc., and examines the backgrounds of scientists who entered x-ray astronomy.
TranscriptThis is a telephone interview with G.W. Clark at MIT July 15, 1976, by Richard Hirsh of the National Aeronautics and Space Administration History (NASA) Office, Washington D,C,
Well, the first thing I'd like to ask is this: Dr. Giacconi said that you were a consultant to A.S. and E., and that you, along with Dr. Rossi, suggested that he do work in X-ray astronomy. Now, one of the things I've been working on is the field of X-ray astronomy before 1962, before Giacconi made the discovery of the first X-ray sources. What I'm interested in finding out is: what were the motivations for people to go up into space and look for X-ray sources? I suppose I can ask you, generally, why did you suggest for Giacconi to go into X-ray astronomy?
Clark:Let me suggest a slightly different nuance for that initiation. At least, I believe it's important -- I think Riccardo would too, I think the best way is to recite the history, as I perceive it, and I think it does slightly change the flavor of how you put it there.
As I understand it, and I think this is borne out by the response which Rossi made to the questionnaire that Colin Dean gave to various people in the field, including of course Rossi, Giacconi and myself- I think there's good consistency among our replies. As I understand the history, after Sputnik there was convened by the National Academy of Science a committee which evolved into the Space Science Board, which was charged with the responsibility of examining what might be done in space from satellites. Things that were considered were communication with satellites, weather satellites, and so on. And in the group there were people, primarily concerned with each one of these different areas, and I think each one took it upon himself to examine the field and make a report, or contribute to the report of the committee. And I gather that in the area of scientific investigations, there were a number of topics which Rossi early on felt were potentially of considerable importance, and as I understand it, he found that if there was to be proper consideration of at least two areas, that he would probably have to consider them himself. In fact, I think he presented to the committee his views of the potential importance of two areas in particular-one of which was the study of the interplanetary plasma, which was known as the solar wind, about which there had been some preliminary conclusions, based upon cometary tails and so on, i.e. The observations of Bierman and the theory of Parker. In that area, he stressed the possibility of making direct observations and measurements of the solar wind. The other area was X-ray astronomy. He pointed out [that there was a window beyond the hydrogen-helium absorption, and that X-rays in the energy range above a kilovolt should be transmitted freely through space, and that it might be worth looking for cosmic X radiation.] Of course, at that time it was known that the sun was, of course, a strong source of X radiation based largely upon the 10-year work of the NRL group.
In any case, I think he found that there was not very wide interest in these areas, and he came to the conclusion that it these areas were going to be developed, he was going to have to do something about them himself. At MIT, where he was and of course is professor now emeritus but still active -- we already had under way a fairly heavy program of investigations in extensive air showers with possible applications in study of ultra-high energy gamma rays cosmic gamma rays, and also studies in gamma ray astronomy, initiated by Bill Kraushaar, now at University of Wisconsin.
Clark:And so, when Rossi came back to Cambridge, he undertook two initiatives. First of all, he suggested to Herb Bridge, who is now Professor of Physics and Associate Director of our Center for Space Research, that he consider the possibilities of direct measurements of the interplanetary plasma. That suggestion led to the development of the MIT Plasma Cup, and ultimately, to the first direct measurements of the solar wind, in Explorer X. [At the time, I was a consultant at AS&E, AS&E had been formed the previous year by four of us, including myself, Giacconi joined the company as the chief of the space physics work that evolved into a major division of the company,] The other initiative that Bruno took when he returned from this meeting was to have the three of us look into the question of the possibilities of experimental and theoretical work in X-ray astronomy. So the nuance that I'm suggesting is that in fact, it was not I -- it was not Rossi and I that suggested to Giacconi, but rather, Rossi who suggested to the two of us, and joined with us, in studying the potentialities of the field,
Hirsh:Right. I see.
Clark:I think if you come to describe it, I would say that the proper wording would be that Bruno Rossi, chief consultant of AS&E, professor at MIT, suggested to Giacconi and Clark that we undertake a theoretical study of the possibility for experiments and observations in Xray astronomy.
Hirsh:I see, Now, you were a consultant at the time, or were you actually part of the company?
Clark:Well, I was one of the founders and investors and a consultant, and at the same time, a professor at -- at that time, I guess that I was an [associate] professor at MIT.
