It’s a sweltering Tuesday in Washington, D.C., the kind of day that stretches the definition of Earth as a “habitable” planet. But on an eighth-floor terrace near the U.S. Capitol building, dozens of people are outside anyway, talking and watching as passersby dart between pools of shade on the sticky streets below. Besides the heat, the onlookers are sweating something else, too—an audacious project to learn, at last, whether we have any neighbors living on Earth-like planets in our neck of the Milky Way.
Back inside, a boisterous reception is underway. Hundreds of scientists and engineers, NASA leaders, lobbyists, congressional staffers and Senator Mark Kelly of Arizona are also discussing the search for ET.
Surveilling alien worlds for signs of life is a lot more than a twinkle in this community’s eye. It’s the reason why everyone is here for a four-day science conference in late July, at the height of D.C.’s arguably habitable oppressive summer heat. They are prepping for NASA’s next flagship space telescope, a multibillion-dollar machine that could launch as early as the mid-late 2030s and reveal whether Earths are as universal as sand on a beach or if our watery world is instead a lonely island in a quiet sea. Called the Habitable Worlds Observatory, the telescope’s core mission is to search for signs of alien biospheres on Earth-size planets orbiting sunlike stars—worlds, at least in theory, that could be twins of our own.
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This search demands something conceptually simple but breathtakingly difficult: we need to actually see those faraway planets. To do that, the telescope must separate each world’s almost impossibly faint gleam from the overpowering glare of its star.
“HabWorlds is not out to answer your typical science question,” said astrophysicist Chris Stark of NASA’s Goddard Space Flight Center (GSFC), using what’s become the preferred nickname for the mission, during a presentation the day before. “It is arguably attempting to answer the single most profound unanswered question in the history of science and in the history of humankind.”
As Stark spoke, “Are we alone?”—in huge letters—loomed over the audience from one of his slides, almost daring anyone to deny the question’s importance. Absent any answer, it’s entirely reasonable to think that our planet is, and that we are, either as common as dirt or astronomically implausible enough to represent life’s sole spark throughout the entire observable universe.
The truth may well lie between those immense extremes, and HabWorlds’ planned search for life in multiple star systems could be our best way to find out—if the observatory ever gets off the ground. As hard as the problem of finding ET may be, overcoming the many terrestrial obstacles on HabWorlds’ path to the launchpad may yet prove harder. Some of those obstacles are technical, such as choosing the right targets to investigate and developing innovative but affordable instruments to get the job done.
Others are more grossly mundane and political. During the meeting and in the weeks afterward, power brokers in the White House and Congress were hashing out budget plans that could devastate federally funded U.S. science. For NASA, those plans would lower the curtain on dozens of its iconic space missions and make it very difficult (if not impossible) to launch new ones. But space science is just one small stitch in the fabric of life for most Americans, and its potential ruin pales against this country’s rising tide of socioeconomic disruption, civil unrest and political violence. So for HabWorlds, another, more pressing question looms: Can a nation so busy ripping itself apart come together to achieve any big, bold aspiration—especially one as lofty as solving life’s cosmic mystery?
To the conference-goers, this moment was all the more reason to spend a few enlightened, unflinching days immersed in the challenges and opportunities of their chosen task. “We needed to be reminded that we have the power to create a world we want to see,” said Janice Lee, an astronomer at the Space Telescope Science Institute and one of the meeting organizers, to Scientific American afterward.
Astrobiology’s Apollo Moment
In the late 1990s, Shawn Domagal-Goldman was home in Chicago, on break from college. Normally, city lights—and stray moonlight—made it difficult to see the twinkling tapestry overhead. This night was different because a total lunar eclipse brought deeper darkness to the sky. With the moon in Earth’s shadow, Domagal-Goldman and his younger brother found themselves on their front lawn, gazing up at the stars.
“Do you think there’s anyone out there?” his brother asked. “I remember freezing for a solid couple of minutes,” Domagal-Goldman recalls. “And I was like, ‘I don’t know, man. That’s a really tough question, but it seems like one worth thinking about.’”
Fast-forward more than a quarter-century, and Domagal-Goldman is now an astrobiologist in charge of NASA’s entire astrophysics division, still transfixed by the abiding mystery of our apparent solitude. It’s his job to manage and oversee NASA’s astrophysics fleet, including such flagship missions as the venerable (and beloved) Hubble Space Telescope, the James Webb Space Telescope (JWST) and the upcoming Nancy Grace Roman Space Telescope. Each of these projects has a special place in his heart, but the Habitable Worlds Observatory gets him particularly excited.
“I cannot hide my passion for this mission,” he admits. “In any field, there aren’t many opportunities where you get to be a part of something that can literally change the world—and that’s not hyperbole. For many people who work on this, including myself, we view this as an Apollo-like moment. I think about those pictures of Earth from the moon and what it meant to see ourselves that way, and I think this is a similar chance to change our view.”
