Atacama Large Millimeter Array, Radio interferometer in Atacama Desert, Chile

The Atacama Large Millimeter/submillimeter Array (ALMA) is an interferometer of 66 radio telescopes on the Chajnantor plateau at 5,000 m in northern Chile. Fifty 12-metre dishes form the main array, while the Atacama Compact Array adds four 12-metre and twelve 7-metre antennas. Each dish is a shallow circular reflector on a white base, set on dark paved pads; a curved track arcs across the reddish-brown plain.

ALMA grew from three projects: the US Millimeter Array, Europe's Large Southern Array and Japan's Large Millimeter Array. In 1997 NRAO and ESO agreed to merge the first two, and the name ALMA was chosen in March 1999. The ALMA Agreement was signed on 25 February 2003, with Japan joining Enhanced ALMA on 14 September 2004. Observations began in 2011 and the array was fully operational in March 2013.

The 1.4 billion dollar observatory emerged through cooperation between Europe, the United States, Canada, Japan, South Korea, Taiwan, and Chile. Scientists from participating nations use the facility to investigate fundamental questions about star, galaxy, and planet formation while contributing to advances in international radio astronomy research and understanding of the universe.

The site sits approximately 31 miles (50 kilometers) east of San Pedro de Atacama via unpaved roads. Due to extreme elevation and health risks, public access is severely restricted; tours require advance booking and occur only on weekends at a lower elevation facility. The dry desert environment and cloudless skies enable research operations year-round under optimal atmospheric conditions with minimal water vapor interference.

The telescopes can be distributed and repositioned across 10 miles (16 kilometers), making the installation function like a zoom lens. This flexibility allows astronomers to switch between wide-angle views of large sky regions and extremely detailed close-ups of individual objects by simply adjusting the distance between antennas. Specialized transporter vehicles move the antennas to their respective positions for different observation campaigns.