Ask astronomers what question they most want to answer, and you will get scattered responses: How did the first stars, galaxies and black holes form? What is the nature of dark matter and dark energy? Are we alone?
Each question demands its own large telescope: no ultimate, one-size-fits-all instrument will ever exist, for none can be made to gather each and every kind of cosmic light. Black holes sometimes shine in X-rays, for instance, whereas Earth-like exoplanets are best studied in optical and infrared light. Yet such projects so strain the fraction of public and private funds allocated to astronomy that only a few — perhaps just one — can be prioritized at a time, leading to pileups of also-ran proposals and anxious researchers awaiting a rare chance to open new windows on the universe.
In the U.S., astronomers have managed these competing ambitions by devising a process that has become the envy of the scientific world: the Astronomy and Astrophysics Decadal Survey, a once-in-10-years exercise that recommends and ranks the community's priorities for the next decade — embodied, eventually, by major new federally-sponsored observatories on the ground and in space. Projects such as NASA's Hubble Space Telescope owe their existence, in part, to coveted endorsements from Decadals of yore, and the practice has spread to several other disciplines that now undertake Decadal Surveys of their own.
Organized by the National Academies of Sciences, Engineering, and Medicine, six Decadal Surveys have set the course of U.S. astronomy since they began in the 1960s. The results of the seventh, dubbed Astro2020, will soon be announced after two years of exhaustive deliberations led by a 20-member steering committee. And just like its predecessors, Astro2020 will reveal where major new investments and discoveries are most likely to be made — and where neglect, disinterest or even fear may block progress for generations to come.
Few people know the power of these surveys better than Joel Parriott. More than 20 years ago, he got his start in federal politics as a staffer at the National Academies, where his initial assignment was to assist the scientists crafting the first astronomy Decadal of the new millennium. Then he served a 10-year stint in the White House Office of Management and Budget (OMB), where he weighed and implemented Decadal recommendations for federal science agencies. Today he is director of public policy at the American Astronomical Society, the leading advocacy group for U.S. stargazers. At the OMB, where the nation's policy objectives often brutally intersect with its fiscal realities, the Decadal Survey offered Parriott and his colleagues a foolproof way of dismissing overly solicitous astronomers. "If a project wasn't highly ranked, we knew it didn't have the community's endorsement," he says. "That's really helpful for folks on Capitol Hill and in the White House who need to make hard choices.”
For more than a few astronomers, a drab name like "the Decadal" does not properly capture a process that holds such sway over their destiny. Instead they sometimes just call it "the voice of God." In coming weeks, when Astro2020's final report is released, that voice — that supposedly communal voice — will once again speak. Yet outside of a chosen few, sworn to secrecy, no one in the community knows in advance what it will say. Everyone, though, agrees Astro2020's conclusions arrive at a time of peril.
"We are right now on a knife-edge," says John O'Meara, chief scientist of the W. M. Keck Observatory on Mauna Kea in Hawaii. "I do believe this Decadal is existential for astronomy in the United States. When you consider the facilities and the science topics that are under discussion, it will influence whether or not we become a second-place player in global astronomy…. When the [federal] agencies and Congress receive the Decadal report, they will hold in their hands the decision of whether or not we wish to have leadership in this field of science."
U.S. telescopes in twilight
From the outside looking in, one would not realize the enterprise of U.S. astronomy is teetering on the edge of crisis. Several new ground-based telescopes have recently come online, each bringing a bumper crop of celestial discoveries — and even more ambitious projects are waiting in the wings. Consider the Vera C. Rubin Observatory, a high-ranked priority of the past two Decadals (which are naturally called Astro2000 and Astro2010). Sited on a mountain in Chile, the Rubin Observatory should begin operations in late 2023 to generate a high-definition, decade-spanning time-lapse movie of the entire overhead sky.
But among large upcoming U.S. projects, Rubin is a rare, healthy exception. The other highest-profile ground-based recommendation from Astro2000 and Astro2010 — building an optical Extremely Large Telescope (ELT) with a mirror circa 30 meters in size — remains in limbo. One selling point for ELTs, among many, is that they offer the best hopes of ever studying Earth-like exoplanets from the ground. Astro2020 will probably decide if the U.S. ELT efforts sink or swim — or if other projects, such as a "next generation" upgrade to the nation's Very Large Array of radio telescopes or expansions of gravitational-wave observatories, take priority.
