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Credit: Johns Hopkins University
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Interview of Paul Feldman by David Zierler on May 1, 2020,Niels Bohr Library & Archives, American Institute of Physics,College Park, MD USA,www.aip.org/history-programs/niels-bohr-library/oral-histories/45289
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In this interview, David Zierler, Oral Historian for AIP, interviews Paul Feldman, professor emeritus of physics at Johns Hopkins. Feldman recounts his childhood in New York, his education at Brooklyn Tech, and his undergraduate work at Columbia, where he studied with Polykarp Kusch and worked at Brookhaven Lab during the summers. Feldman describes his decision to stay on at Columbia for graduate school to work in high energy physics, his work at the Naval Research Laboratory, and he provides a broad overview of atomic physics going back to the 1940s. Feldman details his longtime collaboration on projects with NASA during his career at Johns Hopkins, and he describes the significance of the Hubble telescope. In the last portion of the interview, Feldman shares his views on what he considers to be the most important current and future topics of research in astrophysics.
Okay. This is David Zierler, oral historian for the American Institute of Physics. It is May 1, 2020. It’s my great pleasure to be here with Professor Paul Feldman. Paul, thank you so much for being with me today.
It’s a pleasure to be here, David.
Okay. So let’s start with your title and your institutional affiliation.
Okay. My present title is Professor Emeritus. I got that title about eight months ago. For many years I worked my way up the ranks at Hopkins, retiring in 2010 as a full professor. I remained as a research professor in order to participate in a number of space projects, including the Lunar Reconnaissance Observer and the European Rosetta mission to Comet Churyumov-Gerasimenko.
Okay. All right, and now let’s go right back to the beginning, terrain I’m very familiar with. Let me hear about your childhood in Brooklyn.
Oh, my childhood in Brooklyn. Are you from Brooklyn?
My parents are from Brooklyn. My dad is from Coney Island and my mom is from Flatbush.
Mm-hmm [yes]. Well, I was from Borough Park, which is very different from what it is today.
[Laughs] I can't say very much about my background in Brooklyn, except that when I was ten, my parents bought a house way out at the end of Queens and I went to Brooklyn Technical High School, which is one of the city… I don't know if they’re called magnet schools or whatever it is, but special schools that you take an entrance exam for. So I had--
Are your parents native New Yorkers? Were they born in New York?
No. They were both immigrants.
Mm-hmm [yes], and when did they get to New York?
My mother got there in 1935, and then she went back and married my father in Paris in 1938. Then they got to New York in that year.
Was your father from Paris?
No. Both of them were from… Well, it’s hard to tell where they were from. When they were born it was Russia. After World War I, it was Romania and after World War II it’s Ukraine.
Right. Right. I assume they were they were… You're Jewish? Your parents were Jewish?
Were they refugees? I mean this is before it got so bad.
No, they were not refugees. They left. They used various means to get out beforehand.
What was your father’s profession?
He trained as a pharmacist. His father had run a little pharmacy in, I guess, a medium-size town at the isthmus of the Danube River. So he trained. He went to school in Bucharest, Romania, but when he came here, he obviously didn't have…you know, couldn't use those skills. But he worked with… Again, through family connections, he worked with a company that was exporting sugar from Cuba, Haiti--
Did your mom work outside the house?
Mm-hmm [yes]. So when did you decide to go to Brooklyn Tech? How did that play out?
Oh, those are hard things to remember.
You must have been good at science at an early age.
I think I was. I remember one thing that when I was… One of the things in my life, which you might touch upon later, is that I consider myself now an astronomer, but I didn't start out as an astronomer. I was actually a physics major. I wasn’t really a physics major when I started out; I was an engineering major. But basically before we moved to Queens, I was a member of the junior astronomy club at the Hayden Planetarium in New York.
And that was a long haul even from Brooklyn to go. I don't know whether I still have it or not, but I used to make my hand-drawn sky maps and all sorts of stuff like that. It was great. I was really turned on by astronomy.
Was this a program for gifted students? How did you get involved with Hayden?
[Sighs] I can't really remember. It may have been through the school. It may… You know, at that time I would have been in elementary school, and I’m not really sure how. It may have been just something that my parents discovered. No, I can't really give you any information on that.
Did you have a good time at Brooklyn Tech? Was it a good experience?
No. It was an awful experience. I mean I did very well; I won a lot of prizes. But it was a three-hour a day commute, a bus and two subways, and there were no girls there at the time. So I had a really sheltered social life. I guess I did very well because I was able to get a scholarship to go to Columbia College, which at that time, the tuition was something like $800/year, which my parents could not afford if I had not had the scholarship.
Now was Columbia the best and nearest school because you wanted to stay close, or you would have gone to Columbia if you were from Iowa?
No. Actually, my first choice was Cooper Union, which at that time was free and just as close, or just as far. I think at Cooper Union I would have had to commute. Columbia, there were dormitories and the scholarship--
Oh, so you lived up there? You lived at Columbia?
I lived up there, yeah. That was an even longer commute than going to high school in Brooklyn.
Right, right. Now did you declare physics as a major from the beginning? Did you know you wanted to study physics when you started school?
No, I went in in a five-year engineering program. So I really didn't take physics until my sophomore year. I was taking engineering mechanics. One of the reasons I got into engineering is in high school I had a summer job working for a naval architect. I think the company was Rosenblatt. In high school… You're familiar with Brooklyn Tech and those other schools.
So I learned all sorts of skills like drafting and sheet metal working and machine shop work, none of which was needed at the naval architect except for the drafting.
But they had a project at the Brooklyn Navy Yard. They had one of these new aircraft carriers, the Saratoga class or the Forrestal class, of which Theodore Roosevelt is one of the later members of that class of large aircraft carriers. They wanted to find out how stable it was, so what they did… The naval architect had all the drawings of how the insides, how the guts of the ship were done, so what they wanted to know is what happened when water got into the bottom, in the bilges, and sloshed around. So what they would do is they had some heavy machinery on the deck and they would run the machinery from one side of the ship to the other—not the length of the ship, but across the ship. They would measure how much the water moved and how much things changed. So they needed lackeys to run down these ladders five stories down with a yard stick and measure water levels. That was my job. [Laughs] No drafting, no machining.
But I guess I was really taken with engineering. I mean it seemed like a really good field, and I was good in math in high school. I won some… I was on a math team. I won a math prize. I won a chemistry prize. One of the things in moving from my house to here is I found all these old medals and things. I realized I was pretty smart in those days—maybe not smart enough, but I got those medals.
So anyway, I started at Columbia and I enrolled in a five-year engineering program, and I really hated engineering mechanics. All you did is you did analyses of bridge struts, you know, the weight on one… You know, how do you keep a bridge from falling down? Well, that wasn’t very exciting. So in my sophomore year, I was one of the first… The second half of the year is the first bit of physics, the first introduction to physics, and I had a fantastic teacher. I had Leon Lederman.
Oh, yeah. Yeah.
I’ve run into a lot of Nobel Prize winners in my life, especially early on. So Leon Lederman--
Was he famous already? Was Lederman famous at that point already?
Well, he hadn't done the two-neutrino experiment yet.
This is 1957, I guess. Is that right? The two-neutrino experiment was ’64, I think. I almost worked on that, too. That’s another story.
Oh, wow! Okay.
Okay. So let’s see. Where am I? So the second… Now I’m a junior, a third-year student, a junior, and you do electricity and magnetism and modern physics. The instructor for that was Polykarp Kusch, another Nobel Prize winner.
He passed away maybe ten years ago. I still have his obituary on my desk in my office or on my bulletin board. So he was one of my first mentors. It turns out that I needed to work part-time in order to support myself.
Now at this point, had you already switched over to physics, or you took a physics course as part of your engineering program?
No, I switched to physics. I was behind.
