Interview with Alan H. Barrett, 1979

Sullivan: 00:00

I'm with Barrett on 15 August '79. So you knew about these other lines, but what made you pick OH?

Barrett: 00:07

Yes, we knew about the other lines, but at that time the conventional thinking was that one would only find diatomic molecules in the interstellar medium in detectable amounts anyway. And OH was the only one that had lines at a very convenient frequency range. 18 centimeters isn't that much different from 21. There's good transmission through the atmosphere, and the antenna works well there. So that's what we went for.

Sullivan: 00:37

Was that the main disadvantage of CH was that you didn't have a radiometer around 9 centimeters?

Barrett: 00:43

No, not at all. At that time, CH hadn't been studied in the laboratory, in the microwave.

Sullivan: 00:52

Oh, I see.

Barrett: 00:54

It was much worse. Uncertainty on the CH frequencies. In fact, I made a CH search many years later, when still the frequencies weren't known.

Sullivan: 01:05

In the '60s are you talking about now?

Barrett: 01:07

Yeah, sometime then. Jim Moran and I did that as part of-- well, I don't know if it was part of his thesis, because it was negative, but we had found it.

Sullivan: 01:17

That's the late 60s. Okay. What else did you do in NRL besides this OH search?

Barrett: 01:25

Well, that was pretty much it, I think, other than to educate myself in radio astronomy. And I also looked into some work which I started many years later at MIT, and that was studying the Earth's atmosphere with microwave radiometry.

Sullivan: 01:47

But you didn't actually do any observations at NRL?

Barrett: 01:51

No, I wasn't there long enough. I had a one-year appointment, which they wanted to renew, but I went to the University of Michigan.

Sullivan: 01:58

And what was the attraction at Michigan?

Barrett: 02:00

My initial reaction is to say to get out of NRL. I don't know. I was anxious for more of a university environment. Haddock had moved to Michigan one year earlier.

Sullivan: 02:19

Did you overlap a little bit in NRL then?

Barrett: 02:21

We overlapped about a day or so, I saw. But Haddock was starting on the building of an antenna, and so I went there.

Sullivan: 02:33

So that was just an 85 foot that was being built?

Barrett: 02:37

That's right.

Sullivan: 02:38

Can you tell me how that proceeded? Was that a smooth operation? No trouble building it?

Barrett: 02:44

There wasn't much difficulty. One of the jobs I had to do there, however, was line up the polar axis, and the Michigan machine shop built a very heavy camera that I was given to do that job with. And the results were very inconsistent from night after night. In the meantime, we go out at night and check the alignment of the polar axis and find it off. So we'd have the crew move the antenna the following day, and then we'd go back out again. It would still be off in a different direction. Well, it ended up that what was happening was the camera was so heavy and mounted in such a way that it was flexing the members on which it was mounted.

Sullivan: 03:33

And you were mounting it on different places?

Barrett: 03:35

Well, no. We were mounting at the same place every time, but we were looking at it with the dish pointed at different hour angles.

Sullivan: 03:42

And probably when you took the camera off, it would be different yet again.

Barrett: 03:47

That's right. So we had some problems with that.

Sullivan: 03:50

But then what was the goal of this dish? What sort of observations was it--

Barrett: 03:53

At Michigan.

Sullivan: 03:55

Yeah, that Michigan designed to do.

Barrett: 03:57

Fred was primarily interested in short-wavelength continuum observations. And the initial receiver, which went on the antenna, was at 3.75 centimeters traveling wave tube wide bandwidth receiver. And I made some 2 centimeter observations. I also built a little radiometer myself at 2 centimeters to find out how well the dish worked there. So I made some 2 centimeter observations of the planets and the strong radio sources.

Sullivan: 04:31

But this is a mesh surface.

Barrett: 04:33

No. No

Sullivan: 04:34

Oh, solid surface. I'm sorry.

Barrett: 04:35

Solid. It's [inaudible].

