NuSTAR Principal Investigator Fiona Harrison discusses NASA’s Nuclear Spectroscopic Telescope Array.
NuSTAR is the first space telescope capable of taking focused high-energy X-ray observations of the cosmos, providing unprecedented information on the dynamics of black holes, exploding stars, and the most extreme active galaxies. The small but powerful telescope detects high-energy X-ray light and studies some of the most energetic objects and processes in the universe. Celebrating its 10th anniversary in June 2022, NuSTAR is a Small Explorer mission managed by the agency’s Jet Propulsion Laboratory for NASA’s Science Mission Directorate. It was developed in partnership with the Danish Technical University and the Italian Space Agency.
In this episode of Small Steps, Giant Leaps, you’ll learn about:
- NuSTAR’s most fascinating discoveries
- Accomplishments and lessons learned during a decade of high-energy X-ray observations
- What’s next with NuSTAR
Fiona Harrison serves as the Principal Investigator for NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR). Harrison is the California Institute of Technology Benjamin M. Rosen Professor of Physics, and the Kent and Joyce Kresa Leadership Chair of the Division of Physics, Mathematics and Astronomy. Her research is focused on the study of energetic phenomena ranging from gamma-ray bursts, black holes on all mass scales, to neutron stars and supernovae. Harrison was awarded the Presidential Early Career Award in 2000; was named one of America’s Best Leaders by U.S. News and World Report and Harvard’s Kennedy School of Government in 2008; received the NASA Outstanding Public Leadership Medal in 2013; was awarded the Bruno Rossi Prize of the High Energy Astrophysics Division of the American Astronomical Society in 2015; won the Harrie Massey Award from the International Committee on Science’s Committee on Space Research in 2016; and was the recipient of the American Physical Society’s 2020 Hans A. Bethe Prize. She holds a bachelor’s degree in physics from Dartmouth College and a doctorate in physics from the University of California, Berkeley.
Fiona Harrison: It’s the first telescope that can actually focus high energy X-rays.
NuSTAR has been able to survey the sky to find black holes that are hidden to low-energy X-ray telescopes. And so, this has really expanded our view of what I would call the demographics of black holes.
Putting all this information together in many different realms has been really transformative.
Deana Nunley (Host): Welcome to Small Steps, Giant Leaps, a NASA APPEL Knowledge Services podcast where we tap into project experiences to share best practices, lessons learned and novel ideas.
I’m Deana Nunley.
NASA’s Nuclear Spectroscopic Telescope Array – or NuSTAR – is celebrating its 10th anniversary. Launched in June 2012, the telescope has observed some of the hottest, densest, and most energetic regions in our universe. Our conversation is with NuSTAR Principal Investigator Fiona Harrison.
Fiona, thanks so much for joining us.
Harrison: Oh, it’s a pleasure to be here.
Host: Congratulations on NuSTAR’s 10-year anniversary.
Harrison: Thank you. It’s hard to believe it’s been 10 years, but the mission we thought might last a couple years — and here we are.
Host: Wow. And you’ve been the PI from the beginning. What do you think are the most fascinating NuSTAR discoveries?
Harrison: Well, my favorite, I have to say, is the discovery that objects called ultraluminous X-ray sources, which everybody thought were black holes, are not black holes at all. What they are is the compact core of a star called a neutron star, about the mass of the Sun and the size of the Los Angeles area. And these sources, these X-ray sources, are in nearby galaxies and they’re incredibly bright. And everybody thought to be that bright, you had to have a black hole, and you had to have matter falling from a companion star onto the black hole.
But NuSTAR, studying these sources, found pulses, which showed that these are not a-hundred-times-the-mass-of-the-Sun black holes, rather just wimpy one-times-the-mass-of-the-Sun neutron stars. And it was completely unexpected. I can still remember when I announced it at a conference, and just the gasps from the audience. It was one of the most amazing moments of my career, I have to say.
Host: Wow, that is so fascinating. And for anyone who may be unfamiliar with NuSTAR, could you provide a quick overview of the mission?
