Atacama Large Millimeter Array

• 1. Introduction

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  The Atacama Large Millimeter/submillimeter Array, known as ALMA, sits on a plateau 5,000 meters above sea level in the Atacama Desert of northern Chile. At that altitude, the air is so thin that the truck drivers who move the giant antenna dishes must have oxygen tanks built into their seats just to breathe. That detail alone tells you something about the lengths humanity will go to in order to see the invisible universe.

  ALMA is made up of 66 radio telescopes working in concert, picking up millimeter and submillimeter wavelengths of light that human eyes cannot detect. At a cost of roughly 1.4 billion US dollars, it is the most expensive ground-based telescope ever built. And the partnership behind it spans seven entities: Europe, the United States, Canada, Japan, South Korea, Taiwan, and Chile.

  What draws so many nations to this patch of high desert? The answers lie in what ALMA can see that nothing else can: the cold, dark gas clouds where stars are being born, the rings of material gathering around infant suns to form planets, and at the extreme edge of what it can observe, a direct image of a black hole. What follows is the story of how ALMA was built, who built it, what it found, and why a strike, a pandemic, and a cyber attack each tested the limits of an institution designed to outlast them all.

• 2. Why This Desert

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  The Chajnantor plateau sits near the Llano de Chajnantor Observatory and the Atacama Pathfinder Experiment, placing ALMA in one of the driest, most isolated corners of the planet. High elevation and extremely low humidity were the decisive factors. Both work to cut down on atmospheric noise and reduce the signal loss that Earth's air inflicts on faint millimeter and submillimeter waves.

  Prototype antennae were already being tested at the Very Large Array site in Socorro, New Mexico, as early as 2002. Site testing with ESO, NRAO, and their Chilean counterparts began in 1995, a full eight years before a shovel touched Chajnantor soil. The decision to place the main array at 5,000 meters, with the Operations Support Facility at 2,900 meters below, created a two-tier infrastructure that would later define the logistical challenge of the whole build.

  The choice also had a technical payoff that numbers make concrete. ALMA's spatial resolution reaches 10 milliarcseconds, which is 10 times sharper than the Very Large Array and 5 times sharper than the Hubble Space Telescope. That performance is only possible because of where the array sits. The thin, dry air that makes breathing difficult is precisely what allows millimeter-wave signals to travel from space to the antenna dishes with minimal distortion.

• 3. Three Projects Become One

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  ALMA did not spring from a single vision. It was assembled from three separate national ambitions: the Millimeter Array of the United States, the Large Southern Array of Europe, and the Large Millimeter Array of Japan. Each project had its own goals and its own community of advocates.

  The first concrete step toward merging them came in 1997, when the National Radio Astronomy Observatory and the European Southern Observatory agreed to combine the US and European proposals. The merged design took the sensitivity of the European plan and the frequency coverage and preferred site of the American one. Canada and Spain joined the coordination work at this early stage, with Spain later becoming a full ESO member.

  The name "Atacama Large Millimeter Array" was formally chosen in March 1999. The ALMA Agreement between the North American and European parties was signed on the 25th of February 2003. For those keeping track of the etymology: "alma" means "soul" in Spanish and "learned" or "knowledgeable" in Arabic. Japan joined through a separate high-level agreement signed on the 14th of September 2004, bringing with it the Atacama Compact Array and three additional receiver bands. Taiwan entered the collaboration in September 2005 through the Japanese partnership. What had started as three disconnected national projects was now a single coordinated instrument spanning four continents.

• 4. Building the Array

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  Constructing ALMA required splitting the antenna contracts across the partner regions, a decision that was made partly for political reasons. General Dynamics C4 Systems, through its SATCOM Technologies division, was contracted by Associated Universities, Inc. to build 25 of the 12-meter diameter antennae for the North American contribution. European manufacturer Thales Alenia Space built the other 25 principal antennae, a contract described as the largest-ever European industrial contract in ground-based astronomy. Japan's Mitsubishi Electric assembled the 16 antennae that make up the Atacama Compact Array: four 12-meter dishes and twelve 7-meter dishes.