Sullivan: 04:37

I was going to say that it was a bit short of a wavelength. And so this is really a continuation of the continuum work he had done in each of the two regions and so forth--

Barrett: 04:46

That was a--

Sullivan: 04:46

--at NRL.

Barrett: 04:47

That was very much a continuation of that.

Sullivan: 04:50

And is this how you got your interest in planets, was through this testing at 2 centimeters?

Barrett: 04:56

No. I wanted to do some science as well as oversee the building of the antenna, which I suppose you'd say I was kind of the field supervisor for that antenna under Fred, but I wanted to do some science also. And when I was at NRL, Connie Mayer and his group had detected Venus, and it looked as if the temperature was on the order of 600 degrees, the well-known result, very hard to understand at that time, because people regarded Venus as a sister planet of the Earth. And measurements extended roughly between 10 centimeters and 8 millimeters, and it looked as if the brightness temperature was falling off at 8 millimeters. So I began asking, how can one explain that in terms of the properties of the atmosphere, and since I knew a little bit about spectroscopy, why it worked out rather well? There's a non-resident CO2 absorption which I pointed out if the pressures were high enough, would explain the results, and if the surface was hot enough.

Sullivan: 06:05

Was that that JGR paper [crosstalk]?

Barrett: 06:07

Well, that was the initial short note I published, because I published rapidly, and I was interested in getting my name on something. There was a follow on ApJ paper of some length which had all the details in the calculations. The conclusion from all that was that to explain the microwave measurements on the basis of absorption in the atmosphere by CO2, one needed something on the order of 100 atmospheres pressure on the surface.

Sullivan: 06:42

[laughter] Sounds like a good number.

Barrett: 06:43

As you know, that's about what has been found now from-- so we had an American spacecraft actually going there.

Sullivan: 06:50

But now, is this the right reason to get the 100 atmosphere figure?

Barrett: 06:54

Mm-hmm. Pretty much.

Sullivan: 06:56

Well, we'll have to discuss that later. I'm not sure I follow that connection. But in any case, so this was really an extension of your work that you had thought about for the Earth's atmosphere, and you said, "I can do the same kind of work in another atmosphere [crosstalk] remote sensing,"?

Barrett: 07:10

That's right. That's right.

Sullivan: 07:12

Now, did you do any other planetary observations? Did you try to detect some other planets while at Michigan? [crosstalk].

Barrett: 07:19

Oh, I think we detected Jupiter. And yes, eventually - that's right - we had the first measurement of Mercury. Bill Howard and Fred Haddock and I were involved in that.

Sullivan: 07:32

That's right. Bill Howard was there, too. Were there any other radio astronomers in that group at the time?

Barrett: 07:37

No. Not at the time. Not at the time.

Sullivan: 07:40

Okay. And then how long were you in Michigan? I know, eventually, you went to MIT.

Barrett: 07:46

Oh, I went to Michigan in January, '57, and I left in June of '61.

Sullivan: 07:53

Okay. And were there any other projects that you worked on in Michigan before '61?

Barrett: 08:00

Not that I can remember. Well, the Venus thing led to my participation, being asked to be a principal investigator on the Mariner 1 and 2 spacecraft which were scheduled to go to Venus and the microwave experiment was picked to be the prime experiment.

Sullivan: 08:24

For the whole spacecraft? [crosstalk].

Barrett: 08:26

[crosstalk]. Well, I'm sorry. The prime planetary experiment. Now there're experiments en route.

Sullivan: 08:33

They really detected the planetary medium for the first time.

Barrett: 08:36

Yeah, what they call the [cruise?] experiments. But ours was the prime planetary experiment. In fact, they relied on the microwave's radiometer to actually tell them that they had acquired the planet.

Sullivan: 08:48

And this was the first interplanetary US probe anyway. Was it the first one considering the Soviets also?

Barrett: 08:56

I think so, yes. But I’ve got to check that.

Sullivan: 09:00

Yeah, I can check that. Yeah. Now what was the goal of this radiometer on the spacecraft?

