interviews by Summer Bowie
photography by Damien Maloney
Carl Sagan once said, “Somewhere, something incredible is waiting to be known.” From the moons of Jupiter to the icy Kuiper Belt where Pluto and other dwarf planets orbit to the event horizons of distant black holes and colliding galaxies light years away, space is the place where our understanding about ourselves and the universe collide in spectacular wonder.
Everything we know about where we are and when we are comes back to the invention of the lens. This single innovation in human technology has broadened our understanding of everything; from what we know about the contents of an atom to the farthest stretches of the universe. The Nimrud lens is largely regarded as the first known example of an optical lens. Used by the Assyrians in the 8th century BCE, it was made from rock crystal and was likely employed for magnifying and burning. The first wearable glasses were crafted by Italian monks in the late 12th century CE, allowing for those with compromised vision to continue reading. Finally, the invention of the telescope is attributed to German-Dutch spectacle-maker Hans Lipperhey who submitted his patent for a refracting telescope using a convex objective lens and a concave eyepiece lens to the States General of the Netherlands in 1608. His application was denied since there were other spectacle-makers who laid claim to the same invention, but news of his idea traveled far and wide, catching the attention of Galileo Galilei who constructed his own version and used it to make his most pioneering astronomical discoveries. Today, technologies like adaptive optics, pioneered at observatories such as Keck in Hawaii, correct for the blur of Earth’s atmosphere in real time, delivering space-like clarity from the ground.
As of October 2024, the California Institute of Technology is the institute with the highest number of Nobelists per capita in America. The private research university also manages and oversees NASA’s Jet Propulsion Laboratory (JPL), the research and development center responsible for designingand building spacecraft and robotics for the US government’s space agency. Rocket engineer and occultist Jack Parsons, one of the principal founders of the Caltech rocket group that evolved into JPL, helped lay its foundations. In order to gain a better understanding of where and when we are in the space-time continuum, we reached out to three scientists from Caltech who are on the frontlines of both observational and theoretical astronomy. We learn that everything we know about ourselves depends on where we are looking, when we are looking, and how our vision is mediated
MIKE BROWN
When planetary astronomer Mike Brown killed Pluto in 2006, he earned both esteem and vitriol. His discovery of Eris, along with a handful of other dwarf planets, challenged our entire understanding and definition of a ‘planet.’ Now classified as a dwarf planet, Pluto is recognized as one of several trans-Neptunian objects orbiting the farthest reaches of our solar system, in an area known as the Kuiper Belt. Numbering in the thousands, these objects hold clues that point to how the outermost planet in our solar system—Neptune—formed. While his discovery set in motion Pluto’s demotion, Brown believes that a hitherto unknown Planet Nine is out there, and he has spent the last two decades looking for it.
SUMMER BOWIE: Do you remember what inspired you to become an astronomer?
MIKE BROWN: I grew up in northern Alabama in the seventies, which was where the Saturn V rockets were being built at Marshall Space Flight Center and Redstone Army Missile Command. In the 1940s, over a thousand kidnapped Nazi rocket scientists were brought to Huntsville to engineer this rocket program. Growing up as the Apollo rockets were being built and as the astronauts were landing on the moon, I was always surprised that everybody didn’t want to do this. I would see these rockets going up, and I would just want to study the moon and understand what was out there.
BOWIE: You’ve essentially lived through and seen the entire history of humanity getting out into space.
BROWN: I haven’t thought of it that way, but it’s true. You would be playing outside, and suddenly the ground would shake, which is familiar to us in LA, but it was the Saturn V rockets that were strapped down and lit for testing.
BOWIE: Can you talk about the three criteria that are needed to meet planetary status?
BROWN: Yes, I can, but first let me explain why I hate them. The criteria were forced onto astronomy during the Pluto debate. Really, it was just an excuse to reclassify Pluto as not a planet—which was the right call. But the excuse took the form of this lawyerly checklist definition. That’s not how science usually works; we don’t have three-part rules for what counts as a star or a galaxy. The idea they were trying to capture is simple: planets are the gravitationally dominant bodies in a system. When you say “planet,” you’re thinking of a handful of big objects. Pluto clearly didn’t fit—it’s not gravitationally dominant, and plenty of similar objects aren’t either. So instead, we got this clunky three-part definition that I hate even repeating. And notice: I just explained it without actually saying the checklist. (laughs)
BOWIE: There was such an emotional reaction on the part of wider society, which is interesting because Pluto is not the only celestial object that has been considered a planet at one point and then later reclassified. There have been many...
BROWN: The sun is a good example. (laughs)
BOWIE: And Pluto wasn’t discovered until 1930, which is so recent. So, it didn’t even spend a full century as a planet, but we have grown very attached to it. Were there such reactions to other celestial bodies and their reclassifications?
BROWN: The first asteroid, Ceres, was discovered in 1801. Asteroids are small rocky bodies that orbit the Sun, mostly between Mars and Jupiter. When Ceres was found, people thought, ‘Oh, it’s a planet!’ At the time, Neptune hadn’t been discovered yet, so Ceres was considered the eighth planet. A few years later, another asteroid was found and called the ninth planet, then a tenth, and then an eleventh. These were the four brightest asteroids, which is why they were spotted first. But unlike the other planets, these “new planets” didn’t move in neat circles—they had tilted, overlapping orbits. By the mid-1800s, discoveries exploded. So many more asteroids were being found that, by around 1860, it was clear it made no sense to call them planets. That’s when they were reclassified as asteroids. Honestly, I doubt the public cared much either way—people weren’t really educated about what counted as a planet or how many there were. When Pluto got demoted, though, I understood why people reacted differently. We’d all learned about Pluto in third grade, memorizing the planets in order like the houses on your street. It was part of your celestial neighborhood. Then suddenly, scientists said, “That house at the edge of your block? It’s not actually part of the neighborhood.” So, of course, people felt emotional about it—that made sense to me.
