WASHINGTON — Recently, in what amounted to a kind of cosmic Supreme Court hearing, two giant telescope projects pleaded for their lives before a committee charged with charting the future of American astronomy.

Either of the telescopes — the Thirty Meter Telescope, slated for the top of Mauna Kea in Hawaii, and the Giant Magellan Telescope in Chile — would be roughly three times larger and 10 times more powerful than anything now on Earth. Working in concert, they could tackle deep questions about the cosmos. But they are hundreds of millions of dollars short of the money needed to build them.

Failure to build them, American astronomers say, would cede dominion over the skies to Europe, which is building its own behemoth observatory in Chile, and which will be available only to European researchers. The prospective builders fear an echo of a moment in the late 20th century when scientists in the United States lost ground in particle physics to European researchers, and never really recovered in producing path-making discoveries in that field.

“Europe is utterly indifferent to what the U.S. does,” said Matt Mountain, in a rousing introduction to the hearing, which was held in a low-ceilinged, windowless conference room on the ground floor of a National Academy of Sciences building here. Dr. Mountain is president of Association of Universities for Research in Astronomy, which manages observatories for the government.