Why was the Chajnantor site chosen to build ALMA?
ALMA is being built in the Chilean Atacama desert – the driest place on Earth, 5,000 meters (16,500 feet) above sea level. This is the perfect place for a new telescope capable of detecting radio waves just millimeters in wavelength, because the atmosphere is extremely dry. Radio waves penetrate a lot of the gas and dust in space, and can pass through the Earth’s atmosphere with little distortion. However, if the atmosphere above ALMA contained water, the radio signals would be heavily absorbed – the tiny droplets of water scatter the radio waves in all directions before they reach the telescope, and would degrade the quality of the observations.
