American astronomers have been hoping for government assistance in building two enormous telescopes on the ground for several years. However, the National Science Board (NSB), a group of scientists responsible for managing the National Science Foundation (NSF), has recently stated that only one of these telescopes can be afforded by the field. On February 22, the NSB announced that the budget for the U.S. Extremely Large Telescope Program (US-ELTP) will be around $1.6 billion. They have also given the agency until May to develop a process for selecting one of the two 30-meter class telescopes.
With a rival European telescope rapidly taking shape on a mountaintop in Chile, the NSB decision is a relief to those who want U.S. astronomy to unite behind a realistic plan and catch up. “I think the decision was long overdue,” said John Monnier, a professor at the University of Michigan. But for Richard Ellis, a professor at the University College London, “It’s a tragedy, given the investment made in both telescopes.” He also added, “There were many opportunities to merge or down-select. Now, the U.S. has lost a couple of years trying to keep up with the European Southern Observatory.”
These advanced telescopes will provide the ability to zoom in on habitable planets located outside of our Solar System and explore the formation of the first stars and galaxies.
The latest top telescopes have apertures of 8 to 10 meters and have shown that segmented mirrors or several larger ones could be combined to create an even larger effective mirror. They also demonstrated the use of adaptive optics, which involves the use of rapidly deformable secondary mirrors to cancel out the distortions caused by Earth’s atmosphere and capture images as sharp as those taken from space.
The development of these technologies has led to the creation of two major U.S.-led projects: the Giant Magellan Telescope (GMT) in Chile and the Thirty Meter Telescope (TMT) in Hawaii. These projects are backed by consortia of universities, philanthropic foundations, and international partners. However, the privately funded approach that led to groundbreaking instruments during the 20th century stumbled when it came to multibillion-dollar projects. Despite design work and mirror casting forging ahead, both projects failed to secure enough funding.
In 2018, these historically rival projects joined forces as US-ELTP and made an offer to the NSF. In exchange for public funding, all U.S. astronomers would have access to the telescopes, which would provide unprecedented views of the night sky from both hemispheres. This is something that Europe’s Extremely Large Telescope (ELT) will not offer. The 2020 decadal survey in astrophysics, which defines the field’s priorities for funders and Congress, put US-ELTP first among ground-based projects, in line with the recommendation of a panel led by Timothy Heckman of Johns Hopkins University. “We felt this made a compelling case,” Heckman said.
In early 2023, the National Science Foundation (NSF) carried out preliminary design reviews of two telescopes, the GMT and the TMT. The GMT is estimated to cost $2.54 billion, of which existing partners have pledged $850 million. On the other hand, the TMT has a $3.6 billion price tag, and its partners have so far offered $2 billion. However, the costs of both telescopes are much higher than what NSF is used to, and the US-ELTP proposal would consume 80% of the NSF’s entire funding for major projects.
Michael Turner, an astrophysicist at the University of Chicago, argued that insisting NSF fund two telescopes puts both projects at risk in an editorial in Science in November 2023. NSF said it would announce its decision on how to proceed in the coming months, but it is not an easy choice. Turner said, “Neither is a slam dunk. Both have risks. I don’t envy the NSF.”
The TMT’s mirror is larger than the GMT’s, measuring 30 meters and made up of 492 segments, making it a sharper instrument. However, the TMT’s chosen site, the summit of Mauna Kea on Hawaii’s Big Island, is opposed by some Native Hawaiian groups who consider the summit sacred. They have blocked any construction work since 2015. TMT officials hope that construction will proceed under the management of a new state-appointed authority that governs the mountaintop and includes both astronomers and Native Hawaiians. “We’re working on our relationships in Hawaii,” said TMT Executive Director Robert Kirshner. “We’re learning how to do that in a humble and straightforward way,” Turner said the impasse may not be solved anytime soon. “I’m sure a solution will be found, but it may take longer than people like,” he said.
The GMT is a lower-cost and less risky option compared to other telescopes. Its construction is underway on a mountaintop in Las Campanas, Chile, with support structures for its mirrors currently being built in the United States. The telescope features seven 8.4-meter mirrors, which together create an equivalent of a 25.4-meter-wide mirror. Three of these mirrors have already been completed, while the remaining four are still undergoing the polishing process.
Because of the risks attached to the TMT, Monnier and Ellis suspect NSF will probably back the GMT. But with a mirror less than 40% of the size of its 39-meter European rival, the GMT “is no match for ELT,” said Ellis, a former TMT board member. Monnier stated that the GMT will probably be good enough in key astronomy areas, but NSF will need to judge whether those areas are important for U.S. astronomers.
Abandoning either of these very capable telescopes will harm U.S. astronomy, said Wendy Freedman at Chicago, one of the GMT’s partner organizations. “The science that will come out really does justify two telescopes.” Upcoming survey telescopes such as the 8.4-meter Vera C. Rubin Observatory in Chile will identify a wealth of interesting objects in need of follow-up observations by instruments on the GMT and the TMT that can split the light into information-rich spectra. “That’s what these big telescopes give you,” Wendy added.
Language in a spending bill passed by Congress this week “strongly encourages” NSB to build both telescopes, even though lawmakers cut NSF’s 2024 funding by more than $800 million, to $9 billion. Freedman hopes the congressional direction will prompt a rethink. “The United States will sit out the future of astronomy if we don’t get these telescopes,” Wendy continued.