Bruno Rossi

Hirsh:

The following is a telephone interview with Dr. Bruno Rossi, currently at M.I.T., formerly the chief consultant of American Science and Engineering, (AS&E) Inc., Cambridge, Massachusetts, interviewed by Richard F. Hirsh at NASA^[1], July 21st, 1976. Hello, Dr. Rossi; do you mind if I tape-record our conversation?

Rossi:

Oh, no. As I say, I don’t know if I’ll be able to answer all your questions, but whatever...

Hirsh:

I think they’ll be fairly general questions about your involvement with AS&E and X-ray astronomy. I’m very interested in AS&E as an independent company doing research and getting so involved in this new science — actually doing a lot in creating this new science. It seems to be a unique thing that this one company would do so much. Can I ask you first: how did you get involved in X-ray research?

Rossi:

Well, what happened is that, in 1958 the National Academy of Science established its Space Science Board, whose function was to help in the planning of the scientific program of the national space activities. I was one of the original members of the Board, and while I didn’t have any specific assignments, since I was not an astronomer, I was asked just to participate in the meetings and look around and see if there were areas that had not been included in the original program.

Hirsh:

Who set up this Space Science Board in NAS? Do you remember?

Rossi:

I think it was Lloyd Berkner, wasn’t it? [Rossi’s editorial note: I am sure that L. Berkner was the chairman of the Space Science Board when I joined. I also believe that he was responsible for setting up the Space Science Board, but of this I am not 100% sure.]

Hirsh:

And did he invite you to...?

Rossi:

He invited me to be a member. My participation in the activities of the Space Science Board was instrumental in focusing my interest and my attention upon the new possibilities that were opened up by the availability of high flying rockets and space platforms. And I felt that two fields of research, which had not been actively pursued up to that time (1958), deserved attention. The first was the detection in situ of an interplanetary plasma of solar origin (solar wind). As you know, at that time there was only indirect evidence available, mainly from the analysis of the comet tails. This prompted me to initiate at M.I.T. a research project directed at the detection of the solar wind. As you probably know, this led to a number of experiments, of which the first in which the solar wind was detected was Explorer 10, launched in 1961. Now, the second field of research was a search for X-ray emission by celestial objects other than the sun, which was the only celestial X-ray source known at that time.

I must admit that my motivation did not stem from any theoretical consideration. Rather I had a long experience in cosmic ray work, and this experience had taught me that whenever a scientist ventures into a previously unexplored field, he’s likely to be confronted with entirely new and unexpected phenomena. In other words, I did not know anything (or I knew very little) about astrophysics, and at the time when I first became interested in this field, I didn’t have any specific reason to anticipate the emission of strong fluxes of X-rays from stellar objects. The only thing I knew is that no one had looked for them. Or at least no one had looked with sufficiently sensitive detectors. It was now possible, without any major technical improvement, to increase by a couple of orders of magnitude the sensitivity of the detectors, so that I thought that one should begin to look at the sky with such detectors. And this was the origin of my first involvement. At that time I was the chief consultant for American Science and Engineering.

This company up to that moment had worked mainly on projects related to the Atomic Energy Commission program of atomic tests. And this program involved experiments for the measurements of X-ray and gamma rays from nuclear explosions. Now, for many reasons, I considered it desirable for the company to expand its activities in other fields, in particular in the field of scientific space research, for which the company was well equipped because of its previous work on experiments on atomic tests. So in 1959, I believe, I began to discuss the possibility of extra-solar X-ray astronomy. We had Dr. Annis^[2], the president of the company, and also Dr. Giacconi^[3], who had recently joined American Science. Very soon other scientists from this community joined in discussions, particularly Dr. Olbert^[4] and Dr. Clark^[5], who were also consultants for American Science as well as other members of our cosmic ray group at M.I.T. So that’s how American Science got involved. Does that answer your question?

Hirsh:

Yes. Let me ask you this about your background. You say that when you joined the Space Science Board, you really didn’t know much astrophysics.

Rossi:

Well, I had been involved in cosmic rays, which is in a way a part of astrophysics, too.

Hirsh:

And what kind of work were you doing with cosmic rays?

