Planetary Radio • Mar 30, 2022

X-raying the universe with Martin Weisskopf

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Martin C. Weisskopf

Chief Scientist for X-ray Astronomy in the Space Sciences Department at NASA's Marshall Space Flight Center

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Mat Kaplan

Senior Communications Adviser and former Host of Planetary Radio for The Planetary Society

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

He helped invent X-ray astronomy more than 50 years ago. Martin Weisskopf still leads the field as project scientist for the spectacular Chandra X-ray Observatory and principal investigator for the brand new Imaging X-ray Polarimetry Explorer or IXPE. He’ll help us zero in on the most energetic and enigmatic objects in the cosmos. NASA’s fiscal year 2023 budget proposal has just been unveiled. Chief advocate Casey Dreier will break it down. We’ll close with the first words from the Moon in this week’s What’s Up.

Martin Weisskopf and Aerobee-350
Martin Weisskopf and Aerobee-350 NASA’s Martin Weisskopf and colleagues from Columbia University in 1971 pose with the Aerobee-350 sounding rocket they used to detect X-ray polarization from a celestial object – the Crab Nebula – for the first time. Left to right are Robert Novick, Gabriel Epstein, Weisskopf, Richard Wolff, and Richard Linke.Image: NASA
Imaging X-Ray Polarimetry Explorer
Imaging X-Ray Polarimetry Explorer Artist's representation of NASA’s Imaging X-Ray Polarimetry Explorer (IXPE) in Earth orbit.Image: NASA
Supernova remnant Cassiopeia A in x-ray
Supernova remnant Cassiopeia A in x-ray This image of the supernova remnant Cassiopeia A combines some of the first X-ray data collected by NASA’s Imaging X-ray Polarimetry Explorer, shown in magenta, with high-energy X-ray data from NASA’s Chandra X-Ray Observatory, in blue.Image: NASA/CXC/SAO/IXPE

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What is the sum of the following mission numbers: the first Apollo mission to orbit the Moon, the only space shuttle to land at White Sands, New Mexico and the first Mars orbiter?

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The 20x36-inch Mars Science screen poster from Chop Shop, featuring Curiosity, Perseverance and Ingenuity on the Red Planet.

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What was the first European Space Agency to use ion (electric) propulsion?


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Question from the Mar. 16, 2022 space trivia contest:

What were the first words spoken from the Moon? Who said them? These were the words spoken when any portion of the Lunar Module made contact with the surface.


Buzz Aldrin uttered the first words spoken from the Moon when he said: “Contact light.” This happened when a light on the Apollo 11 Eagle lunar module control panel indicated that one of the long probes extending from three of the lander’s footpads had touched the surface.


Mat Kaplan: Our spectacular x-ray universe this week on Planetary Radio. Welcome. I'm Mat Kaplan of The Planetary Society with more of the human adventure across our solar system and way, way beyond. Martin Weisskopf says you can't do astrophysics without studying the sources of x-rays across the cosmos.

Mat Kaplan: He should know, he helped invent x-ray astronomy from space telescopes, which is the only way it can be done. With decades as project scientists for the great space observatory known as Chandra behind him, he now also leads work with a new and unique instrument called IXPE.

Mat Kaplan: We'll enjoy a conversation with this NASA Marshall Space Flight Center Experimental Astrophysicist, right after we check in with Planetary Society, Chief Advocate, Casey Dreier. Casey has been poring over the brand new fiscal year, 2023 NASA budget proposal. He'll share his impressions with us. Later, we'll hear the very first word spoken from the Moon when humans made first contact. They may surprise you.

Mat Kaplan: We'll also roll out yet another space trivia contest. Speaking of humans on the Moon, is that a beaming Buzz Aldrin we see in the March 25 edition of our weekly newsletter? Yeah, that's Buzz having the time of his life at the annual Yuri's Night celebration. As one of the founders of Yuri's Night, I can hardly wait for this year's party.

Mat Kaplan: It has been three years since we last gathered in person under Space Shuttle Endeavour in Los Angeles's California Science Center. I'll be there to once again interview some of the space stars in attendance for the 61st anniversary of humankind becoming a spacefaring species.

Mat Kaplan: The Planetary Society is, once again, a sponsor. There are at least 113 events planned in 30 countries across seven continents. And you can plan your own if there isn't one near you. Information and tickets are at That's Y-U-R-I-S

Mat Kaplan: You can also learn more at Be sure to say hi if you see me at the LA party. Casey Dreier is The Planetary Society's Senior Space Policy Advisor and Chief Advocate. Casey, as we speak, it has been barely 24 hours since we saw the Biden administration's proposed NASA budget for 2023, the federal fiscal year that doesn't get underway until October. And we'll remind everybody that this is just a proposal. But what are your initial thoughts?

Casey Dreier: Well, overall, this is a pretty good budget. Just looking at the numbers, it's proposing a $26 billion topline for NASA. That's about an 8% increase over what Congress provided just a few weeks ago for this fiscal year that we're currently in.

Casey Dreier: It gives great funding for NASA science programs, it's about eight billion. That's, as they claim, the most ever spent or proposed for NASA science. And 3.16 billion for planetary science. A wonderful number, for those of you who remember when we were aiming for one and a half billion just a few years ago. We're now well into the threes for the second year in a row.

Casey Dreier: We see major program initiatives that The Planetary Society supports. They're all funded. We're looking at Mars sample-return getting 822 million for next year, we're looking at Europa Clipper continuing, we're looking at Artemis with the Human Landing System now with funding to support a second provider, in addition to SpaceX.

