A web of steel girders is rising from the flattened summit of Cerro Armazones, 3000 meters above sea level in Chile’s Atacama Desert. The dome it will support will be vast—with a footprint as big as a soccer field and almost as tall as the Statue of Liberty— and unexpectedly nimble: It will smoothly rotate on rails as a giant Telescope inside tracks stars through the night.
Everything about the aptly named Extremely Large Telescope (ELT) inspires awe. Its main reflector will be a bowl of silvered glass 39 meters across—the size of three IMAX screens—and the patchwork of 798 hexagonal Mirror segments that comprise it must together form a perfect parabola down to a few tens of nanometers. The biggest optical telescope ever built, it will take pictures of Earth-like worlds around others stars and look for signs of life in their atmospheres.
A few months ago, the European Southern Observatory (ESO), which is building the $1.5 billion telescope, declared that construction had passed the halfway mark. Next month, the first batch of polished mirror segments should set off from France by ship in a temperature-controlled container. “It really is going to happen,” says astronomer Richard Ellis of University College London.
As ELT marches toward completion in 2028 or 2029, U.S. astronomers can only look on in envy. A pair of similarly ambitious U.S.-led projects—the Giant Magellan Telescope (GMT) and the Thirty Meter Telescope (TMT)—once vied with ELT to be first on the sky. Although the projects are also polishing mirrors, they have not begun construction—and success is not assured. Unable to find enough funding from private and international sources, they are waiting for the National Science Foundation (NSF) to bail them out by paying at least 25% of their combined cost of about $5 billion. Earlier this month, NSF Director Sethuraman Panchanathan, speaking to a Senate committee, said the agency was looking to make a budget request for construction funding— but not until fiscal years 2025 or 2026. “There is some urgency,” says John O’Meara, chief scientist of the W. M. Keck Observatory. “They need investment by NSF to move forward or the projects are in serious jeopardy.”
ESO, with 16 member nations and a treaty that guarantees steady annual income, can pay for ELT without serious sacrifices, although a few years ago it had to shrink the mirror from an originally planned 42 meters. But for TMT and GMT, led respectively by the California Institute of Technology and the University of California and by the Carnegie Institution for Science, it’s been a struggle. Fundraising has stalled even though the projects have attracted international partners in Australia, Brazil, South Korea, Japan, India, Canada, and China. Ellis, a former TMT board member, thinks it was unrealistic for private institutions to think they could manage multibillion-dollar projects. “This is well beyond any individual university or consortium,” he says.
The two projects acknowledged as much in 2018. Burying the hatchet after years of competition, they joined forces to propose that NSF take a share in the two telescopes, giving publicly funded U.S. astronomers access. In 2021, that proposal, dubbed US-ELT, was named the highest priority in ground-based astronomy by the field’s “decadal survey,” a community exercise that is meant to guide NSF and Congress.
Both projects passed NSF’s preliminary design review earlier this year. But that is just the first of many hurdles. Next week, an NSF panel will assess whether the projects are ready to move to final design and will make a recommendation to Panchanathan. Only after the projects are in the final design stage will NSF consider requesting construction funding from Congress.
Even for the U.S. government, the ask may be too big. Just one of the telescopes would be NSF’s biggest ever outlay for a facility, and Ellis isn’t sure NSF’s astronomy division can convince others that contributing to both at a cost of well over $1 billion is justified. Another problem is operating costs, which at NSF come out of the same account as grants to researchers. Funding one-quarter of US-ELT’s day-to-day operations—likely to be more than $100 million annually—would create a painful squeeze for the astronomy division. Ellis thinks NSF’s slowness to act is a sign of reluctance. “After 2 years there’s nothing to show,” he says. “It would be a tragedy for U.S. astronomy if they don’t get built.”
In the case of TMT, there is an added complication. TMT’s chosen site, near the summit of Mauna Kea on Hawaii’s Big Island, has been opposed by local people including Native Hawaiians who want no more telescopes built on what they consider a sacred place. Protesters blocked an attempt to begin construction in 2015. A second attempt in 2019 was also stymied after protesters built a camp along the summit access road.
Now, says TMT Executive Director Robert Kirshner, “We’re doing a lot of community work in Hawaii. … We have to find a solution that is agreeable to everyone.” NSF, too, is legally required to assess the project’s environmental, historical, and cultural impacts before endorsing it. Officials held public meetings on the Big Island in the summer of 2022, some marked by fractious debates. The agency is now evaluating what it learned. Linnea Avallone, NSF’s Chief Officer for Research Facilities, says the process will take 2 years to complete.
At the GMT site at Las Campanas Observatory in Chile the seven mirrors will be combined to form a 25.4-meter main reflector. It will give GMT a wide field of view, perfect for surveys, says Chief Scientist Rebecca Bernstein. “Some kinds of science need to get large numbers of objects very efficiently,” she says. The support structure for this mirror is fully designed, Bernstein says, and the team is preparing to begin construction.
Like ELT, TMT will consist of a mosaic of smaller mirrors, an approach pioneered on the existing 10-meter Keck telescopes. At the Richmond, California, plant of the company Coherent, an assembly line is polishing three 1.5-meter mirrors each month before trimming them into hexagons. While Coherent is making 230 Mirror Segments, other teams in India, China, and Japan will produce the remainder of TMT’s required 492 (plus 80 spares). Kirshner says 97 are done so far. Canada is poised to build the telescope’s enclosure and Japan has committed to building its support structure—once the site dispute has been settled.
ELT has had hiccups of its own. The €400 million contract for its structure and enclosure—the biggest ESO has ever awarded—went to a consortium of two Italian companies, Astaldi and Cimolai. When Astaldi went bankrupt in 2018, Cimolai had to take over, which led to delays that ESO says it has recovered from. French company Safran-Reosc is producing up to four mirror segments a week at a dedicated factory in Poitier and is ramping up, says ELT program manager Roberto Tamai. Once the first 45 have passed inspection, they’ll be packaged up and dispatched to Chile. Tamai says: “Now you can touch with your hand things that were only on paper for so many years.”