Barrett: 09:04

Well, it was now well established in the early '60s that the brightness temperature of Venus at 3 centimeters to 10 centimeters roughly, was indeed 600 degrees. And there were two conflicting theories for that. One, that the surface was indeed that hot, which has horrendous implications for spacecraft if you want to land them on the planet because lead melts at about that temperature. And the other one was that it was a very dense ionosphere around the planet, which was opaque at 3 centimeters, which makes it quite dense. And at that electron temperature was about 600 degrees. Well, that also has a big impact if you're going to the planet because if you're going to put things down on the surface of the planet, the ionosphere is dense and you got to use millimeter wavelengths to communicate with what's on the surface. So NASA had the problem of establishing just what is the origin of the high brightness temperature from Venus. And it looked as if a microwave experiment would do it. Because as one scanned the planet, as you flew by in a spacecraft, if you got limb darkening, then it would be an origin on the surface. Whereas if you got limb brightening, then it would be an origin high in the ionosphere. And indeed, as we went by, we got three scans across the planet, and they all showed limb darkening.

Sullivan: 10:47

Was this a single channel, single band thing you had?

Barrett: 10:49

No, it was two channels. Started out to be four channels. And it was going to be launched by the Agena launch vehicle. But I think this was supposed the initial launch of the Agena. Anyway, the Agena slipped. Anyway, something slipped and they had to cut the spacecraft to two essentially in weight and power and what have you. A lot of experiments got dropped. But as I say, the microwave was the prime experiment that got cut back to only two channels.

Sullivan: 11:25

Are these different frequencies or these comparisons?

Barrett: 11:27

Yes, one was on the water line and one was about 1.8 centimeters, so I don't remember the exact wavelength. And one was on the water line. And it was the original plan to have two others, one in eight millimeters and one four millimeters I think.

Sullivan: 11:43

Did you see a significant difference between the two K band channels there?

Barrett: 11:47

It was those millimeter channels that got knocked off. As I remember, there was, but not enough to be significant in terms of saying anything about water vapor. That was one of the prime motivations initially, not only was-- well, it was the secondary, not the prime one being to establish the origin of the high temperature, but the secondary one was to detect water vapor in the atmosphere if it existed. But as it turned out, the radiometers weren't near sensitive enough for that job.

Sullivan: 12:27

So it sounds like then that the thing was quite successful, this microwave experiment.

Barrett: 12:33

Well, there were many problems. It was supposed to make many scans across the planet.

Sullivan: 12:37

Oh, I see.

Barrett: 12:38

But the way the scan logic was rigged up and some component failures en route, it only made three scans across the planet. But that was a major step at that time.

Sullivan: 12:51

It was sufficient to prove. So you got the main information, right.

Barrett: 12:54

And it accomplished the prime objective. Exactly.

Sullivan: 12:56

But now I don't think you stayed in space astronomy after that. Did you sort of switch back to ground based?

Barrett: 13:02

No. And in fact, there hasn't been a microwave experiment in space other than Earth orbiting microwave radiometers, and I was involved in those, but.

Sullivan: 13:13

Did you push for more experiments at that time?

Barrett: 13:16

Well, there wasn't any more experiments planned for some time. The next experiments were not for Venus. They were Mars, Jupiter, and Mercury. And the need for a microwave experiment in those cases is not near as strong as it was for Venus. There are now microwave experiments on the Venus Pioneer. There's microwave radar and things of that sort. But I think I'm right in saying that microwave radiometers haven't flown in space after that Mariner 1 and 2.

Sullivan: 13:54

I didn't realize that.

Barrett: 13:55

Mariner 1 went in the drink and Mariner 2 made it. Our best equipment was on Mariner 1, by the way. [laughter]

Sullivan: 14:04

Is that right? In terms of just working best in the lab?