Casey Dreier: Serious continued investment in the plans being established now that NASA's been pursuing at the Moon. This is really happening, right? This is what we take from this budget. So it's again, all round, very good. There's a few exceptions that we can talk about, but lots of positive growth throughout the agency.

Mat Kaplan: Good news for Earth science as well. But let's talk about those downsides because at least one of them is something we take pretty seriously.

Casey Dreier: Yeah. The biggest problem, and I've been doing this for 10 years, Mat. And there's no such thing as a perfect budget. They always have to do something either irritating or just outright foolish sometimes. The biggest flaw I'd say in the '23 proposal that we're seeing is the serious decline of investment in the NEO Surveyor asteroid hunting space telescope.

Casey Dreier: The one that we really need out there looking for these asteroids before they surprise us and potentially slam into Earth, as we've all decided, and I think we can all agree on, one of the few areas of agreement in America and in global politics, is that that would be bad.

Casey Dreier: And so we need to look for them and find them in order to prepare any potential deflection. NEO Surveyor does that. It's endorsed by the national academies, it's endorsed by a lot of folks in Congress and has been receiving strong support in both budgetary appropriations and in authorizations, the kind of legislative mandates from Congress.

Casey Dreier: However, despite getting $140 million last year, which is what it needed to stay on track for a mid-2020s launch, this budget proposes a mere 40 million, basically putting the program into a deep-freeze and delaying its launch by at least two years.

Casey Dreier: And their argument is, is that they don't have enough money in what they were given to do NEO Surveyor at this rate and pursue Mars sample-return and Europa Clipper, both of which were highlighted as having budget overruns this year. And this is the consequence.

Mat Kaplan: And we are also looking at Mars sample-return being delayed another two years and split into two landers. It's easy to see where a good piece of that money that could have gone, should have gone, maybe to NEO Surveyor, is ending up.

Casey Dreier: Yeah, it's a bit tricky. The 2028 now deadline for Mars sample-return, that was always in the mix. In fact, that was recommended by the Independent Review Board that evaluated the program multiple years ago at this point.

Casey Dreier: Anyone really looking at it, looking at a potential 2026 launch for this brand new technology of a Mars ascent vehicle and Mars fetch rover, that was an almost wildly ambitious timeline. Four years to build those. It makes total sense. And I always assume '28 would be the likely launch. You can characterize it as a delay. I'd say it's a more realistic assessment of the program. You don't want to have a mad rush to an impossible deadline. You just end up wasting resources that way.

Mat Kaplan: Something you've never tried to do before, right?

Casey Dreier: Yes, exactly. No one ever is happy about maintaining that wild rush. And so it's a reasonable deadline. There's a bigger issue at Mars, which is again, why they're still throwing 800 million. That's a huge amount. That's 25% of the entire Planetary Science Division budget is now going to Mars sample-return in '23 if this budget goes through.

Casey Dreier: What that says is that there is still a ticking clock and there's these larger cycles at Mars of dust storms that will seriously hamper ground operations and starting in the early 2030s, that if we don't launch by 2028, we'll seriously complicate efforts from Mars sample-returns.

Casey Dreier: So '28 is about as late as you can push it. And even then, we're still having to have huge increases in spending for this massive program. Again, wildly exciting, worth doing, but you can see why NEO Surveyor, which does not have a ticking cosmic time clock dictating when it can launch and when it can't. You can see why that one was maybe singled out as the one to delay, versus Mars sample-return.

Mat Kaplan: Any other last thoughts in this brief segment? I did note that NASA is once again, going to try and ground SOFIA, that big infrared telescope cut into the side of a 747.

Casey Dreier: Yeah. That's the great bipartisan effort to cancel SOFIA. We've been seeing that for years. The National Academies even came out and said it's not worth running anymore because it's the third most expensive astrophysics mission that there is after JWST and Hubble.

Casey Dreier: It's just not worth the investment. Congress has, so far, for years, refused to do it. I anticipate they'll probably do that again. Big picture though, there's two things to keep in mind that are worth remembering. One, is that this is a midterm election year. Very unlikely we'll see any real action on this budget or any other US budget for any federal agency really happening before the elections in November.

Casey Dreier: If we're lucky, we'll get something done in the lame-duck session, but it could easily push back into next year. Members of Congress, generally, don't like to take votes before elections. So they push a lot of things off. So that's going to take up a lot of political oxygen in the next few months.

Casey Dreier: And beyond this, one other thing to keep in mind, again, this is an overall 8% increase to NASA. That's great. But of course, we're seeing serious inflation for the first time in a generation, two generations here that are going to eat into NASA's buying power.

Casey Dreier: We don't know what the ultimate impact is going to be, but I would argue very likely that this 8%, in reality, in terms of just maintaining buying power, will probably only turn out to a couple a percent at the end of the day. It's another reason why we need this increase is just to maintain NASA's ability to provide and secure and procure all of its materials, technology, and people to achieve these missions.

Mat Kaplan: Casey, what have you got available perhaps at for people who want to dig deeper?

Casey Dreier: We do have our tracking page for the fiscal year 2023 budget. You can compare a lot of the topline NASA programs to what have been passed by Congress, key aspects of analysis, some I mentioned here and also links to source documents, which I just love to always do.

Casey Dreier: So if you want to read the NASA 2023 president's budget request, which I actually always really recommend doing, it's kind of a fascinating document. It might take a few days, it's a 700-page PDF. I always do it, it's fascinating. It's all linked to on there on

Mat Kaplan: I think I'll wait for the movie and rely on you, Casey, to give us this great kind of report that you regularly provide. There are a couple of other things that we should mention before we go. One, anybody expecting to hear the Space Policy Edition at Planetary Radio?