Rossi:

Well, you see, I’d been involved with cosmic rays since 1928 and ‘29. I’d done all sorts of work. It’s a little difficult to summarize it in a few words. I became involved in cosmic ray research at a time when most people still thought that cosmic rays were gamma rays of very nigh energy. It was in 1928 when a paper came out by Bothe and Kohlhorster^[6] which advocated the idea that they were charged particles. So my first involvement was in experiments trying to determine whether primary cosmic rays were in fact gamma rays or charged particles. If you want to know more about it, there is a little book which I wrote about cosmic rays published by McGraw-Hill, which describes all this early work.^[7] Then I was involved in many aspects. One of the most recent was the measurement of the lifetime of mu-mesons. That was just before the beginning of the war. And then after the end of the war I started a cosmic-ray group at M.I.T. which did many different things: experiments on the nuclear-active component of cosmic rays at various altitudes and experiments on extensive air showers. You see, what happened then with the development of high-energy machines, was that most of the work that used to be done using cosmic rays as a source of high-energy particles, was transferred to the high-energy machines. And so some of us went into machine work and others instead became interested in the astrophysical aspects of cosmic rays; I was one of them.

Hirsh:

Possibly could I get back to the Space Science Board? When you made these suggestions to do work in X-ray astronomy, how were these suggestions received by other people besides the AS&E group?

Rossi:

I made the suggestion to the American Science group, and they tried to get a contract for this work. NASA was not willing to do it; they (at NASA) were not willing to enter into the field because all the astronomers felt that the possibilities of success were exceedingly small. Of course, we, too, when we began looking into those aspects of astrophysics that might have to do with X-ray astronomy, found that all known celestial objects were very unlikely to be sufficiently strong sources of X-rays to be detectable with instruments at the present state of the art. What happened, however, was that there were some very strong sources, but they were objects that were not known at the time. The first activity in extra-solar X-ray astronomy was not supported by NASA but supported by the Air Force^[8]. And the reason was that the official purpose of our first experiment was to try to detect X-rays excited in the lunar surface by solar X-rays. The sun was the only known source of X-rays at that time, and one could figure out that these solar X—rays should excite X-radiation on the moon. Besides there was another possible source of X-rays on the moon, and this was the impact of high-energy electrons.

When we started, we had not yet measured the solar wind, and the previous estimates had predicted that the solar wind had a much greater velocity and a much greater density than it was actually found to have. In that case, the electrons from the solar wind could have been a source of X-rays. But in any case, the choice of the aim for the first experiment, which was the detection of X-rays from the moon, was dictated in part by the fact that from all that was known, the moon appeared to be the strongest source of X-rays after the sun — which it wasn’t — and also the fact (which was not certainly a minor factor) that we could get support for this particular experiment because the Air Force, through the Cambridge Center, had a program in lunar research and was willing to support this particular experiment because of information that it could possibly have given on the nature of the lunar surface. At the same time as we were trying to obtain support for the detection of extra-solar X-rays with conventional detectors, which were just Geiger counters and proportional counters, we started a program of developing a focusing X-ray telescope using grazing incidence reflection^[9], and eventually NASA supported that program. Of course, the first rocket flight that was launched in June 1962 did not detect the moon, but detected SCO X-1, which is a much stronger X-ray source than the moon. X-rays from the moon have been detected since. So there were those two programs. One was the detection of extra-solar X-rays with more or less conventional detectors of increasing sensitivities and sophistication. And the other was the development of the grazing incidence telescope which was flown in several sounding rockets and in the Sky Lab. for the study of solar X-rays.

Hirsh:

Why do you think NASA rejected your initial proposal?

Rossi:

Because it was so extremely unlikely to give positive results.

Hirsh:

And the Air Force thought differently?

Rossi:

The avowed aim of that first experiment was not to look for X-rays sources outside the solar system, but to try detecting X-rays from the moon. And that, for one thing, looked more promising. Also, lunar exploration was part of the Air Force program. To astronomers who knew anything about astronomy, it looked extremely unlikely that there would be sources outside the solar system that would be detectable. I heard it many times said that it was known that the sun is the only, possible celestial X-rays detectable from the earth. And as I said before, the only reason why the astronomers was wrong was not because they miscalculated but because they didn’t take into account the possibility of other objects that weren’t before, which were many orders of magnitude stronger X-ray sources than previously known objects. I’m sure that Dr. Newell^[10] knows all of that very well. I had a long correspondence with him.