NASA’s existing great observatories—like Hubble, JWST and their siblings—have helped us to read our cosmic origin story, revealing how the big bang’s blaze of energy infused the newborn universe with the physics needed to forge atoms, stars, galaxies, planets and eventually life. The Habitable Worlds Observatory could tell us how often that story unfolds beyond Earth.
“My expectation is that there is life out there, and the question is: How frequent is it? It could be very rare. It could be very common,” says Evgenya Shkolnik of Arizona State University, an astrophysicist and co-chair of HabWorlds’ Community Science and Instrument Team. “I have no idea. But one thing in astronomy we’ve never done is discover just one of something.”
In its search for extraterrestrial life, HabWorlds will target exo-Earths—rocky worlds in temperate orbits around sunlike stars—within about 100 light-years of the solar system. One of the mission’s primary tasks will be to find a few dozen of those worlds that are ripe for in-depth study. But sunlike stars can be 10 billion times brighter than any orbiting exo-Earth would be. To find those dusky worlds, the telescope will employ a starlight-blocking instrument called a coronagraph; once the star is masked, the light from an orbiting Earth-size planet can be seen.
The observatory will then study the composition of the planet’s atmosphere in infrared and ultraviolet light. As the telescope dissects those almost inscrutably faint planetary gleams, it will look for the spectral signatures of molecules that, here on Earth, are intimately associated with life. Short of receiving an interstellar radio transmission, finding wriggling Martian microbes or Europan cephalopods or watching a starship touch down next to the U.S. Capitol, a HabWorlds detection of gases such as oxygen, ozone and methane in the skies of a warm, wet, rocky planet would be the best evidence for alien life we could get.
Biology’s atmospheric fingerprints are of course still somewhat of an open question on alien worlds, but the data harvested by HabWorlds should at least give scientists something to debate. “We know what we’re looking for with respect to the signatures of life that we can already identify on a living planet, such as our own,” Shkolnik says.
But that’s not all HabWorlds will be doing. As it sniffs for whiffs of alien biospheres, the observatory will also perform transformative studies of the distant universe and of objects in our own solar system. Its uniquely powerful optics will help astronomers learn more about dark matter and dark energy, the evolution of galaxies, the lives and deaths of stars, our sun’s rich retinue of planets and moons, and even potentially Earth-threatening comets and asteroids. Such capabilities are crucial for getting broader buy-in from the entire astronomical community, not all of whom see a search for aliens as their guiding star. (Technically, much of that buy-in already occurred years ago, when a once-every-10-year Decadal Survey of the U.S. space science community anointed HabWorlds as the nation’s highest new priority in astrophysics.)
“This is the strongest science case of any flagship I’ve ever been a part of, whether I’ve helped build it, fix it or review it,” says Lee Feinberg, HabWorlds’ principal architect and an engineer at GSFC. “That gets me really excited.”
The Need for Speed
In March 1930, almost immediately after the stock market crash that rang in the Great Depression, crews began building a hulking skyscraper on Manhattan, N.Y.’s Fifth Avenue. Each week, those workers added more than four floors to the rising structure of glass, concrete and steel until it soared to be superlative: this was the world’s tallest building, the first to top 100 stories.
On May 1, 1931, U.S. president Herbert Hoover turned on the skyscraper’s lights.
A mere 21 months had elapsed since architects drew up the first plans for what’s now known as the Empire State Building—an art deco edifice that dramatically changed New York City’s skyline and, in many ways, Americans’ conceptions about what we as a nation could achieve.
For Feinberg, a veteran of multiple flagship space telescopes, that story is less of a parable and more of a blueprint. It demonstrates, he says, that with careful planning and concerted action, we can achieve great things in almost no time at all—great things such as building the first space telescope to find alien biospheres.
“If we could match their level of planning and thought, I think this could be done a lot faster and for a lot less money. But we also need to solve hard technical problems,” Feinberg says. “That’s what we’re trying to do with HabWorlds—maybe not in a year, although that would be amazing.”
Impatience is not the main reason to move quickly; the best reason to go fast is that speed can save money. (Delays during the development of JWST cost NASA more than $1 million a day.)
“It’s all about doing it less expensively, and the way you do that is to do it fast,” Feinberg says.
One of the secrets to speed, Feinberg and his colleagues argue, can be found in the Hubble Space Telescope. Launched in 1990, Hubble was designed to be serviceable, with instruments that could be swapped out and tinkered with by human hands. Between 1993 and 2009, astronauts visited Hubble five times to repair, upgrade or replace components, each time greatly improving the telescope and extending its life. Had Hubble’s planners instead waited for technology to catch up with all their aspirations, the observatory might never have launched in the first place.
