Some places in the Chilean Andes are exquisite for astronomy (clouds are rare, rain rarer) and yet very inhospitable to humans. One such location is a plateau called Chajnantor, perched in the Atacama Desert at an altitude of 16,400 feet (5,000 meters).The stark quiet of this incredibly remote plateau was broken recently when a massive, 14-wheel transporter trundled slowly along a dirt road, eased into position, and gently placed a 100-ton radio dish onto a concrete pad. It's the first of 66 precision antennas, with diameters of 40 and 23 feet (12 and 7 meters), that will eventually make up the Atacama Large Millimeter/submillimeter Array (ALMA).
The air over Chajnantor is so thin — roughly half the pressure at sea level — that ALMA's dishes are being constructed 17 miles away at a relatively breathable altitude of 9,500 feet (2,900 m).
But the rarefied air is the key to ALMA's eventual success. The energy that these sensitive dishes will collect, between infrared and radio wavelengths, is absorbed strongly by water vapor. By picking such a high, arid site, ALMA's managers will have the clearest view possible of the cosmos at millimeter and submillimeter wavelengths.
More of the big dishes will be arriving soon. ALMA scientists hope to link three antennas by early 2010 and to make their first scientific observations about 15 months later. They'll be probing a largely unexplored band of the electromagnetic spectrum, at very high resolution, to study some of the coldest objects in the cosmos. Among these are the dense clouds of gas and dust where stars form, and distant, dusty, star-forming galaxies near the edge of the observable universe.