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Dava Sobel hears from telescope operators at ALMA, the remote observatory high in the Atacama Desert in Chile, talking to us with their oxygen tanks at the ready. Episode two of five.\\nAs we hear, the \\u2018radio sky\\u2019 presents an alternate universe, in which the Moon and planets are barely detectable. In their place are clouds of interstellar gas and other exotic celestial sources which reveal different aspects of our history and astronomy. At ALMA, the radio astronomers do not need to wait until dark to make their observations but can work at any hour, day or night.

(Photo: The ALMA Observatory is located in the Chajnantor Plateau over 5,000 metres above sea level. Credit: ALMA/ESO/NAOJ/NRAO)