Four centuries ago Galileo pointed his spyglass toward the heavens and astronomy changed forever. As the world celebrates the 400th anniversary of the telescope, another cosmological revolution is coming: The Giant Magellan Telescope (GMT), Thirty Meter Telescope (TMT) and European Extremely Large Telescope (E-ELT)—all expected to see first light by 2020—will dwarf the biggest observatories in use today.

The largest, the 42-meter (138-foot) E-ELT, will gather 15 times more light than today's 10-meter (33-foot) optical telescopes. TMT, with its 30-meter- (98.5-foot-) diameter primary mirror, and GMT, delivering the resolving power of a 24.5-meter (80-foot) reflector, will also outclass any current optical telescope.

Astronomers have always wanted bigger telescopes to resolve ever fainter objects. Large-diameter telescopes, essentially big light buckets, collect more photons for a given amount of observing time. Bigger mirrors also boost a telescope's angular resolution, or its ability to measure the separation between two close objects.

This next generation of big telescopes follows the leap in technology achieved with the W. M. Keck Observatory on Mauna Kea, Hawaii, in the 1990s.

"We've been using the Keck telescopes for 10 years and we saw … the limitations of 10-meter apertures," says Chuck Steidel, professor of astronomy at the California Institute of Technology and chair of the Science Advisory Committee for TMT. "So we started thinking: there's going to be a next-generation ground-based telescope, (and) we need to start thinking about it now."

The race to build the observatories is driven in part by "not wanting to be left behind," Steidel says. "There's plenty of science to occupy multiple versions of these telescopes. … Having one of these to be shared for the whole world—that would be a shame, actually, because there's more than one telescope worth of things to be doing."

But size isn't everything. These telescopes will rely on adaptive-optics technology that incorporates a deformable (shape-changing) mirror to cancel out atmospheric turbulence. That is expected to make these scopes perform as well as any space-based visible-wavelength observatory—including the Hubble Space Telescope. Collectively, they are expected to illuminate an epoch when the first stars and galaxies lit up the void more than 12 billion years ago, probe the atmospheres of exoplanets, and unveil the exotic physics of supermassive black holes at the center of the Milky Way as well as distant galaxies.

For Avi Loeb, a theoretical physicist and professor of astronomy at Harvard University, these instruments will allow researchers to add the missing pages from astronomy's "photo album of the universe."
"In order to probe deeper into the universe, you really need bigger and bigger telescopes," Loeb says. "Modern telescopes allow us to see galaxies when the universe was a billion years old. We would like to see the very first galaxies. … This is sort of the first chapter of Genesis—'Let there be light.'"

Closer to home, obtaining spectra from planets orbiting other stars could be within the reach of these next-generation telescopes, says Paul Kalas, the University of California, Berkeley, astronomer who used Hubble to take the first ' visible-light image of an exoplanet called Fomalhaut b—25 light-years from Earth. "We're lucky enough to see it as a point of light, but to disperse that light and to obtain even a low-resolution spectrum seems beyond the capabilities that we have today," Kalas says.

With unprecedented angular resolution and light-collecting capacity, the new telescopes should be able to focus the light of stars so sharply that astronomers will be able to discern planets close enough to the star where Earth analogues might be found, Kalas says. Spectral analysis of such planets may reveal biomarkers such as oxygen, methane and nitrous oxide.
GMT, TMT and E-ELT are expected to work in partnership with space telescopes and other ground-based observatories. For example, NASA's James Webb Space Telescope, scheduled to launch in 2013, will catalogue some of the universe's most distant objects in the infrared. The three optical telescopes will then examine those objects in more detail.

Modern astronomy is inherently a multiwavelength enterprise, with distinct instruments deployed to characterize objects according to the types of photons they emit across the electromagnetic spectrum, from radio to gamma rays. That's making large-aperture optical instruments increasingly relevant, Steidel says. "You're going to be extremely relied upon to understand things that are discovered with these other facilities, because there's really nothing more sensitive than spectroscopy in the optical or near-infrared with a large-aperture [telescope] from the ground," Steidel says.

E-ELT, projected to cost 950 million euros, or about $1.2 billion, is sponsored by the European Southern Observatory. Candidate sites are in Chile, Argentina, Spain and Morocco.

TMT, at $754 million in 2006 dollars, is a partnership among the Caltech, the University of California, and the Association of Canadian Universities for Research in Astronomy (ACURA). There are two candidate sites: Mauna Kea in Hawaii and Cerro Armazones in Chile's Atacama Desert.

The GMT, estimated at $650 million, is supported by the Carnegie Institution of Washington and eight other astronomical research organizations in the U.S., Australia and South Korea. It's scheduled to be built at Las Campanas Observatory in the Atacama.

E-ELT and TMT are being designed similarly with thin, segmented mirrors arrayed as a contiguous primary mirror. The 42-meter primary mirror of the E-ELT will be composed of 984 segments, whereas the 30-meter primary for TMT will comprise 492 segments.

The GMT opted for seven, 8.4-meter (27.6-foot) honeycomb mirrors mounted together as a primary. Mechanically the mirrors will be very stiff, and because of their largely hollow structure they'll be relatively light, GMT Acting Director Pat McCarthy says.

The three projects, in a race to first light, have nonetheless shared some collective knowledge about their science goals, the quality of potential sites, and other challenges.

"There's always competition, a sportsman's spirit," says Markus Kissler-Patig, an E-ELT project scientist. "We want to make the first discoveries…but despite the competitive spirit my wish is that all three projects really happen."