So many of my junior physics majors were ahead of me. I tried to catch up a little bit, but it wasn’t so hard because then you’ve got all your elective courses in your senior year and you can do…you know, once you get past the introductory courses. It still strikes me as strange to start in the spring semester and make it a three-semester sequence, but that’s the way it was then.
So I needed a job and I was working on 125th Street in a factory that made garage doors. I was looking for a job for the summer, and Brookhaven had a program for undergraduates. I needed a reference, so I went up to see Kusch, knocked on his door. Well, his door was usually always open, but I went into see him, trembling, and asked him if he would write a reference for this summer job.
Now you trembled because that was the kind of personality he had or just you were just shy?
Oh, he was an incredible personality. He had a big stentorian voice.
Was he approachable, though?
He was approachable, yeah. He turned out to be a very kind person. So he’s asking me what sort of skills do I have? Basically, I assumed he was wanting to know what kind of a person I’d be in this job. I said I’d been to a Brooklyn Technical High School and I had drafting skills. I had some machine skills and sheet metal skills. He said, “You know which way to turn a screw?” I said, “Yes.” He said, “I have an Indian post-doc named Rao [?], and when he comes to a screw, he tries it first one way and then the other way. I need someone to work with this post-doc to help him.”
So he had a little machine, a very short vacuum chamber, in which he basically had an atomic version of a Fizeau toothed wheel experiment. You know how they measure the velocity of light? You have two wheels spinning somewhat asynchronously, and light is separated by distance. So in this case, a particle goes in one…gets past one slot or past one tooth. Then if it travels and finds another tooth, it’s stopped, but if there’s a space, it goes through. By rotating the wheel and knowing the separation, you can tell what the velocity is. This was done in the 19th century to determine roughly the velocity of light. So Kusch was doing this with velocity distributions with atoms. He was measuring the Maxwell-Boltzmann distribution of various atoms. So I got to work on that experiment with Rao. I quit my job with the garage factory and worked in the lab.
We were actually in sort of borrowed space. The rest of the lab was occupied by I. I. Rabi, who I think this was his last hurrah. He had one student. He was trying… Again, I’m trying to recollect what he was trying to do. He was trying to measure the electric dipole moment directly by looking at the repulsion between a neutral molecule and its image in a reflecting surface, which is an extremely difficult experiment. I’m not sure whatever happened to the graduate student who was working on it, but he would show up occasionally. Generally I would see Rabi at the departmental colloquia. He would sort of come in, sit in the front row, read his mail, and then either ask a question and get up and leave, or just get up and leave without asking a question. So he was pretty old by that time.
So what happened? So a year later, I’d been to Brookhaven. Thanks to Kusch, I’d worked in his lab. I was getting ready to apply to graduate school. I went to Kusch and asked him for a recommendation for graduate school. I was applying to Princeton. Don't ask me why I picked Princeton, but I was applying to Princeton.
Only Princeton? That’s the only place you were considering?
No, I applied to Columbia, but I wanted a recommendation for Princeton. The place was relatively close to New York. He said, “You don't want to go to Princeton. You're not the Princeton type.” I said, “Okay. What do you recommend?” He said, “Well, if you stay at Columbia, I’ll give you a fellowship so you won't have to teach the first year.”
Now what did he mean, do you think, when he said, “You're not the Princeton type”? Were you too New York for Princeton?
No, I wasn’t preppy enough, I think. [Laughs] Well, I have a number of colleagues who are Princeton types, and so that’s another story, right? [Chuckles] So I stayed in New York. I worked at Brookhaven again in the summer.
Now do you think Kusch specifically wanted you to stay because he had specific ideas of what he wanted you to work on based on what you had already done?
He might have. Right, right, but there was a monkey wrench, which is that summer, the summer I graduated, he became vice president of the university, a provost. I’m not sure what the position was. He brought in a former student, actually a student of Willis Lamb, Bob Novick, who took over his labs and took over basically running… This was the tenth floor of Pupin Hall. In addition to Townes… I mean in addition to… I gave the story away. In addition to Kusch and Novick, Charlie Townes was on the 11th floor, and I had some interesting experiences with my graduate school friends who were working for him in the sense that one of them missed out on making the first ruby laser because of bad advice from Art Schawlow, who was visiting Columbia at that time. But he had the fourth ruby laser in operation then.
Do you know what the bad advice was?
Yeah. The bad advice was that there wasn’t enough amplification to get the system to mase. Basically, a factor of 2 error in the calculation and it--
Mm-hmm [yes], when in fact there actually was enough amplification.
There was enough amplification to mase.
Mm-hmm [yes], mm-hmm [yes].
I say mase, but you know, to oscillate. So he had the fourth one. I remember working with him, one night helping him in an experiment in which a flash lamp blew up—it made a lot of noise—and another time in which he did a very fundamental experiment. His name was Isaac Abella. He did a very fundamental experiment in which there are sufficient photons coming out of a laser that he could tune by changing the temperature of the cryogenic bath that it was in. You could change the frequency, the wavelength, somewhat, and you got a coincidence where two photons were absorbed at the same time by a single atom. That was a major breakthrough that appeared in Physical Review Letters in, I think [overlapping voices].
What was the breakthrough exactly?
Oh, just technical… I mean it was predicted; it was never observed. It was the first time it was observed.
Anyway, I’m digressing, because we’re still back in Pupin. So I start working for Bob Novick, who is a wonderful guy. He had some ideas for experiments involving atoms with two excited atoms rather than single excited. I mean, he was doing basically magnetic resonance spectroscopy—you know, basically what I. I. Rabi had pioneered. So I took over this little apparatus and made a detector that worked and was pretty successful at it. I guess that took a while, but I basically finished my PhD in four years.
But at the end of my first year, I’d been planning to go back to Brookhaven. It was a congenial group of people. I’d hooked up with somebody in building a little instrument to measure the beam intensity in the Cosmotron, which was at that time the accelerator that they were running. One of my friends in graduate school, who had a job lined up in Paris, was going to get married to a Frenchman, who was again working in high energy physics, and she asked me if I wanted to take this job in Paris. So I said, “Why not?” Again, it was a low-level job. Her name was Mary Kate Ralph, who is now Mary Kate Gaillard. She just, a couple of years ago, published a book on her memoirs.
She was married to Jean-Marc Gaillard.
Yeah. I interviewed Mary a few weeks ago.
Oh, you did?
I did, yeah.
Yeah. She’s somewhere… Her second husband was Bruno Zumino and he died a couple years ago, I guess.
Mm-hmm [yes]. Yeah, yeah.
Yeah. She did very well. I saw her once a couple decades ago when she was visiting Hopkins, but I haven't really kept in touch. But I did get a copy of her book from World Scientific Publishing. An interesting book. A lot of her early reminiscences are by that crowd from Brookhaven.
So anyway, I decided I would take the job in Paris, never having been abroad, and it was an interesting experience. I spent two months working in the College de France doing basic…the lowest level of work, which is sitting in front of a screen with bubble chamber pictures and tracing them and doing calculations of the trajectories. It wasn’t completely a horrible job because I got to take a trip to Geneva, where they were building… I can't remember the name of the… It was one of the first accelerators at CERN that they were building. I got to ride a bicycle through the tunnel that they were building. But that was two months. I worked for two months and spent one month traveling around to music festivals in Europe and visiting my uncle in Algeria for a week during the Algerian War.
You had an uncle in Algeria.
An uncle in Algeria.
How did you have an uncle in Algeria?
Well, my parents came to the US. My mother’s brother went to France to study medicine, and he married a woman who was from Algeria. They call them pieds noirs, black feet.
So they were the French settlers. I mean that was ’61, so by ’63, he stayed for a couple years after Algerian independence because they needed physicians. But then he resettled to Marseilles and lived there till the end of his life.
I take it you had a good time on this trip in Europe and Algeria?