Hirsh:

Did you have any contact with the NRL group^[11]? Apparently in 1957 there was a rocket shot by James Kupperian.^[12]

Rossi:

Yes, yes. The feeling at NRL was that they had not detected any extra solar X-rays.

Hirsh:

So that had actually no effect on your research?

Rossi:

No, that had no effect whatsoever. (refers to notes) Yes, I remember talking with Dr. Kupperian about that, but I’ve forgotten now whether it was before or after we had our first success. I really don’t remember when I heard for the first time about the daylight time flight of Friedman^[13] which I think he had in 1956. But I think it was after we had discovered the first celestial X-ray source. In fact, my contacts with NRL began only after this discovery. I know that there was a tentative claim by Friedman that there were some X-rays coming from outside the solar system, but apparently they did not believe it very strongly.

Hirsh:

Well, they never published anything.

Rossi:

They never published anything. Perhaps you know more than I do. I had a conversation with Kupperian in 1959. Kupperian was already at NASA at that time. And it was on a personal visit that I went to NASA to seek support for our X-ray astronomy program. In this conversation, Dr. Kupperian told me about a night-time flight performed in 1957, so that wouldn’t be the same as the one in which Friedman had made his claim. And the main purpose, I remember, of that flight, was to observe Lyman α emission from the night sky. Now, I remember that during that flight they detected a signal which might have been due to X-rays coming from a direction near the horizon. However, the other members of the NRL group became convinced that the signal was much too strong to be ascribed to a celestial X-ray source and must be explained by some atmospheric effect. So it is my belief that really there was no evidence for any extra-solar X-ray source, before our flight in 1962.

Hirsh:

You said earlier that you had really no theoretical expectations, about celestial X-ray sources.

Rossi:

Not when I got the idea first. As soon as I got the idea I began to study a little bit and, not only I but as I said, Dr. Olbert and Dr. Clark and especially Dr. Giacconi, and we examined the possibility that unusual X-ray objects, such as the Crab nebula or stars with high magnetic fields could be X-ray sources. And the results of this analysis were rather discouraging in the sense that, yes, they probably were X-ray sources, but very weak. In the case of the Crab nebula, well, the expectation was not proved to be correct. The Crab nebula is a much stronger X-ray source than anyone had the right to expect, and the reason, of course, is that: electrons are accelerated by the pulsar in the Crab to much higher energies than was expected at that time. But, as I say, the main reason why X-ray astronomy became such a big field of research is that there were objects whose existence had not been expected before and which are particularly strong X-ray sources. SCO X-1 puts out a thousand times more energy in the form of X-rays than in the form of light.

Hirsh:

I have in front of me AS&E’s technical note from 1960 which you and Riccardo Giacconi and George Clark wrote.^[14]

Rossi:

Yes, I remember that.

Hirsh:

In them I see you have a section on future observations and the possible mechanisms of X-ray production. There were two possibilities for X-ray production. One was just bremsstrahlung radiation and the other was syncrotron radiation. How come there was no mention in here about just very hot black-body radiation in the X-ray range?

Rossi:

Let me clarify this point. We did not consider black-body radiation, i.e., radiation from a very hot dense object. But we did consider radiation from a thin hot plasma, transparent to its own radiation. This is just a particular form of bremsstrahlung, not known as thermal bremsstrahlung, which is bremsstrahlung from electrons with Maxwellian velocity distribution. And in fact it very soon became clear that this kind of bremsstrahlung was one of the most likely sources. I remember I mentioned the possibility at the Solvay conference in Brussels in 1964. Well, I don’t know if I was the first one to point out, I think other people did it more or less simultaneously with me — but I remember a meeting at Holland^[15] a few years later at which I stressed the likelihood that bremsstrahlung from a hot plasma would be a major source of X-rays. And the reason is that If you consider electrons of energy necessary to produce X-rays, i.e., electrons In the energy range of kev’s or tens of kev’s — let us say up to 100 kev, they will radiate a very small amount of energy by direct bremsstrahlung and use the rest to heat up the gas. And this will happen until… the electrons acquire the same average energy as the photons. At that time the electron collisions will not on the average lose energy through a heating mechanism, but will lose energy by radiation. I don’t know if I made myself clear. Even if you inject what might be called high-energy electrons — electrons of 100 kev or so — they will lose only a small amount of energy by direct bremsstrahlung and most of their energy will be used to heat the gas, which then will become a source of thermal bremsstrahlung.