Mat Kaplan: Yes, usually first Friday of the month, but we are delaying it one week this time. So you will hear it on Friday, April 8th. And the other one though, the other point to make though, Casey, is one of congratulations. You have your very first peer-reviewed published article.

Casey Dreier: I do. Thanks, Mat for bringing that up. Some of you listening may remember that I published a big data set on Apollo cost. Reconstructing the cost of all of the programs within Apollo, not just the topline, which really improved the ability to do more refined inflation adjustment, one-to-one comparisons with modern programs, all that good stuff.

Casey Dreier: And I took that work that I first published at, and now worked it through the peer-reviewed journal at Space Policy, the Space Policy journal. That just came out, it was a long process, as it should be, to get through peer review because The Planetary Society is committed to engaging as many people as possible in these issues with space exploration.

Casey Dreier: We sprung for open access. That means anyone, whether or not you're an academic or a subscriber to the Space Policy journal, has access to this piece for free. And we linked that on our website. But if you just search for an improved cost analysis of the Apollo program, you will find it at Space policy.

Mat Kaplan: And we will put a link up on this week's show page, of course,, along with Casey's great resources to understand the NASA budget and this new FY '23 proposal from NASA and the Biden administration. Thank you, Casey. I look forward to talking again in, what, about nine days, April 8.

Casey Dreier: Of course, Mat, always a joy to pop into the regular weekly show too.

Mat Kaplan: He's the Senior Space Policy Advisor and our Chief Advocate at The Planetary Society. That's Casey Dreier. The Earth's magnetic field is no slouch, but it measures at something less than a single gauss in strength. The magnetic field generated by objects called magnetars can reach as much as 10 to the 15th gauss. That's a one followed by 15 zeros or a quadrillion.

Mat Kaplan: A field that powerful does weird things to physics, much as black holes do. Both are mind-bogglingly huge sources of energy. And much of that energy is emitted as x-rays. And though x-rays are a far more energetic form of electromagnetic radiation than the visible light your eyes can see, they thankfully can't penetrate Earth's atmosphere. No. To see and analyze them, you have to put your telescope in space. That's what Martin Weisskopf has been doing for over a half century. His new instrument is an international effort called the Imaging X-ray Polarimetry Explorer, or IXPE, fondly referred to as IXPE.

Mat Kaplan: It can determine the polarization of x-rays that have traveled millions or billions of light-years to reach it. And understanding that polarization may help us unlock deep secrets of the cosmos. On top of his work with IXPE and the Chandra X-ray Observatory, Martin is the Chief Scientist for X-ray Astronomy at NASA's Marshall Space Flight Center in Huntsville, Alabama. That's where he was when we talked a few days ago. Martin Weisskopf, welcome to Planetary Radio.

Mat Kaplan: I also want to congratulate you on the release of IXPE's very first science images. I think the first were released publicly only about six weeks ago as people hear this. Some of my favorite images of the universe are those beauties that overlay images from more than one instrument. You must be very proud of one that was in the press release that combines an image of the Cassiopeia, a supernova remnant, taken by Chandra with a brand new one from IXPE. Both instruments that you have a lot of responsibility for.

Martin Weisskopf: That is correct. It's really nice to see them merge together. I've been the Project Scientist for Chandra actually since the beginning in 1977. They made me an offer at NASA that I didn't refuse. And I got this fantastic opportunity to help build a scientific cathedral. Really an amazing opportunity.

Martin Weisskopf: And as people may or may not know, Chandra has been one of the most successful science missions that NASA has ever flown. It's still operating after 23 years now this year. It has thousands and thousands of papers, paradigm shifts et cetera. And it's the best damn x-ray telescope that's ever been built. IXPE, on the other hand, is really... although it's the Imaging X-ray Polarimetry Explorer, its prime reason for existence is it does something that Chandra can't do, which is measure the polarization or attempt to measure the polarization from astrophysical and astronomical sources.

Martin Weisskopf: For me, I helped start that field of x-ray polarimetry with the first measurements from sounding rockets in 1971. That's 51 years ago. And now we have a much more powerful polarimeter. And although we didn't release these results too in detail yet, we're seeing polarization, measuring polarization from several different classes of sources.

Martin Weisskopf: So that's wonderful to me again. And the Crab Nebula, which is another star that exploded like Cas A supernova remnant. That was the first and only real positive detection of polarization that we made with the satellite called, Orbiting Solar Observatory number eight in the mid '70s. And we got a 19 standard deviation result, which is pretty big time.

Mat Kaplan: Not bad.

Martin Weisskopf: Now, in our Quicklook data for IXPE, we got a 65 sigma result.

Mat Kaplan: Wow.

Martin Weisskopf: Yes. They're like gangbusters and there'll be a lot of a very fascinating data that comes from it. I'm really excited because there's things that I wanted to do in the '70s and we didn't have enough events. Getting polarimeters flown has always been difficult because it's not easy to do polarimetry.

Martin Weisskopf: So IXPE gives us a dedicated satellite mission so we don't have to worry about the fact that if we did this type of experiment, it would be much more efficient and we could be 20 of those as opposed to one polarization measurement. But yeah, that polarization measurement may tell us something new astrophysically. And that's where the real excitement is coming up in the next few months.

Mat Kaplan: I want to back way up. I found a photo of you and your colleagues back from 1971. It was a decidedly furrier era.

Martin Weisskopf: Yes.

Mat Kaplan: And you were all standing around an Aerobee sounding rocket, as you mentioned, that got that first measurement of a polarized x-ray source from something out there in the sky, a celestial object, which is impressive as the work being done now is by Chandra and IXPE and so on. That was quite an accomplishment back then, wasn't it?