Well, yeah. I was 21 years old and it was discovery of the world. It’s pretty interesting. Okay. So let’s get back to physics. So here I am, just about ready to come back to the US, and I find out that Mel Schwartz… You know Mel Schwartz. He was with Lederman and the two-neutrino. They meet me… He meets me at Orly Airport. I think this was before they built Charles de Gaulle. He says, “Would you like to join my group? We’re going to do some interesting experiments.” I didn't know they were going to do the two-neutrino experiment, which they both… You know, Schwartz and Lederman got the Nobel Prize for that.
But I, being relatively shy and antisocial and being at the bottom of the totem pole in some of these high energy physics labs, had found that I was really enjoying working on my own experiment, my own tabletop experiment, which is more and more rare these days for someone to be able to have a tank this big with some diffusion pumps hanging off it and a whole bunch of wires coming out of it. You stick things in it and you make beams and you see results. So I declined. Maybe a mistake, maybe not. But the interesting thing is that staying in atomic physics for my PhD, Novick, my advisor, got bitten by the Sputnik bug.
He wanted to make x-ray telescopes. He was way ahead of his time because x-ray telescopes were notoriously difficult to fabricate. The technology for making grazing incidence I think is like… You know, the Chandra Observatory is the latest and most modern of the series.
But it was a long way to get there.
Now Paul, when you say that he got bit by the Sputnik bug, what does that mean exactly? In terms of the kinds of new projects he wanted to work on or his collaborators? What does that mean?
Well, it means that there were opportunities to use the techniques and skillset for doing atomic physics in the laboratory in attacking astrophysics problems. So you could build an instrument that would survey the sky and look for x-rays, which is what Herb Friedman at the Naval Research Lab was doing. So Friedman had a program at the Naval Research Laboratory… That’s another story. It’s intertwined.
Friedman was a 1940 graduate of Johns Hopkins and he went to work for the Navy. After the war, they retrieved V2 rockets and they brought them back to New Mexico and launched them from White Sands while they were developing their own small rockets for scientific research use. That’s where the first spectrum of the Sun below 3000 angstroms was obtained, and it was relatively early, like 1947. It was a different group at the Naval Research Laboratory.
But anyway, he had a program at the Naval Research Laboratory and I met him when he came up to Columbia once, because he was doing exactly what Novick wanted to get into the business, only he wasn’t using x-ray optics. He was using basically x-ray proportional counters. In other words, you measure… An x-ray comes in. You get an electrical signal proportional to the energy of the x-ray, a very crude technique for doing spectroscopy, but for doing astronomy, it works fine because you can make wide area scintillation counters and put them on a rocket and scan the sky. In those days, the rockets didn't have very good guidance, so you would just sort of scan the sky and then reconstruct where you were looking.
Did they actually have any guidance?
Well, they were spin-stabilized.
Later on, not too much later on, NASA developed the attitude control systems that gave you pretty good pointing at particular regions of the sky. So Novick wanted to get into that, and he suggested that I should look at opportunities to do that and do a post-doc in Washington at the Naval Research Laboratory. So I went there and got in with a group that was trying to build a cooled infrared telescope, which in 1965 was a horrendous job.
What was so bad?
Well, it’s the question of building a container that will hold a cryogen and launch it, cool the telescope and keep it cold… I mean ultimately the problem was solved. They built the IRAS satellite in 1983. I’m not sure there were any… There were a couple groups that were trying to build rocket-borne telescopes, but again, you needed longer observation times. You’ve got five or six minutes with a rocket.
Anyway, so I went to this group, and while I was in Washington, there was a position at Johns Hopkins. They were looking for people to work with Bill Fastie in a rocket program, which was at the other end of the spectrum (which was the UV). That was a very fortuitous move that I made because sort of at that time UV optical techniques were pretty well advanced. You could build telescopes and spectrographs, and pointing came along. So I got into the business.
Actually, the first thing was Apollo 17 where… Again, this is a whole new story now, shifting to the people at Johns Hopkins and their history. Their history was in spectroscopy, their background…
…going back to Rowland and Wood and then in the ’30s… Well, Wood retired in ’37 or ’38, but he was still around…not into my time. He died in ’55. But John Strong had just retired from Hopkins and moved to the University of Massachusetts. He was flying infrared instruments on balloons, and he actually flew with them. He was a character. But he had retired, so they had an open position and they were interested in building up an astronomy program that would take advantage of the spectroscopic expertise at Hopkins. I wasn’t a spectroscopist at that point. Still I’m not very good. I can't design a good instrument, but there were plenty of people who could. So I had Novick as a mentor at Columbia. I had Friedman as a mentor at NRL, and then Bill Fastie, who was another character. You must have a history from Fastie.
Oh. Yeah. I’ll look. I’ll look.
You must. Okay. Anyway, I managed to get into my office yesterday and I picked up some stuff that was very good to refresh my mind about the history of how Hopkins got into the sounding rocket business.
It has to do with a spectrograph design or a spectrometer design that looks like a rocket. I mean it’s basically a cylinder and the optics are very symmetrical in it. It’s called the Ebert-Fastie spectrometer. It’s sturdy. It’s rugged. It will withstand the vibration of liftoff, and it can be… It’s not a multiplexing spectrograph like we use today, but it had fairly high throughput because if you curve the entrance and exit slits, you got a fair amount of throughput.
So my first project there involved using an instrument like this to look at the Earth’s atmosphere, but in the course of doing these experiments, Fastie had written a three-page letter to NASA Headquarters saying, “Hey, I have an instrument that would be great to put on your Apollo command module. We’re going to find the atmosphere on the Moon.” He made a very powerful argument. The argument was the following, that if you're in orbit—and the Apollo orbits were all retrograde, nearly equatorial about the Moon. So as you're going across, twice a day you cross the terminator. You know, you go from sunlight… You go from day to night. So his idea was the following. If you're at 100 km and you're looking down at the surface, you're looking through the densest part of the atmosphere. The atmosphere is barometric, so it follows some sort of an exponential law. You look down and when you get to the terminator, you look against the dark surface. But if you point the instrument ahead of the terminator—you know, you don't look straight down, but you look, say, this is 23 degrees—then part of the atmosphere is illuminated. As you move through, every time you cross, you get a certain… You get five minutes of time. It was a two-hour orbit. At each terminator you got five minutes of illuminated atmosphere against a dark surface.
Now what would you see? You would see hydrogen atoms. Why? Because the Sun is emitting protons. It’s emitting solar wind. The solar wind has to go somewhere. When it hits the Moon, the protons are going to strike the surface, neutralize, and come off as neutral atomic hydrogen. So that was the purpose or the goal of the experiment, to detect that hydrogen, which we didn't, but that’s beside the point.
What’s significant of the fact that you didn't detect it?
I’ll get to the punch line. [Chuckles]
It recombines, is what happens. You get molecular hydrogen. In other words, it stays on the surface long enough for another one to come along. Okay. So he proposed this experiment. It was based on one of these instruments that he’d been flying now for a decade, the Ebert spectrometer, and it was accepted. It was accepted for Apollo 20, and as you probably remember, Apollo 20 never flew, nor did 18 or 19.
So they managed to put us on Apollo 17, which was really nice. [Chuckles] We were in the command module. We did a lot of stuff besides looking at the surface of the Moon and searching for this atmosphere. In between we did some observations of stars and the cosmic background as we were traveling between the Earth and the Moon. At one point they dumped the hydrogen fuel cells and we got a beautiful spectrum of hydrogen fluorescence excited by the Sun, which was one of my classic papers, but which turns out to be a very valuable technique because we used that to look for molecular hydrogen on Mars and various other places, comets. Okay.
So what happens? So we didn't detect the hydrogen, but we did detect… And we couldn't detect the molecular hydrogen because the fluorescence efficiency was much lower. Hydrogen, you use Lyman-alpha, which is the brightest thing in the ultraviolet coming from the Sun, 1216 angstroms. So it has a tremendous efficiency for scattering from atoms. In other words, you get a big signal. But we got upper limits on hydrogen and various other species that fluoresce in the far-ultraviolet, and we got an upper limit on the molecular hydrogen. Okay, so that was all the lunar work that I did.
