ALMA is the highest observatory in the world. The Chajnantor Plateau, where the 66 ALMA antennas are located, is at five kilometers above sea level. It would take you about an hour to walk that distance – straight up!
Only one building in the world is slightly higher – a train station in the Himalaya Mountains in Tibet.
Many big telescopes and observatories are at high mountaintops. The reason is that the atmosphere of the Earth hampers astronomical observations. In the mountains, the air is thinner and clearer, so you have a better view of the stars.
But in the case of ALMA, it is even more important to be at a very high site. That’s because ALMA studies millimeter waves from the Universe – a special form of ‘invisible light’. Millimeter waves from outer space can only be seen from very high mountaintops. At sea level, ALMA would be completely blind!
But if you’re at a very high altitude, there’s not so much atmosphere above your head anymore. Not all the millimeter waves from the Universe are absorbed. That’s why ALMA has to be at a very high site. And that’s why the air above the ALMA antennas has to be very dry, so it contains little water. Chajnantor is the perfect spot!