At 16,400 feet in the driest desert on Earth, 66 antennas work together as one telescope. It captured the first-ever image of a black hole's shadow in 2019.
ESO/B. Tafreshi (twanight.org), CC BY 4.0
A telescope in the clouds
On the Chajnantor Plateau, 5,000 meters (16,400 ft) up in the Andes Mountains, sits the Atacama Large Millimeter/submillimeter Array (ALMA). This location in the Chilean Atacama Desert was selected for its high elevation and extreme dryness, factors that minimize the atmospheric water vapor that otherwise absorbs the faint millimeter and submillimeter waves from space. ALMA is not a single telescope but an array of 66 high-precision antennas that work in concert. The main array consists of fifty 12-meter antennas, complemented by a smaller, more compact group of four 12-meter and twelve 7-meter antennas known as the Atacama Compact Array.
The entire project is an international partnership between Europe, North America, and East Asia, in cooperation with the Republic of Chile. The construction, which began in the early 2000s after decades of planning by separate international groups, was difficult to build due to the remote and harsh environment. The observatory was officially inaugurated in March 2013 and has been conducting scientific observations since 2011. The individual antennas, weighing over 100 tons each, have surfaces that are accurate to within 20 micrometers—less than the width of a human hair.
A lens 16 kilometers wide
ALMA achieves its remarkable resolution through a technique called interferometry. By combining the signals from all 66 antennas, the array is a single, virtual telescope with a diameter equal to the largest distance between the individual antennas. These antennas can be moved to 192 different concrete pads, creating configurations that range from a compact 150 meters to an expansive 16 kilometers (nearly 10 miles) across. This "zoom lens" capability allows astronomers to study both the broad structure of large gas clouds and the fine details of planet formation.
The data collected by each antenna is transmitted via optical fiber to a central building, where one of the world's most powerful supercomputers, the ALMA Correlator, processes the information. To combine the signals accurately, the system requires synchronization with a precision of one-millionth of a millionth of a second. The result is an angular resolution up to ten times sharper than that of the Hubble Space Telescope, capable of distinguishing a golf ball from 15 kilometers away.
Seeing the cold universe
ALMA is designed to observe the "cold universe"—the faint radio waves emitted by molecular gas and cosmic dust, often at temperatures just tens of degrees above absolute zero. Scientists use this to study the building blocks of stars, galaxies, and planets. In 2014, ALMA captured a revolutionary image of the young star HL Tauri, showing a protoplanetary disk with distinct gaps created by forming planets. This provided clear evidence that planets form faster than many theories had predicted.
The observatory was also an important part of the Event Horizon Telescope (EHT) collaboration, which in 2019 released the first-ever image of a black hole's event horizon—the supermassive black hole at the center of the galaxy Messier 87. ALMA's high sensitivity was essential for calibrating the data from the global network of telescopes. Beyond these milestones, ALMA has detected complex organic molecules like sugars and methyl isocyanate in interstellar clouds, showing the chemical origins of life. It continuously maps the gas and dust that fuel star formation in galaxies near and far, including some of the earliest galaxies in the universe, seen as they were over 13 billion years ago.
💡Fun Facts
Two custom 132.5-ton transporters, named Otto and Lore, are used to move the 100-ton antennas between different observing pads.
The data rate from the array can reach up to 150 megabytes per second, with about 100 terabytes of data being archived each year.
The high-altitude site on the Chajnantor Plateau is so dark that the Milky Way can cast a visible shadow.
The international partnership that created ALMA is so extensive that the project would have been impossible for any single nation to achieve.
Free guided public tours of the Operations Support Facility (OSF) at 2,900 meters are typically held on Saturday and Sunday mornings. Advance registration is mandatory and fills up months in advance. The high-altitude site (5,000m) is not open to the public for safety reasons.
Admission
Free
Accessibility
The OSF is generally accessible, but the high altitude can pose health risks. Visitors must meet age and health requirements. The tour does not go to the 5,000-meter antenna site.