Then in 1999—20, almost 30 years later—Alan Stern put together a little panel to assess where we were on the knowledge of the lunar atmosphere because by then, there were the measurements that were made on the surface of the Moon in Apollo 16 and 17 where they detected argon. The argon was outgassing from the interior of the Moon. There was some from the solar wind also, and there was sodium produced by micrometeorite impacts on the Moon.
So there was a proposal for a mission to the Moon called Lunar Reconnaissance Orbiter, which would fly in a polar orbit. It would fly along the terminator, or fly from north pole to south pole. The Apollo missions were equatorial orbit, so there wasn’t any good information about whether or not there might be ice deposits near the poles. If there were ice deposits, there would be water which might be sufficient for habitability if you were to try to build a station. So that led to some interest in new lunar missions like Clementine and LRO (Lunar Reconnaissance Orbiter) and Lunar Atmosphere and Dust Explorer (LADEE).
So Alan Stern proposed a small instrument—not an Ebert—an instrument similar to what we proposed for the European Rosetta mission for this mission. Again, it was a small, compact spectrometer with roughly the same throughput as the Apollo 17 instrument. It included a spectral line of helium at 584 angstroms, which was way outside of what Apollo 17 could do. I used the technique that Bill Fastie had come up with, except that instead of looking… Since you're not in a circular orbit and you don't see the dark surface, just at the terminator… When you're flying close to the terminator by… Depending how far you're off the terminator, the surface is constantly dark, but the atmosphere above is illuminated. They also changed the orbit so you had an elliptical orbit. So as you went from north to south, you went from something like 200 km apolune to 40 or 50 km perilune. In other words, your atmospheric column changed so you could actually use a model to… You could make an atmospheric model that you could try to fit to the data. So that was very nice. That was done in 2012, and it just shows you that old things that you try once, if it doesn't work once, when you get further on and the technology changes, those ideas sometimes are worth preserving.
So how did the technology change that allowed this research to happen?
Well, one is that it was a long mission. It’s still going on, in fact.
But the instrument is smaller, so it can be part of a…with the same sort of throughput. It went to much shorter wavelengths than the original, than the Apollo instrument went. Again, none of these things can be done from Earth or Earth orbit very easily. So it was just the right geometry and the right mission for accomplishing this. I see I’m squinting. I guess I should… I’m trying to see you and look at what’s going on.
That’s okay. It’s an audio file only; it doesn't matter. [Chuckles]
Good, good! Okay, so…
So at this point, Paul, what is your status at Hopkins? Are you on the tenure track line at this point or it’s more like a post-doc?
No, I was on the tenure. I came as an assistant professor.
I was promoted… Okay. So having these tools and having been through the Apollo experience in 1972, we were then looking at… Warren Moos was also a member of the group, and a very important optical designer was there called Murk Bottema, who ended up going to Ball Aerospace in Boulder, Colorado, but who was responsible for the major design of the piece of hardware that rescued the Hubble Space Telescope. That’s another story. There are lots of stories! Okay. So as I said, Bill Fastie was sort of the guiding light and would sort of come up with the right instrument design for the right problem. We got…
In 1973, there was a comet discovered pretty far out, Kohoutek, and it was going to be the comet of the century according to the early predictions. So NASA decided to invite people to propose rocket experiments. Now usually a rocket experiment takes a year or so if you're preparing it from scratch. The way we had a successful program in our rocket program was that we would basically take an instrument, fly it, recover it in reasonable shape—it comes down on a parachute in the desert—refurbish it, maybe change the instrument around, change the target, and fly again. In this way we were able to have students go through the system relatively rapidly. Basically one per year would have a rocket which would ultimately be a PhD thesis.
So we had no payload to look at this comet, but Bill had a very, very simple idea where you just take two of his Ebert spectrometers that existed and we put two off-axis parabolic mirrors at the base of a cylinder and do prime focus spectroscopy of this comet. You needed a pointing control. You needed a pointing control to point to the comet, and the problem was that… What made the comet very intriguing is that it came close to the Sun. So you were going to be looking close to twilight, close to either sunset or sunrise, and you had to baffle against the sunlight getting into the instrument. You also had to have a… The star tracker had to be able to distinguish the comet from any stray light.
So we put this together very quickly. We went to White Sands in January of 1974 and we launched it. It worked like a charm, except when the rocket got higher, the horizon drops. In other words, the rocket is going up, so the horizon is changing and you start getting more sunlight. In other words, you're not… It was a very clever idea, again, to use the limb of the Earth to baffle out the Sun so you could observe the comet, which was something like 23 degrees from the Sun. So it was close to the Sun both the elongation, and physically it was close to the Sun where it would be very active. Again, you’ve got five minutes to observe it, so you want to look at an active comet.
So not surprisingly, our star tracker lost track of the comet, but as it came back down, we were able to get 20 or 30 seconds where it locked onto the comet and we got some spectra. But the interesting thing is that that comet was sort of interesting, but two years later--
What makes a comet interesting or not?
Well, interesting in the sense that we were looking for things that hadn't been seen in comets before. We were looking at, again, in the ultraviolet using spectroscopic signatures. A comet is basically water, and water is… It’s water ice, and when water is dissociated, you get hydrogen and oxygen and hydroxyl. Hydroxyl had been seen from the ground for a long time, but the hydrogen and oxygen you don't see. You can see the forbidden oxygen transition, which is produced by breaking up the water. In other words, it’s a prompt emission. But basically the hydrogen images turned out to be very extended because the hydrogen comes off at a high velocity, being a light species. But this was all new. No one had done this before, so that was sort of the game at the time, is you're doing problems or you're doing observations that obviously could not be done without being above the atmosphere and later on would be done very easily by Hubble Space Telescope or by the IUE (the International Ultraviolet Explorer). But anyway, at this time it was very, very exciting doing this.
So Comet West came along in 1976 when we had the payload ready to go. Now there were actually… In the Kohoutek campaign, there were three separate payloads—one from NRL which got a nice image of the oxygen and hydrogen coma of the comet; there was a Colorado experiment; and then there was our experiment. So for Comet West we combined the NRL imager with our spectrograph, and we got spectacular spectra—not very high spectral resolution, but enough to detect carbon monoxide for the first time in a comet. So I basically spent the rest of my career measuring carbon monoxide in comets. It’s a very variable species. It sort of gives you an indicator of the temperature of formation of the comets in whatever the protoplanetary system is. Even just recently, in collaboration with a group led by Dennis Bodewits, I did the analysis of carbon monoxide seen in this interstellar comet Borisov. It just was published online in Nature Astronomy.
So anyway, it was sort of an interesting way to do science. I mean basically you had an attitude control system that you programmed in a particular region in the sky. You gave it a guide star, and then it would go from the guide star to where you wanted to point. But the comet is changing every day, so if you don't launch one day because the weather… You have to go in, get on the tower. It was a tower then; now it’s a rail that they launch from. You had to go in and adjust a bunch of screws you turn to change the program.
How does the comet change day to day? What does that mean?
Its position. Its position on the sky changes. It’s not fixed like the stars or…
And it moves. These comets that were close to the Sun are moving very fast.
Paul, let me ask. In terms of the relatively narrow interest of measuring carbon monoxide in comets, what are some of the larger questions that result from this research about larger ways of understanding how comets work or where they come from?
