Where is Chajnantor?

ALMA is the biggest astronomical observatory on Earth. It was built by countries in North America, Europe and East Asia. You might expect that ALMA is located in one of these three continents. But you would be wrong. ALMA is in South America.

The best place to build your observatory would be in space, high above the atmosphere. That’s because it’s very difficult to detect faint millimeter waves from the Universe through the atmosphere of the Earth. But so far, no one has figured out how to launch 66 big antennas into space for a reasonable price.

The next best place for an observatory like ALMA is a very high mountaintop on Earth. The sky should be as clear as possible. The air should be very, very dry. And the less people live in the neighborhood, the better.

Such places are hard to find in North America, Europe and East Asia. But in South America, specifically in Chile, astronomers have found the perfect spot for ALMA. It’s called llano de Chajnantor – the Chajnantor Plateau. That’s where the 66 ALMA antennas are studying the Universe.


Chajnantor is in Chile, a country in South America. Chile is the longest and thinnest country in the world. The northern part of Chile lies in the Tropics; the southern tip of the country is close to Antarctica. Chajnantor is in the north of Chile, high in the Atacama Desert.

The Atacama Desert is one of the driest places on Earth. That’s good, because ALMA needs very dry air. Moreover, the Chajnantor Plateau is very high: 5,000 meters above sea level. That’s also good, because an observatory for millimeter waves needs to be at a very high altitude. Finally, the sky above Chajnantor is usually very clear, with no hint of clouds, and there are only a few small villages in the area, most of them at least 40 kilometers away. Chajnantor is the perfect place for ALMA.

It’s not easy to get to ALMA, because it is so remote. First, you have to take the plane to Santiago, the capital of Chile. From there, a smaller plane takes you to the town of Calama. In Calama, you have to take the bus to the small village of San Pedro de Atacama – a laid-back oasis in the desert.

From San Pedro, it takes almost 40 minutes to reach to the ALMA base camp, called the Operations Support Facility (OSF), located at an altitude of 2,900 meters. There you find the offices, laboratories, the antenna control room, dorms and the cantina. From that point, a 28 kilometer-long steep mountain road finally gets to the Chajnantor Plateau, at an altitude of 5,000 meters – the home of ALMA.