Hirsh:

I see.

Rossi:

You know, all this idea developed gradually over the years.

Hirsh:

You say that AS&E was originally working with defense contracts — AEC contracts. Was this in conjunction with that Project Vela?^[16]

Rossi:

No, no, that was much before. That was in the time of the Pacific tests in the ‘50s.

Hirsh:

So did AS&E have anything to do with Project Vela?

Rossi:

No.

Hirsh:

Can you tell me possibly what type of general feeling that space scientists had toward NASA? And don’t be kind just because I’m with NASA right now. I’m only here for a short while and I have no long-term affiliations with NASA. Was there any antagonism? Was there hope perhaps that NASA would pick up a lot of projects that people had given up?

Rossi:

You know how it is: we always wanted NASA to do more than NASA was prepared to do. In retrospect I must say that once NASA had been convinced of the viability of this kind of research, it did support us quite well. However, it’s still true that from the moment the first X-ray satellite was proposed to the moment it was launched, too many years went by. It went through in 1970. But then the situation changed, and I don’t think we can, in all honesty, complain about NASA, knowing, of course, the restrictions which NASA is subjected to.

Hirsh:

What restrictions are those?

Rossi:

Well, I mean financial restriction. NASA cannot do everything that they would like to do, obviously. But the cooperation that we had from people like Homer Newell...

Hirsh:

It comes to my attention when I look over the people who were involved with X-ray astronomy and perhaps space science in general, that the people who got into these fields were not astronomers.

Rossi:

That’s right. That’s very true.

Hirsh:

Why do you think that’s so?

Rossi:

Well, most of the people that were involved were cosmic ray people, as you may have noticed. I don’t think there are many who came from any other fields, are there? Whom are you thinking about? Giacconi was an X-ray man; I was an X-ray man; Clark was an X-ray man. Excuse me, not X-ray — cosmic ray. And the reason was partly practical that the detectors used in X-ray astronomy were counters not very dissimilar from the counters used in cosmic ray research, so these people were very familiar with that kind of instrumentation. The other reason is that astronomers are used to doing their observations with the instruments that other people build. They don’t build their own telescopes. They’re not accustomed to develop instruments, or at least not the major parts of the instruments which they are using in their experiments, like the cosmic ray people were. The first and most important reason is the one I mentioned before: that cosmic ray people were used to expecting the unexpected, while optical astronomers were not. In fact, many of the radio astronomers also came from physics rather than from astronomy at the beginning of radio astronomy.

Hirsh:

But it also seems that many astronomers did not get involved with X-ray astronomy, at least after discoveries were made.

Rossi:

Not for a while. Either they did not believe our results or they thought they would prove to be not important aspects. There is some sort of — how shall I say — laziness involved and this will change. In effect, — I don’t know for sure — but I imagine it must have been disturbing to some classical astronomers to see that objects would behave in a manner different from what they expected. There must have been a psychological difficulty there. I think Herb Friedman was the only one who really jumped into this right away because he was used to stiff X-ray experiments, and he had done a lot of excellent work on X-rays from the sun.

Hirsh:

Would you say that the astronomers were excessively cautious or overly conservative, something like that?

Rossi:

I wouldn’t like to say that specifically about any one of them, but in general that’s correct.

Hirsh:

I don’t want to put words in your mouth, of course, but that’s just the feeling I’ve been getting.

Rossi:

Well, look, you know that book, Science in Space that was published by Berkner and Odishaw?^[17] Look at that, and you’ll see that X-ray astronomy is disposed of in a few lines.

Hirsh:

Yes, I have that somewhere in my desk here. What was the really exciting part about the discovery in 1962 of what was later shown to be SCO X-1? Was it the discovery of something that you did not expect?