“When we sent up Hubble, it was not a great observatory. It was an okay observatory,” says John Grunsfeld, a veteran astronaut, former NASA science chief and self-proclaimed Hubble hugger, who flew on three of those servicing missions. Grunsfeld is now working to ensure that the observatory can also be serviced—albeit robotically because the telescope will operate from the same location as JWST, a blissfully dark deep-space spot 1.5 million kilometers away called the Earth-sun Lagrange Point 2, or L2.
“As much as I love human spaceflight, Earth-sun L2 is not a fun place to go. Orbiting the earth is amazing. Going to Mars would be amazing. Going to the moon would be cool. Going to L2 is like two months of travel to spend a few days at a telescope and two months back in a really hazardous place,” Grunsfeld says. “I think we can build a telescope that will be so easy to service it’ll be trivial for a robot to open a door, pull an instrument out, put a new instrument in and close the door. We just have to design it that way.”
Astronauts John M. Grunsfeld (right) and Richard M. Linnehan (left) service the Hubble Space Telescope in low-Earth orbit on March 8, 2002. The possibility of robotic servicing could significantly boost the science and accelerate the development and launch of NASA’s Habitable Worlds Observatory, mission planners say.
Making HabWorlds robotically serviceable means the telescope can launch even if all the instruments aren’t ready. As it stands, some of the concepts Feinberg and his colleagues are considering even include an empty instrument bay for future filling-in. And the instruments that launch with HabWorlds don’t need to be the fanciest, most sophisticated things we can build; they only need to be good enough until we replace them with more capable, next-generation hardware. In many ways, making the observatory serviceable could pay for itself. But if the telescope’s instruments can change, this makes what must stay fixed—namely, the size of HabWorlds’ mirror, or aperture—all the more important.
“Let’s focus on aperture, and if we have to compromise on the instruments in the first round, that might be the smart way to go,” says Matt Mountain, an astronomer who has helped to plan multiple large telescopes and is the current president of the Association of Universities for Research in Astronomy.
The Mirror of Our Dreams
Beyond the possibility of answering existential questions about extraterrestrial life, all the scientific excitement really boils down to one big thing: HabWorlds is likely to boast the largest, most stable starlight-gathering mirror ever sent into space. But size doesn’t always play nicely with other considerations, including cost.
More aperture equates to more light, more resolution, more planets and more science. A bigger mirror can also mean fewer tough engineering challenges down the road. “There’s an obvious trade that exists between how many technologies you want to advance [to support high-contrast coronagraphy] and how large your aperture is,” Stark said in his June presentation. With a larger mirror, “you only have to advance a few of the difficult ones.”
And in theory, it’s easy to make a mirror very big. You can break it into segments, as engineers do with ground-based telescopes, which then can dynamically adjust to function like a single, solid reflective surface. That’s what Feinberg and his colleagues did with JWST, and they’re eyeing a similar design for HabWorlds. But with space telescopes, the crucial question is: Just how big of a mirror can we reasonably launch?
“We are ultimately limited by our launch vehicles,” says Breann Sitarski, an optical engineer at GSFC and deputy principal architect of HabWorlds. Right now that means teams are looking at rockets with payload fairings between seven and nine meters wide—SpaceX’s Starship, Blue Origin’s New Glenn and NASA’s Space Launch System.
Given those constraints, one option is to design a segmented mirror to fit as-is within the fairings; another is to make a mirror that, like JWST’s, folds up for launch and then unfurls in space. Teams are now studying two designs for HabWorlds. One includes a mirror that would be at least 6.5 meters in diameter, and the other uses a foldable mirror that would be at least eight meters across.
“You don’t want too many potential points of failure, but we do have heritage from JWST on folding mirrors, so that’s one of the reasons we’re looking at it,” Sitarski says. “The launch vehicle size is one thing, but it’s also the complexity of the deployment.”
Additionally, for HabWorlds to image exo-Earths, the telescope must be ultrastable. Regardless of the mirror’s size, to sweep away unwanted starlight, each of its segments must precisely align to within picometers of one another—that is, within about one trillionth of a meter. This level of precision is the equivalent of measuring the distance between the Earth and moon to within the length of a grain of rice. Adding to the difficulty, each second the system will be making minuscule adjustments to all the associated parts to maintain clear, crisp imaging. As far-fetched as this requirement may seem, the members of the HabWorlds team are confident that they’ll meet it. “We’re on the verge of being there,” Sitarski says.