Hirsh:And when was AS&E created?
It began, I believe, in 1957, And began with a couple of small steady contracts with the Air Force Cambridge Research Laboratory. (AFCRL)
Hirsh:And what was the organizing principle? Why was the company founded? Why was it founded instead of having the individual scientists working through the universities?
Well, it was a private enterprise, initiated for the purpose of research of interest at that time initially to the Air Force in the area of weapons effects at high altitude, having to do in part with radiation effects, and the founder of the company is the present president, Martin Annis, who had done previous work with a company called Allied Research Associates in Boston, in a related area and was himself an expert in the effects of X-rays, from nuclear explosions at high altitude. And he promoted this company and convinced a number of us that it was an interesting enterprise. And I think, right from the start the emphasis was on good quality scientific research. Very early there developed an interest in the development of apparatus for the teaching of physics, and at that time there was developing in Cambridge, under the leader ship of Zacharias, the physical sciences study committee that led to quite a reform in high school science, and the -- that new company, ASLE was an early bidder for development of the demonstration apparatus. And in fact, it took the lead in developing the production designs for the equipment in that high school physics program. And that was perhaps the principal reason why Professor Rossi took an interest in the company, because of his own interest in education, and the possibilities of developing the field, Then early on, he saw the possibility of building on the base of interest and background in Xray physics at that company, with the exceptional talents of Giacconi, in building an entirely new field of science, X-ray astronomy. And one thing led to another. As you perhaps, I'm sure you know, that study, which is an AS&E document, that I suppose is available in the files there at NASA. --
Hirsh:W ell, Dr. Giacconi said that he would send me copies of those two reports.
Clark:Oh yes. Well, they must be somewhere, lost in the
Hirsh:well, more than just lost. They were burnt.
I have been trying to get some files through Nancy Roman's office.
Clark:-- oh yes --
Hirsh:And from AS&E -- There's a whole list of the files that have been retired and put in some warehouse around here, and I requested that they find them for me, and I just found out this morning that they have been destroyed..
Hirsh:You can laugh.
Clark:Well, I think it's kind of funny.
Hirsh:Yes, in a way it's kind of funny. In a way, it's terrible.
Clark:To the historian, it's a little bit of a nuisance, I would think.
Hirsh:Right, because there are a few controversies, I think, or conflicts between Giacconi's story and Nancy Roman's story about how NASA funded or didn't fund Giacconi's projects. And I thought that those files might be able to help me.
Clark:Well, those files could have been certainly very important in that resolution, Yes, I do know that there's difference in the perception of those early days.
There's no question of course about the contents of that original study, that the three of us did, because in that, which was included, I assume, most certainly, in the proposal that was made to NASA -- that among the objects that we considered were the Crab Nebula, flare stars, the peculiar A stars which we came to know had rapidly changing magnetic fields, and therefore might be the seat of particle acceleration through X-ray generation; very bright nearby stars, like Sirius, which we considered in a very naive way, just simply by completely naive scaling, though we were aware of the fact that Sirius presumably lacks a convective zone and therefore may not have a hot corona; and also, just the collection of stars in the Milky Way. I remember doing that calculation myself. What if all the stars were like the sun, and you put them all together in the Milky Way, would you see a galactic ridge, or something of that sort? Most of those results came out of course with quite low estimates. The Crab, we based upon a simple extrapolation of the synchrotron, the fact that the visible synchrotron radiation was know at the time, and it was possible to extroapolate to Xray, and indeed, that is the case. And then of course, there was the famous estimate that we could make, based upon lunar fluroescence of solar X-rays. And that has acutally now been detected, you probably know, in the Apollo experiments, There were a number of interesting orginial ideas using the chemicals analysis technique, which I think was exploited in the Apollo series. So in any case, that study went together with a proposal to NASA, and I think it was in the spring of '58, to maybe '59, I'm a little hazy about those two years proposing a series of rocket experiments, and indeed, NASA turned it down. The company did have this relationship with ARCRL. And there was there a very bright lieutenant who was in charge of the office who had responsibility for lunar research, and he saw in this lunar Xray idea, some possibilities for a new approach to the problem of lunar composition. And he did indeed fund the first four rockets. The first one failed, the second one was the June 1962 rocket that dealt with the bright source in the Southern sky, and then there was a follow-on, I think in October, looking at another part of the sky, and then a fourth rocket which I think was the next June. And there was in April of '63 the experiment of Friedman and his NRL group, which, with a narrower field of view, showed that the initial suggestion that that Southern source might be the galactic center was not correct, but that rather it was a source some 25 degrees away, which was SCO X-1.