This is an image of the dwarf planet Eris (center) and its satellite Dysomia (at 9’o'clock position) taken with NASA/ESA’s Hubble Space Telescope on Aug. 30, 2006. Hubble observations were obtained on Dec. 3, 2005 and Aug. 30, 2006 using the Advanced Camera for Surveys. The Hubble images were combined with images from the Keck telescopes taken on Aug. 20, 21, 30, and 31 to measure the satellite’s orbit and calculate a mass for Eris, which is the largest dwarf planet in the solar system. Credit: NASA, ESA, and M. Brown (California Institute of Technology)
BOWIE: You became famous for demoting Pluto to a dwarf planet and for discovering another dwarf planet called Eris. Is there a reason why Pluto was discovered so much earlier, even though it’s only slightly larger?
BROWN: It turns out to be just slightly bigger than Eris, which irritates me to this very day, but it is less massive by about a factor of 20%. The reason Pluto was discovered in 1930, and it took until the 2000s to find the rest of these dwarf planets, is dumb luck. Pluto is the closest of the large objects in this region of space. On average, it’s the closest. But these other large objects are on these very eccentric orbits. So, Eris was discovered while it was on the inner part of its eccentric orbit. When it’s way out on the other end, it’s faint, but when it comes closer, it’s brighter than Pluto. Eris takes about 500 years to go around the sun. If it had been on the part of its orbit that it will reach 250 years from now, it would’ve easily been found by Clyde Tombaugh, who was searching for and found Pluto. He would’ve found Eris, and he could have also found Makemake and the others. If he had found the other four back in 1940, everybody would’ve said, “Oh yeah, it’s just like the asteroids, and there must be many more of them.” But instead, there was just that one for sixty-two years, which led to a lot of confusion about what was going on out there.
Clyde Tombaugh, a 24-year-old student and the discoverer of the planet Pluto, looks over a Newtonian reflecting telescope he built in 1928. The mount for this telescope was built from part of the crankshaft from a 1910 Buick and discarded parts from a cream separator. It was with this telescope that Tombaugh made the observations responsible for a job offer from the Lowell Observatory in Flagstaff, Arizona. Bettmann and Getty
BOWIE: You talked about Eris being more massive than Pluto. How do we measure the mass of a planet?
BROWN: The easiest way to figure out the mass of something in space is if it has a moon. If the object is more massive, its moon will orbit faster. If it’s less massive, it’ll be going slower. So, we can use the distance and the speed of the orbit to figure out the mass. We were very excited when Eris was discovered, and we then found a moon for it, which enabled us to measure its mass.
BOWIE: When you discovered Eris and its moon Dysnomia, you were calling them Xena and Gabrielle, after characters from Xena: Warrior Princess. What was it about the show that made such an impact on you, and why were they renamed in the end?
BROWN: I just really liked that show. And, when we discovered what was to become Eris, we always used code names for the objects so that we could talk about them before they got real names. People had talked about Planet X for a long time. Something mythological seemed good, and I didn’t mind that it was TV mythology instead of Greek. And, I thought that there were not nearly enough planets named after female deities. Then, when we found a moon, there was no question of what the moon was going to be called.
BOWIE: How did they decide on the renaming process?
BROWN: Official names are approved by the International Astronomical Union. And when it was time for the official name, we didn’t propose Xena and Gabrielle. My wife was like, “You want to be known forever as the guy who named this thing after some campy 2000s TV show?” And I’m thinking, Yeah, maybe. But in the end, I thought, it deserves a Greek or Roman name, just like all the other things that were planets at the time. So, we did this search for good mythological names that had not already been used by asteroids, and there was almost nothing left. The only major god or goddess that had not been used was Eris, goddess of discord and strife. And then her daughter, Dysnomia, is the demon spirit of lawlessness. It’s named for my wife, who is not a spirit of lawlessness, but one of Pluto’s moons, Charon, was sort of named after the discoverer’s wife, Charlene, and my wife’s name is Diane. I think she likes that she has a moon named after her, but she doesn’t admit it.
BOWIE: It’s interesting to think about the way that astronomers and other people in the field feel about space movies. Do you watch them, and are there frustrating aspects of the way that space is portrayed?
BROWN: I am completely capable of suspending disbelief and just watching some dumb space opera. I’m not more or less interested in a movie because it takes place in space for the most part. I will read science fiction if it’s a good piece of literature. But I don’t read science fiction because I find it more appealing than real literature. I would much rather read a good modern novel than a bad, modern science fiction piece any day of the week.
BOWIE: What is the Kuiper Beltand what does it tell us about the history of our solar system?
