U.S. astronomers are wary that their next big space telescope, a mission to study cosmic acceleration and exoplanets, could balloon in cost and scope just like the budget-busting $8 billion James Webb Space Telescope (JWST). So says a National Academies of Sciences, Engineering, and Medicine panel tasked with taking the temperature of the field midway between “decadal surveys”—the regular reports in which astronomers list their funding priorities for the next 10 years. Given the recent success in detecting gravitational waves, the panel also says the United States should rejoin a partnership with the European Space Agency (ESA) to build the Laser Interferometer Space Antenna (LISA), a mission to study gravitational waves in space.

“The community very much wants to see LISA go forward,” says panel chair Jacqueline Hewitt, an astronomer at the Massachusetts Institute of Technology in Cambridge. “The partnership [with ESA] dissolved, but it will have to be rebuilt,” she says.

Overall, the panel was pleased with progress made on a variety of scientific fronts, particularly in the study of exoplanets and gravitational waves. But they worried that stagnant budgets at the field’s main funding agencies—the National Science Foundation (NSF) and NASA—along with pressure from overspending on large projects like the JWST are threatening many of astronomers’ most sought-after projects. “Budgets have been different from what we assumed” in 2010, she adds.

The 14-member panel reviewed progress in the priorities set by the 2010 decadal survey known as New Worlds, New Horizons in Astronomy and Astrophysics as well as recent scientific successes. It highlights the first detection of gravitational waves by the Laser Interferometer Gravitational-Wave Observatory, and notes the advances made in the study of exoplanets, much of it made possible by NASA’s Kepler spacecraft, which revealed the ubiquity and rich diversity of planetary systems. “No one expected the exoplanet field to burgeon in the way it has,” Hewitt says.

There have also been notable successes in new facilities. The Atacama Large Millimeter/submillimeter Array, built in collaboration with Europe and Japan in northern Chile, is nearing completion and has already notched up some significant observations. Others, including the Daniel K. Inouye Solar Telescope, the JWST, the Transiting Exoplanet Survey Satellite, and the Large Synoptic Survey Telescope, are on track to come into operation over the next few years.

Some of the highest priorities identified in the 2010 survey, however, have been delayed, reduced in scope, or canceled altogether, the panel notes. This is partly because of flat budgets that were not forecast when the 2010 decadal report was drawn up. The astronomy budget at NSF has been steady in real-year dollars which, with its commitments to large new facilities, has meant that funding for medium-scale projects and individual investigators has been squeezed. At NASA’s astrophysics division, budgets have kept pace with inflation, but the increasing costs of the JWST have delayed the funding of new projects by up to 5 years and reduced the number of midsized Explorer missions being commissioned.

Hewitt says that the community is concerned about large projects squeezing out smaller ones and that both NSF and NASA need balance in their programs. “It doesn’t make sense to invest in amazing facilities and not to have the money to do science,” she says. The report suggests that NSF should divest from older facilities with “lower scientific impact” to free up funding for individual astronomers. NSF has conducted reviews of its ground-based facilities in 2006 and 2012, and has recommended closures to observatories such as the Arecibo radio telescope, but astronomers have resisted some of those moves.

The suggested projects that have suffered because of tighter budgets include two of the largest planned telescopes on the books, the Giant Magellan Telescope and the Thirty Meter Telescope. The 2010 survey suggested NSF should support one of them but the agency has been unable to do so. (They are both progressing with private and international backers, but neither is fully funded.) A millimeter-wave survey telescope known as the Cerro Chajnantor Atacama Telescope was not funded by NSF. NASA pulled out of collaborations with ESA to build LISA and the International X-ray Observatory. ESA is proceeding with scaled-down versions of both projects (dubbed the Evolved LISA and Athena).

The 2010 survey’s top-ranked space project for the decade was the Wide Field Infrared Survey Telescope (WFIRST), which was first developed to measure cosmic acceleration, but over time became more of a multipurpose telescope that would also study the demographics of exoplanets, and survey the galactic plane of the Milky Way. WFIRST did get the go-ahead this year (3 years later than expected) for launch in 2025 (5 years late)—delays that were due to the JWST’s budget problems.

Although the panel welcomed this advance, it urged caution from NASA in case the mission goes the same way as its predecessor—the JWST—with cost and schedule ballooning out of control and threatening other projects. Since winning the endorsement of the 2010 survey, WFIRST has acquired a new, larger mirror; larger detectors; and a coronagraph (a device to block out the light of a star so its planets can be seen directly)—add-ons that will probably add to its more than $2 billion cost. “NASA needs to manage it very carefully,” Hewitt says. “Tremendous advances are being made in exoplanets, but some other areas may suffer” if costs get out of control, she says.

In addition to its call to rejoin the LISA project, the panel also advocates NASA taking a role in ESA’s x-ray mission Athena and calls on the agency to beef up its midsized Explorer program. “I hope we can make those happen,” Hewitt says.