Was that what it was that you saw on the June flight? Was it SCO X-1.
Oh yes. Sure, SCO X-l accounted for about 75 or 80 percent of the total radiation that would be detected by the large solid angle detector. So there's no doubt that that was the discovery of SCO X-l. It was done in, what I would say is the typical Rossi exploratory style, that always characterized his work that he has had any relation to, and that is, in the first place, the imagination to go after something interesting, and then, to do it with a broad enough net so you have a fair chance to catch it. That's what with his suggestion, Giacconi's group did there (with Gursky and Paolini). They developed the largest area X-ray detector that they could, and certainly the largest to date, a large Geiger tube it wasn't a proportional counter but a Geiger tube, a pancake shape of about 20 square Centimeters in terms of area. And a thin window I think they had mica windows. And they gave it a very large solid angle, so that they had a chance of seeing something. Of course, the experiment was always conceived, for the purpose of detecting extra-solar sources, as well as the moon. As I say, it was a very broad net, that they cast. And I think that's why NRL missed the discovery. They had a decade of brilliant leadership in the solar field, and the detectors that were appropriate for that study were rather tiny areas, and small solid angles. I think if one of them had by chance swept over SCO X-l they would have detected it.
Right. Well, in 1957, Jim Kupperian, working with the NRL group did fly an X-ray detector, and he did see something. He got some data from that detector. But he never published anything because he didn't know what the X-rays were from.
Hirsh:Had you heard about that at the time?
Clark:No, we did not know about that at the time, though we of course since then have come to know about it. I've had a number of conversations with him about that And I think indeed, as you say, the problem was that they had a signal. I think they were up to the point of a press conference, which they called off I think Herb Friedmam felt it was premature, because as you say, they didn't really know what the interpretation of it was. The evidence was not of sufficient clarity to prove that it was an extrasolar source. And so they missed the discovery. I think in retrospect Jim's feeling is that it might in fact have been one of the transient sources, We know that this recent one that flared up a year ago, in Orion, -- actually in Monoceres -- was even brighter than SCO X-l. And indeed such a bright source could, I think, readily have been detected by their detector.
Hirsh:Right. That's the impression I get from him, He believes that it might have been some very transient phenomenon, some pulsating X-ray source possibly.
Clark:But of course, as these things go in science, it is only when you do an observation in which you have sufficient confidence and which has intrinsically the control aspects that lend confidence that it constitutes discovery. I'm sure you've read that first paper of Giacconi, et. al.
Clark:And you, I'm sure, appreciate the extraordinary lengths to which they go to consider all the alternative hypotheses, particularly the possiblity of particle contamination, and finally they are driven to the conclusion that it's an extra-solar source. Now, that is of course what makes it a discovery.
Yes, it was quite impressively written, I think. Let me ask you this: there seems to be a conflict about this fluorescence, X-ray fluorescence from the moon. From Nancy Roman I get the feeling that the theories would not justify the actual experiment, because from what she says, there would not have been enough solar winds for any fluorescence to be detected from near the earth's surface by the rocket.