Well, it’s not only… Okay, let’s… You have a very good point. Basically, what we were doing is we were using UV spectroscopy to probe the atmospheres of any object in the solar system—I mean, just like UV is used to probe the atmospheres of stars. But you learn a lot about the processes that go on. You learn about the composition. You learn about the population of energetic particles. So you use UV to look at the aurora on Jupiter or Saturn and you're being able to untangle the spectra based on what the charged particle composition is. You study the interaction between the solar wind with the Jovian magnetosphere and so you get an insight into what’s going on. Mars and Venus have been studied a lot in terms of their composition: Mars with regard to what the primordial Mars might have looked like in terms of what we can measure now in terms of the loss rates of water. Basically, water doesn't escape, but you break up the hydrogen… You break it into hydrogen and oxygen. The oxygen has excess energy, and the hydrogen, of course, is very light and they escape. So you try to make measurements in the current epoch and go back to what the primordial Mars… Let’s see. So what I should… Let’s see.
So we’re in the middle 1970s. This is my own little corner of this group. So we’re getting to the point where, at Hopkins, the small rockets are serving as seeds for projects to go on the upcoming space shuttle. We’re also using our expertise in this area to get involved in the Hubble Space Telescope, or as it was known then, the Large Space Telescope. There’s a lot of literature on this. I gave a talk a couple of years ago in Padua in which I described… They had… I’d better not jiggle my laptop. They had a symposium in…I guess it was 2016. I’m trying to remember. Anyway, it was based on… Yeah, right. It was the 30th anniversary of the Giotto flyby of Halley’s Comet, which was 1986. So the theme of the symposium was 30 years of cometary research from Giotto to Rosetta, and I gave a talk on the history of the Hubble Space Telescope during that period. I’m skipping a lot of what went on, but originally, both Hubble and the space shuttle were scheduled to observe Halley. But again, the Challenger accident delayed the launch by four years, and so there were no comet observations until 1991. Halley was long gone. But there were a lot of good observations—split comets, Comet Shoemaker-Levy 9 which broke into many pieces. It looked like a string of pearls on the sky, and very complementary to what could be done by the planetary missions. Yeah, by the planetary missions.
So from a personal point of view, getting involved in the Rosetta mission with a small UV spectrometer was a great way to spend my retirement. [Laughs] But I mean I’m missing a lot if you want to know the things that went on in the meantime.
…what went on at Hopkins because in addition to the people who were using the sounding rockets for relatively simple experiments like pointing at a comet, you have a star tracker where the comet is bright enough [that] the star tracker locks up on it and the whole payload looks at it. Or what Warren Moos was doing was looking at Venus and Mars, and this could be done because… I’m sorry, Venus and Jupiter—because they’re very bright targets and you could lock up on them, and again, because not only are they bright and visible where you track them, but they’re bright in the UV. So you get data with a reasonably short rocket flight.
But when Arthur Davidsen joined the group, I guess at the end… When did he join? ’72, ’73? He and Bill Fastie wanted to look at the UV spectrum of a quasar. Now a quasar is faint in the visible but is expected… You know, it has a fairly high x-ray emission and would have a very interesting UV spectrum if you could measure it. There was at this time… Let’s see. 1978 was the launch of the International Ultraviolet Explorer. The earlier observatories launched by Goddard, the OAOs, had again limited… They were good, but I don't know too much about them in terms of sensitivity and… There is a literature of them. There are some, again, publications summarizing their results. I think they were mainly useful looking at stars rather than… I don't think people were trying to look at faint galaxies and quasars.
So they came up with a very interesting concept, Bill Fastie and Arthur Davidsen, which is to use two star trackers and to basically have these two star trackers aligned roughly…both of them lined 90° to the target they wanted to look at and to each other so that you basically had a three-axis system where you pointed at two guide stars in one direction, and then your target was in this direction. So they were able to be able to look at… The quasar they chose was the brightest known quasar at the time, 3C 273, and that made a big splash. That was very successful and that gave them a fair amount of visibility within a community and led to their becoming part of a proposal by AURA (the Association for Universities for Research in Astronomy) to host the Space Telescope Science Institute, which again, there’s a whole story about that. You can read it. I think it’s Bob Smith’s book, which…
Again, I was only peripherally involved in that because at the same time while they were doing that, we wanted to keep the sounding rocket program going. We had students whose PhD theses depended on it, so we--
Was there a concern that that program would not be able to continue going?
What, the rocket program?
Yeah. It’s been threatened several times both budget-wise and… You have to realize that once you have an orbiting observatory, you have to find a raison d’être for a small rocket program.
The training of students and the development of new instrumentation that would ultimately find its way into orbiting spacecraft are the two main reasons, but you also have to have targets that will give you some scientific information and not just, “Hey, this works great.”
So comets, which are targets of opportunity, were one class of object. The other were—
By targets of opportunity—because you mean they’re not always there like a planet?
They’re not always there. Right.
Especially if one of these comes and breaks into many little pieces, or if it’s extremely bright or unusual and gets a lot of publicity in the press, a naked-eye comet… We had Hale-Bopp in 1997, Hyakutake in 1996. We had a really great sounding rocket campaign with Hale-Bopp, a beautiful spectrum. There were three separate teams that launched, and Hubble Space Telescope was working. But there were two problems. One is it couldn't look closer to the Sun than 50 degrees because you can't let the Sun get into the surfaces. So we were looking at Hale-Bopp much closer, you know, at 30 degrees or 28 degrees from the Sun. The other thing is the instrument that we wanted to use, that we would ideally use, from Hubble was the Space Telescope Imaging Spectrograph (or STIS), and that was broken at the time. They ultimately… They had fixed it, but they had not commissioned it for use from the servicing mission.
So in this case, we weren't competing with anybody. We just had a unique opportunity to do this. Hale-Bopp was incredibly bright. I have a nice photograph on my wall, which I would show you if I was in my office, taken of the rocket launch with the comet in the background taken by Alan Hale. So both Hale and Bopp came to see our launch, which was sort of fun. [Laughs]
How is brightness measured? Is that simply a function of size or are there other factors?
It’s a function of size and activity. So if you have a very volatile comet, if you have a lot of… When you look at a comet in the sky, you're seeing dust. You're seeing mainly the particulate matter, and comets will vary in how much…whether they’re dusty or not, depending on how often they’ve been around the Sun, whether they’re brand new. But it’s also size because the larger… Hale-Bopp was large. It was on the order of 30 km in diameter, whereas most comets, like the ones we’ve visited, are of the order of a couple of kilometers in diameter. So you’ve got more dust and gas off a larger comet than a smaller comet.
So anyway, the story about the Space Telescope is a completely other story that runs parallel because it’s very important for the history of Hopkins. I like to think that the future of UV astronomy is sort of bleak. I mean what’s on NASA’s… Excuse me. Let me just get a drink.
What’s on NASA’s plate? The next is the JWST, which only goes down to about 5,000 angstroms. There’s no UV capability. Hubble, surprisingly, keeps on working. They’ve got plans to operate it even if one or two of the gyros fail. They do have three operational… I think they have four operational gyros right now, and if any instruments fail, they’ll just take it out of circulation, but Hubble is still a great workhorse. But I like to think that I was very fortunate to show up at Hopkins at the beginning of a golden age of ultraviolet astronomy and see the progress from the small rocket-borne instruments through the first generation of observatories, the International Ultraviolet Explorer, and ultimately the Hubble. But mixed in with that, we had two flights on the space shuttle of the Hopkins Ultraviolet Telescope, which were shorter missions. The shuttle is not a good place to do astronomy. The platform is presumably stabilized, but with a certain amount of difficulty. You couldn't do observations while the astronauts were doing their treadmill exercises.
And the Far-Ultraviolet Spectroscopic Explorer, which was originally going to be part of the NASA Explorer series, it got to be very expensive and they said, “You either descope it by a factor of 2 or we cancel it.” My colleague Warren Moos was the Explorer investigator.
I’ll be talking to him later this month.
Oh, okay. Very good. Yeah, he’s got even more stories than I do. He said, “Well, the way we can descope it and cut the cost by a factor of 2 is to cut NASA out of it,” cut all the paperwork that Goddard, all the… In those days, they were the plastic viewgraphs, cut out all the—
Yeah, but surely you need NASA, right? You can't actually cut them out.