In the quest for an ELT, the U.S. has managed to produce two competing projects, the Thirty Meter Telescope (TMT) and the Giant Magellan Telescope (GMT). Both are short on funding, and neither appears likely to begin operations before the decade is out, each having served to stifle the other. And the TMT's early stages of construction on the Hawaiian volcano Mauna Kea — a site unrivaled for pristine views of the Northern Hemisphere sky — sparked protests from activists who see telescopes there as an occupying affront to the mountain, which Native Hawaiians hold sacred. Construction on the TMT ceased after protesters repeatedly blocked the road to the mountaintop; the conflict remains at an impasse. "If [Astro2020] says, 'Forget the ELTs; let's prioritize something else instead,' then it's quite possible that both the TMT and the GMT will die," says a senior ground-based astronomer familiar with the situation.
Europe, in contrast, took the lead over the U.S. in ground-based optical astronomy years ago and is well into construction of an ELT of its own in Chile. The European Extremely Large Telescope boasts a 40-meter mirror — and it is projected to come online in 2027.
China is surging ahead as well. For evidence, look no further than the U.S.'s iconic, National Science Foundation-funded Arecibo radio telescope in Puerto Rico: Once the world's largest, the radio telescope catastrophically collapsed last year in part because of budgetary neglect — but not before China's Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) had superseded it in size. And together — without the U.S. — Europe, China and many other international partners are building the Square Kilometer Array (SKA), a breathtakingly powerful collection of thousands of radio telescopes that is set to become fully operational at sites in Australia and South Africa as early as 2030.
The hungry giant and Frankenstein's monster
The outlook is similarly mixed for the nation's space-based astronomy. For now, the U.S. remains at the forefront of off-world observing, but of the four "Great Observatories" NASA launched between 1990 and 2003, only Hubble and the Chandra X-ray Observatory are still operational, and both are nearing their end, with no replacement on the horizon. "Hubble is probably not going to last another decade, and maybe we'll get five more years out of Chandra. But then that's it — they're gone," says Jason Tumlinson, an astronomer heading the community missions office at the Space Telescope Science Institute. "We'll probably have a long gap with no real optical, ultraviolet or X-ray capability in space. And now is the time to decide how and when we might get it back."
Astro2000's top-ranked space project, NASA's flagship-class James Webb Space Telescope, is a technological marvel: a cryogenically cooled infrared observatory with a segmented, 6.5-meter starlight-gathering mirror that folds, origamilike, to fit in a rocket. Early on Webb was projected to cost about $1.5 billion and to launch perhaps in 2011 to study the emergence of stars and galaxies in the early universe. Today those projections seem hopelessly naive. After a staggering number of cost overruns and delays that hobbled planning for other projects, the current best-case scenario is that the telescope will reach space no sooner than this mid-November, operating for just a decade with a total project cost of about $10 billion. Less optimal scenarios, of which there are many, are almost too grim to contemplate. "We can't do science at this scale without taking risks — and I'm confident in our chances of success — but if Webb fails, it will be an unmitigated disaster," says Matt Mountain, the project's telescope scientist and president of the Association of Universities for Research in Astronomy. "It has to work. Because if it doesn't, we aren't going to do another ambitious flagship for, I would guess, two decades."
If Webb was a hungry giant unleashed by Astro2000 and its antecedents biting off more than could be chewed, then the top flagship recommendation of Astro2010, NASA's Nancy Grace Roman Space Telescope, was a different beast entirely — a cut-rate Frankenstein's monster the Decadal committee pieced together from the dismembered remains of multiple competing mission concepts. The committee had hoped to avoid another Webb-style debacle with Roman (initially named the Wide-Field Infrared Survey Telescope, or WFIRST) — and it did. But instead Roman's very existence threatened to tear the community apart from within. "Do you know what WFIRST really stood for, for most of us?" says one leading astronomer. "It stood for 'What the fuck is this ridiculous space telescope?'"