Barrett: 14:06

Yeah. You build two radiometers, one of them is going to work better than the other. So you decide which spacecraft it goes on. We picked the wrong one. [laughter]

Sullivan: 14:12

I see. Well, it's easy while you picked one because that's certainly the highest chance of going I think. Now having done this experiment and not seeing any more microwave experiments going up in the next few years, you had to choose whether to go back to ground-based radio astronomy or were you thinking at all about going back to laboratory spectroscopy? What I'm trying to get at is what at that point--

Barrett: 14:41

I never thought about going back to laboratory spectroscopy. But you don't have to present it as an either/or with regard to doing radio astronomy because being a principal investigator on one of those experiments back in the early '60s didn't take up all one's time, and there was still time for ground-based observations of another nature. I started this Mariner stuff when I was at Michigan. But I was also doing the 2 centimeter observations.

Sullivan: 15:05

I see. I was having more of the idea like it is in today I think where the guy is madly running around from--

Barrett: 15:15

Yeah. Well, our equipment, you see, wasn't built by us. It was built out at JPL. So that makes a big difference. If you're building your own equipment or if you're just an investigator that writes the specs and attends meetings and oversees the tests on the radiometers and things like that, that's different in the amount of time it takes.

Sullivan: 15:36

Yeah. But then what was the next thing you did in radio astronomy once you got to MIT?

Barrett: 15:43

Well, when I went to MIT, I started two programs because there's no astronomy department there. There hadn't been much astronomy done. There was excellent work done by Bruno Rossi and his plasma group. But for ground-based observations, there was practically none. And I went there to start up radio astronomy. But I also had as a backup, I started the program in microwave studies of the Earth's atmosphere. And I had both these programs going at the same time, and they both managed to flourish beyond my wildest expectations. And the satellite, or the microwave studies of the earth's atmosphere, once it reached the satellite stage, why, I let go of it and turned it over to an associate of mine, Dave Staelin. He's one of my first graduate students at MIT.

Sullivan: 16:46

And my undergraduate thesis advisor.

Barrett: 16:48

He is now carrying on that program, there's many radiometers and Nimbus and Tyra satellites spewing back data daily. The radio astronomy program got started. Dave Staelin initially did his thesis in that, and in fact I had him take a look at the OH problem, thinking he might want to do that as a thesis. There'd been one major change in the OH that had happened in the intervening years and many people don't realize it. And that was that the 18 centimeter lines were measured in the laboratory by Townes and one of his students. So that removed any necessity for searching 10 megacycles around the center frequencies and made it much more reasonable to do a deep search.

Sullivan: 17:51

But obviously, Dave decided not to do this. Do you remember what the reasoning was?

Barrett: 17:56

Yeah, it didn't look encouraging to him. And like any graduate student wants a thesis that has relative certainty of panning out. And my other interest was Venus. And so Dave built a multi-channel radiometer and did a series of ground-based observations at about half a dozen microwave wavelengths. Something between 2 centimeters and 8 millimeters for Venus.

Sullivan: 18:30

Right. That was published in '64 or '65, I believe?

Barrett: 18:33

I don't remember when it was published, but once again we were looking for the water vapor dip and pressure broadened line, but the sensitivity from doing it from the ground was not sufficient to bring out anything.

Sullivan: 18:49

So although he decided that it didn't look like a good thesis topic, apparently you were interested enough to keep pushing for OH.

Barrett: 18:58

Yes. At that time let me joined the Lincoln Laboratory staff and Sandy Weinreb came around to see me shortly after I got to MIT. He was doing his doctoral work in the double E department building the spectral line correlator, the first one under Jerry Wiesner. But Jerry went off to be a science advisor to President Kennedy, so Sandy didn't have much in the way of an advisor around. And he was reaching the stage well through the engineering and the hardware and was reaching the stage of making observations. So I offered whatever assistance I was able to give him in the interpretation of his observations. He tried two things with that correlated, neither one of which worked out. They were both done at NRAO in Green Bank. One was to look for the deuterium line at 327 megacycles. The other one was to look for Zeeman splitting I the hydrogen-line absorption [inaudible]. Neither one of those turned out.

