Astrophysics decadal survey recommends a program of flagship space telescopes

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WASHINGTON — A long-awaited report on the future of astrophysics research recommended NASA pursue a series of flagship observatories, starting with a large space telescope estimated to cost $11 billion but which would not fly until the early 2040s.

The “Pathways to Discovery in Astronomy and Astrophysics for the 2020s” report by the astrophysics decadal survey, released by the National Academies Nov. 4, endorsed the future space telescope as part of a broader program that would later support work on infrared and X-ray space telescopes.

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 large “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 600-page report.

That approach, said one member of the steering committee for the decadal survey, is based on the realization that many of the scientific goals extend far beyond the traditional 10-year time horizon of the study.

“We were tasked and encouraged by the funding agencies, including NASA, to really think big, bold, ambitious and long-term,” said Keivan Stassun of Vanderbilt University in an interview. “We took that to mean that we should not be thinking only about that which can be accomplished in a 10-year period.”

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 NASA-funded mission concepts studied for the decadal. 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.

The decadal survey concluded that the eight-meter version of LUVOIR “would be unlikely to launch before the late 2040’s or early 2050’s” and cost $17 billion. HabEx, while less expensive, might fall short of “providing a robust exoplanet census” desired by astronomers and its ability to do other astrophysics might not be worth its cost.

The concept ultimate recommended by the decadal survey would cost an estimated $11 billion, similar to the James Webb Space Telescope after accounting for inflation. 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 mission studies NASA funded to support the decadal survey, the Lynx X-Ray Observatory and Origins Space Telescope.

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 line 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 cost. Making the cost cap $2 billion 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.”

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 funding 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.

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 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 couple 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.”