This bodegón, which surprises with a proposal of traditional dishes, is located in the heart of the Atacama Desert, at more than 5000 meters above sea levelThere, one of the most ambitious scientific projects in recent history rises: the Atacama Large Millimeter/submillimeter Array, better known as ALMA. In this place where the air is so dry that there are almost no clouds and breathing is difficult, a set of gigantic antennas coordinates itself as if it were a single eye capable of "seeing" in wavelengths that our eyes cannot perceive.
ALMA is not just any observatoryIt is the largest ground-based radio astronomy complex ever built, the result of an international collaboration involving Europe, North America, East Asia, Chile, and other partners such as Canada, Taiwan, and Korea. Thanks to its extreme sensitivity to millimeter and submillimeter radiation, this observatory has become a key tool for understanding how stars are born, how planets form, and how the first galaxies in the Universe emerged.
What is the ALMA Observatory and why is it so special?
The ALMA Observatory is a huge radio interferometer Composed of 66 high-precision antennas with diameters of 7 and 12 meters, these antennas work together as a single giant radio telescope, designed to observe wavelengths between approximately 0,3 and 10 millimeters—that is, in the millimeter and submillimeter regions of the electromagnetic spectrum. Instead of “seeing” bright stars in visible light, ALMA detects the faint radiation emitted by cold gas, dust, and molecules in space.
The project rises on the Chajnantor plainIn the Antofagasta region of northern Chile, at an altitude of approximately 5050–5060 meters above sea level, ALMA's observatory is located at an extreme altitude. This extreme location is not arbitrary: at that altitude, the atmosphere is very thin and contains very little water vapor, minimizing the absorption of the millimeter-wave radio waves that ALMA needs to capture. It is one of the driest places on Earth and, precisely for that reason, one of the best locations in the world for this type of astronomy.
The total cost of the complex easily exceeds one billion eurosThis makes ALMA the most expensive and complex ground-based radio telescope ever built. The observatory is a partnership between the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF), the National Astronomical Observatory of Japan (NAOJ), and other collaborating institutions, with the Republic of Chile as the host country and a key partner.
ALMA's scientific operation began in 2011The telescope was launched while the antenna array was still being assembled, and its first images were released later that year. The official inauguration took place on March 13, 2013, formally marking the beginning of a new era in the observation of the so-called "cold universe." Since then, the number of scientific results and published articles has continued to grow.
International collaboration and project partners
ALMA is the result of the fusion of several previous ideas and projects These projects were being developed in parallel in different regions of the world. In the United States, the Millimeter Array (MMA) was being considered; in Europe, the Large Southern Array (LSA); and in Japan, the Large Millimeter Array (LMA). In the late 1990s, it was decided to combine efforts to create a single facility, much more powerful than they could have built separately.
In 1997 ESO and the US National Radio Astronomy Observatory (NRAO) They agreed to collaborate on a joint design that combined the strengths of MMA and LSA: the excellent frequency coverage and high altitude of the North American site, along with the enormous sensitivity planned by the Europeans. Canada and Spain later joined this core group; Spain, at that time not yet an ESO Member State, actively participated in the design and development phase.
The name Atacama Large Millimeter Array (ALMA) It was officially adopted in 1999, and the major agreement between the North American and European parties was signed in 2003, establishing a 50/50 funding scheme between ESO and the North American-Canada consortium. Shortly thereafter, in 2004, Japan and the National Astronomical Observatory of Japan (NAOJ) joined the expanded project with a crucial contribution: the Atacama Compact Array (ACA) and several additional band receivers for the main array.
The international consortium that supports ALMA is large and very diverseIn Europe, funding is provided by ESO and supported by a network of regional support centers. In North America, the NSF coordinates the project through the NRAO with the participation of the National Research Council of Canada. In East Asia, funding comes from Japan's NINS, in cooperation with Taiwan's Academia Sinica and the Korea Astronomy and Space Science Institute (KASI). Chile, for its part, not only provides the territory and legal framework but also participates as a strategic partner.
Unified management of construction and operations The responsibility falls to the Joint ALMA Observatory (JAO), based in Santiago, Chile, which coordinates the work of the three major regions: ESO leads on behalf of Europe, NRAO on behalf of North America, and NAOJ representing East Asia. This multi-level organization allows for the management of an extremely complex project involving hundreds of technicians, engineers, astronomers, and support staff from dozens of countries.