Oh, you need NASA. What you don't need is the…
Is the bureaucracy, or I was going to say the welfare system of providing employment for a large number of engineers.
So what’s the secret? How do you get all the benefits of NASA without all the bureaucracy? How do you do that?
You hire a couple of retired NASA engineers and you bring them up to Baltimore and have them sit with us.
Oh! So you don't really need the infrastructure… I mean, you don't need any of that? You just need the know-how that NASA offers, the institutional memory?
Well, we obviously needed NASA to do the launch, but one of the--
[Laughs] Right! I was going to say unless Hopkins had a rocket program I didn't know about, I figure there had to be some…
Oh, it wasn’t that. It was the Applied Physics Lab that has the facilities and the engineering capability.
Oh, I see. Okay.
They had worked on our shuttle program and they’d worked on Apollo 17 also. So we’re sort of at arm’s length from them, and their scientific expertise is more in planetary magnetospheres and… They’re doing a lot of planetary missions. I mean they pulled off this New Horizons… They’re really building up, and I’m sort of proud of that because some of their people are former students of ours who have gone and done that.
Yeah. That’s great.
So the future… What I want to point out, though, is that after Hubble, there really won't be a UV observatory in space till the 2030s. There are a number of concepts that are being worked on. Again, one of my former students is leading one of them, the LUVOIR, and there are some other former Hopkins students who are contributing to the instrument designs. But these are projects that are not going to be launched until the late ’20s and provide capabilities, and that’s after they… There’s going to be competition and they’re going to have to survive the budget crisis, budget problems. So these are beyond my professional horizon.
So I’m sitting (not here) by, in my office, a number of datasets from Hubble that haven't been published yet and a number of projects from the Rosetta mission, which I guess I’ve had the most fun working on Rosetta and finding all sorts of things. I’m still collaborating with people in the UK and Germany on work, plus some of our original team.
I wonder if you could talk a little bit about, in a broad sense, the partnership between an academic department and NASA. So from your particular perspective, overall what does Hopkins offer NASA and what does NASA offer Hopkins? Is it really a two-way street or are the benefits weighted one toward the other?
Well, in a way NASA has its own scientific workforce, but its scientific workforce is really intended to support the projects that NASA is carrying out, not necessarily to be doing independent research.
So let’s take two different cases. One case is, say, the Hubble Space Telescope, and there’s a very interesting story there because the forerunner of Hubble, the International Ultraviolet Explorer, was a collaboration between ESA and NASA. The Europeans had one eight-hour shift. It was in a geosynchronous orbit, so you had 24/7. The Americans had two eight-hour shifts, but one of them, the orbit was slightly elliptical and we got into the radiation belts and it was a slightly noisier shift. But there were plenty of scientific collaborations. The operations center was at Goddard. The management was at Goddard, and you needed access to Goddard. You had the Goddard scientists—there weren't that many of them—who basically ran the show in terms of organizing the review committees and scheduling, things like that.
So when Hubble was coming up, NASA Headquarters, realizing that it was a much bigger operation… Also, I should point out that IUE was providing grants of the $15,000 to $20,000/year variety to the university community, to individual PIs. Well, they were called guest investigators, guest observers. That’s not really enough to support a student, so you really had to have two or three programs to divide. But again, you got a couple days’ worth of orbits and that’s it, so…
So the powers at NASA Headquarters, mainly Ed Weiler, decided that it would be good to have a much more robust support structure because Hubble was going to be generating boatloads of data, and it’s an asset that these data should be analyzed and published as quickly as possible and disseminated to the worldwide astronomical community. So he advocated for a separate institute, which is how the Space Telescope Science Institute came to be, but not after a long battle between some of the entrenched interests at Goddard and the wider university community, which again had good representation in terms of these consortia. AURA, for instance, was operating Kitt Peak, and so they had a good group of management people who could put together a proposal to build and operate a center (which is the Space Telescope Science Institute) on the Hopkins campus. Okay.
So individual researchers at a university will go through a formal process of proposing. There are peer review panels called TACs (Telescope Allocation Committee). I have chaired a couple of the subpanels. I’ve been on various TACs. It’s, I think, a fair process, but they’re going now to a double-blind system, which I’m not sure how that actually works. How can you write a proposal and completely keep your identity out of the proposal in terms of your ability to do something?
It’s a small world.
Yes, yes. Okay. So that’s one type of interaction between NASA and the academic community. The one you're more interested in are the PI-class missions.
There are the great observatories, things like the Hubble, which are done by committee. They’re managed by NASA and they have various committees like a telescope committee. So I have in this pile of stuff here some recollections from Bill Fastie who was a telescope scientist on how did NASA screw up in putting a faulty primary mirror into space on the Hubble Space Telescope? So there’s a question. He gets into details about how much oversight were they allowing the outside scientists who were on the committee to come into the manufacturers like Perkin-Elmer that ground and polished the glass, because these companies were also doing a lot of work for the military, and they had reasons to keep snoopy people out. They had reasons not to have a lot of outside scientists coming in and looking at it.
The sad part of the story is that NASA in its wisdom decided that they would build two mirrors for the Hubble with two separate contractors. They would grind them down to a rough finish, and in case the primary (which was Perkin-Elmer) failed either to achieve the specs or the glass broke, they would have the backup which was from Bausch and Lomb. It turns out that there was a mistake in the measuring instrument that PerkinElmer, which has been well documented, and they said, “Okay. It works. It satisfies the specifications. We don't need to go do any further work on the second mirror,” which was sitting in a warehouse at Bausch and Lomb in Rochester. The Bausch and Lomb mirror didn't have this error, but they didn't know that. This is one of the problems that, again, the outside committee couldn't really get their arms wrapped around. So it was clear that there was a failure and that they had to investigate what went wrong. Okay.
So those are the large projects. Most of the planetary missions are that way. They’re run by JPL or APL and there are teams. So for instance, this Lunar Reconnaissance Orbiter is run out of Goddard. The instrument teams have their own PIs. The UV spectrometer which is called LAMP, I was a member of that team. So you have teams that get together. They try to give input to the project, which… You know, where do you put the instrument? How do you mount it? How to do you test it? How much data do you get from it? What’s the data rate? How does it sit with… Is there interference with other instruments? Do you need shutters or baffles that need to be provided by the spacecraft? So those are huge projects. They take forever. I mean, the European Rosetta project is even more complicated—more instruments. You had the lander part of the Rosetta, which was another… I think the Europeans did a spectacular job with Rosetta.
They really… Considering that NASA had canceled a similar program called CRAF that I was part of also in 1980[-something]… It was scheduled to go… Okay, I don't have… I think it was canceled in 1991. Rosetta was actually originally intended to be a follow-on to CRAF in which it would be a lander, and then when CRAF was canceled, Rosetta changed and became an orbiter and added a lander, Philae, later on. Philae was not run by… It was not part of the ESA package, but was provided separately by the French and Germans. So that’s another problem.
Mm-hmm [yes]. I wonder if you could talk sort of… I mean, I know Hubble is still tremendously valuable and it’s still giving all kinds of amazing data to scientists. But I wonder if you could talk from your perspective from the beginning of it. To what extent has Hubble exceeded, met, or not met expectations from its inception?
Oh, it’s far, far exceeded. First of all, in the inception, it had a suite of instruments that were 1980s instruments. It was actually launched… The 30th anniversary of the launch was last week, so it was launched in 1990, and the instruments, the original Wide Field Planetary Camera… There was a Faint Object Spectrograph. There was a Faint Object Camera from Europe. I think the Belgians built it. Those instruments were all, I’d say, zero generation, but… The High Resolution Spectrograph. They’ve all been replaced by much more sensitive and capable instruments. But in terms of just the… In fact, I think I can grab… We should grab… When I was in my office, I brought some magazines home with me. I think it’s a recent Physics Today that has a whole section on the achievements of Hubble with a lot of graphics, a lot of nice, pretty pictures. So I think it’s far exceeded the expectation. I remember reading many years ago some Congressmen saying, “Well, Hubble has done such good work in two years that we might as well turn it off and save some money.” [Laughs]
How long do you think Hubble will keep going?