Sullivan: 20:07

I've talked to him about those experiments in some detail. Those are both on the 85 foot. The first 85 foot [crosstalk] I think.

Barrett: 20:14

Probably. Yeah, I don't remember, but--

Sullivan: 20:18

The Haystack dish didn't exist at that time.

Barrett: 20:19

Yeah, they must have been. No, Haystack [crosstalk]--

Sullivan: 20:21

But Millstone [crosstalk]--

Barrett: 20:22

And the NRAO dish didn't exist [the 140 foot]. The Millstone dish existed, but it was still pretty heavily tied up for military work at that point. So--

Sullivan: 20:33

Now, why wasn't the OH line one of these things that you suggested to him? Or maybe you did, and he--

Barrett: 20:40

Well, I did, but he was already well-underway under Wiesner with these other two [crosstalk]--

Sullivan: 20:46

No, I mean, to use his correlator.

Barrett: 20:49

Well, that is exactly what happened [crosstalk].

Sullivan: 20:52

Right. But it wasn't his thesis per se.

Barrett: 20:53

No, no. No, he had his thesis done at that point and he took the job on the staff at Lincoln Laboratories. And so Lit Meeks, and Sandy, and I launched the OH experiment to, using the correlator, and using the Millstone antenna.

Sullivan: 21:15

With a better known frequency and probably a much more sensitive radiometer, I suspect, than you had to have.

Barrett: 21:19

Oh, yes. Oh, yes. We had many things in our favor that we didn't have in the earlier search in the mid-'50s.

Sullivan: 21:26

Have you sat down and gone through-- if you had looked at the Galactic Center at NRL, if you might have had a chance to see the rather strong absorption lines?

Barrett: 21:36

I suppose I did. I don't remember. One of the problems there would have been the baseline, because the absorption is broad at the Galactic Center. But what I did do is sit down and see if we might have seen it in Cassiopeia. And we were just marginally above the detected signal.

Sullivan: 22:05

Couldn't quite have done it [crosstalk].

Barrett: 22:06

Well, had we worked longer, maybe, or had we known exactly where the [crosstalk] concentrated there, instead of spending our time tuning over 10 megahertz. We might have done it.

Sullivan: 22:20

Well, how did it go in terms of the observations at Lincoln Lab? Did it just sort of pop up the first night, or-- ?

Barrett: 22:27

We had some problems tying the correlator into the digital computer that was at Lincoln Laboratories. And the fellow by the name of Henry came on our team, and assisted us, and got that tied in. Then I don't remember whether it was the first night or what, because there's always debugging problems, but very early on, the OH absorption showed up, and it may indeed have been the first night.

Sullivan: 23:01

Which source were you looking at?

Barrett: 23:02

We were looking at Cassiopeia, and that's where we detected it. And then whether it was the first night or not, I don't remember. But once we had hints of absorption dips, we were most eagerly awaiting the following night when we could get back on the antenna and do it all again. Lo and behold, it was there. And over two weeks time, it moved the right amount for the Doppler shift to the Earth. We were very confident. Then we switched and looked at 1665, and that line was there too. So there was no question that we were seeing was absorption. There was a bit of a problem in that we didn't have as wide a spectral window as we wanted, so we didn't have too much baseline.

Sullivan: 23:46

What was the maximum clock rate?

Barrett: 23:49

I don't remember what it was and [crosstalk]--

Sullivan: 23:52

I can check.

Barrett: 23:53

--the spectra have very little baseline to them. And that's why we didn't spend much time, or any time as I remember, maybe once or twice, looking at the Galactic Center.

Sullivan: 24:05

Because you knew from the H I work that it would be that would be quite broad.

Barrett: 24:08

That it would be broad. One would expect it to be broad. And without the baseline well, you're hopelessly lost in trying to piece things together.

Sullivan: 24:16

One final thing that comes to mind is the OH emission story is sort of a fascinating one, the way it was going on at Berkeley and at Haystack, or rather at Millstone. But I can't remember now if you were involved in that or was that another group?