Martin Weisskopf: Yes. That was just me and a graduate student and another professor, assistant professor and his student and the director of the laboratory. And when I showed that picture, as you said, that it's kind of furry.

Mat Kaplan: Yeah.

Martin Weisskopf: Because except for the director of the laboratory, Robert Novick, we all had beards. And when I show that in seminars and stuff, I will say I'm the handsome one, if you want to know which one of them is me.

Mat Kaplan: In the middle, crouching down at the foot of the rocket.

Martin Weisskopf: That's right.

Mat Kaplan: So you've been collecting and focusing x-rays from space for over 50 years, as you said. And you have said that x-ray astronomy is as compelling for you as ever. Why? Why is that?

Martin Weisskopf: Oh, so many reasons. Especially since I'm an experimentalist, and so I like to build things. X-ray astronomy presents several challenges to move forward. Extremely high resolution Chandras half-arcsecond angular resolution. And we need something that competes with JWST and even Hubble at about 0.05, arcseconds. It's a dream that I have to build optics like that.

Mat Kaplan: So an order of magnitude, better than Chandra?

Martin Weisskopf: An order of magnitude better. I think that's extremely important to move the science forward. But the best thing and the most exciting thing to me about x-ray astronomy, is we're probing new phenomena all the time. Every instrument that we put up, Chandra, IXPE has made some very surprising astrophysical discoveries.

Martin Weisskopf: Things don't work like the way we think. My theorist colleagues, bless their hearts, they're very clever. But many of them are only very clever after the fact and not before the fact. And I love that aspect of science. Some people said, "Well, let's slide the Monte Carlo simulation, it always looks so nice."

Martin Weisskopf: But I would prefer to analyze the data and find something different. That gives me a real thrill. And as a scientist, one of the things that has always interested me in science is when something like that first polarization experiment, I realized in analyzing that data with my student, we're the only people in the entire universe that have ever known this.

Mat Kaplan: That we know of.

Martin Weisskopf: Well, yeah.

Mat Kaplan: [crosstalk 00:21:11].

Martin Weisskopf: You're right. You caught me there on a slight sports exaggeration. But certainly in the history of the Earth. And that's just a tremendous feeling. And that's one of the reasons I love science so much, is that very occasionally, you're lucky enough to get that feeling.

Mat Kaplan: Let me ask you an obvious question. Why aren't we doing x-ray astronomy from down here on the surface of our planet?

Martin Weisskopf: Oh, I wish we could. Except, we'd all be dead. X-rays will not penetrate through the atmosphere to reach the surface of the Earth. Even though they're energetic photons, the higher energy end of the electromagnetic spectrum, they don't have this nice cross-section for visible light, which comes down to the surface.

Martin Weisskopf: And so x-ray astronomy is a child of the space program. The first experiments had to be done from rockets. The early experiments were done by the Naval Research Laboratory that took balloons to lift the rocket up partially, and then fired the rocket once it got to a high altitude and got above the atmosphere to be able to look for x-rays from, in that case, the sun, the earliest ones.

Martin Weisskopf: And then Riccardo Giacconi, who won a Nobel Prize for being the father of x-ray astronomy and the father of Chandra, amongst others, did his rocket experiment and discovered the brightest x-ray source in the sky, Scorpius X-1, which all the theorists said couldn't exist.

Mat Kaplan: X-1.

Martin Weisskopf: Yes.

Mat Kaplan: The first thing suspected to be a black hole?

Martin Weisskopf: Actually Sco X-1 is more likely to be a neutron star, but those small objects, black holes and neutron stars are the key to x-ray production.

Mat Kaplan: When we talk about x-ray telescope optics in space, maybe you should say something about this. Because I know we're not talking about glass lenses and shiny mirrors, like I have in my optical telescope downstairs in my house. How did we learn? How did you learn to precisely bend x-rays the way we do light in the optical spectrum?

Martin Weisskopf: Right. Well, actually the physics is there all along. It's just that as you go with visible light, you can come and reflect the x-rays in normal incidents to the mirror. Just look at the mirror in your bathroom, if you send an x-ray at it with much higher energy, it would just be absorbed in the mirror.

Martin Weisskopf: But if you make the angle of incidence much more shallow, then as you get to a certain angle, called a critical angle, the x-ray will reflect. So what you have to do is you have to come into a very shallow angle and then have surfaces of revolution, which will then focus the x-rays down to a point.

Martin Weisskopf: It sounds simple. For Chandra, it took about 20 years of development, the surface roughness of a few angstroms and many mirrors nested. In the case of Chandra, four. IXPE, we had far less angular resolution, about 30 arcseconds rather than half. But that's what we need for IXPE because we need lots and lots of x-rays to get good statistics, and not just trying to detect the source where you just need a few photons there. There's a source there.

Martin Weisskopf: But if the source with those few photons is 10% polarized and there are only 100 photons, you only have 10 photons, that 10% that could be useful for polarimetry. And 10 photons is not a lot. You need millions to get the statistics down. A strongly polarized source is 20% astrophysically.

Martin Weisskopf: The Chandra mirrors were built through using a different technique to get down to that half arcsecond, much more expensive and heavy. The IXPE mirrors we built here at Marshall Space Flight Center using a technique called replication, where we build a mandrel. It's a solid piece of material that has the outside shape of what we want the optic to have, deposit material on it and just a little bit to keep it thin and not weigh so much.