I don't know. That’s a good question. I’m surprised it’s still going.
What would cause it to stop? What would be the most likely thing to cause it to stop?
I guess if there was a catastrophic failure. If one of the instruments fails, it will keep going. If all the gyros fail, then it can't point and it would stop. If the power system fails or if the transmitter… There are a lot of systems on it. There is a lot of redundancy. It’s pretty robust. A lot of the components… The last servicing mission was ten years ago or a little more, and so I can't really say. But it’s certainly exceeded its original lifetime. It’s exceeded all expectations for the science that it’s done. It’s probably the most productive observatory ever designed by man in terms of publications.
And what do we know as a result specifically of the Hubble? If we were to sort of narrow what the Hubble has taught us, what would you say we know as a result of the fact that Hubble exists and it’s out there?
Well, now you have to ask everybody for their individual highlights. I would say the discovery of dark energy is a major… The way we’ve changed our view of how the universe moves or expands.
So let’s start with the dark energy, if you could explain that in detail. What does it mean that the Hubble discovered dark energy?
Well, it was the observations of… You’d have to ask my colleague Adam Riess to get the details, but basically, it was able to make measurements of variable stars further away than you could do from the ground and to extend the distance scale, the distance ladder to greater distances. Now one of the things that JWST is supposed to do, even though it doesn't work at a shorter wavelength, because it goes into the infrared, it’s supposed to take you back further in time. So in principle you can get more and more information about this behavior. But yeah, it’s just…
There are two things about Hubble. The one that I’m most happy with is the fact that we’re above the atmosphere, and so I get a clear view in the far-ultraviolet which is unique. I should mention that, again, it’s not perfect because the ultraviolet goes down to the soft x-ray range, and that’s what… FUSE tried to bridge that gap. There are still regions of the spectrum that have very valuable information that are not covered by Hubble. There’s that, but also in terms of its being above the atmosphere and having the stability and the optical characteristics with the new cameras that have flown since the initial discovery of the spherical aberration, you're getting to the point where you're getting images of the order of 7 milliarcseconds in diameter. So you're able to see light that’s much… Even though it’s only a 2.4-meter telescope, you're seeing light concentrated in a smaller area on your detectors, so you're able to see fainter and farther. So you’ve got beautiful, beautiful images, a lot of detail, and the images are what sells the public, sells Congress. The spectroscopy is an afterthought, but it’s extremely valuable in terms of understanding the physical processes in all parts of the universe, starting with the solar system, and now exoplanets is the next thing that it’s been doing wonderful work on.
Now you mentioned dark energy. What about dark matter? Does Hubble have a role in understanding dark matter?
Not so much. I think the evidence for dark matter is basically the large-scale distribution of galaxies and clusters, and I think it’s… Again, Hubble’s fields are relatively small, so I think you do better from the ground in studying dark matter.
Even though… One of the spacecraft on the drawing board is WFIRST, which again will be a wide field infrared spectrograph and camera, which will again give you bigger pictures that are useful both in cosmology and with dark matter.
Yeah, so that gets me into my next question, which is what is… Do you think that there will be a successor to Hubble on the scale of Hubble or even larger? Is that feasible? Is that on the horizon? Is that necessary? Are the budgetary considerations… You know, does that make something like that possible?
Well, I think the JWST is doing some groundbreaking work in terms of segmented mirrors that you unfold, and so you can make bigger telescopes without having to have bigger pieces of glass or bigger launch vehicles. So the current concept for LUVOIR, the… What does LUVOIR stand for? Lightweight Ultraviolet and Optical Infrared Instrument that I mentioned earlier would be a 2030 type of instrument [and] is either 8 meters in diameter or 15 meters. They are two; they’re carrying two different designs. There are questions of carrying coronagraphs that you could look for, planets around stars with high…you know, being able to actually pick out a planet from the glare of the star. There won't be another Hubble Space Telescope. You're going to do better one way or another.
And what does better mean? The instrumentation?
Better instrumentation, more collecting area… It’s sort of interesting. The UV is a very well advanced field, so you gain by improving your detectors, making the detectors bigger so you get more information, and at the same time make the detectors more sensitive by improving your coatings, make the optics coatings better or extend them to larger range. Those are things that people are working on these days, but they’re incremental. They’re not game-changers. They’re not going to all of a sudden…
Right. Right. So with discovery always, I mean what part of discovery is you learn what you don't know, right? So based on what we’ve discovered with Hubble, what do we know that we don't know, and what might some next generation teach us?
Okay. Yeah. A lot of astronomy is what I disparagingly call stamp collecting. You have to have a large number of objects in order to understand what makes one object unique relative to other objects. You have to understand what’s going on. I mean that’s what makes working on our own solar system so nice, because it’s very limited in what you have and you don't have… There’s a clear difference between all of the planets that we know about, and we understand pretty much… We might not understand how they came to be arranged and ordered the way they are and why the relative sizes and compositions, but we do know that each planet has unique characteristics that differentiate it from all the other planets. I mean Venus might be Earth’s twin, but it’s certainly not habitable the way the Earth is. Mars is smaller than the Earth. It’s more like the Moon, but the Moon doesn't have an atmosphere and Mars does. So those are just, off the top of my head, examples. But now when you go out and look at many more objects—especially I’m thinking of exoplanets—you're going to find a huge diversity, and you have to figure out the ways to be able to actually study the planets in the presence of their host stars. So I think that’s one of the challenges for the next generation of UV. I think the IR is also going to be very important for exoplanets.
Now you said something earlier that intrigued me. You described yourself—you used to be a physicist, but now you're an astronomer, right? So I just want to get the nomenclature clear. Why does that not simply make you an astrophysicist? Why don't you combine the fields as opposed to emphasize the transition?
I could. I think people ascribe unusual powers to astrophysicists. [Laughter] I actually tell people I’m a spectroscopist because that’s basically what I do. I measure spectra and try to interpret them in terms of the physics that’s producing the emission.
You can slice and dice it any way you want. It’s the same thing.
I mean, is there such a thing as an astronomer who is a real astronomer who’s not also a physicist or does not have a strong basis in physics?
What field do they come from? How do you do that?
Well, there are graduate programs in astronomy where you have to have a background in physics up to a point, but you know, one of my gripes—I do have gripes—is that people who are measuring molecular spectra don't really understand molecular spectroscopy. They just know the slogans. They know how to label certain features in a molecular spectrum. But if they were to try to explain to you why you have an A doublet ? [?] state, they would be completely lost. It’s not easy. I mean it’s something I have trouble [with] when you get to more than two atoms in a molecule, but nevertheless, it’s pretty useful for a lot of things. For instance, I’ve seen over the years people looking at a spectrum and saying, “Oh, this matches and that matches and this matches, so we must be detecting such-and-such.” But in fact, if you know what you're doing, you know that you have to have certain ratios between these different features in order to get a positive identification. You have to look at the width of the lines if you can measure them and determine whether or not there’s a temperature effect or what. So it’s not perfectly true. I guess I would then come down on your side of the question and say I’m an astrophysicist.
Yeah, yeah. I wonder if you could talk broadly about your contributions to the field. What do you see is the work that you’ve done that has contributed broadly to the field and how your work has advanced the field?
