Commentary: Astronomy from space after the JWST

Abstract

Last year the James Webb Space Telescope (JWST) trained its sights on the distant universe and began returning stunning images in IR light that captured the imagination of the public around the world. The JWST has only just begun its exploration of the universe, yet astronomers are already looking to the future.Every 10 years, panels of experts convened by the National Academies create a survey in astronomy and astrophysics to assess scientific frontiers and plan the capabilities that will keep advancing humankind’s understanding of the cosmos. The JWST, originally called the Next Generation Space Telescope, was recommended as the top-priority large strategic mission in the 2000 decadal survey, and given its technical ambition and scale, it took more than two decades to be realized. But it was also conceived as costing less than 10% of the eventual price tag, in part because of wishful thinking by NASA and the scientific community and in part because the project began full-scale development before it was ready.The 2010 decadal survey gave the highest priority for a large space mission to the Nancy Grace Roman Space Telescope, then known as the Wide-Field Infrared Survey Telescope, an observatory that will image large swaths of the sky in IR light to make three-dimensional cosmic maps of many millions of galaxies, elucidate the nature of the mysterious dark matter and dark energy, search for and image exoplanets, and explore many other topics in IR astrophysics. Roman is scheduled for launch in 2027, some 17 years after it was recommended as a priority. That delay was a result of funding issues directly related to the cost overruns on the JWST. Like the JWST, Roman was chosen to address the big, outstanding scientific questions identified at the time. But the delays bring with them costs in both taxpayer dollars and scientific opportunities.The most recent astronomy and astrophysics survey, Astro2020 (for which the two of us served as committee cochairs), considered how to accomplish its charge to chart a course for the future of space astrophysics. In previous surveys, the committees have provided a rank-ordered list of concepts for large space missions, and in the decade that followed, NASA began development of the top project on the list as its next priority. Astro2020, however, took a different course.Stephan’s Quintet, as captured by the James Webb Space Telescope. The image, a mosaic constructed from almost 1000 separate IR images, reveals previously unseen details of the group of galaxies. (Courtesy of NASA, ESA, CSA, and STScI.)PPT|High resolutionThe lessons of the past 20 years indicate that with business as usual, the next large mission beyond the JWST and Roman would be two or more decades away from its realization, and the next one after that more than four decades away. In the 1990s and early 2000s, NASA was able to launch a suite of Great Observatories—with four strategic missions, launched within a period of 13 years—that significantly advanced observational capabilities across a broad swath of the electromagnetic spectrum. The scientific impact of that panchromatic suite was enormous, as the universe could be simultaneously probed in many different and complementary ways, and it achieved leverage and insights well beyond what was imagined for any of the individual telescopes.Astro2020 therefore includes a strategy to realize a new generation of Great Observatories: They have intervals between launches greatly reduced from the roughly two decades that is the recent norm yet still include capabilities at the scientific forefront. Far from business as usual, Astro2020 does not forward a ranked list of missions; rather, it imagines the Great Observatories Mission and Technology Maturation Program to study multiple mission concepts in the same decade. Individual mission cost targets would be appropriate for the scientific scope. While the survey prioritizes the first mission to enter the maturation program, it emphasizes that multiple missions should be studied this decade, so that if the one at the top of the list runs into problems, delays, or large cost overruns, backup options are ready.First in the list of missions to be matured is an observatory that spans the wavelength range covered by the Hubble Space Telescope (which is 2.4 m in diameter and covers UV to near-IR) and has the collecting area of the JWST (6.5 m in diameter but mainly IR). The large IR/optical/UV (IR/O/UV) mission would have the ability to image a target planet while blocking out the light of its parent star, even when the star is 10 billion times brighter than the planet. It is an ambitious mission, on the scale of the JWST, yet the survey sets a target that it should not cost significantly more than the JWST.The choice is motivated by the mission’s ability to diagnose the atmospheres of planets outside the Milky Way to search for signatures of life—which, if detected, would change the way humans view their place in the universe. Like Hubble, the IR/O/UV mission would revolutionize our understanding of galaxies and stars and of the interstellar, circumgalactic, and intergalactic gases that give birth to them and would link them in a complex cosmic ecosystem. The IR/O/UV mission is technically challenging, and like the JWST and Hubble before it, it demands a major investment; NASA is the only agency worldwide capable of leading it.Also compelling—and essential to advancing modern astrophysics—are a next-generation x-ray telescope and a mission sensitive in the far-IR. The former, with resolution matching that of NASA’s Chandra X-Ray Observatory but with a vastly greater collecting area, would map hot, diffuse structures that are believed to feed the growth of galaxies and would peer back to find black holes forming in the early universe. The latter would unveil the dense regions of gas and dust enshrouding sites of star formation and the active central regions of many galaxies, and it would reveal the complex chemical processes that give rise to stars, planets, and ultimately life. With disciplined study and technology development, both missions can realize transformative capabilities on a size scale only one-third that of the large IR/O/UV mission. With strategic investment in the coming decade, both could also be ready to launch in quick succession.Our “crystal ball” description of future missions and observatories beyond the JWST has focused on the largest space missions, but Astro2020 also recommends that NASA continue with a balanced portfolio of mission sizes from the large missions or Great Observatories described here down to probe, explorer, and smaller missions. Our committee was only tasked with planning future US-based activities, but in reality many of the projects will involve international partnerships, and implementation of the NASA road map will need to take into consideration missions led by the European Space Agency and other countries.Astro2020 envisions a bright future, with eyes on the universe spanning the electromagnetic spectrum.© 2023 American Institute of Physics.