BROWN: The Kuiper Belt is the icy leftover remnants from when the last planet formed. The last planet to form is probably Neptune. A planet might have been able to form farther out there, but as soon as Neptune formed, it kept on shaking everything around out there, which didn’t let any of the clumps get together and form a real planet. So, they’ve just been sitting out there for the past four and a half billion years, and they are, in some sense, the frozen remnants of the original solar system. They’re a dynamic fossil record. It’s as though at some time, in the early solar system, there was a gory murder and the blood was splashed all over the wall, and then somebody removed the body. But we get to see all those bloodstains and try to figure out what happened. So, maybe that’s what the Kuiper Belt is: the bloodstains on the wall of the outer solar system.
BOWIE: What can you tell us about Planet Nine?
BROWN: Well, I wish I could tell you where it was, but I can’t because I can’t find it yet. But I can tell you what we think we know about it. From the evidence that we see from the Kuiper Belt, we can map its gravitational influence by seeing where these objects are and what they look like. Based on that evidence, we think that it’s about seven times more massive than Earth. So, huge. It’s probably about twenty times farther away than Neptune, which is why we haven’t been able to track it down yet. And it probably takes about five or ten thousand years to go around the sun. I’m always convinced that we’re going to find it within two years. But I’ve been saying that every year for the past nine years, so you really shouldn’t believe me when I say that, but I think we really will this time.
BOWIE: Last year, you wrote a paper titled “Enabling Fast Response Mission to Near-Earth Objects, Interstellar Objects, and Long-Period Comets.” Can you briefly explain what this kind of mission would entail and what kinds of key technology gaps would need to be filled to launch a rapid response to a planetary threat?
BROWN: There are two ways to do it. A notable example came about six months ago when an asteroid was discovered, and at first there was a chance it might hit Earth. Since then, better observations have shown a 2% chance it could hit the moon—which is entertaining to think about, although the dust would cause a worldwide satellite catastrophe. So, if it were going to hit the moon or Earth, we’d want to do something about it. If impact were likely, it would happen around 2032, and we’re not prepared for something that soon. So, you have two options. One is to build a spacecraft, store it in a big warehouse, and wait. Then, when needed, you commandeer the biggest rocket you can find, launch, and deflect the asteroid. Deflection is usually just hitting it—not blowing it up, Bruce Willis–style, but nudging it hard enough to shift its orbit slightly. And that’s why you want to act early. The better option is this cool idea we came up with: a constellation of spacecraft always in orbit around the Sun—say, a dozen of them. Once a month, one would swing by Earth, using Earth’s gravity to redirect itself with very little fuel. That way, once a month you’d have the chance to send a spacecraft to check out or deflect any threatening object. Over time, we’d be building up a permanent capability—ready for anything. If we just wanted to take a picture, we could; if we needed to hit something, we could. It would be a sentinel fleet waiting for whatever comes, good or bad. For example, an object was recently discovered beyond Jupiter, coming from outside our solar system. It looks like a comet, and it will pass by the Sun—but at its closest, Earth will be on the opposite side, so we won’t get a good view. With this kind of fleet, though, we could send a probe to meet it.
BOWIE: Do we suspect this object to be the oldest comet we’ve observed since most come from within our solar system?
BROWN: It’s a comet from another planetary system, and so its age is whenever that planetary system was formed, which we don’t know, but we do have clues based on how fast it’s traveling. It’s coming in at a speed that’s fast in the same way that really old stars move much faster than younger stars. They’ve just had more time to move around through the galaxy. So, it may be older than our solar system. I haven’t thought of it that way, but it’s very possible.
BOWIE: How do you feel about the United States Space Force and SpaceX? Some of our newer centers of space research?
BROWN: I would call neither of them a center of space research. These are engineering projects, defense projects, or industrial projects. They have shown that they have zero interest in scientific research. I have mixed emotions about a lot of it. Of course, it’s hard not to have mixed emotions about SpaceX just for general reasons, but also as an astronomer, they are putting so many things up in the sky that we’re going to have a hard time doing astronomy in the future. And, the head of SpaceX says, “Oh, we’ll just put telescopes in space.” I’m like, Great. We have so many telescopes on the ground because we can do that relatively cheaply. Everything in space costs a billion dollars. How many billions of dollars do you want to give us to put all of our telescopes in space? The answer is zero. That said, I like the fact that my phone now has emergency satellite texting from the middle of nowhere, and I think that’s important. I go on long bike rides every weekend in the mountains, and I can text my wife and say, “I got a flat, but I’m coming home, so don’t worry about me.”
BOWIE: Right. There are some obvious practicalities, and then there are some extremely impractical propositions that they’re putting out there.
BROWN: People talk about going to asteroids and mining them and all this stuff, and I suspect that 99% of what people talk about will not come to fruition. If you make somebody a billionaire and they want something fun to do, space always comes up as a new playground. But it’s a lot harder than most billionaires realize. It’s easy to get billionaires to spend money and talk about building things like giant shields in space. Maybe some of this will happen, but I remain a skeptic until there’s some more progress than we’ve seen so far.
BOWIE: Have you ever had any desire to become an astronaut?
BROWN: As a kid, I mean, how could you not? Growing up, astronauts were living just down the street from me. I wanted to be an astronaut before I wanted to be an astronomer. And even as late as grad school, I thought about doing that application, because every couple of years, you can apply to be an astronaut. The woman I was dating at the time was like, “Great, please let me know if you’re planning on doing this, because I will move on. I’m not going to become an astronaut wife doing my knitting in Houston.” I thought that was valid. So, I never did. Once you’re married and you have kids, you think about things very differently.