Oh, that’s a good question. I mean, if you're looking for eureka moments, I’ve done a lot of things that were sort of really interesting at the time. I’d have to look at my publication list to see, but just doing the first UV comet spectra was something that was brand new. Now it’s sort of done routinely if you have access to the Hubble Space Telescope. I was involved in the first observations of the atmospheres of Europa and Ganymede and did some of the early analysis of that. What else have I done? Hmm. I’ll have to scratch my head. I mean I’ve done some… I’ve written some important papers on Rosetta, and I’m still working on some of those projects. I’m sort of very modest about my accomplishments. I don't go out and advertise that I’ve done this and that.
But you have.
Yeah. Yeah, I’ve done…
I can go through the CV and give you ten important papers.
No, of course, of course, and I can go through the CV, too.
We could just look--
My question is really how you understand those contributions—not just like looking at a particular paper, but how you see how a given paper has advanced understanding in your field.
Yeah, okay. I would argue that taking, again, recent work, the Galilean satellite atmosphere papers—those were based on Hubble observations and the whole… I wasn’t the prime person. It was a post-doc who discovered the plume emission on Europa, even though I was very closely involved working with him on trying to convince him that it was a real effect, but that’s one the whole industry was looking for water plumes on Europa. The Europa Clipper mission is going to go after that. So that had an important effect.
When Rosetta was flying by Mars, we observed the spectrum off the limb and we saw an extended…I saw an extended oxygen component which I attributed to escaping oxygen to go along with the escaping hydrogen which had been measured back by the Mariners in 1970, Mariner 6 and 7. So there’s been a lot of work with MAVEN and some of the other Mars missions, a lot of theoretical modeling of how the water escapes or how the…what the mechanism is to give oxygen enough energy to escape from Mars. But again, that controls the escape over geological timescales. That’s again a small, little paper, but it has an outsized effect on people’s thinking.
Paul, do you ever think about the ways in which there is the possibility of extrapolating what you’ve learned about our solar system in terms of how other solar systems work, how other galaxies work, how the universe works? To what extent is close study of our solar system representative in your mind of much larger questions?
Well, none of the exoplanet systems seem to bear any resemblance to our own. I mean, you’ve got these hot Jupiters which don't exist in our system. You’ve got… You know, as people get more and more sensitive and you see more and more Earth-size planets being discovered and trying to determine what sort of atmospheres they have… When exoplanets hit the stage, some of the colleagues of my age decided this was a new area to jump into. I saw some of my own former students jumping into it and I said, “This is a great field for young people to get into,” and I… So I haven't really followed it very closely.
[Laughs] So does that suggest possibly that our solar system is actually unique?
I don't think anyone’s come up with… I mean there is the TRAPPIST system with five planets or six planets, but I don't think their orbits are similar to our orbits, and I’m not sure what they’ve learned about the atmosphere compositions of those. So far, I think our system is unique.
Do you allow… I mean, obviously as a scientist you're evidentiary-based, but do you allow those kinds of musings to…you know… Do you ever ask those kinds of questions about the likelihood of there being intelligent life elsewhere in the universe?
Yeah. I’ve never mused publicly or published anything about that, but I would argue that yes, there probably could be. But if you look at… Sorry. You're hearing all these people sending me emails.
If you look at how long humans have been on the surface of the Earth and how long they might be in total before they’re wiped out compared to the total lifetime or the age of the Earth, it’s very small, right? So the ability to find intelligent life anywhere else in the universe will depend to some extent on where a planet like ours is in its evolution. So again, from that point of view, I would have to be very skeptical. On the other hand, there are--
You mean skeptical of there being concurrent life?
Yes, of being able to communicate with…like SETI. On the other hand, there are billions and billions and billions of stars, so we don't know where you're going to get a system similar to ours.
What have been… If you look at the sum total of all of your research, do you see any overriding questions or curiosities that inform all of the projects that you’ve undertaken, or do you see these projects as separate endeavors with their own research questions?
Well, I guess from my point of view, or at least from my habit, I would say the latter, separate research projects. I’m not… A fad—I don't know if it’s a fad or a trend—of the last decade or two has been these large-scale surveys in which you try to put your local information into the context of a much broader view of a particular environment. I guess when you mentioned that I was trained as a physicist, we were trained in a way not to make an observation and to try to figure out what’s going on. But as a physicist, we were trained to what is the current problem that you need to solve? What is the question that you want to determine? Then you design an experiment to solve that problem. So if you're working on atomic structure and you understand a particular atomic system, or you don't understand it and you want to know what’s going on, you design a spectroscopic experiment to try to get the data that you need to fill in the gaps that you have in that system. So it’s sort of problem-, object-oriented research and in that way, astronomy is, in many ways, different in which people are now collecting huge surveys, huge samples of information and using data mining and artificial intelligence to try to find out what the interesting problems are. It’s not for me. I think it’s a personal taste. It’s also the way I was brought up.
Yeah. Now on that question of being trained as a physicist, are there any particular fundamental concepts or laws in physics that are close to you in terms of informing the way that you see the world, the way that you see your research, the way that you take your measurements, and the way you understand the data that you collect?
Yes, but I don't work on that. I mean I’m interested in… When I see an article about searches for the fine-structure constant at large redshifts or early in the universe to see if the fine-structure constant has evolved, or any components of the fine-structure constant, I’m very curious about that. I haven't seen anything that says that fine-structure constant isn't a constant, but… You know, fine-structure constant pervades a lot of the atomic physics that I did, and so I’d really like to know if it’s changed with time.
But I’m not doing anything to pursue that question.
Mm-hmm [yes], mm-hmm [yes]. What is still really poorly understood in your field that you're surprised that at this point… You know, maybe 30 or 40 years ago you thought that at this point maybe we’d really have a better handle on a particular thing, but in fact not. Is there anything that sticks out in your mind in terms of things that are still persistently misunderstood or not well understood?
Well, I think the search for oceans in some of the Galilean satellites or Saturnian satellites… You know, are there environments in which you could harbor life? I guess the major question that still haunts everybody is why is there life on Earth, right?
You and I are here. Why aren’t we on Mars, or why isn't someone like us on Mars?
Is the research that you're engaged with… I mean, that’s actually another question that I wanted to ask. I mean, obviously the pursuit of knowledge for the sake of understanding how nature works is by definition “practical” even if it doesn't necessarily translate to, you know, our own day-to-day lives in terms of improving our daily existence. But do you ever dwell on the relationship between the kind of work you do and how it may or may not sort of advance human society?
Very good question. I guess doing my post-doc in a military laboratory, there are obviously pros and cons of working on societal issues, both good and bad ones. So I suppose it’s been a privilege to work in an ivory tower and be there when all the right things were happening, and I think… Well, what worries me most—and I’m not sure that I can contribute or what my profession can contribute—is the lack of scientific literacy in this country. I don't know how there can be so many stupid people if we have such a good education system, so that’s really very disturbing.
Is that a good way to end?
No, I think we’ve got a little more time. I guess for my last question, you know, forward-looking, what are you excited about, either in terms of where your field is headed, where the technology is headed, what can be understood that’s not understood now, and how that might relate to, you know, questions of benefiting our own civilization?
Unfortunately, most of what we look forward to is dependent on some of these very large projects. Like if we were to go outside of astrophysics now and go into high energy physics, the quest for the Higgs boson took a long time, but the parallel to the Higgs boson was the supersymmetric model which would sort of overturn the Standard Model. But that hasn’t been found. Do you build a bigger accelerator? Do you build a different type of accelerator? Do you look at cosmic rays or very high energy cosmic rays or neutrinos at the South Pole? Those are really, really interesting questions, and the question of dark energy is particularly intriguing because you can explain it mathematically, but you can't really explain why… You can't… You can give it a term in an equation, but there’s not much substance to the idea of it, other than being one part of an equation. So yeah, those are very puzzling things. I would certainly be curious to learn about them in the future.
Well, Paul, it’s been a great pleasure speaking with you today. I really want to thank you for your time.
Okay. Well, I hope it hasn’t been too mundane in the sense that--
Not at all! Not at all. It’s been great. Okay, I’ll end the recording here.