Originally envisioned to study dark energy with a barebones instrument package and a mirror scarcely half the size of Hubble's, Roman was projected to launch as early as 2020 on a relatively slim budget of less than $2 billion. To many expert eyes, such a project barely qualified for its supposed "flagship" status. NASA, with bipartisan congressional blessings, ultimately added more instruments and upgraded Roman's mirror to the same size as Hubble's, enhancing its science objectives and assuaging many criticisms — but also nearly doubling its estimated price tag and delaying its launch to no earlier than 2025. Meanwhile Europe and China have each proceeded with dark energy-focused space telescopes of their own, potentially scooping some of the promised scientific discoveries used to justify Roman's existence in the first place.
Although both Webb and Roman may each eventually succeed beyond astronomers' wildest imaginings, some fear the projects' greatest initial impact on the field will be to sharply curtail Astro2020's ability to plan for a prosperous future. "This is, I think, the first Decadal where both of the top space-based recommendations from the previous two Decadals — Webb and Roman — were still on the ground," Tumlinson says. "And if the Astro2020 committee were to say, 'We've got two flagships stacked up; the queue is too long; let's just pause for a while and do some smaller missions and catch up later,' that would be a mistake. The idea that you can just take a decade off from ambitious things to come back and do them later is not valid when you consider how our government actually works."
Most astronomers, Tumlinson says, seem to misunderstand what the Decadal's "governing dynamic" actually is. "A Decadal report is the beginning of a multiparty, multiyear negotiation between the scientific community, NASA, the aerospace contractors, Congress and the White House," he explains — which is why aiming high at the outset is in astronomers' best interest. "I would hope, with Astro2020, we temper our natural desire to mitigate risk and cut costs," Tumlinson says, "because all the other forces in this system will be doing that for us anyway.”
Whether because of the COVID-19 pandemic, soaring deficit spending or the increasingly dire global climate emergency, some might question the wisdom of U.S. astronomers reaching for the stars just as the sky seems set to fall. Then again, the reply comes, where is the wisdom in limiting the science of the 2040s or 2050s based on the troubles of the 2020s? "At the end of the day, appropriators are still going to spend money," says a former congressional staffer who dealt with high-level appropriations for federal science agencies. "They're going to get an allocation, and their job will be to use it wisely. If it doesn't go to astronomy, maybe it will go to a new flagship mission for NASA's planetary science division instead — or maybe it will go to a new FBI building. But that money will be spent, regardless of what astronomers do."
The main menu
Responding to the woes from Webb and Roman, years ago NASA began revamping its approach to future flagships, demanding greater certainty about technological challenges and costs. In 2016 the agency assembled four Science and Technology Definition Teams (STDTs), each examining a separate mission concept for Astro2020's consideration. Two of the four concepts — the Large Ultraviolet Optical Infrared Surveyor (LUVOIR) and the Habitable Exoplanet Observatory (HabEx) — would focus on the quest to learn more about planets orbiting other stars, with an emphasis on studying potentially habitable worlds. A third, the ultracold and far-infrared Origins Space Telescope, would also perform some exoplanet studies as part of a broader investigation of the formation of galaxies, stars and planetary systems. The fourth concept, the Lynx X-ray Observatory, would be the most powerful X-ray astronomy facility ever built, offering intimate views of black holes, active galaxies and violent supernovae across cosmic time. Each project has profound potential — but also one or more Achilles' heels to make astronomers antsy.
LUVOIR's strength — and weakness — has always been the enormity of its segmented mirror. If deployed and maintained with picometer-degree stability (which is very hard), this mirror would allow astronomers to discover and study hundreds of exoplanets while also performing revolutionary observations across a wide swath of general astrophysics. Assuming any true-blue Earth-like worlds exist around the sun's neighboring stars, LUVOIR should offer the best odds of finding them. But whether considering either a 15-meter "deluxe model" or an eight-meter "budget size" one, putting such a demanding deployable mirror into space translates to an astronomical cost. "When I started working on this, I closed my eyes and said, 'It's gonna be $1 billion a meter. But if LUVOIR realizes its vision, that would be a bargain," says O'Meara, one of the leaders of this mission concept's STDT. Estimates from two separate groups at NASA's Goddard Space Flight Center have arrived at slightly higher figures, placing a deluxe LUVOIR somewhere in the realm of $15 billion or $20 billion and finding the budget version between about $12 billion or $15 billion.