Harrison: Sure. So NuSTAR, it’s a Small Explorer. That’s the smallest standalone astrophysics platform that NASA launches. And it is a high-energy X-ray telescope. Previous telescopes, like you may be familiar with the Chandra X-ray mission or ESA’s XMM-Newton, those operate in low-energy X-rays. But by developing special technologies, new optics, new detectors, we were able to push out the region of the X-ray spectrum that we could observe into much higher energies. This is the same energy X-rays that your doctor and dentist use when they X-ray your teeth and your bones. And so, this enables us to both observe higher energy phenomenon, and because these X-rays are very penetrating, just like your dentist’s X-rays penetrate through your skin and are only stopped in your bone, we can see objects that are embedded in large amounts of dust and gas, because these high-energy X-rays can get out.
Host: You touched on some ways that NuSTAR is different from other X-ray telescopes. Are there other differences?
Harrison: Yeah. So, besides the energy range, NuSTAR was one of the first telescopes that was, when we launched, it was launched in a very compact configuration. Now, the way X-ray telescopes work, you have to have a very large separation between the X-ray mirrors and the X-ray detectors, which register the image from the mirrors. And to achieve this, NuSTAR had a tinker-toy-like structure of tens of thousands of piece parts, that after launch unfolded to achieve the 10-meter separation. That’s 33 feet, or the length of a school bus, between the mirrors and the detectors, so that was a rather unique aspect of the mission.
And it’s the first telescope that can actually focus high energy X-rays. And prior to NuSTAR, we could only detect these by very crude means. And NuSTAR’s unique optics and detectors enable us for the very first time to build a real telescope, a telescope that focuses light onto a detector, which registers an image. And this makes it hundreds of times more sensitive than any other telescope that is operated in this part of the X-ray spectrum.
Host: You talked about kind of the big surprise and what was so fascinating with NuSTAR. Are there other things that you’ve seen with NuSTAR that you weren’t expecting to see?
Harrison: Yeah. Well, I think the way I would characterize it perhaps, is that some of the places that NuSTAR has had the biggest impact are not areas that I anticipated before launch, for example, observing the Sun. We didn’t plan to do that before we launched, and it turns out there’s no reason NuSTAR can’t look at the Sun. And the Sun has giant flares, the ones that you’ve probably heard about, but there’s also these tiny, very weak flares. And because of the sensitivity of the telescope, the first focusing high-energy X-ray telescope, we are able to look at these and see that the surface of the Sun is actually popping off these tiny little flares all the time. And what we’re trying to understand is how the corona is heated.
You’ve probably seen images of the Sun taken with low-energy X-ray telescopes designed to look at the Sun, and you see this sort of plasma extending out from what looks like the surface or the photosphere. And it’s been a mystery how that plasma is heated. And so, NuSTAR’s observations are helping us answer that question.
Host: Fiona, what would you say are NuSTAR’s biggest accomplishments in the black holes arena?
Harrison: There’s several different areas where NuSTAR has really made advances. The first is, I mentioned that these high-energy X-rays are very penetrating, so they penetrate through dust and gas. And so, NuSTAR has been able to survey the sky to find black holes that are hidden to low-energy X-ray telescopes. And so, this has really expanded our view of what I would call the demographics of black holes.
And the other thing NuSTAR can do is measure the spin of black holes. So black holes are pretty simple, in a way, in that they are just characterized by a mass and a spin. And the spin, the black hole can acquire when it’s born, or it can acquire it over time as matter falls onto the black hole and spirals in and transfers what we call angular momentum, which spins up the black hole. And so, NuSTAR, by looking at the matter falling onto the black hole and diagnosing its geometry, essentially, can measure how fast black holes are spinning.
Host: What else has NuSTAR observed?
Harrison: So, one of the interesting observations that we’ve made is of the debris of exploded stars. So, when a massive star multiple times the mass of the Sun burns all its nuclear fuel, it will explode in an event called a supernova. And if the supernova is fairly young, you can still see radioactive debris. So in this explosion, there’s lots of particles and neutrons and things that create radioactive material that will decay over time. And this has not been done before, that this radioactive debris has been imaged so that we can look at it and understand its distribution.
And so, NuSTAR has done this, and with a famous supernova remnant called Cassiopeia A. And by looking at the distribution, we can actually tell what went on in the core of the star just before the explosion. And we’ve been able to determine that the core of the star really was sloshing around, not spherically symmetric at all. And we think that mechanism actually helps the star explode. So that’s a rather unique observation.
Host: Are there other unique observations with this mission?