BOWIE: It’s such a fascinating existence to be like any other human who’s got their feet firmly planted in the ground with a family and a life. But then, to also have your brain out in the farthest reaches of space, imagining all that exists.
BROWN: I like to think I spend my days exploring. I’m not idly theorizing about what’s out there. I’m actively looking to expand our understanding of the solar system’s neighborhood. I want to see that edge and understand what’s beyond it.
BOWIE: What would you say is your most controversial theory about the universe?
BROWN: The easy answer to that question is also the controversial one: Planet Nine. Some people do not believe it and are adamantly against it just because they don’t want to believe it. And some people are not convinced by the evidence, which I find overwhelmingly convincing. So, until the day it’s found, it will be controversial and hypothetical. It’s a little lonely out here. But I think that if we keep looking, we will eventually find this, and the decade of searching will have been worth it.
BOWIE: As a professor, what do you hope to impart upon the next generation of astronomers?
BROWN: The sense that what we do is exploration. Some people lose that, and they get into the weeds of: I do this and I do this and I do this. I’m exploring the edge of the solar system and trying to tell stories about how it got to be. We tell these stories to our colleagues and we tell them to the public. We are privileged to get to do that. It’s an amazing job.
KATHERINE DE KLEER
The Goldilocks principle explains why Earth has such a singular ability to host life as we know it. We are close enough to the Sun that all of our water doesn’t freeze solid, yet distant enough that it doesn’t boil away. We’re massive enough to hold onto our atmosphere, but not so massive that we are crushed by an impending gravitational force. Combine that with the fact that we have an ideal balance of nitrogen, oxygen, and carbon dioxide, and you have the base ingredients for a planet that allows us to breathe, protects us from solar radiation, and keeps us warm via its greenhouse effect. Knowing the why is one thing, but how our planet (and others) formed and how they evolve remains a mystery. Planetary astronomer and extraterrestrial geologist Katherine de Kleer studies solar system bodies—their surface atmospheres and typographies—to further our understanding of how planets work.
In 2019, de Kleer and Caltech cohorts Mike Brown and Samantha Trumbo discovered sodium chloride on Jupiter’s moon Europa, suggesting that its subsurface ocean may be chemically similar to our oceans here on Earth. Last year, she was awarded the Harold C. Urey Prize for her outstanding achievements in planetary science as an early-career scientist. De Kleer’s work has taken her from educating the incarcerated population of San Quentin to the next generation of Caltech’s finest, from observing the volcanic activity and auroral emissions of Jupiter’s moons to the thermal structure and composition of its Great Red Spot.
SUMMER BOWIE: Do you remember what first inspired you to become an astronomer?
KATHERINE DE KLEER: I always loved the stars as a kid. My dad had a telescope that he taught me how to use, and I ended up using it more than he did. It wasn’t computerized, so you had to know the constellations and ‘star hop’ to find what you were looking for. That puzzle-hunt aspect fascinated me. At the time, I didn’t consider that it could be a career. But over the years, my school interests converged with that passion, and I realized I could actually do this for a living.
BOWIE: What was your first major question about the universe?
DE KLEER: Early on, I was fascinated by how galaxies come to be, and why some galaxies have a spiral structure while others have an elliptical structure. Later, my focus shifted to planetary science, and one object in particular captured me—Io, a moon of Jupiter and the most volcanically active body in the solar system. The Keck telescopes in Hawaii have adaptive optics, which correct for Earth’s atmospheric turbulence in real time, giving sharper images from Earth. I pointed one of the Keck telescopes at Io to look at infrared wavelengths, where you see heat, and you can see these little spots of heat coming from each volcano on Io. Until then, I hadn’t realized you could get such detailed geological information about another world from Earth. That moment sealed it for me.
BOWIE: What is it about Io that challenges our understanding of geology beyond Earth?
DE KLEER: There aren’t many places in the solar system with active volcanism, and volcanic activity requires enough heat to melt rock. The early solar system was hotter, and most planets had volcanoes hundreds of millions or billions of years ago. Most have cooled by now, so Io is an anomaly. It’s tiny, yet has hundreds of active volcanoes and new eruptions every few weeks. We think we understand the tidal forces that power it, but it’s still an outlier.
BOWIE: Because Jupiter casts such a large shadow over its moons, you’ve said that Io’s auroral emissions are the only reason we know we’re pointing the telescope in the right direction. Can you describe what you’re seeing through the telescope and how you imagine it would look from Io’s surface?
DE KLEER: Moons appear bright in the sky because they reflect sunlight. But when they pass into Jupiter’s shadow, they become invisible at the wavelengths our eyes can see, except for their aurora, which emit light at specific wavelengths. Some of these colors match Earth’s aurora exactly, like the green and red from oxygen in the upper atmosphere. We’ve seen them on Europa, Ganymede, and Io.
BOWIE: Would they look similar to the aurora we see on Earth?
DE KLEER: That’s a good question. From here, they’re visible because all the light from the aurora is concentrated into a tiny point in our sky. On Io, it would be spread out over the sky, which might make it too dim for the human eye. Ganymede, one of Io's neighbors, has a magnetic field. This means that, like on Earth, the aurora only appears in particular ‘auroral ovals’ around the poles, although on Ganymede those ovals extend to much lower latitudes than on Earth. Because of this, the aurora are much more concentrated in particular locations on Ganymede than on the other moons, so that is probably your best chance of actually seeing the aurora from the surface of a moon.