Martin Weisskopf: And then cool it off and take it off and assemble and align various different sizes of them, one inside the other. And that's now the 30 arcsecond IXPE optics. Now the image isn't as great as the Chandra image, but it's one of the best images out there other than two satellites that are flying two or three that have anything close to 30 arcseconds.

Mat Kaplan: Could one of those be NuSTAR? I'll mention that your colleague, Fiona Harrison, was a previous guest on our show.

Martin Weisskopf: Yes, she's wonderful, isn't she? I had dinner with her the other night and have known her for quite a while. No, actually NuSTAR is an arcminute or so. And so it's actually four times worse.

Martin Weisskopf: Well, the angularization is twice as bad, but sensitivity is four times lower. But they go to higher energy. It does different science than IXPE. And it's beautiful for what it was trying to do and is doing. It's still up there and flying, which we're very happy about.

Mat Kaplan: I want to make sure people caught that, that NuSTAR is designed to work with even more energetic photons than Chandra and IXPE are.

Martin Weisskopf: That is correct. Absolutely correct.

Mat Kaplan: More of Martin Weisskopf is seconds away, but we've got a special invitation first. I'm a big fan of Radiolab. I often say it's the best produced public radio and podcast series anywhere.

Mat Kaplan: Radiolab has just come up with a story that a lot of us space geeks can connect with. We were honored when they asked us to help get the word out.

Speaker 4:

What the hell are we doing? Oh, my God.

Mat Kaplan: On Radiolab.

Speaker 5: Get ready to rock.

Speaker 7: Rock and roll.

Speaker 6: And with the blind crew.

Speaker 8: What's going to happen when you put a bunch of people who use wheelchairs, who are deaf or who are blind and zero gravity?

Speaker 7: Making our way up the switchback ramp here.

Speaker 4: This could really change a lot of things.

Speaker 5: Oh, my goodness.

Mat Kaplan: The right stuff from Radiolab.

Speaker 11: Something out of this world.

Mat Kaplan: Listen wherever you get podcasts. Since IXPE's whole reason for being is to detect this polarization of x-ray light, obviously that is an important priority for a good reason. What are those reasons? What is it that polarization can tell us about the object that is emitting those x-rays?

Martin Weisskopf: Right. We've come a long way in x-ray astronomy from the early rocket days where we were very happy to detect a source and ecstatic when we detected a source that was out of the galaxy. And it was an extended source, which was a cluster of galaxies.

Martin Weisskopf: But now we have detailed models and often the case, competing models, to explain the x-ray emission that we see from these various objects. Well, if you put polarization, polarimetry, into the astrophysical bag of tools, you have another constraint on what's going on.

Martin Weisskopf: And then you have to explain the polarization too in addition to the energy distribution, the time variation et cetera. And so that's what polarimetry brings to the table. We used to measure energy, location, time variability.

Martin Weisskopf: Now we have polarization to add to that. And you got to be able to predict it. Theoretically, we've done a lot of studies in preparation for IXPE and other scientists interested in polarimetry. And find all kinds of neat things. So for example, there are neutron stars called magnetars, this neat name because their magnetic fields are supposed to be 10 to the power 15 gauss.

Martin Weisskopf: And all kinds of things happen very interestingly at those magnetic fields. You don't use standard physics anymore, you have to use quantum electrodynamics to see how x-rays propagate in the atmosphere of these stars.

Martin Weisskopf: If we look at the magnetars and see what the energy dependence is of the polarization... And by the way, these things pulse two at a few seconds per pulse period. Looking at the polarization as a function of pulse phase can tell you something about whether or not the field really is 10 to the 15 gauss.

Mat Kaplan: You say they pulse. Does that mean that magnetars are also pulsars because they're spinning or is something else [crosstalk 00:30:28]?

Martin Weisskopf: Yes. No, that's absolutely... That's just [inaudible 00:30:33] but theoretically quite complicated. But yes, neutron star spinning. These beasts happen to have these huge magnetic fields.

Mat Kaplan: Well, let me drag you back, not quite to the event horizon, but back to black holes as well. Another, as you said, very tiny source of enormous energy.

Martin Weisskopf: Yes, x-ray. We always make a mistake in talking to people because the x-rays don't come from the black hole because you can't see the black hole. But they come from very close. And what's happened is that particles are accelerated to extremely high energies and then radiate x-rays.

Martin Weisskopf: And one of the neat things we can do with x-rays polarimetry is look at one of these black holes, the microquasars, as we call them. And see what the energy dependence is. How does the polarization vary with energy? IXPE can do some energy resolution. It's not wonderful, but it's not terrible. The way that polarization varies with energy is directly related to the spin of the black hole.

Martin Weisskopf: Polarimetry is a function of energy of these systems, can tell you what the spin of the black hole is. And it's not the only way to tell the spin. There are other techniques that have been used, and it will be very interesting to see whether or not we agree. And if we don't agree, why don't we agree?

Mat Kaplan: So that takes us back to how you experimentalists sometimes tweak your friends, your colleagues, the theorists. I love when distinguished scientists and mission leaders and others come on our show and say that all of our thinking about some basic physical property or feature of the solar system or the universe, turns out we got the data and we were wrong. Isn't that about as exciting as science gets?

Martin Weisskopf: Yes, it is. And it's kind of funny. We keep doing experiments to really try to understand things better. And what we're doing is working to put ourselves out of business because once we understand everything, there's nothing for us to do.

Martin Weisskopf: But the reality seems to be is that the more we understand things and the better our experiments, we have to tweak everything we find out. We didn't understand it at all, as you said, or it all may be a little bit of an exaggeration, but it wasn't quite right.