Of course, there are two approaches. One is the impact of the solar wind, The other was the fluorescence scattering of solar X-rays themselves. I don't recall I don't have a copy of that study. The one I had somehow leaked away from my office. I must have loaned it to somebody a good many years ago, and I have to get another copy from Riccardo. I think that we looked at two mechanisms. One was the bremsstrahlung of electrons in the solar wind, which depends critically upon the temperature of the solar wind the velocity. But that's on the one hand. The other hand is the fluorescent reflection, the fluorescent scattering of solar-generated X-rays by materials on the surface of the moon, And I think in both cases, they were somewhat long shots, I guess I ought to really redo those calculations. I don't think that Nancy's correct in dismissing the thing as an impossibility. But in her judgement, of course, it was not possible. On the other hand, the study did consider other possible sources; needless to say none of us were clever enough to think of the mass transfer neutron stars. I must say, in retrospect it is amazing that no theoretician thought of that, of even thought of it until about 1967, at the -- I think the Soviet Conference, when a number of people jumped on that idea. Shklavsky wrote an article in '67 and it suddenly became a very popular model, [I. Shklovskii, Astrophysical Journal Ap.J. (Letters) 148, LI (l967).] But there was a tremendous amount of speculation, leading in unprofitable directions, right from the moment of the announcement, until that idea of the accretion onto a compact object hit, I think the first person, incidentally, to mention the possible role of mass transfer in close binary systems was Hayakawa in an article in the 1964 issue of the PROGRESS OF THEORETICAL PHYSICS, the Japanese journal. But that remark of Hayakawa's does not include the concept of the neutron stars. But it involves some, I think, production of shock waves in this mass transfer.
Clark:But as I say, nobody imagined the possibility of these high luminosity X-ray sources, and of the fundamental mechanism of acceration onto compact bodies, and therefore, anything we could think of was something of very low luminosity. I think really, the basic philosophy that was successful, was that one of Rossi's which was: since space is transparent to X-rays, and since one can think of a number of ways in which X-rays can be generated in space and by stars, one should go and look and see what you find, That philosophy was what led to success. And I think it was a philosophy which was not appreciated; that exploratory philosophy was what Nancy (Roman) failed to appreciate.
Hirsh:I see. Again, that's slightly different than what she tells me, but --
Clark:-- yes, because in fact I've discussed it with Nancy once or twice over cocktails, and I know that she I think she does feel that her evaluation, based upon her judgement of the lunar ideas, was correct, and that she did indeed do the right thing there.
Hirsh:Right. Can I ask you a little more about this lunar fluorescence? What theories were there to support your looking for fluorescent X-rays? Who suggested, or who had predicted the solar wind?
Clark:Well, as I say, I think that that was only one of two. The other was the simple knowledge, based largely upon NRL measurements, of the flux of solar X-rays. We did the simple geometrical factors, and the factor of about a tenth of a percent, for the fluorescent scattering efficiency.
Clark:Then you arrived at a flux at the earth.
Clark:Anybody could do that, and I think it led to the conclusion that it might be just barely perceptible. As for the solar wind, both of those ideas I think did originate, as far as I know, in our studies. As for the solar wind, the only information I believe that was available at the time was the estimates based upon the deflection of cometary tails, as used by Bierman, and I think the Parker theory of the solar wind was just making an appearance then. But I don't recall what we referred to in that study. But I do emphasize that there are these two separate mechanisms. I don't think that the solar wind mechanism is as likely as the fluorescent scattering of solar X-rays, and unfortunately as I say, I haven't re-done that calculation. I'm not quite sure where that stands.
Hirsh:OK. Let me get back to something else also. Do you know when Rossi wrote that report, or submitted that report to the Space Science Board of the NAS?
Clark:I really don't know what form his comments on the possibilities for scientific investigation took, whether it was a contributed paragraph,or whether they each submitted a paper. I would suggest that you might call you might call Dick Hart at the National Academy, and -- Richard Hart -- and he may be able to dig out of the files of the Space Science Board the very first, I would think it would be one of the very first documents that they generated. It would be a report of that committee, and in it, there may be something identified with Rossi. Or it may just appear as a paragraph. You know how those things are.
Clark:To which he contributed, I don't know.
Hirsh:Well, that's what I was getting at. They're just in town here, so I thought I could give them a call, or drop by and see what they had.
Clark:I think Richard Hart would be the man to speak with.
Hirsh:OK. Also you mentioned something about Defense Department and Air Force high altitude studies. Did this have anything to do with that project, VELA? That satellite that was supposed to be looking for nuclear explosions in space or on the surface of the earth?
Clark:Well, not at that time. Of course, high altitude bomb testing became very popular, starting around '59 to '62 or '63. During that time (when I was a consultant there) AS&E made a very major proposal to the Air Force to construct a dodecahedral shaped staellite with gamma ray and X-ray detectors, (omnidirectional, omnisensitive) to monitor bomb tests in space, and that was very early in the business. I think in fact it was unsolicited, an initiative of AS&E. Later on, there grew up the VELA project, and Los Alamos in fact constructed those dedcahedral satellites, with gamma ray detectors aboard. But AS&E was not involved in the project.