BOWIE: Another one of Jupiter’s moons that you study is Europa. It has recently been discovered that there’s sodium chloride (table salt) in its subsurface oceans, which could mean life, or at least habitability. What sits above those oceans, and is it possible that life already exists there? Europa has an energy source from tidal heating, liquid water, and chemicals. So, it is absolutely possible that there is life in that ocean right now.
DE KLEER: Europa, like other icy ocean worlds, probably has a global ocean beneath a thick ice shell that’s anywhere from a few kilometers to 30 kilometers thick. Some scientists think that the salt we see on the surface came from the ocean beneath because it is concentrated where exchange through the ice is greatest. We haven’t sampled the ocean—that would require drilling through miles of ice—but if there is salt, then there are also other chemicals that have come from the rocks at the base of its ocean, which suggests many things. Anything from bacterial life to some kind of extraterrestrial whales could be there, in theory. Life needs three things: a heat source, water, and nutrients. Europa has an energy source from tidal heating, the same mechanism thatdrives Io’s volcanoes, liquid water, and chemicals. So, it is absolutely possible that there is life in that ocean right now.
BOWIE: You’ve also studied the atmosphere on Saturn’s largest moon, Titan, and believe that due to its dense atmosphere and methane-rich composition, it’s one of the most intriguing places in our solar system for astrobiology, and that life could possibly evolve there. Do we have any idea how that life might differ from life on Earth?
DE KLEER: Europa is a stronger candidate for life than Titan. Titan’s atmosphere has chemical processes we would consider prebiotic, but there’s no evidence of life. Europa’s water ocean is a much more favorable environment. Life as we know it benefits from liquids, especially water, and because it’s a polar molecule, it pulls apart other molecules and facilitates reactions. Titan’s liquids, like methane, are symmetric molecules and therefore less reactive. So, while Titan’s atmosphere contains precursors to amino acids, the environment is more challenging for life.
BOWIE: What made you specifically want to study the atmospheres of other planets and moons?
DE KLEER: I’ve always been interested in comparative planetology: for example, asking why Earth, Venus, and Mars turned out so differently. Why is Earth habitable, Mars nearly airless, and Venus smothered in atmosphere? What in their formation and histories led to such different outcomes? My focus shifted from planets to moons because the moons are also dynamic, unique worlds in their own right, as was shown by spacecraft missions like Voyager, Galileo, and Cassini. There are more moons than planets, and their atmospheres are like fingerprints of their internal processes and histories. Studying their atmospheres helps us understand how each became the way it is.
BOWIE: Looking at the scope of your work, what prompts you to set one question aside and begin exploring another?
DE KLEER: I get excited about new ideas. I might read something in a paper or hear it at a conference, and that plants a seed. Over time, I follow that topic more closely, and my focus naturally shifts. Another factor is that I’m an observational astronomer. Some people focus purely on computational work, but observational scientists get new opportunities roughly once a decade, whenever a major telescope is built. As soon as that happens, there’s all kinds of new things you can do: you can see fainter objects, in higher resolution, or different wavelengths, and that opens up entirely new projects.
BOWIE: Do you come back to earlier questions later, or are you juggling them all at once? Do you have teams working on different projects with you?
DE KLEER: Yes, now that I’m a professor. I’ve only been in that role for six years, but before that, I could manage one or two projects at a time. Now, I have about ten people in my research group. Some are working on asteroids, others on moons, others on planets. Mentoring students and postdocs allows me to be involved in multiple scientific areas at once.
BOWIE: You’ve also taught at San Quentin Rehabilitation Center [formerly San Quentin State Prison]. What drew you to that work?
DE KLEER: I think it’s important for scientists to communicate with the public, not just with students in universities. Much of our research is funded through the federal government, and everyone should have opportunities to learn science throughout their lives. I wanted to give a very different population access to science education from a practicing scientist, and the program at San Quentin was excellent.
BOWIE: Did their questions ever surprise you?
DE KLEER: Absolutely. It was a more engaged classroom than any college class I’ve taught. They wanted to be there and to interact—not just take notes for a grade. Their questions were rooted in their personal experiences. So, I’d introduce a concept, and they’d have examples from their daily lives, asking, “Is that why this happens when I do this?” It made me connect the physics I was teaching to contexts I hadn’t considered before.
BOWIE: There’s something interesting about how people approach work and learning when it feels novel and not at all compulsory. If the material is entirely new, the questions can flow freely. However, when your ego is tied to grades and the opinions of your peers, enthusiasm is easily diminished.
DE KLEER: Exactly. The class I taught there was supposed to be a lab class, but bringing equipment into a prison was almost impossible. Sometimes students had to bring equipment from their cells—one even brought a water heater, since I wasn’t allowed to bring in a boiler.
BOWIE: What do you hope to pass on to the next generation of astronomers?
DE KLEER: It’s something I think a lot about, not so much about the science I hope the next generation does, but how I want them to operate as scientists. One of my goals is to train the next generation of solar system observers using big radio telescopes like the Very Large Array—the one from the movie Contact (1997). That’s an uncommon specialty, and it’s important to preserve that expertise. However, there are a couple of attributes that I value as a scientist and hope to instill in the next generation. I value finding new ways to look at things and being ethical citizens of the scientific community. I try to lead by example in those areas.