Mat Kaplan: I want to go back to the spacecraft itself, IXPE, in this case. Because I hope that people will explore the website. There are some great images there. I'll start with this, a shot of the lens or one of the lenses. It is a beautiful piece of engineering. And it sounded like you're using pretty state-of-the-art techniques. What additive deposited material?

Martin Weisskopf: Yeah. There are not too many groups in the world that do replicated optics, and we're one of them and we're darn good at it. And the other group is in Italy and they're very good at it too.

Martin Weisskopf: In fact, we collaborate a lot on techniques and the discoveries we make of how to do something better. For example, removing the shell from the mandrel, we discovered many years ago that the best way to do that was not to cool down the mandrel but to pour water on it.

Mat Kaplan: Oh, no kidding?

Martin Weisskopf: Yeah. The shell and the mandrel, the shell would just pop off.

Mat Kaplan: Almost as if you were blacksmiths of old quenching something.

Martin Weisskopf: Yes, indeed. Indeed.

Mat Kaplan: This is where I was going to bring up NuSTAR, if it hadn't come up earlier. Because for all of their differences, I remember how amazed I was looking at how NuSTAR deployed itself in space, in two parts. And IXPE doing exactly the same thing. I watched the animation from the clean room at Ball Aerospace. And I thought, "Oh my God, this is like watching with white knuckles as the James Webb Space Telescope unfolds."

Martin Weisskopf: Yes.

Mat Kaplan: Was that anxious?

Martin Weisskopf: Yes. Darn right. Moving parts are always the thing you really worry about for space systems because they have to work then. You don't know if they're going to work. You've tested it on the ground, but you've tested it under one gravity. There's no gravity. And as you saw, ours twirls around three and a half times before it gets to its final extended position.

Martin Weisskopf: And that's a pretty scary moment. I put that one right up there with the solar panels have to flip out. That happened right away. So we didn't have time to get too nervous. The deployment of, what we call, the boomer optical bench. It took a little longer, a few minutes, and we were holding with bated breath what the indicators were after that happened.

Martin Weisskopf: There's always something you worry about. Chandra, we worried about the various instruments. The most important instrument had a door that had to open. It was vacuum sealed until it was up there. And during tests prior to launch, it failed. And we never really found the root cause. And so we put in several different approaches to try to make sure that it wouldn't fail. But then when that happened, when that door was supposed to open, we were all sitting there fingers and toes crossed et cetera. But it worked like a charm.

Mat Kaplan: Thank goodness. Why was it important for both NuSTAR and for IXPE to separate the components of the telescope? And I keep saying the telescope; IXPE is really three telescopes, isn't it?

Martin Weisskopf: Three independent telescopes. Yes. With three independent detectors. We need a certain focal length that is the distance between the telescope and the detector. Now, the launch vehicle that NuSTAR did launch on and IXPE was supposed to design to, NASA hadn't selected the final thing. Did not have the room for us to launch with the boomer optical bench extended.

Martin Weisskopf: So both of us had to deploy the bench. Turned out that in the end, IXPE launched on the Falcon 9, which would've had enough room to put the bench in without that. But changing the design et cetera, et cetera, at that stage of the program was not feasible cost-wise in schedule.

Mat Kaplan: Well, thank goodness that it worked regardless. What's ahead? What is on the order of business for IXPE?

Martin Weisskopf: For IXPE, there's a whole year's worth of sources that we plan to look at, plus six or seven targets of opportunity where some wonderful x-ray sources goes bump in the night and really extends its flux so that we can get a good shot at measuring something.

Martin Weisskopf: And so we're going through those sources for the first year and that'll design what we're going to do in the second year. It'll guide us. And then in the third year, assuming that IXPE is deemed wonderful by senior reviews and continues, which I have no doubt that it will, we then open up a general observer program where scientists throughout the world will be making proposals to look at that particular target for their science and their particular reason. I should say that IXPE, my science advisory team, has over 100 scientists from 12 countries.

Mat Kaplan: Wow.

Martin Weisskopf: Internal group of IXPE is not confined to just the United States, but it's truly collaborative. And I should say that one of the reasons that IXPE is so beautiful and so sensitive to polarization, comes from these beautiful polarization-sensitive detectors that were provided by Italy and developed in Italy. Italy has played a major role in the success of IXPE.

Mat Kaplan: I was about to ask you about the international involvement because I had read a little bit about this. In fact, you have an Italian colleague, right? Who is also a PI, a principal investigator on the project?

Martin Weisskopf: Yes. There are actually two PIs in Italy because the Italian Space Agency will recognize two: Paolo Soffitta from Rome and Luca Baldini from INFN in Pisa are the two Italian PIs. NASA doesn't recognize more than one. So I'm stuck with it.

Mat Kaplan: I see. Okay. Before we wrap up, you should give us a status report on Chandra. As you said, I think you're headed toward the 23rd anniversary of your launch being carried into space on Space Shuttle Atlantis. Still going strong, right?

Martin Weisskopf: Yeah. Still going strong. We're having thermal issues. As the observatory gets older, the thermal insulation is degrading. Things are getting hotter. This complicates our operations, but still doesn't prevent the basic science that wants to be done. We're having troubles with one of the instruments right now.

Martin Weisskopf: It's the high-resolution camera microchannel plate device, but we're looking at ways of trying to bring it back alive again. But our principal detectors or the charge-coupled detectors provided by MIT and Penn State, they are working good enough. We have some contamination issues, again, that are in there, building up through launch.

Martin Weisskopf: These are angstroms of material but above one kilovolt, essentially, no change in the response from when we launch. That's somewhat of an exaggeration. But 5% here, 10% there is not the end of the world. Because it can't be the end of the world because we typically get 500 proposals every year for use of Chandra. And those are scientifically active. And we go through senior reviews. We're having one this year and we got a good... They always ask, "Oh, what have you done for us lately?" And we've got quite a bit.

