The high-desert telescope is the product of two decades of work. In the 1990s astronomy teams from Europe, North America and Japan were all planning new radio arrays but, in an unusual move, decided to sidestep the normally competitive world of telescope construction and collaborate on one massive project.
Finding a site was the next challenge. Unlike their optical siblings, radio telescopes can operate in daylight. But to collect the frail radio waves emitted by objects across vast space—particularly the water vapor-vulnerable short wavelengths that defied detection by other observatories but that were the primary aim of the new telescope—ALMA would need an exceptionally parched and transparent atmosphere. Few places fit those criteria better than the 5,000-meter Chajnantor Plateau—a blustery Andean mesa perched over Chile's Atacama Desert, one of the driest places on Earth, averaging just 1.5 centimeters of precipitation a year.
Construction at such altitudes is challenging: Workers need medical monitoring and carry oxygen tanks on every trip from the 3,000-meter elevation operations center to the vertiginous plateau. Also, the strong radiation from the sun at this altitude can distort the dishes' accuracy, an effect space-based telescopes have always battled, says ESO representative Massimo Tarenghi.
"It's like building 66 space telescopes," he says, but "we also have to consider the wind, the snow, the rain, the humidity, the dust."
Of the 43 saucers now roosting on Chajnantor, most are 12 meters in diameter—broad enough to comfortably cap the Washington Monument. As scientists turn their attention to a new celestial target, the dishes roll their gaze in unison across the lucent Andean sky, their synchronized dance witnessed by snow-capped volcanoes.
That they work at all is an impressive feat: If a dish's surface has an imperfection measuring as small as the diameter of a human hair, it will not perform. The antennas' receivers are chilled to just a few degrees above absolute zero—any warmer and their own radio emissions would interfere with those they receive.
To make things more complicated, no fewer than 20 countries contribute financially and scientifically to ALMA, a collaboration that has undoubtedly slowed the project—but also resulted in an excellent final product, Tarenghi says. Because many nations' science budgets are shrinking, such collaborations are on the horizon—including a radio telescope with a square kilometer surface area to be located in South Africa or Australia. That project could be complete by 2024.
In the meantime, astronomers will make due with ALMA, whose entire array should be online by the end of 2013. That's when the really big discoveries will come, Tarenghi says. "Then—I am exaggerating just a little bit—we can point to any part of the sky and see something new," he adds. "We have a window to the universe that's not available any other place in the world."