NASA is planning four of the largest space telescopes ever. But which one will fly?

For NASA astronomers, this was not a good year. In June, a review board found that the agency's prized observatory—the already overdue and vastly overbudget $8.8 billion James Webb Space Telescope (JWST)—was still years away from taking flight and capturing the faint light of the universe's first stars. The holdup: torn sunshields and loose bolts. Also in trouble was the next big astrophysics mission in line, the Wide Field Infrared Survey Telescope (WFIRST), intended to pin down the nature of mysterious dark energy by surveying wide swaths of the sky. Not even off the drawing board, WFIRST was predicted to burst its $3.2 billion budget by $400 million, another review panel found—not a plus for a mission that the administration of President Donald Trump was already thinking of canceling.

Yet astronomers are about to look skyward and dream even bigger dreams. The decadal survey in astrophysics, which sets priorities for future missions by NASA, the Department of Energy, and the National Science Foundation, began last month. Dozens of astronomers, broken into committees, will identify science goals and develop a wish list of telescopes, both on the ground and in space, that could best address them. One of the toughest tasks will be to decide which—if any—of four proposed successors to the JWST and WFIRST most deserves to fly as a NASA flagship observatory. It would be launched in the 2030s to L2, a gravitationally balanced spot beyond Earth’s orbit, on the far side of Earth from the sun.

In a special online presentation, Science examines those dream telescopes. The Large UV Optical Infrared Surveyor (LUVOIR), a 15-meter-wide giant with 40 times the light-collecting power of the Hubble Space Telescope, is a bid to look back at the universe's first galaxies, and to answer the question: Is there life elsewhere in the universe? The Habitable Exoplanet Observatory (HabEx) would also focus on that question, but with a smaller mirror. HabEx would fly in tandem with a separate spacecraft carrying a starshade the size of a soccer field. By blocking the glare of a star, the starshade would reveal Earth-like exoplanets, enabling HabEx to scrutinize their faint light for signatures of life. The Lynx Xray Observatory would gather x-rays from the universe's first black holes to learn how they help galaxies form and evolve. And the Origins Space Telescope, with machinery to chill its telescope to just 4° above absolute zero, would study a little-explored kind of infrared radiation emanating from the cold gases and dust that fuel star and planet formation.

Whichever concept rises to the top, researchers hope it has a smoother path to space than the missions chosen in previous surveys. The 2001 survey picked the JWST as its top priority, but that telescope will be lucky to meet its scheduled launch in 2021, 2 decades later. WFIRST was the top pick of the 2010 survey, but it won't fly before 2025. There's a general sense that the initial proposals were immature and unrealistic, says Roger Blandford of Stanford University in Palo Alto, California, who chaired the 2010 survey. "There's frustration all around."

This time, NASA wants the concepts on a firmer footing. Not only did the agency identify the four flagship concepts early, back in 2015, but it has since funded teams to work up rough designs for each one. In June 2019, the teams will deliver to NASA a report that includes two concepts—one expensive and big, the other constrained and relatively affordable at less than $5 billion in most cases. (Here, Science examines the larger concepts.)

"This prepreparation will put the survey in a better situation to evaluate the possibilities," says Fiona Harrison, a high-energy astrophysicist at the California Institute of Technology in Pasadena who was named last month as co-chair of the survey along with Robert Kennicutt of Texas A&M University in College Station. The product of the decadal survey—a prioritized list of missions delivered in 2020—is supposed to be consensual, in part so that agencies and scientists can lobby Congress for funding with a unified voice. But competition among the four flagships will be fierce.

LUVOIR's backers tout its wide appeal as a general-purpose observatory in the mold of Hubble. LUVOIR's instruments cover the parts of the spectrum where the universe is brightest, and the huge size of its mirror means it can peer the farthest, at the faintest objects, with the sharpest vision. "It transcends astrophysics," says Jason Kalirai of the Space Telescope Science Institute (STScI) in Baltimore, Maryland. Critics argue that LUVOIR's huge mirror will lead to a huge price tag and inevitable delays, as the JWST's 6.5-meter mirror already has.

Proponents of the cheaper HabEx hope it will ride high on surging enthusiasm for exoplanets—and a concern for simplicity and thrift. But flying in formation with a distant starshade is an untested technique. And though HabEx can study a few nearby planets in detail, its smaller mirror—4 meters compared with LUVOIR's 15 meters—means more distant worlds will be out of reach. LUVOIR and HabEx will compete head-to-head for the committee's attention, and HabEx and LUVOIR team member Chris Stark of STScI says there won't be a need to launch both. "There are only so many nearby stars."

A race to the stars

Four NASA space telescope concepts targeting different wavelengths and goals are competing to fly in the 2030s. Astronomers are now picking a favorite.

Sciencetargets Spectrum Firstgalaxies Firstsupermassiveblack holes Planet-forming disks Earth-likeexoplanets Visible Ultraviolet Infrared X-ray ORIGINS HABEX LUVOIR LYNX
C. Bickel/Science

Origins would look back in time to see how dust and molecules coalesced to create the first galaxies and black holes and how the disks around young stars clump into exoplanets. But the JWST and the Atacama Large Millimeter/submillimeter Array in Chile can capture some of the same wavelengths, squeezing Origins's discovery space.

Lynx would take up the mantle of NASA's aging Chandra X-ray Observatory, zooming in on hot gas swirling into a black hole or jetting from the center of a galaxy. That would placate x-ray astronomers still smarting from the low rating their International X-ray Observatory proposal received in the 2010 decadal survey. "We got robbed at the last decadal," says STScI x-ray astronomer Rachel Osten. "Is it time for x-rays?"

Whichever mission wins the decadal's favor, funders will ask: How do we know it won't be another JWST, swallowing up budgets and delaying other projects? Study director Dwayne Day of the National Academies of Sciences, Engineering, and Medicine (NASEM) in Washington, D.C., which organizes the decadals, says the survey is taking a sophisticated approach to estimating costs, hoping "to avoid sticker shock, committing to something that is too expensive to afford."

Day says project teams usually estimate costs by tallying labor, materials, and testing. "It's good, but it leaves out unforeseen circumstances, threats." So, for the past decade NASEM has been paying The Aerospace Corporation of El Segundo, California, to apply a cost model called CATE (for Cost And Technical Evaluation) to any proposals a decadal wishes to consider.

CATE draws on a database that goes back decades and contains details of cost and performance for more than 150 NASA missions and 700 instruments. When presented with a new mission, CATE can say how similar missions have fared in the past. The model is particularly powerful in assessing the things that can go wrong. "The best forecasters can't have hands on all the unknown unknowns," says Debra Emmons, a senior manager with Aerospace in Chantilly, Virginia. For example, if a sensor takes longer than expected to develop, or if an international partner delivers an instrument late, the project can be delayed and costs can rise. "[CATE] assesses technical threats, monetizes them, and makes a forward projection," she says. Paul Hertz, NASA's astrophysics chief in Washington, D.C., calls it "a great addition to the tool set."

The project teams are wary of the exercise, fearing that if they produce a scientifically bold and technically challenging proposal, CATE might judge it to be risky and expensive, Emmons says. And NASA wants the four project teams to be ambitious. "The missions had better be hard to do because the questions are hard," Hertz says.

But with the still-grounded JWST on everybody's mind, astronomers are eager to ensure that the winner of the great space telescope bake-off is at once dreamy and real. Blandford says: "It gives a rationale for making these terrible decisions."

For more on these telescopes, also see here.

*Correction, 2 January, 1:15 p.m.: An earlier version of the story misstated the location of L2.