Mat Kaplan: Chandra was, of course, one of the great observatories.

Martin Weisskopf: Yes.

Mat Kaplan: Those are all aging. Hubble, still going fairly strong. Had another close call there just in the last month or so. I know you look forward to the future because you've already talked about needing that instrument, which will have 10 times better resolution in the x-ray domain. We did just see a new astrophysics decadal survey recommendations released. What are your hopes in this area? Do you see a good opportunity to build this new, bigger and better x-ray telescope?

Martin Weisskopf: Well, I prefer to put it this way: we have to. We have discovered with Chandra and the other missions, the European XMM-Newton, which is also an x-ray observatory that you can't do astrophysics without the x-ray data. If you just try to study any class of objects, you need the full tools. That's why JWST is important because it provides the infrared. But you also have to... We used to have this terrible analogy, you can't understand a human by studying the foot.

Mat Kaplan: It's the blind men and an elephant.

Martin Weisskopf: Right. So scientifically, there's absolutely no question that we need a bigger and better with more chrome, as it were, x-ray telescope mission for in the future. When is it going to happen? How much will it cost? These are all issues. These things take time.

Martin Weisskopf: The Chandra was 22 years before launch. We started it in '77 and launched it in '99. If you look back at Hubble and Spitzer, even Compton took over a decade. And the Compton really, in some sense, it was a great observatory after the fact to complete the great observatories.

Martin Weisskopf: It was a mission that was already going. And they said, "Well, let's call it part of the Great Observatory program. That way, we don't have to build another one."

Mat Kaplan: Compton, of course, the Gamma Ray Observatory.

Martin Weisskopf: The Gamma Ray.

Mat Kaplan: The Gamma Ray telescope.

Martin Weisskopf: Yes. Which brought us the fabulous science.

Mat Kaplan: I got one more question that only occurred to me a moment ago.

Martin Weisskopf: Okay.

Mat Kaplan: When you go into the doctor's office and he says, "We better get an x-ray of that," does that hold any special fascination? And does he know how you earn your living?

Martin Weisskopf: Yes. Well, I tell them all that whenever I run into an x-ray machine, and unfortunately at my age, I have a lot of doctors that try to keep me going, I'm always amazed at the crudeness of the medical instrumentation. I could build them a system that subjects the human to a far less dose than what they're doing. It's more brute force.

Mat Kaplan: You've made me very glad that I asked that question. I think you need to become an entrepreneur.

Martin Weisskopf: I should tell you that I'm 80-years-old and I'm a little bit past entrepreneur.

Mat Kaplan: Well, I hope though that you have many more years of leadership ahead of you and great science ahead of you in this x-ray domain.

Martin Weisskopf: So do I. I'm planning to formally retire and apply to become an emeritus, which will allow me to play with data and not attend meetings. It sounds wonderful.

Mat Kaplan: A little bit of heaven. Martin, thank you so much. This has absolutely been delightful. I so look forward to seeing the results keep flowing from IXPE and from Chandra, and really across the spectrum that you and your colleagues are contributing so much to.

Martin Weisskopf: Thank you. It's a pleasure. They even pay me.

Mat Kaplan: I won't tell. Time for What's Up on Planetary Radio. Here's the Chief Scientist of The Planetary Society. Bruce Betts is back. Welcome.

Bruce Betts: Hi, Mat. How are you doing?

Mat Kaplan: I'm doing good. I hope our connection holds up here. We've been having a little trouble the last few minutes. But right now, I'm looking at your, I was going to say, smiling face. But you're not smiling.

Bruce Betts: No, it's because I'm trying to read your lips because I can only hear about every half of the words you say. But that actually makes as much sense as when I can hear all the words. So I think we're good.

Mat Kaplan: Like I didn't know that was coming. Hey, before this thing goes belly-up again, tell us what's up?

Bruce Betts: In the evening sky, no planets but a lot of constellation goodness. Still Orion, catch it while it's hot. You've got that over in the early evening in the southwest. And then in the pre-dawn sky, it is indeed still a planet party. Venus, looking super bright, Mars looking reddish and Saturn about the same brightness as Mars, looking yellowish.

Bruce Betts: And here's the really exciting part, April 4th or April 5th, check them out. Mars and Saturn about the equivalent of one lunar diameter apart from each other. So very close together. And Venus very nearby. So a cluster of three hanging out in the pre-dawn east.

Mat Kaplan: They couldn't stretch it one more day to hit my birthday? That's a real shame.

Bruce Betts: Did you just casually drop your birthday?

Mat Kaplan: You noticed?

Bruce Betts: Happy birthday, almost, Mat.

Mat Kaplan: Almost, almost. Keep going.

Bruce Betts: Next week's episode, we'll have a birthday celebration.

Mat Kaplan: Oh, boy.

Bruce Betts: I'll order party hats. That will be meaningless for a radio show. All right, we go onto this week in space history, 1997 Comet Hale-Bopp reached periapsis around the sun, visible to folks on Earth around that time. In 1973, Pioneer 11 launched out to the outer solar system, joining its sister craft, Pioneer 10, on what would be the first explorations of the giant planets.

Mat Kaplan: When Hale-Bopp made its appearance, one of those nights I was on a plane, headed to Florida and had the presence of mind to bring my binoculars in the cabin. We were on the correct side of the plane. There was a crowd of us going to a conference before my Planetary Society days. And sure enough, we had a great view of the comet out of the window next to our seats.

