Astro2020: A decadal survey that spans decades
Since the summer, the astrophysics community has been eagerly, if also a bit nervously, anticipating the release of the latest decadal survey for astronomy and astrophysics, called Astro2020. The report was expected in the summer, after the pandemic and a shift to online-only meetings delayed work on the study last year, but summer crept into fall with no sign of it.
The agencies that sponsored the decadal, including NASA, also were in the dark about the report. At a mid-October meeting of NASA’s Astrophysics Advisory Committee (APAC), Paul Hertz, director of the agency’s astrophysics division, showed the results of an office pool predicting when the decadal would be released. All but two people thought the report would have already been released. The other two predicted something of a worst-case scenario: the release of the decadal the week of Thanksgiving.
Fortunately, astronomers avoided having the report spoil their turkey dinners. The National Academies published the 614-page report, titled “Pathways to Discovery in Astronomy and Astrophysics for the 2020s,” Nov. 4, describing in detail its priorities for both space-based and ground-based astronomy. What astronomers got was a remarkably ambitious report that charted the future of the field not just for the next decade but, in some aspects, out to the middle of the century.
GREAT OBSERVATORIES PROGRAM
One of the biggest outcomes of the astrophysics decadal survey is its recommendation for the highest priority large, or flagship, space mission. Past decadals have recommended NASA pursue missions such as the Hubble Space Telescope, James Webb Space Telescope and, in the most recent report in 2010, the Roman Space Telescope.
As part of the preparations for the decadal, NASA funded studies for four mission concepts, ranging from a space telescope with a mirror 15 meters across to an X-ray observatory. The survey was free to select one of them as the top-priority flagship mission, or ignore them and go in a different direction.
“We realized that all of these are visionary ideas but they require timelines that are pan-decadal, even multi-generational,” said Fiona Harrison of Caltech, one of the co-chairs of the steering committee for the decadal survey. Independent assessments of the concepts projected costs in the range of $10 billion to $20 billion, and development times approaching two decades each. “We really think a different approach needs to be taken.”
Rather than develop those missions independently, the report recommended NASA establish a Great Observatories Mission and Technology Maturation Program that would oversee initial studies of flagship astrophysics missions as well as invest in the technologies needed to enable them. “
The survey committee expects that this process will result in decreased cost and risk and enable more frequent launches of flagship missions, even if it does require significantly more upfront investment prior to a decadal recommendation regarding implementation,” the committee concluded in the report.
The first flagship mission that would emerge from that new program is a space telescope six meters in diameter designed for observations at ultraviolet, visible and infrared wavelengths. Such a telescope would be particularly well suited for characterizing potentially habitable exoplanets but could also be used for a wide range of other astrophysics research.
The concept described in the report is a compromise between two of the NASA-sponsored mission concepts. One, called LUVOIR, offered a telescope between 8 and 15 meters in diameter for ultraviolet, optical and infrared observations. The other, called Habitable Exoplanet Observatory or HabEx, proposed a telescope between 3.2 and 4 meters across that could be combined with a separate spacecraft, called a starshade, for direct imaging of exoplanets.
That compromise was a “Goldilocks approach,” said Robert Kennicutt of the University of Arizona and Texas A&M University, the other co-chair of the steering committee. “We thought that six meters provides assurance of enough target planets, but it’s also a big enough gain in capability over Hubble to really enable general astrophysics.” The committee concluded LUVOIR would cost $17 billion and not be ready until as late as the 2050s, while HabEx fell short of science goals for exoplanets and other astrophysics.
The concept recommended by the decadal survey would cost an estimated $11 billion, similar to the James Webb Space Telescope after accounting for inflation, and be ready to launch in the first half of the 2040s. Work on it would start late this decade after several years of work to mature the mission concept and key technologies.
The report recommended that, five years after starting the new large space telescope, NASA begin studies of far-infrared and X-ray flagship missions, each with estimated costs of $3 billion to $5 billion. Those concepts are similar to the other two NASA mission studies, the Lynx X-Ray Observatory and Origins Space Telescope.
It also enables NASA to go in a different direction if a proposed observatory runs into technical problems in its early development. “If the progress appears to be stalled or delayed, then we can rapidly on-ramp another one of the compelling, exciting ideas,” said Keivan Stassun of Vanderbilt University, another member of the steering committee. “We can be phasing in multiple great ideas.”
That approach, he said, reflects the fact that the questions astronomers are seeking to answer, like searching for signs of life on exoplanets, won’t be answered in a decade. “We were tasked and encouraged by the funding agencies, including NASA, to really think big, bold, ambitious and long-term,” he added. “We took that to mean that we should not be thinking only about that which can be accomplished in a 10-year period.”
