And both groups are looking over their shoulders at the Europeans, who in April approved a site for the European Southern Observatory's Extremely Large Telescope (ELT), whose 40-meter light-gathering mirror would make it the largest of the three projects.
Even in an era of space-based telescopes, the case for more glass on the ground is compelling, astronomers say. The light-gathering mirrors on space telescopes tend to be relatively small compared with their ground-based counterparts. So they can't perform the detailed studies of distant objects as efficiently as the larger, ground-based observatories.
But these new telescopes are expected to cost between $700 million and $1 billion just to build. Operating costs are likely to range from $70 million to $100 million a year, several researchers say.
In the United States, the choices revolve not only around which, if either, of the two projects to support.