Hirsh:There was concern that there would be nuclear explosion tests in space?
Clark:Well, there's no problem, of course, in detecting clandestine tests in our own atmosphere, because they make such a racket. But with the test ban treaty that Kennedy finally succeeded in negotiating, there was concern that if one could test bombs far enough from the earth so that it had no terrestrial effects, that would give useful data on the performance of bombs, radiation damage and anti-ballistic missile defense, That was, and I guess still is, the motivation of the VELA satellites; that if a bomb were to be exploded anywhere within practical distance of the earth, on a space probe, these would detect it. That is of course why there was such a delay in the announcement of the discovery of gamma ray bursts, Gamma ray bursts are just fair approximations of what was expected in the way of a nuclear bomb burst.
Hirsh:What I'm trying to see also is, what kind of impact this project VELA might have had on people looking into areas of X-ray astronomy?
It was quite the other way around, At least from our point of view, it was our interest in X-ray astronomy that led us to consider, at AS&E possible methods of surveillance of bomb explosions. And that came second. The thing that came first, before the X-ray astronomy investigations, as far as the work at AS&E was the problem of the effect of X-rays from bombs exploded at high altitudes on incoming missiles, as a possible kill mechanism, in ballistic missile defense. And Annis who as I indicated was the founder of AS&E -- was the first physicist to remark, and to point to the X-ray flux from bombs as a possible destruction mechanism for ballistic missile defense, the idea being that the blast of X-rays would produce a rapid evaporation from the surface of the incoming missile, and the back pressure would cause a mechanical failure, what's called blow-off pressure. He pointed to that mechanism, and it therefore became very important to measure the flux of X-rays from bombs at high atitiude, and then also to measure the strength of this blow-off pressure, and to determine whether or not it would be a useful or possible mechanism for defense against attack by ballistic missiles. So that laid a certain foundation of X-ray physics and X-ray technology - a level of activity and interest, which led quite naturally then to the development in the scientific area. Hirsch: One of the things that's really fascinated me, in this study of X-ray astronomy, is that most or practically all the people involved in X-ray astronomy and the space sciences were physicists. They came out of a training in particle physics, or cosmic ray physics instead of traditional astronomy. Do you have any explanation for why that might be so?
Well, yes. I think that the techniques that were involved were very foreign to classically trained astronomers. Exactly the methods of detection of radiation, electronics, remote sensors, telemetry methods and so on, were completely outside the experience -- student experience or professional experience of astronomers. I think also there is quite a difference in the general tradition, perhaps of astronomy, which has always been to order the contruction of instruments from instrument makers or commercial organizations, and then the use of those in scientific investigation. I think the tradition, at least in American physics in this century, has been to tinker, and to build at least prototypes of new instrumentation concepts in the laboratory and test them out, And that was what was called for in these new fields of high energy physics: innovation in the construction of X-ray detectors with large sensitive areas and the development of new forms of mechanical collimators. The measurements and development of reflection objects research required techniques that were unknown to industry, or to any standard instrument makers, and in the tradition of experimental physics, those were things that were attractive, to be done by experimental physicists. It's still the case. Almost everybody who's in X-ray astronomy (now, 90 or 95 percent of them) are from a physics background.
Hirsh:Well, I think I've run out of questions right now Do you possibly have ony of these old papers, or any proposals that AS&E would have had? As I said, we've lost them here. But do you know of any papers of historical value?
Clark:Well, the papers you don't mean the published papers, because that's easily available?
Clark:No, unfortunately, I never did maintain an extensive or carefully managed file of those early documents. The most important one that we've been talking about, I seem to have loaned to somebody, and never did get back, I would think that -- well, you perhaps have found this Riccardo has most of them, but I would think that a call to Martin Annis at AS&E would probably produce from their files, copies of these things. I should think he would be most eager to cooperate, because I think the role of that private company in the development of that whole field of science is quite unique. I think he's very proud of it, and rightly so.
Hirsh:Oh yes. Yes.