Ultimately, size is not something that scientists are keen to compromise on. Many are pushing for the larger option of an eight-meter-class mirror. Such a big, starlight-catching beast would make it easier to characterize the atmospheres of Earth-size worlds and, in particular, to search for telltale molecules in the near-infrared. In some ways, a larger mirror means the difference between finding potential biosignatures and simply finding life—having more light and more resolution makes ruling out potential “false positives” much easier. And we are learning that studying Earth-size worlds is hard to do with anything smaller. Even the mighty JWST is having trouble meeting that challenge as it scrutinizes rocky planets orbiting smaller, dimmer red dwarf stars. Compared with Earth-sun analogues, such systems are easier to observe—but also so utterly alien that they’re very difficult to interpret and understand.
“Nothing would be more depressing than only finding one Earth-like planet where you’re not 100 percent sure whether it has life and you don’t have the sensitivity to do any more,” Mountain says.
Avoiding that frustrating result is one argument for building a bigger mirror. “The other big argument for aperture is: What if you find nothing? What does it tell you about the abundance of life?” Feinberg says. “If you don’t have a big enough aperture, you don’t have the confidence to say much statistically about how rare life is—whereas if you have a bigger telescope, and you survey enough stars and Earth-like planets yet don’t find life, you can statistically say that life is pretty darn rare.”
Bigger mirrors are more expensive, however, and in NASA’s politically charged project planning, even marginal budgetary fluctuations can mean the difference between enduring support and abrupt cancellation. Thomas Zurbuchen, NASA’s former chief of science, was an early supporter of HWO and initiated its first technical study weeks after the release of JWST’s first images. He urges teams to be pragmatic, to defend against the allure of bigger-is-better—a mistake, he says, that almost killed JWST. “Do a mission you can be proud of, but don’t prematurely fall in love with the wrong one,” he says. “I’d rather have a HabWorlds that looks at 15 exoplanets than one that looks at zero because I couldn’t afford it.”
There’s more behind such caution than just the lessons of JWST. NASA has pursued a HabWorlds-style mission before—a 2000s-era effort called the Terrestrial Planet Finder (TPF) that was ultimately abandoned after astronomers, policymakers and agency officials failed to win broader support from the astrophysics community and couldn’t entirely agree on a preferred design, let alone a realistic price tag. Although now just a historical footnote, to those who remember, TPF remains a cautionary tale, a warning of how a project’s internecine fighting can cause the entire effort to falter and fail.
An Uncertain Future
The challenges for HabWorlds, at least right now, seem to be more political and less technological.
No shortage of dire portents exist to make the sunny optimism of this summer’s conference seem like little more than futile coping. The fate of federally funded U.S. science has yet to be decided, but if the Trump administration gets its way, the national research enterprise will be transformed into a flimsy husk of its former self, scarcely capable of supporting HabWorlds and other visionary projects. No science agency, not even NASA, will be spared; research at universities will sputter and decline; scientists and students will look for opportunities abroad. Macroeconomic effects from other policy decisions—such as runaway deficit spending and the White House’s tariff-fueled trade wars—could further accelerate and amplify the downward spiral.
“I’m not worried about the science [of HabWorlds]; I’m not worried about the experiments we design,” Shkolnik says. “I’m worried about losing people. If we lose the people with expertise and experience, then we will just be relearning the same lessons. We’ll figure out how to do it, but it won’t be the fastest, most efficient way—and it won’t be the cheapest.”
Congress, which carries the constitutionally granted power of the purse, has signaled its intent to avoid this bleak future—to fight, in fact, for the putatively pro-science goals the president himself occasionally proclaims and celebrates in executive actions, public speeches and social media posts. The ruinous treatment of federal science, many researchers and policymakers say, arises not from Trump but from one of his most disruptively ideological appointees: Russell Vought, the ultraconservative bureaucrat in charge of the White House’s Office of Management and Budget (OMB). In their proposed science appropriations bills, both congressional chambers largely rejected Vought’s machinations, although the OMB is pursuing ways to evade congressional oversight and set U.S. science on a collision course with mediocrity.
In truth, just as the scientific process can’t prove a negative statement, it’s hard to argue that a world without HabWorlds would be materially worse off. The same could have been said in the 1930s of suggestions to construct the Empire State Building or in the 1960s of proposals to send U.S. astronauts to the moon.
But history tells us, time and time again, how even seemingly small acts can have outsize effects that profoundly shape reality and our place within it. If nothing else, the Apollo lunar missions gave Americans a reason to look up again in unison, as enormous sociopolitical unrest wracked the U.S. The Empire State Building’s spire became a towering beacon of progress on New York City’s skyline, even as almost unbearable economic hardships unfolded at its base. Such grand projects expand the horizons of our hopes and dreams.
Maybe in the fullness of time, HabWorlds and its quest for exo-Earths will be the same—bright points of light shining against a very dark background, illuminating and forever changing our sense of where—and what—we really are.
“I think it’s that kind of enthusiasm which allows us to carry these projects through all of the ups and downs that we’re going to face between now and actually getting this launched,” Mountain says, “because it’s going to be a journey.”