Barrett: 24:34

Well, I don't think it's Millstone or Haystack that you're thinking about. It was--

Sullivan: 24:38

Or Agassiz, I'm sorry [crosstalk].

Barrett: 24:40

Yes. [crosstalk] Berkeley and it was Agassiz.

Sullivan: 24:41

Agassiz, yes. Gundermann and so.

Barrett: 24:42

Ellen Gundermann and Ed Lilley-- and I forget who else. [inaudible] or somebody else.

Sullivan: 24:48

But so you weren't involved in that?

Barrett: 24:50

No, we weren't involved in that. I don't remember what OH we were doing at that time or even what year that was.

Sullivan: 24:59

Apparently, having discovered the OH absorption just didn't do a random survey of different kinds of sources around the galaxy.

Barrett: 25:08

Well, we were limited for antenna time at that point because we didn't have a radio astronomy antenna at our disposal. We had a military antenna--

Sullivan: 25:15

Which is still being mainly used for Air Force.

Barrett: 25:17

Absolutely. Haystack was not in existence then. And maybe that's when I went off to Russia. I've forgotten just what happened in between. Oh, I know what happened in between. We looked at even higher resolution in Cassiopeia and we resolved what everybody had thought had been one line into two lines at OH because of the heavier mass. And then we could deduce the kinetic temperature of the clouds from the splitting of the lines and the lack of splitting at 21 centimeters. And that, of course, was [inaudible] published also. And by that time, the OH and emission broke.

Sullivan: 26:08

Right. [So?] [crosstalk]--

Barrett: 26:09

I also went off to Russia for four months. Actually, observed OH with the Soviets.

Sullivan: 26:17

Oh, really? Okay.

Barrett: 26:17

It was the first time OH had been observed in the Soviet Union.

Sullivan: 26:20

And which dish was that?

Barrett: 26:22

This was using the Pulkovo antenna in Leningrad.

Sullivan: 26:29

[inaudible]--

Barrett: 26:32

We hurriedly put together-- or rather they hurriedly put together an 18 centimeter front end. What they hadn't heard about which was the news I brought with me when I went there was that the Australians had found OH in the absorption in the Galactic Center. And it was very, very broad. And with the noisy equipment they had they could open up the bandwidth and reduce the noise and still see the line. So once they heard that news they went to work building an 18 centimeter front end and I helped them with the observation [crosstalk]--

Sullivan: 27:08

With some broad filters or something-- yeah.

Barrett: 27:09

Yeah. Yeah.

Sullivan: 27:10

So basically you couldn't follow this up as much as you might have otherwise because of the lack of antenna time. Do you think [crosstalk]--?

Barrett: 27:17

Well, that was part of it. But then we certainly followed it up in other things. Polarization of OH we discovered, the O18H we discovered [crosstalk]--

Sullivan: 27:27

Now emission you were talking about with polarization, yes.

Barrett: 27:29

That's right. That's right. The absorption from O18H. We discovered OH from [inaudible].

Sullivan: 27:39

So now this is getting onto--

Barrett: 27:40

[crosstalk]--

Sullivan: 27:42

--three or four years-- three or four years later, right, yeah. But the astrophysics of the OH absorption line seemed to be making sense.

Barrett: 27:49

Oh, yes. Oh, yes. It tied in rather well with what we knew about 21 centimeters at that point.

Sullivan: 27:55

And of course, completely unlike the maser emission. Well, unless there's some other things that we missed along the way this is about where I'm ending things. Any more general comments about--

Barrett: 28:07

Only other program I've started is the cancer detection which you've probably heard about it [crosstalk]--

Sullivan: 28:11

Right. But that's been in the '70s. Right.

Barrett: 28:12

That's in the 70s. [That's?] [crosstalk]--

Sullivan: 28:14

Okay. Thank you very much. That ends the interview with Al Barrett on 15th August, '79.