ALMA Dish Takes the High Road

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).

Delivbery of first ALMA dish
A transporter vehicle named Otto carefully carries the first radio antenna of the ALMA array to its permanent site in Chile's Atacama Desert.
European Southern Observatory
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.

5 thoughts on “ALMA Dish Takes the High Road

  1. Orville

    Aviators are required to have supplemental oxygen above 10,000 feet (about 3,000 meters) MSL. Will the technicians and scientists working at 5,000 meters (16,400 feet) need extra oxygen?

  2. Fred ShumanFred from Laurel, Md

    Yes, by my calculations, the mean pressure up there is around 540mb (sea level P is 1013mb). This would be like being at sea level in air that was 11% O2 instead of the usual 21%. I doubt that this has been overlooked, and if people are going to be physically onsite, I think they’ll be provided for. On the other hand, a professional astrophysicist who got time on Mauna Kea (~13000 ft, and no O2 is supplied there) once told me how difficult it is up there even to remember what you just did 5 sec ago, so maybe they’ll just have to deal. It would take a couple weeks to a couple months at some intermediate alt. to get better accustomed to low O2 levels (the body gets stimulated to make more RBCs), and that’s difficult or impossible for a busy researcher. Of course, humans (even excluding Sherpas!) have functioned at much higher altitudes (think of the # of people who’ve climbed Everest — 29000 ft!), but they haven’t been tasked with anything as intricate and demanding of concentration as scope operation at those altitudes. Another possibility is that the site will be equipped for remote operation. Anyone know?

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