  The European components crossed an unusual logistics path. A specialist aerospace and astrospace logistics company called Route To Space Alliance transported 26 components manufactured across Europe, delivering them to Antwerp for onward shipment to Chile. Antenna deliveries to the Chajnantor site ran from December 2008 to September 2013.

  The Atacama Compact Array carries its own quiet history. In 2013, it was named the Morita Array in honor of Professor Koh-ichiro Morita, a member of the Japanese ALMA team who designed the ACA and died on the 7th of May 2012 in Santiago. The smaller dishes serve a different purpose from the main array: by placing antennae closer together, the ACA can image sources of large angular extent, such as nearby galaxies and molecular clouds, that the main array's wider configurations might miss.

• 5. Moving Mountains of Steel

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  Each ALMA antenna weighs 115 tonnes. Getting them from the assembly building at 2,900 meters to the observing plateau at 5,000 meters, and repositioning them around the site to change the zoom level of the array, required machinery that did not yet exist when planning began.

  The solution was a pair of custom 28-wheel self-loading heavy haulers built in Germany. Each vehicle is 10 meters wide, 20 meters long, and 6 meters high, and weighs 130 tonnes empty. Twin turbocharged diesel engines producing 500 kilowatts each power the transporters. A driver's seat designed to hold an oxygen tank comes standard, given the altitude at which the vehicles operate.

  The first transporter was completed and tested in July 2007. Both were delivered to the ALMA Operations Support Facility in Chile on the 15th of February 2008. The first antenna movement using a transporter took place on the 7th of July 2008, when a single dish was rolled from the assembly building to a nearby test pad for holographic surface measurements. By autumn 2009, the first three antennae had made the climb to Chajnantor. The story of those transporters was significant enough that it became the subject of the television documentary Monster Moves: Mountain Mission.

• 6. Science at the Edge

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  Early Science observations began in the second half of 2011 with 16 of the eventual 66 antennae in place. The first images ALMA released to the press, on the 3rd of October 2011, targeted the Antennae Galaxies, a pair of colliding galaxies with dramatically distorted shapes. The image was the best submillimeter-wavelength view ever made of that system, revealing cold dense gas clouds invisible to optical telescopes.

  In 2014, ALMA produced an image of the protoplanetary disc surrounding HL Tauri, a very young T Tauri star in the constellation Taurus. The disc was between 100,000 and 1,000,000 years old, and most theories at the time did not predict planet formation could begin so early. The image showed concentric bright rings separated by gaps, suggesting protoplanet formation was already underway. One proposed explanation centered on the complex magnetic field of the disc accelerating the accretion rate.

  Also in 2014, ALMA was used for the first time to map the distribution of molecules, including HCN, HNC, and H2CO, as well as dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON). In 2019, ALMA was a key participant in the Event Horizon Telescope project, which produced the first direct image of a black hole. A claimed detection of phosphine, a potential biomarker, in the atmosphere of Venus followed, though later reanalyses left the finding controversial and awaiting further measurement. In 2022, ALMA launched exoALMA, a detailed survey of 15 protoplanetary disk systems in search of still-forming exoplanets.

• 7. Disruptions and Resilience

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  In August 2013, workers at the telescope went on strike, demanding better pay and improved conditions at high altitude. The strike was among the first ever to affect an astronomical observatory. After 17 days, an agreement was reached that provided reduced schedules and higher pay for work performed at elevation.

  In March 2020, ALMA shut down entirely because of the COVID-19 pandemic. The closure delayed the Cycle 8 proposal submission deadline and suspended public visits to the site.

  On the 29th of October 2022, ALMA suspended observations after a cyber attack. It took 48 days for operations to resume, with observations restarting on the 16th of December 2022. Together, these three interruptions, a labor dispute, a global health crisis, and a digital intrusion, form an unusual record for any scientific facility. ALMA's thousandth published paper appeared in June 2018, well before any of those disruptions ended. The 66th and final antenna was accepted on the 23rd of September 2013, completing an array whose management is now unified under the Joint ALMA Observatory, led since February 2018 by director Sean Dougherty.