2.09.2021

“Decadal survey” considers $1.8-billion bailout for Thirty Meter Telescope and Giant Magellan Telescope

The Giant Magellan Telescope (left) and the Thirty Meter Telescope, shown in artists’ concepts, are making a joint pitch for federal funding.GIANT MAGELLAN TELESCOPE/GMTO CORPORATION; TMT INTERNATIONAL OBSERVATORY

Roughly every 10 years since the 1960s, U.S. astronomers have provided a valuable show of consensus to U.S. funding agencies and Congress by agreeing on which questions and new facilities are critical to the field. But the current “decadal survey,” known as Astro2020 and scheduled to be published at any time, faces a particularly knotty question, one that could settle whether the United States stays in the front rank of ground-based observing. Should the National Science Foundation (NSF) come to the rescue of two struggling private projects to build giant optical telescopes in exchange for a chunk of observing time?

The future of the Giant Magellan Telescope (GMT) and the Thirty Meter Telescope (TMT) likely depends on whether the survey recommends that NSF spend what sources put at $1.8 billion to support a recently forged partnership between the projects. If it does, other proposals could lose out, such as a ­continent-spanning radio array and detectors for neutrinos and other cosmic particles. (Space missions are ranked separately.)

Understandably, astronomers are divided. The GMT-TMT proposal “is critical for the field to thrive,” says John O’Meara, chief scientist of the W. M. Keck Observatory. With Europe pushing ahead with its own giant telescope, “If a federal partnership does not happen, I believe that the U.S., which has been the international leader in the field of astronomy for a century, will pass that role on to Europe,” says Wendy Freedman of the University of Chicago. But Richard Ellis of University College London, former director of the Palomar Observatory, believes rescuing both telescopes would cost “too much money and would eclipse so many other things.”

For ground-based optical astronomers, giant telescopes with mirrors about 30 meters across are the obvious next step after the huge advances made with today’s 10-meter scopes. O’Meara says there is no other way to image an Earth-like planet around a red dwarf star, for example. “No matter how tricky you get, the laws of physics overrule you,” he says. “Aperture is king.”

Telescope designers have been planning such behemoths since the 1990s, and the European Southern Observatory is laying the foundations for its 39-meter Extremely Large Telescope (ELT) on the summit of Cerro Armazones in Chile, with first light due in 2027. But divisions over technology and funding have hampered the two U.S.-led projects. The TMT, to be built in Hawaii by the California Institute of Technology and the University of California (UC), will have a honeycomb mirror built of 492 hexagonal segments. In contrast, the GMT, led by the Carnegie Institution for Science, will arrange six 8.4-meter mirrors around a seventh, giving the Chile-based telescope an aperture of 24.5 meters. The projects had early talks about joining forces, but “there was no desire to abandon their telescopes,” Ellis says. “Once money started flowing, it was impossible to merge.”

The 2000 decadal survey rated a giant segmented-mirror telescope (essentially the TMT) as the No. 1 U.S. priority in ground-based projects. NSF started discussions with the TMT about partnership in the project, but backed away after the GMT objected. In the 2010 decadal a giant ground-based scope dropped to No. 3, behind a large survey telescope and an instrument innovation program—a decision that essentially killed federal involvement for another 10 years. “That really damaged the momentum of the project,” says Garth Illingworth of UC Santa Cruz.

Both projects set out to raise their own funds, but neither has sufficient money so far. The TMT has also faced continued opposition, including legal challenges, from Native Hawaiians to building such a large structure on  Mauna Kea, which they consider sacred.

For the current decadal, the projects have finally united into a two-telescope package that would give all U.S. astronomers access to at least 25% of the observing time in exchange for the $1.8 billion (a figure that is not officially confirmed). This deal, dubbed US-ELT, would give U.S. astronomers an advantage over Europeans: front rank telescopes in both the Northern and Southern hemispheres. “The U.S. really does need [giant telescopes] to follow up on other investments on the ground and in space,” Illingworth says.

Those include the James Webb Space Telescope (JWST), set for launch later this year. The JWST should revolutionize astronomy by picking out objects including the earliest galaxies with its pin-sharp infrared eyes, but fine-grained spectrometers on the ground will be needed to follow up on some discoveries. Then there is NSF’s Vera C. Rubin Observatory in Chile, a survey telescope that from 2023 will carry out a census of the sky nearly every night, identifying thousands of objects demanding closer investigation. “It would be an admission of defeat to let Europe take over this area,” Illingworth says.

Others say the U.S. giants are too far behind to avoid that outcome, and the costs are prohibitive. “How can you make a sales pitch for two telescopes when the rest of the world has one?” Ellis asks. Astronomers also worry about the consequences of funding US-ELT for projects such as a next-generation upgrade of the Very Large Array Radio Telescope (ngVLA) in New Mexico into a continent-spanning network of 263 dishes. “The ngVLA is no silly idea, it’s something we must do,” O’Meara says. The science cases behind upgrading the IceCube Neutrino Observatory at the South Pole and building a next-generation detector for the cosmic microwave background radiation are similarly compelling.

The decadal may also decide that U.S. astronomy doesn’t need to pursue an endless quest for ever-greater expanses of glass on the ground. Some astronomers think the greatest potential for discovery lies in modest new telescopes, or upgrades of older ones, with multiobject spectrographs that can scrutinize thousands of objects at once. “It’s a different vision,” says Ray Carlberg of the University of Toronto, and one that requires a team-based approach more familiar to particle physicists. “So much more science can be done with large groups of collaborators working together on a range of projects,” says Jennifer Marshall of Texas A&M University, College Station, project scientist of the proposed Maunakea Spectroscopic Explorer.

The 20-strong decadal committee, after a delay of more than 6 months because of the COVID-19 pandemic, should soon deliver its verdict, bringing joy to some and misery to others. “You need really big pockets” to build giant telescopes, Carlberg says. “The only people who can build them now are nations or consortia of nations.”

Quelle: AAAS