Harrison: The NuSTAR Science Program is very broad, and I think that working in tandem with low-energy X-ray telescopes, for the first time, we’ve been able to provide a very broad look at the X-ray emission from many, many different types of objects, from black holes to neutron stars to giant structures called galaxy clusters. And putting all this information together in many different realms has been really transformative. It’s enabled astronomers to understand much better the distribution of energetic particles in these objects, or understand better what the magnetic fields in neutron stars are. And so, it really is a very broad science program.
Host: What are some of the key lessons learned during this mission that might be helpful to others?
Harrison: Yeah, so first off, I’ll start by saying Small Explorers are a challenge. You have a cost cap that you can’t go above. You have a very tight schedule, and this may sound somewhat obvious, but one of the things that I really put NuSTAR’s success, pin it to, is the fact that we had a very cohesive team. From our industry partner to the project manager to myself to the system engineer, we were just in constant communication. And there were times we disagreed for sure, but it was always necessary to keep in mind we’re doing something really hard, and we were always able to resolve it. And if it took getting everybody in the same room when we hadn’t planned it, we did that so that we could iron out any disagreements or any differences. And so, I would say the cohesion of the team for these small missions is really essential.
And the other thing is you have to be willing to make compromises. In the large strategic missions, it’s often necessary to say, ‘No. We signed up for this performance,’ and you take the time to do it right. With these small missions, you just have to be nimble, and you have to say, ‘OK. Maybe we can’t implement something the way we originally planned. Maybe there will be a small compromise, but we can make it up in some other way.’ And so, the constant trades happening sort of every day were really essential.
Host: Did that happen more in the early phases, say in the development phase, or did that continue once you had launched?
Harrison: That was mostly in the development phase. Once we launched, there were some nail-biting moments as we were trying to get the spacecraft working and commissioning the telescope and deploying that long mast that I told you about before. But since then, I have to say, NuSTAR has been very, very well behaved, and we really have had very few problems. And so, I guess I’ll just put that down to luck.
But our biggest challenge, by the way, is especially recently, space debris. We do get warnings that some satellite is headed towards NuSTAR, and we didn’t design it with any propulsion or any mechanism to sort of get out of the way, so all we can do is try to orient the telescope in a way that we minimize the chance that we get hit. And that’s more and more of a reality in low-Earth orbit.
Host: How quickly can the team react when you receive a warning?
Harrison: Well, usually we get notified days in advance, and then we can watch and see whether as they track the satellite or the debris, whatever it is, and make their predictions more accurately. Sometimes they say, ‘Oh, well, it’s not going to come that close after all.’ But if it does appear to be on a trajectory to come uncomfortably close, then we reorient in hours. We don’t have to do it very quickly, because we know when the encounter is going to happen well in advance.
Host: In addition to that challenge that you face periodically, have there been other challenges along the way?
Harrison: Yeah. As I said, the mission has really operated very seamlessly on orbit. And we haven’t had too many technical challenges. We have had challenges with the ground station. We rely on a ground station in Kenya that was provided by the Italian Space Agency. And occasionally, if that has a technical problem, then we have to scramble around and try to find other ways to get the data down. But they’re really pretty minor in the scheme of things, I would say.
Host: What’s next with NuSTAR?
Harrison: So, we just went through the Senior Review. So this is the process whereby every three years, NASA looks at all the operating missions that are beyond their prime mission phase, and evaluates their scientific productivity. And based on that, this is done by a panel, and the panel recommends whether the mission should be continued for another three years. And we just went through that process, and the panel recommended that NASA extend NuSTAR’s mission for another three years.
And what are we going to do in that three years? Well, the great part about it is, I can’t tell you. Why can’t I tell you? Well, because it’s the community’s telescope now. The community writes proposals every year. They submit ideas for what NuSTAR should do. And some of these ideas are very creative. They’re not things that I would’ve thought of, and then we carry them out. And so, I’m always excited every time we get the results from one of these reviews, proposal reviews, saying, ‘Here are the things that the Review Team thought were the most interesting, the most impactful.’ I look through them, and I pay attention to the ones that are just things that I wouldn’t have thought of. And that is a really fun part of the mission right now.
Host: Many thanks to Fiona for joining us on the podcast. You’ll find her bio, links to related resources, and a show transcript on our website at appel.nasa.gov/podcast.
We’d love to hear your suggestions for future guests or topics on the podcast. Please share your ideas with us on Twitter at NASA APPEL – that’s app-el – and use the hashtag Small Steps Giant Leaps.
As always, thanks for listening.