Thanks to its smaller four-meter mirror, HabEx would be cheaper than either version of LUVOIR, with an estimated cost approaching $10 billion. But it would yield far fewer exoplanets — delivering details on perhaps 10 potential exo-Earths rather than dozens. It represents an all-in bet on a novel technology: a secondary sunflower-shaped spacecraft called a "starshade" that would unfurl to more than 50 meters across and fly more than 75,000 kilometers in front of the four-meter mirror to blot out a target star's light, revealing comparatively dimmer accompanying worlds. "The starshade concept, although potentially very powerful, scares a lot of people for whom it is a relatively new idea," says Scott Gaudi, an astronomer at the Ohio State University, who co-chaired the HabEx STDT. Yet he maintains that scientists have a realistic plan for managing risks and developing the starshade on budget.
Origins is, in most respects, the "safe" choice: it features a large but nonsegmented mirror of nearly six meters and is based mostly on preexisting technologies, netting a cost estimate of around $7 billion. Chilled to less than five kelvins above absolute zero, the telescope would offer a 1,000-fold increase in far-infrared sensitivity over previous missions, allowing astronomers to map the inner workings of galaxies across the observable universe while also studying a handful of small exoplanets as well as water in protoplanetary disks around nearby stars. But Origins' tried-and-true approach makes it relatively bland: It would not answer the burning questions about Earth-like worlds that LUVOIR or HabEx might. Its infrared optics would not deliver the crisp, colorful images of Hubble. And a top ranking from Astro2020 would make it, after Webb and Roman, the third infrared flagship in a row recommended to NASA. Some might call Origins a space telescope only an astronomer could love. "We have a real PR problem in the infrared," says Cara Battersby, an astronomer at the University of Connecticut, who served on the Origins STDT. "But if you look at the specs of each [STDT] concept and the science questions they have in common, such as planets and the coevolution of black holes with galaxies, I challenge you to not conclude that Origins is the most well-rounded and safest of them all."
Lynx is the oddball of the four, with a projected price tag slightly shy of that of Origins but a radically different design and goal. Its "mirror" would be a first-of-its-kind three-meter-wide assembly of nearly 460 nested shells of polished silicon, all densely packed and angled to reflect and focus high-energy X-rays. That design would provide far better performance than Chandra or other earlier X-ray telescopes, allowing Lynx to unveil new details of the universe's oldest and biggest black holes. Some astronomers fret about potential budget-busting difficulties in making Lynx's exotic mirror, but most instead worry about duplicating the efforts of a similar, already approved project: the European Space Agency's Advanced Telescope for High-Energy Astrophysics (Athena), which is set to launch in 2031. "For many people, the existence of Athena is an insurmountable argument against Lynx," says Grant Tremblay, an astronomer and a contributor to the Lynx STDT at the Center for Astrophysics at Harvard University and the Smithsonian Institution. "But I sincerely believe there is a very compelling argument for why these two missions are scientifically complementary to each other."
Collectively, these four STDT concepts make up the main menu of flagship options that the Astro2020 steering committee is most likely to choose from — presuming, that is, the committee picks one at all. All four projects are subject to Astro2020's Technical Risk and Cost Evaluation (TRACE) process, a brand-new, behind-closed-doors checkup by the Aerospace Corporation on each STDT concept's estimates. If the TRACE deems all four concepts far more expensive than the STDTs' in-house estimates, the steering committee could opt to choose none.
The new great observatories
Astronomers have named the four likeliest outcomes of the Astro2020 deliberations: "Scenario one, we call 'the shit sandwich,' which is if they recommend no flagships," says one senior scientist. "Most of us think that would be disastrous. The 'shit sandwich with a side of pickle' is when they choose no flagships but recommend technology development for whatever could come next — which is close to what happened with Astro2010. The 'nice lunch' is what we get if they pick a true flagship. And the 'perfect meal' is their picking a flagship and setting priorities for technology development to enable a few more."
For now, most members of the STDTs are lining up for a "perfect meal." Remarkably, after years spent extolling the virtues of their chosen missions, they have almost universally concluded that the ideal future for U.S. astronomy in space is one in which none of their pet projects triumph over all but rather where multiple flagships are somehow built and launched in rapid succession. Such an approach would effectively create a "New Great Observatories" program for the 21st century, much like the one that produced Hubble and its epochal kin. "I've promised — and many of my colleagues have, too — that if the Decadal chooses any flagship, then that is what I'll be cheering for," O'Meara says. "Something like the New Great Observatories can only happen if we stop shooting into each other’s backyards."