BOWIE: Speaking of space films, do you enjoy watching them?
DE KLEER: I actually don’t watch many films. I read a lot, including science fiction, but I’ve never been much of a movie watcher.
BOWIE: I find it fascinating that you’re not particularly interested in seeing how others visualize the worlds that your work asks you to imagine. Do you prefer to create the whole image from text and calculations, rather than taking in someone else’s imagined world?
DE KLEER: That might be true. Both from the reading of my own data and from reading text, I prefer my own visualizations over a filmmaker’s.
BOWIE: Do you have a favorite science fiction author?
DE KLEER: There are many science fiction authors I love. A hard science fiction example would be Kim Stanley Robinson’s Mars trilogy: Red Mars, Green Mars, and Blue Mars. It’s a realistic attempt to portray human settlement and the terraforming of Mars, with rigorous science, plus explorations of human psychology and politics. It considers how Earth’s national politics might play out on another planet.
BOWIE: I watched your interview with Lex Fridman, and I agree with your take on colonizing Mars: that tourism might eventually be possible, but long-term settlement poses challenges that far surpass some of our most intractable issues here on Earth. If we could solve the problems involved in settling Mars, surely we could solve our problems here on Earth and make it habitable for more people.
DE KLEER: Yes. If we could solve the problems involved in settling Mars, surely we could solve our problems here on Earth and make it habitable for more people.
BOWIE: How do you feel about SpaceX and the rise of the private space industry?
DE KLEER: SpaceX and the private space industry have a lot of potential for collaboration with scientists and NASA. There’s already collaboration—for example, NASA instruments hitching rides on private launches. The private sector can move faster and more cheaply, though with higher risk tolerance. NASA missions are expensive and slow, but extremely high caliber, with rare failures. Ideally, we’d have a broad portfolio: both high-quality, long-term NASA missions and faster, riskier private missions. My hope is that national priorities don’t shift entirely toward human settlement-related science, but continue to support exploration for its own sake.
BOWIE: Have you ever wanted to leave Earth yourself?
DE KLEER: It hasn’t been a major ambition. If the opportunity arose to be an astronaut, I’d probably take it, but it hasn’t driven my career.
BOWIE: What do we ultimately learn about ourselves in the process of learning about the solar system and the universe we inhabit?
DE KLEER: At the heart of it, we gain context for ourselves; a better understanding of our place in the solar system and the universe. We’re one species on one particular planet, in one solar system, in one galaxy. We want to understand why we’re here; what’s special about Earth. We still haven’t answered how unique intelligent life is. Maybe it’s everywhere; maybe it’s rare. Understanding how Earth became the right place for us, and how it fits into the larger universe, affects people differently. Some feel small and insignificant in the vastness. Others feel special. It can be humbling or uplifting, depending on your perspective.
CAMERON HUMMELS
Similar to the way Mike Brown compares the Kuiper Belt to the blood spatters of a crime scene, computational astrophysicist Cameron Hummels describes galaxies as cannibals that pull smaller galaxies in with their gravitational force and then consume them. When this happens, massive amounts of matter are fed into the greater galaxy’s supermassive black hole at its center, which can lead to jets and outflows of excess gas back into space in what’s playfully referred to as a ‘galactic burp.’ Despite the macabre nature of all of these space metaphors, Hummels explicates the immortal nature of galaxies and the way that they move along the cosmic web. This web is made up of all the traditional matter that we know of in the universe. However, it consists primarily of dark matter, a mysterious ghostlike force in the universe that has perplexed scientists since it was first hypothesized by Swiss astronomer Fritz Zwicky in 1933. When Hummels is not observing and modeling the behavior of galaxies, he and his wife, Katherine de Kleer, are pushing the limits of what their bodies can endure in the wilds of planet Earth. In 2022, he set a world record with the fastest known time completing the Death Valley Traverse, but whether he will ever explore beyond Earth’s atmosphere is yet to be determined.
BOWIE: Do you remember what first inspired you to become an astronomer?
HUMMELS: Many astronomers have a similar entrée into the field, but when I was in first or second grade, my father took me to a local astronomy event in the parking lot of our elementary school, and I had the opportunity to see the rings of Saturn and Jupiter through a telescope. It was pretty sublime.
BOWIE: You work in the fields of galaxy evolution, cosmology, and computational astrophysics. Do many of your colleagues work at the boundary of theory and observation, or do most usually belong to one camp or the other?
HUMMELS: Most astronomers will be either observers or theorists. However, a lot of my work is in using computer simulations to model the behavior of different physical systems, like galaxies, so that we can predict how radiation would transfer through and get an idea of what it would look like through a telescope. That way, you can make a kind of apples-to-apples comparison between what the simulations produce and what we see in reality. The thing about observation alone is that it’s just the light signals that are traveling from that distant object to your telescope at any given moment, and what we’d like to know is how the object is moving, how it’s living its life, how it’s transforming. Computer simulations allow us to try to model the behavior and see how it might be changing according to the laws of physics.
BOWIE: So, it’s a back-and-forth conversation where you make the simulations, then you observe the object. And from there, do you modify aspects of the simulation?