PROBES AND GROUND-BASED TELESCOPES
In addition to backing a program of flagship missions, the decadal survey recommended NASA pursue a line of medium-class “probe” missions. Such missions, costing up to $1.5 billion each and launching once a decade, would fill a gap between the more expensive flagship missions and the smaller Explorer-class astrophysics missions NASA launches every few years. It would be similar to the New Frontiers series of missions in NASA’s planetary science program.
NASA, anticipating the interest in probe-class missions, conducted studies of nine concepts. The decadal found that the original cost cap of $1 billion for probe missions was too constraining, with only one of the nine concepts fitting within that cap. Making the cost cap $2 billion, though, would prevent NASA from flying such missions at the desired rate of once a decade.
Those probe-class missions, Stassun said, can help fill the gap in flagship missions between the Roman Space Telescope, scheduled to launch in 2027, and the large space telescope projected for no earlier than the early 2040s. “They’ll allow us to have a significant infrared observatory or X-ray observatory,” he said, “that we can deploy during that period of time to ensure we have full panchromatic capability.”
Harrison said that one of two leading concepts for probe missions was a far-infrared telescope similar to the Space Infrared Telescope for Cosmology and Astrophysics (SPICA), a concept studied by ESA and JAXA but abandoned last year because of funding problems. SPICA’s cancellation “really opens a void worldwide” that a probe-class mission could fill, she said. The other concept would be a “targeted” X-ray observatory to complement ESA’s Athena X-ray mission under development.
In ground-based astronomy, the decadal survey recommended that the National Science Foundation fund development of two so-called extremely large telescopes (ELTs), the Giant Magellan Telescope in Chile and the Thirty Meter Telescope proposed for Hawaii. The two observatories would be far more powerful than existing telescopes in service today and comparable with the European Extremely Large Telescope being built in Chile.
Both the Giant Magellan Telescope and Thirty Meter Telescope, funded so far by private organizations and other governments, had lobbied for support from the NSF to secure the money needed to complete the observatories and to give U.S. astronomers broader access to them. The report recommended NSF invest roughly $800 million in each telescope in exchange for at least 25% of the observing time.
“These instruments provide six times the spatial resolution of Hubble when used with adaptive optics,” said Kennicutt. “They will be workhorses as follow-ups to James Webb observations. The scientific case is just compelling.”
Work on the Thirty Meter Telescope has been blocked by legal action and protests by Native Hawaiians who oppose construction of the telescope on top of Mauna Kea, a mountain with cultural significance to them. The report noted that any decision on investing in that telescope needs to come after resolution of that issue, and that if only one of the two observatories proves to be viable, the NSF should increase its investment in that one to obtain a larger share of telescope time.
In radio astronomy, the report called on the NSF to begin studies for the Next Generation Very Large Array, a radio observatory that would ultimately replace the Very Large Array in New Mexico. Those studies would support construction of the $3.2 billion observatory beginning late in the decade.
Besides backing large ground- and space-based observatories, the decadal survey emphasized the need to invest in what it called “foundational activities,” from supporting early-career researchers to funding more sustainable ways of operating facilities. The report placed a particular emphasis on issues regarding the state of the profession, such as how to retain more students to study astrophysics to the threats posed by climate change and satellite megaconstellations.
A key point behind those recommendations, Stassun said, is making “a serious reinvestment in human capital” in the field. Supporting researchers, he said, fell behind investments in observatories and spacecraft in the last few decades.
“In doing this report, it really hit the committee, in a very important and sobering way, that these facilities don’t invent and build themselves. Discoveries made with them don’t get made by themselves,” he said. “We need to bring the human capital investment back into balance with the infrastructure investment.”
WORTH THE WAIT?
The report is now in the hands of NASA, the NSF and other agencies to review and implement. The delay means that it’s too late for it to influence budget proposals for fiscal year 2023, which are in advanced development at the agencies ahead of release next February.
Hertz, at the APAC meeting in October, said a full NASA response to the decadal likely won’t be ready by early January, when NASA holds its usual town hall meeting at the American Astronomical Society conference. “We’ll do that at a standalone town hall later on,” he said. “It would have been great to have it in September.”
The delay in the final report, Harrison said, was caused by the pandemic. “When the committee started doing its heavy lifting was about the same time that the pandemic hit,” she said, forcing the committee to move meetings online while its members dealt with all the other personal and professional disruptions the pandemic caused. “Yes, the survey was delayed, but we were determined not to compromise the final product.”
Stassun agreed. “COVID affected not just the 20 members of the decadal steering committee, but the scores of people who were involved in the different panels and the reviewers,” he said. “That’s just the reality. Things got slowed down.”
Another factor, though, was the scale of the task. “We’ve laid out some concrete decadal priorities, but we’ve done it within a longer-term pathway into the 2030s and 2040s,” he said. “The fact that we took 20 months instead of 12 maybe doesn’t feel quite so unreasonable.”
This article originally appeared in the November 2021 issue of SpaceNews magazine.