Or rather the New Great Observatories can only happen if astronomers become more savvy at what Gaudi has termed "astropolitics." "I'm utterly convinced a 'New Great Observatories' program with Lynx, Origins, and LUVOIR or HabEx — a 'LuvEx,' so to speak — could be done with a single phone call to the right person," Tremblay says. "Because on Capitol Hill, it's not about total cost — it's about annual appropriation. A couple hundred million dollars a year added to NASA's astrophysics line would suffice."
Such hopeful speculations are not necessarily just wishful thinking. "We're talking a 1 or 2 percent increase in real dollars to NASA's budget to enable another Great Observatories program," says one Beltway insider. "These are the perturbations concerted advocacy can create. Only about 30 senators are really involved in appropriations, and the annual discretionary budget of the federal government is running at about $2 trillion. So divide $2 trillion by 30 and then factor in the staffers working for each of those senators. You'll find, perhaps to your horror, that anything much below about half a billion dollars a year is essentially left to staffers and lost in the margins." Tremblay puts it more bluntly. "NASA does not really work for the Executive Office of the President," he says. "It works for the 25-year-olds a few years out of college who serve on appropriations committees. A flagship mission — or a whole new series of Great Observatories — could be green-lit over lunch by some low-level staffer while they're eating a burrito."
A single question
To secure a multiflagship future, astronomers will likely need an overarching goal that resonates not only with professional scientists but also with policy makers and the general public they serve (burrito-munching congressional staffers included). And for that, arguably no topic has broader appeal than humankind's long, unrequited search for alien life.
"If the Decadal wants to get the most public buy-in, they should distill the next 30 years of U.S. astronomy down to a single question: How does the universe enable biology?" O'Meara says. "Think about what's needed to get answers for that. It's not just going out and taking pictures of small, temperate exoplanets. You need to track the creation of atoms and molecules from the big bang all the way to planetary biosignatures, and you need to understand how galaxies and stars arise so that the universe could make planets in the first place. You also need to understand where life can and cannot exist, which means deeper exploration of the solar system and probably even sending astronauts to Mars. Addressing that single, fundamental question could speak to all of NASA while bringing in the NSF and international partners as well."
"This is another reason to consider Astro2020 'special,'" Tumlinson says. "For the first time, we can say with a straight face that we're conceptually designing missions with reasonable expectations of finding life on planets outside the solar system. And to not do that when you have the opportunity to — well, that seems a little crazy to me."
Yet many astronomers are also uneasy about promising more than they might be able to deliver. "This mentality, that we all must be behind one big thing and that this one thing has to be so big to justify us all being behind it, is exactly how we've gotten into trouble before," says Sara Seager, an astrophysicist at the Massachusetts Institute of Technology, who, alongside Gaudi, co-chaired the HabEx STDT. "Of course looking for other Earths has public support! But if you put a dollar figure on it — if you tell the public it may cost a half-billion or a billion per planet, what do you think they'll say then? It just so happens that we live on a really hard planet to find — Earths are small and faint, and they're right next to big, bright stars. That makes seeing them — or searching for signs of life on them — nearly impossible."
At least, that is the case if astronomers try to achieve those goals within existing budgets and a Decadal's 10-year time frame. "For us to decide on the future of astronomy based on some arbitrary length of time that just happens to be the number of fingers we have on our hands may not be the best way to go about things," Gaudi says. "Maybe we should decide on a different, longer timespan that actually corresponds to the missions we're considering — missions that are getting more ambitious, complicated and technologically challenging over time. These really aren't 'Decadal' surveys anymore, and they haven’t been for a while. They're multidecadal surveys, and we just need to start being honest about that."
After experiencing the process and its imperfections from within the National Academies, the White House and now the largest advocacy group for U.S. astronomers, Parriott offers a simple recommendation of his own. "We need to support the Decadal, because it's probably the best we can hope for in planning and executing a program of benefit to all of us," he says. "You know that old saying about democracy? Well, maybe it applies to the Decadal, too: it's the worst way to do things — except for all the other ways."