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Atacama Large Millimeter Array
ALMA Antennas on Chajnantor.jpg
Alternative names Atacama Large Millimeter and Submillimeter Array Edit this at Wikidata
Part of Event Horizon Telescope
Llano de Chajnantor Observatory Edit this on Wikidata
Location(s) Atacama Desert, Antofagasta Region, Atacama Desert, Chile Edit this at Wikidata
Coordinates 23°01′09″S 67°45′12″W / 23.0193°S 67.7532°W / -23.0193; -67.7532 Edit this at Wikidata
Organization European Southern Observatory
National Institutes of Natural Sciences, Japan
National Science Foundation Edit this on Wikidata
Altitude 5,058.7 m (16,597 ft) Edit this at Wikidata
Built March 2013; 10 years ago (2013-03)
Telescope style radio interferometer Edit this on Wikidata
Website Edit this at Wikidata
Atacama Large Millimeter Array is located in Chile
Atacama Large Millimeter Array
Location of Atacama Large Millimeter Array

The Atacama Large Millimeter/submillimeter Array (ALMA) is an astronomical interferometer of 66 radio telescopes in the Atacama Desert of northern Chile, which observe electromagnetic radiation at millimeter and submillimeter wavelengths. The array has been constructed on the 5,000 m (16,000 ft) elevation Chajnantor plateau – near the Llano de Chajnantor Observatory and the Atacama Pathfinder Experiment. This location was chosen for its high elevation and low humidity, factors which are crucial to reduce noise and decrease signal attenuation due to Earth's atmosphere. ALMA provides insight on star birth during the early Stelliferous era and detailed imaging of local star and planet formation.

ALMA is an international partnership amongst Europe, the United States, Canada, Japan, South Korea, Taiwan, and Chile. Costing about US$1.4 billion, it is the most expensive ground-based telescope in operation. ALMA began scientific observations in the second half of 2011 and the first images were released to the press on 3 October 2011. The array has been fully operational since March 2013.


The first two ALMA antennas linked together as an interferometer
Three ALMA antennas linked together as an interferometer for the first time
The ALMA correlator

The initial ALMA array is composed of 66 high-precision antennae, and operates at wavelengths of 3.6 to 0.32 millimeters (31 to 1000 GHz). The array has much higher sensitivity and higher resolution than earlier submillimeter telescopes such as the single-dish James Clerk Maxwell Telescope or existing interferometer networks such as the Submillimeter Array or the Institut de Radio Astronomie Millimétrique (IRAM) Plateau de Bure facility.

The antennae can be moved across the desert plateau over distances from 150 m to 16 km, which will give ALMA a powerful variable "zoom", similar in its concept to that employed at the centimeter-wavelength Very Large Array (VLA) site in New Mexico, United States.

The high sensitivity is mainly achieved through the large numbers of antenna dishes that make up the array.

The telescopes were provided by the European, North American and East Asian partners of ALMA. The American and European partners each provided twenty-five 12-meter diameter antennae, for a subtotal of fifty antennae, that compose the main array. The participating East Asian countries are contributing 16 antennae (four 12-meter diameter and twelve 7-meter diameter antennae) in the form of the Atacama Compact Array (ACA), which is part of the enhanced ALMA.

By using smaller antennae than the main ALMA array, larger fields of view can be imaged at a given frequency using ACA. Placing the antennae closer together enables the imaging of sources of larger angular extent. The ACA works together with the main array in order to enhance the latter's wide-field imaging capability.

ALMA site under the arc of the Milky Way, photo by Stéphane Guisard, 2012

Scientific results

Photos from initial testing

Antennae Galaxies composite of ALMA and Hubble observations
Antennae Galaxies composite of ALMA and Hubble observations
HL Tau protoplanetary disk
HL Tauri protoplanetary disk.

By the summer of 2011, sufficient telescopes were operational during the extensive program of testing prior to the Early Science phase for the first images to be captured. These early images gave a first glimpse of the potential of the new array that will produce much better quality images in the future as the scale of the array continues to increase.

The target of the observation was a pair of colliding galaxies with dramatically distorted shapes, known as the Antennae Galaxies. Although ALMA did not observe the entire galaxy merger, the result is the best submillimeter-wavelength image ever made of the Antennae Galaxies, showing the clouds of dense cold gas from which new stars form, which cannot be seen using visible light.

Comet studies

On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).

Planetary formation

An image of the protoplanetary disc surrounding HL Tauri (a very young T Tauri star in the constellation Taurus) was made public in 2014, showing a series of concentric bright rings separated by gaps, indicating protoplanet formation. As of 2014, most theories did not expect planetary formation in such a young (100,000-1,000,000-year-old) system, so the new data spurred renewed theories of protoplanetary development. One theory suggests that the faster accretion rate might be due to the complex magnetic field of the protoplanetary disc.

Event Horizon Telescope

ALMA participated in the Event Horizon Telescope project, which produced the first direct image of a black hole, published in 2019.

Phosphine in the atmosphere of Venus

ALMA participated in the claimed detection of phosphine, a biomarker, in the air of Venus. As no known non-biological source of phosphine on Venus could produce phosphine in the concentrations detected, this would have indicated the presence of biological organisms in the atmosphere of Venus. Later reanalyses cast doubt on the detection, although later analyses confirmed the results. The detection remains controversial, and is awaiting additional measurements.

Project timeline

The final ALMA antenna
The final ALMA antenna.
Date Activity
1995 ESO/NRAO/NAOJ joint site testing with Chile.
May 1998 Start of phase 1 (design & development).
June 1999 European/U.S. memorandum of understanding for design & development.
February 2003 Final European / North American agreement, with 50% of funding from ESO, and 50% of funding shared between USA and Canada.
April 2003 Testing of first prototype antenna begins at the ALMA Test Facility (ATF) site in Socorro, New Mexico.
November 2003 Groundbreaking ceremony at ALMA site.
September 2004 European, North American & Japanese draft agreement, with Japan providing new extensions to ALMA.
October 2004 Opening of Joint ALMA office, Santiago, Chile.
September 2005 Taiwan joins the ALMA Project through Japan.
July 2006 European, North American & Japanese amend agreement on the Enhanced ALMA.
April 2007 Arrival of first antenna in Chile.
February 2008 Arrival of the two ALMA transporters in Chile.
July 2008 First antenna movement with a transporter.
December 2008 Acceptance of the first ALMA antenna.
May 2009 First interferometry with two antennae at the Operations Support Facility (OSF).
September 2009 First move of an ALMA antenna to Chajnantor.
November 2009 Phase closure with three antennae at Chajnantor.
2010 Call for shared-risk Early Science proposals.
September 2011 Start of Early Science Cycle 0. Sixteen 12-m antennae in the 12-m array.
February 2012 First paper published with ALMA data
January 2013 Start of Early Science Cycle 1. Thirty-two 12-m antennae in the 12-m array.
March 13, 2013 ALMA Inauguration.
September 23, 2013 66th and final antenna arrived and accepted.
June 2014 Start of Early Science Cycle 2. Thirty-four 12-m antennae in the 12-m array, nine 7-m antennae in the 7-m array, and two 12-m antennae in the TP array.
June 2018 ALMA 1000th published paper
March 2020 ALMA shut down due to the COVID-19 crisis

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See also

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