Rossi:

That’s right, that’s right. I mean SCO X-1 was about 30 degrees from the moon. I remember we spent months and months to make sure that there wasn’t some mistake in our aspect determinations and convince ourselves that the X-ray source wasn’t the moon actually.

Hirsh:

So you were working on trying to make sure that what you received was actually X-rays from something besides the moon...

Rossi:

Something outside of the solar system, yes.

Hirsh:

And was there internal dissention opposed to this conclusion?

Rossi:

I wouldn’t say so. We were all skeptical at the beginning, and a lot of work was done to try to evaluate all possible sources of error. Then another flight was carried out, you know, in October at the time when the presumed source would have been below the horizon, and we didn’t find anything at that time. And that confirmed our conclusions. And then, as you know, the year after, 1963, Friedman got his first flight, which also detected SCO X-1, in fact he located it much more accurately than we had done.^[18] So in June 1963 we repeated our flight and got the same result. However, we did commit ourselves I mean we asserted the existence of this source — already in the fall of l962.^[19]

Hirsh:

The company, AS&E, seems like it’s a rather liberal company in allowing you to do basically anything you want.

Rossi:

Yes, it’s right.

Hirsh:

What kind of company was it?

Rossi:

It’s a very strange company. It was founded by Martin Annis, who had been a student of mine, and almost all the people in the company working as consultants were old friends of ours. So the company was at the beginning directed by scientists, and it applied itself, as I told you, to experiments of the Atomic Energy Commission, and then to educational problems. Now, of course, it has changed a great deal. It’s much more of a commercial company now, and it’s making much more money than it used to at that time.

Hirsh:

Well, I suppose it was financially stable in its early years. It got most of its money from what — from contracts?

Rossi:

Yes.

Hirsh:

And who else did it do contract work for?

Rossi:

Well, as I say, in the beginning it was mainly the Atomic Energy Commission and then NASA. And until fairly recently a large proportion of the business was with government agencies. Now this has changed and there’s a lot of commercial work going on, as you probably know. You’ve probably seen the X-ray machine that examines the baggage of passengers. That was a great success. And it was a direct fall-out from the X-ray astronomy.

Hirsh:

No kidding.

Rossi:

Yes. And now there is the medical application. And they all came out of the expertise which the company had developed in scientific enterprises, so it was not even from a financial point of view, a dumb thing to do.

Hirsh:

And how many scientists were there in the years when X-ray astronomy was moving along?

Rossi:

At the beginning there were only a few, a dozen or so. Now, while it’s still doing some scientific work, it’s main activity is commercial. You probably have seen the reports. All the original people — I mean Giacconi and I and Olbert, and Clark are no longer connected with the company.

Hirsh:

Let me ask you this: there was a conference in Cambridge at the Smithsonian Astrophysical Observatory in 1960.

Rossi:

Yes, I remember that.

Hirsh:

On X-ray astronomy. You were there.

Rossi:

I was there, yes.

Hirsh:

And what was the general feeling of the people there? Did they really expect to see something? I know that they were obviously interested in the field of X-ray astronomy.

Rossi:

Well, on that occasion the various possible sources of X-rays were examined. Have you seen the report?

Hirsh:

Yes, I have.

Rossi:

You probably know better than I do since you have read it recently. If I remember correctly, there was evaluation of the various possible sources, and none of them was very strong but could be detectable if the detecting equipment was sufficiently improved. At that conference also X-ray optics was discussed, wasn’t it?

Hirsh:

Yes, indeed.

Rossi:

Baez^[20] was there.

Hirsh:

When you went up in 1962, obviously your detectors were more sensitive than they really had to be to detect SCO X-1. Now, what kind of effect did this have on further development? You must have obviously been pleased that there were sources stronger than you expected.

Rossi:

Yes.

Hirsh:

How did that affect further instrumental developments?

Rossi:

Oh, yes, of course. As I said, the detectors continued to be and still are for the most part, proportional counters. The construction of the counters became more and more sophisticated. That meant larger windows and particularly better collimaters. That was a very important development. Of course, a very good source for all of that is the book edited by Giacconi,^[21] you know — you have seen that.

Hirsh:

Right, indeed. Actually I think that finishes my major questions here.