Clark:So I would recommend that you call Martin Annis directly. Stating your problem you need some of the copies of the early documents, to establish the role of AS&E in the early development of the field.
Hirsh:Right. I know there should be some copies of these around here in different people's files, I just checked Nancy Roman's files, and those are the ones that have been destroyed. Of course, that's where everything was directed.
Clark:(laughing) They'd have been glad to see the last of them.
Clark:I think it was unfortunate from the NASA point of view, that the discovery wasn't their first discovery. I think it may have ranked all these years.
Hirsh:Maybe. It does seem like there is a bit of embarrassment over here about that.
Hirsh:Well, thank you very much for all your answers and everything.
Clark:You're very welcome.
Hirsh:Let me ask you, is it all right to quote you in my work here from what we've discussed?
Clark:Well, essentially. I would like, however, to see the quote, if I may. I would appreciate the right to edit.
Clark:At least for checking that I didn't misspeak myself.
Hirsh:Well, that's reasonable.
Clark:And say some nonsense.
Hirsh:And finally, would it be okay to deposit this tape and a transcript of our conversation with the American Institute of Physics. The AlP has a Center for History of Physics there, headed by Spencer Weart, who is trying to build up archives in the History of Modern Astrophysics.
Clark:Well, I guess so. I don't think I've said anything that's in error, but, as I say, as far as printed citation, I would like the opportunity to review what I said so that nonsense doesn't get into it.
Hirsh:Right. Well, that's what I'm trying to avoid. So it would be to my benefit, too, to let you see it beforehand. Okay, well thank you very much for giving me this much time.
Well, you're very welcome.
Also of the Dept. of the History of Science, University of Wisconsin, Madison, Wisconsin.
 Dr. Riccardo Giacconi (born 1931) of the American Science & Engineering Inc. (A.S.& E).
Dr. Bruno Rossi (born 1904), a consultant to AS&E and professor of Physics at MIT.
Another researcher into the history of x-ray astronomy, at the University of Edinburgh, Scotland.
Sputnik I was launched October 4, 1957.
 [ See, for example, L. Biemann, “KOMETENSCHWEIFE UND KORPUSKULARSTRAHLUNG”, ZEITSCHRIFT DER ASTROPHYSIK 29 (1951) :274; L. Biermann, “PHYSICAL PROCESSES IN COMET TAILS AND THEIR RELATION TO SOLAR ACTIVITY" , PHYSICAL MEMORIES DE LA SOCIETE DES SCIENCES , Liege 4th ser., 13 (1953): 251; and E.N. Parker, “THE SOLAR WIND”, in William Liller, ed., Astrophysics (New York: McGraw Hill Book Co., 1961), pp. 157—70.]
The company was incorporated in late 1958.
Actually, late 1958.
Jerrold R. Zacharias (born 1905) was a professor of physics at MIT (now emeritus professor) and director of the Institute's Educational Research Center.
Riccardo Giacconi, George W. Clark, and Bruno B. Rossi, A Brief Review of Experimental and Theoretical Progress in X-ray Astronomy, AS& E.
Note #49 (Cambridge, Mass, 1960). See Apendix: Dr. Nancy G. Roman is currently chief of the Astronomy and Relativity section in NASA’s office of Space Science.
Actually, February 1960.
Dr. John W. Salisbury (born 1933).
Dr. Herbert Friedman (born 1916) of the Naval Research Laboratory (NRL) in Washington D.C. was the chief scientist in charge of many rocket astronomy experiments. He is currently the chief scientist of NRL’s E.O.Hulburt Center for Space Research.
Dr. Herbert Gursky (born 1930) of AS& E.
Dr. Frank Paolini of AS& E.
Dr. James E. Kupperian (born 1925) worked at the NRL from 1954 to 1958. He then transferred to NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
R. Giacconi, H. Gursky, F.R. Paolini and B.S. Rossi, “Evidence for X-rays from Sources Outside the Solar System”, Physical Review Letters 9 (1962): 439-43.
I.S. Shklovskii (born 1916) of the Sternberg Astronomical Institute of Moscow.
S. Hayakawa and M. Matsuoka,"Origin of Cosmic X-Rays" Progress of Theoretical Physics, Supplement 30 (1964): S 204-228.