HUMMELS: That is usually how it goes. Sometimes over weeks, sometimes over decades. You’re iterating on the process to improve the ultimate interpretation of what’s going on. It’s a tit-for-tat kind of thing. For instance, with a galaxy, we might make the insightful deduction that it isn’t a single galaxy but a merger of two galaxies that we are just witnessing at this moment. Maybe that extra clump on the side that we thought was a star cluster is actually a totally different galaxy that’s merged with this system. Once we know that, we can go back and hone the observations around that region to better understand it, and then go back to the model.
BOWIE: About a hundred years ago, Edwin Hubble discovered that there are galaxies outside of our own, and Mike Brown is still looking for Planet Nine. Can you explain how discoveries at that distance were made with comparatively primitive technology?
HUMMELS: The primary thing is the brightness of the objects, as opposed to the distance to them. If Planet Nine exists—and I think Mike, Konstantin [Batygin] and others have shown pretty strong evidence that something out there does exist at the magnitude and at the size that they’re suggesting—its distance to us is minuscule compared to the distance of galaxies and other structures that we’ve certainly been observing for hundreds of years. The stars, of course, are light-years away, but they’re powered by a nuclear fusion process in their core, and that causes them to illuminate, and thus we can see them from great distances.
BOWIE: Do galaxies have an average lifespan?
HUMMELS: That’s a difficult question to answer because galaxies are born at different periods of time, but they don’t really die.
BOWIE: Okay, so they have a birth, but they don’t have a death. But then, what happens to all the matter getting sucked into black holes? Is it all just replaced by more matter within the galaxy?
HUMMELS: Well, not that much stuff from a galaxy is going into that supermassive black hole at its center. The Milky Way, for example, is about a million times more massive than the black hole at its center. Stuff does get pulled in, but there’s no risk of it swallowing something that is that much larger than itself. You have to get quite close and travel at a reasonably slow speed to fall in. Astronomers sometimes describe galaxies themselves as ‘cannibals,’ because as they grow more massive, their gravitational reach expands, and a large galaxy will gradually pull in and merge with smaller neighbors. We refer to this as hierarchical formation. Our Milky Way formed this way, and in about eight billion years, it is predicted to merge with our nearest galactic neighbor, the Andromeda Galaxy. This will probably only happen a couple more times for our galaxy, because the universe is expanding. As it expands, galaxies are getting farther away from each other, and mergers occur less frequently. But to answer your question about a galaxy’s lifespan: Do those smaller galaxies die when they merge? Not really, you just can’t tell them apart from the Milky Way anymore. Similarly, when we humans die, we will become part of the Earth, and then we’ll grow into other things in the future. There isn’t really a death, per se; it’s just a ... reincarnation.
BOWIE: So, as the universe expands, our galaxy and others start moving out along the cosmic web. Can you explain what that is exactly? The cosmic web describes the distribution of matter within the universe ... galaxies appear to be arranged like pearls on a necklace in long filaments. These filaments connect to form a larger three-dimensional web, much like a spiderweb on acid.
HUMMELS: The cosmic web describes the distribution of matter within the universe. What we see is that galaxies are not randomly placed, but they appear to be arranged like pearls on a necklace in long filaments. These filaments connect to form a larger web, much like a spider web, but one in three dimensions. So, it’s like a spiderweb on acid. Scientists believe that these galaxies are the visible counterparts to the much larger amount of mass that we cannot see, the dark matter that composes most of this cosmic web.
BOWIE: And what exactly is dark matter?
HUMMELS: Dark matter is a hypothetical type of matter, which cannot be seen or touched, but it interacts with other matter through its gravitational effects. So, we can only detect it through its indirect effects on matter we cansee, like stars or gas. Incidentally, it appears to make up the bulk of the mass of the universe, much more than the “normal” matter we are familiar with, like electrons, protons, atoms. We scientists hate having to invoke some unseen ghostly matter that we cannot directly observe. Scientists have gone through every which way to avoid invoking dark matter, but there’s been growing evidence over the last seventy yearsthat points toward its existence, based on the shape and dynamics of the structures we can see. For example, spiral galaxies, as their shape suggests, are rotating. If they spin too fast, they’ll fling themselves apart. The force that holds them together is gravity, and it’s directly related to the total mass of the galaxy. Therefore, we can measure how much mass is in a rotating galaxy, based purely on its rotation speed to estimate the gravity holding it together. When we do this, these mass estimates appear to be much, much larger than the amount of “normal” matter we can see to be present in a galaxy in the form of shining stars and glowing gas. The inferred mass for these galaxies is ten, one hundred, or one thousand times larger than the “normal” visible mass. So, we say these galaxies have a lot of dark matter, since we don’t see the mass directly, but we know it’s there, holding the galaxies together. But it is concerning that we don’t have a way of seeing dark matter directly, and a big part of modern physics is trying to do just that.
BOWIE: What does it mean for a galaxy to be dark-matter-deficient?
HUMMELS: Basically, every galaxy that we observe in the universe has substantially more mass than the stuff that we can see, but there are a handful of systems that have been observed in the last five years, which look like they don’t have any dark matter in them. This is prompting scientists to question what makes these systems so distinct from the billions of other systems we’ve observed. And it may hold the key to understanding what the heck dark matter is and how it is formed.
BOWIE: You worked on a project that led to a paper called “Galaxies Lacking Dark Matter Produced by Close Encounters in a Cosmological Situation.” Can you talk about the seven dark-matter-free galaxies that were found and how they were named?