Bruce Betts: That is so cool. And what an awesome nerdly thing to do, bringing the binoculars. Were you popular? Did you share eye infections with a bunch of people?

Mat Kaplan: We actually did. Yes. We handed the binoculars all around. If I remember correctly, I think even one of the flight attendants took a peak.

Bruce Betts: Wow. That's cool. Glad I mentioned that. Let's move on. I got a good one for you. I got a good random space fact.

Mat Kaplan: Scooby's got a throat problem, I think.

Bruce Betts: Well, it just happens to have to do with dogs this week. Okay. It's not a coincidence. I like dogs. And so here's your comparison. The mass of Mercury, compared to Earth, is about the same as the mass of a Chihuahua compared to the mass of a large German Shepherd.

Mat Kaplan: That's great. I love it. You do love dogs, don't you?

Bruce Betts: I do. And I was actually a little disappointed that the difference wasn't enough to include my giant mastiff in terms of mass. So we'll keep working on that. Although, it's pretty similar to maybe the pit bull. Let us move on to the trivia contest. And I asked you what were the first words spoken from the Moon based upon the words spoken after any part of the lunar module touched the surface and who said them? How'd we do, Mat?

Mat Kaplan: I'm going to play the actual audio. And it begins with the first words when the first portion of the lunar module touched the lunar surface. Here it is.

Buzz Aldrin: Contact light. Okay, engine stop. ACA out of detent. Modes control both auto, descent engine command override, off. Engine arm off. 413 is in.

Mission Control: We've had shutdown. We copy you down, Eagle. [crosstalk 00:50:22]. Standby D1.

Neil Armstrong: Tranquility Base here. The Eagle has landed.

Mission Control: Roger, Tranquility. We copy you on the ground. You got a bunch of guys about to turn blue. We're breathing again. Thanks a lot.

Mat Kaplan: What were those first words, Bruce? And who was that we heard say them?

Bruce Betts: It was "contact light" by Buzz Aldrin. There were sensors that were below the landing pads, three of the four landing pads that dangled down. And when those hit the surface, then the light on the equivalent of a dashboard, that said contact, the contact light went on. So Buzz announced contact light. And that was the first word spoken when something with humans on board touched the Moon.

Mat Kaplan: So as you heard, "Houston Tranquility Base here" was several seconds later. Those words, of course, spoken by Neil. And a few of you said that's one small step for... You know the rest. You know who got it right? Timothy Myers, who has not won for almost three and a half years.

Mat Kaplan: December of 2018 was his last win. Timothy is in California. Congratulations, Timothy. We are going to send you... it's one of those great Chop Shop prizes that we are in the midst of giving out. We've got another one coming up in a moment.

Mat Kaplan: It's the 20 by 36 screen poster of Juno above Jupiter that, as I said before, and will say again, it is gorgeous. It's from Chop Shop's robotic spacecraft series. You can see it at And I got other stuff too.

Bruce Betts: Yay.

Mat Kaplan: I got that audio that you just heard courtesy of... Well, courtesy of the NASA History site. But it was Mark Moffitt in Georgia who reminded us of it. And we will put up a link to this great page with all kinds of multimedia resources related to the Apollo 11 landing. Pretty exciting stuff.

Mat Kaplan: Dave Durden represented most of us who were around at the time. He said, "One of the great thrills of my young, at the time, life was hearing these words because little nerd that I was, even then, I knew it meant that Eagle had landed. I was jumping for joy so much, I almost missed Neil Armstrong's announcement of the landing Tranquility Base here, of course."

Mat Kaplan: Finally, this poem from our poet, Laureate Dave Fairchild, "What were the first words pronounced on the Moon? There were just two. Am I right? They were some jargon from Aldrin, not Armstrong. A simple, concise Contact Light. It showed that the probe from the lander made contact, a comment made somewhat offhanded. And shortly thereafter, we heard from the surface that Houston, the Eagle has landed." Thank you, Dave.

Bruce Betts: All right. This week, I've got the following for you. It's once again, and you'll be happy to know, Mat, the requests for Planetary Radio math are being answered once again. And it's simple math. There basically are going to be three things you need to have answers to, and you'll add them together and submit that number.

Bruce Betts: So here we go. What are the mission numbers? So for example, Apollo 11 would be 11, last shuttle mission, STS-135 would be 135. What are the mission numbers of the following added together? The first Apollo to orbit the Moon, the only space shuttle to land at White Sands, New Mexico and the first Mars orbiter. Get those numbers, add them together. Submit your answer to

Mat Kaplan: Shouldn't be too difficult for you. You've got until the 6th. April 6th, someone's birthday. I can't remember who. At 8:00 AM Pacific Time, to get us this answer. And you might win yourself another terrific poster from Chop Shop.

Mat Kaplan: This one is Mars Science from the historic robotic space craft series. He has redone it. It now features both Perseverance and Curiosity and a cute little helicopter, named Ingenuity on the surface of the Red Planet. That could be yours if you are chosen by this time around. With that, we are done.

Bruce Betts: All right, everybody. Go out there, look up at the night's sky and think about whether you prefer the term math or maths. Thank you. And good night.

Mat Kaplan: I'll just go with Mat, Mat Kaplan, that is. He's Bruce Betts. Not Bruce's just singular. The Chief Scientist of The Planetary Society who joins us every week here for What's Up.

Bruce Betts: Hey, Mats, nice show.

Mat Kaplan: Planetary Radio is produced by The Planetary Society in Pasadena, California. And is made possible by its members who have x-ray vision. You can see what they see at Mark Hilverda and Rae Paoletta are our associate producers. Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. Ad astra.