HUMMELS: A colleague of mine, Jorge [Moreno], wrote this paper. It was an important paper because about two years prior, there had been the first observations of what looked like these dark-matter-free or dark-matter-deficient galaxies. But we didn’t yet have any theoretical models or computational models that could back it up. And so, we took a state-of-the-art astrophysical simulation, which follows thousands of galaxies co-evolving from just after the Big Bang to the present day, and in that environment, we searchedfor systems that might not have a lot of dark matter. Out of the thousands of galaxies in the simulation, we found seven systems that appeared to have little to no dark matter. In each example, it was a small galaxy orbiting around a more massive galaxy, like our Milky Way. These systems came in and made a really close orbital passage to that massive system, which essentially stripped away a lot of their dark matter and absorbed it into the primary system. Seven different systems were identified in this particular simulation, and Jorge chose to name them after the seven Cherokee clans: Bird, Blue, Deer, Long Hair, Paint, Wild Potato, and Wolf. It was important for him to give them Native American representation because people tend to name things solely after concepts from Western civilization.
BOWIE: Can you talk about some of the outreach work you do?
HUMMELS: I’m super passionate about getting people to incorporate science into their lives. At its base, science is a mindset of skepticism and experimentation, and I think that the world would be a much better place if people were intrinsically skeptical of the claims that other people make. I love to lecture in universities, but I also try to reach a broader demographic of people by organizing events in non-traditional places. So, we have something called Astronomy On Tap that takes place at a bar here in Pasadena, where we have public-level science presentations, astronomy-themed pub trivia, we set up telescopes, and there’s live music. We’re doing one in Sequoia next month. We do one in Death Valley in February every year, and also visit the Grand Canyon, Bryce Canyon, and others. Scientists don’t get paid a lot of money to do this. We do this because it benefits all of us to push back the veil of uncertainty about nature in all of its different forms. I also work with a number of national parks and set up Dark Sky festivals. It’s crucial that we, as scientists, engage with the public and rebuild trust in science. Biology, sociology, physics, and chemistry. I think it's incredibly promising that so many people have gone down this path to try and help humanity better understand the world in which we live.
BOWIE: Speaking of Death Valley. What is the Death Valley Traverse, and what motivated you to do it?
HUMMELS: There is a loosely-defined route that goes from the very northern tip of Death Valley to the southern tip; right down the middle of the valley. It doesn’t have a proper trail or anything, and it’s about 170 miles. There’s a contest associated with it, where you’re supposed to cover its distance totally unsupported—no assistance from other people, no food caches, no nothing. You just pretend like you’re the only person on the planet and get from one end to the other under your own power. It’s called the Death Valley Traverse, and I backpacked it a few years ago. I love Death Valley because of all the extremes associated with it—it being one of the hottest, driest, and lowest elevation places in the world. So, I’ve been going out there for a while, and a few years ago, someone set a record for the fastest known time on the Death Valley Traverse. It took him a week, and when I read about his accomplishment, I felt like it could be done much more quickly. As I investigated, I realized he had carried all his water for the entire journey on his back, which is reasonable, but also crazy. You don’t want to carry eighty pounds of water on a long backpacking trip! I figured I could go much faster if I kept a light pack and only drank from wild water sources along the route. So, I did a bunch of research using satellite maps, and speaking with the park hydrologist to find all the potential water sources on the way, I identified four small water seeps where I could resupply my water along the route, each roughly forty miles apart, and I relied upon those. The hike itself was a crazy experience that included unrelenting heat, a sand storm, cramps, exhaustion, hallucinations, and despair, but in the end I finished in a little under four days and set the world record. There was more misery than fun in the moment, but it’s fun to talk about afterwards.
BOWIE: I mean, you were drinking arsenic-laden water after that first day.
HUMMELS: Whenever you drink from wild water sources, you want to avoid getting sick. Before I began, I sent small samples from each Death Valley spring to a water testing service to ensure the water was safe. They identified a substantial amount of arsenic, uranium, and sulfur dioxide in them. With the arsenic, the water that I was going to be drinking was about three times higher than the FDA’s maximum level for potable water sources. So, I called someone at the poison control line, who unsurprisingly advised me not to drink it. But then, I spoke to a doctor who said it would probably be okay to drink for my short time in Death Valley, and I ended up doing just that.
BOWIE: How did Katherine feel about all this?
HUMMELS: She probably thought it was an unnecessary risk, but she’s also taken some unnecessary risks with hiking and backpacking herself. I don’t think all partners would be quite so patient with such a frivolous, risky adventure, but she was supportive.
BOWIE: In this situation, you’re made to feel like you’re the only person on Earth, and in the past, you’ve applied a couple of times to be an astronaut. Do you still have aspirations to be an astronaut?
HUMMELS: Absolutely. Kat and I both applied this last period to become NASA astronauts. But sadly, I think I have aged out. I’ve applied four times since 2012, and got through the first round of cuts before, but there are several rounds from there. I suspect I came at a bad time in history in terms of crewed spaceflight. The shuttle program ended the first year I applied, reducing the need for astronauts, but now with the rise of commercial spaceflight, the number of astronauts is likely going to go back up. I certainly don’t have the kind of money that you need in order to do space tourism, but if there were an opportunity, I would absolutely go. Who doesn’t want to go to space?