Location: the Chajnantor Plateau and its extreme conditions
Chajnantor is a high mountain plateau in the heart of the Andes Mountainsabout 50 kilometers east of San Pedro de Atacama. The area combines altitudes of around 5000 meters, exceptionally clear skies, and an atmosphere with extremely low water vapor content. These harsh conditions for human life are, paradoxically, perfect for observing millimeter and submillimeter radiation.
The main reason for building ALMA there The reason is that water vapor in the atmosphere very effectively absorbs signals at these wavelengths. The drier and higher the altitude, the less the atmosphere "filters" the radiation arriving from space. Chajnantor is higher in altitude than other large observatories such as Mauna Kea (Hawaii) or Cerro Paranal (where the VLT is located), which translates into exceptional atmospheric "windows" for this frequency range.
Working at over 5000 meters altitude is no jokeThe air is so oxygen-poor that personnel ascending to the Antenna Site (AOS) must undergo rigorous medical checks and use supplemental oxygen systems when necessary. For this reason, most of the living and control facilities have been built lower down, at the Operations Support Site (OSF), located at an altitude of approximately 2900 meters.
The contrast between the people of San Pedro de Atacama and the Chajnantor Plateau It's enormous. Although they're relatively close as the crow flies, walking among the ALMA antennas feels like being on another planet: an almost Martian landscape, dominated by volcanoes, salt flats, rocks, and an intensely blue sky. It's not a welcoming place to live, but it is a privileged environment for observing the Universe with unparalleled clarity.
The area around Chajnantor is home to other great observatories dedicated to millimeter and submillimeter astronomy, which has made this region a true world-renowned "astronomical district." ALMA, however, stands out for the scale of its infrastructure, the complexity of its systems, and its scientific impact. For those who wish to find context for other centers, lists can be consulted on astronomical observatories that include relevant facilities.
Antenna design and the concept of interferometry
ALMA functions as an interferometerThat is, it's like an array of antennas that simultaneously observe the same astronomical object and combine the received signals to obtain an image with a much higher resolution than any of the antennas could achieve individually. Instead of building a single dish kilometers in diameter—something technically unfeasible—many antennas are spread across the terrain, and correlation algorithms are used to reconstruct the information.
The main array of ALMA consists of about 50 12-meter antennas These antennas, which are in diameter and can be reconfigured in different arrangements, have separations (baselines) ranging from about 150 meters to about 16 kilometers. The greater the maximum distance between antennas, the finer the detail that can be distinguished in the resulting image. This variable "zoom" allows the study of everything from very compact structures to extensive regions of gas and dust.
The Atacama Compact Array (ACA) complements the main array with 16 additional antennas: 12 seven-meter antennas and 4 twelve-meter antennas. The seven-meter antennas, because they can be placed much closer together, are ideal for capturing the global emission of objects with a large angular extent in the sky, preventing the interferometer from "losing" large-scale information. The four twelve-meter antennas of the ACA are also used in total power mode to measure absolute flux.
The mechanical and surface precision of the antennas is spectacular.Although they may appear to be enormous metal plates, the reflective surface must be kept polished and aligned to tolerances thinner than a sheet of paper so that millimeter and submillimeter waves are correctly reflected to the receivers. These wavelengths don't require the same extreme polishing as an optical telescope, but the technical demands are still extremely high.
The level of detail that ALMA can offer This is illustrated by a simple physical relationship: the approximate angular resolution is given by θ ≈ λ / B, where λ is the observed wavelength and B is the maximum separation between antennas. In the best configurations, ALMA achieves resolutions on the order of a few milliarcseconds, surpassing the Hubble Space Telescope's sharpness in the visible range by up to a factor of ten and outperforming other radio telescopes such as the VLA in terms of sensitivity to millimeter wavelengths.
Megatransport and construction of the complex
The construction of ALMA involved a huge logistical challengeThe antennas, which weigh around 100-115 tons each, are assembled and tested at the Operations Support Site (OSF), at an altitude of about 2900 meters, where conditions are somewhat more manageable. Once ready, they must be hoisted up to the Chajnantor Plateau, at over 5000 meters, without damaging equipment that costs millions of euros.
Two special conveyors were designed in Germany for this task.Manufactured by Scheuerle Fahrzeugfabrik, these self-propelled vehicles are approximately 20 meters long, 10 meters wide, and 6 meters high, with 28 wheels and a weight of around 130 tons. They are equipped with diesel engines of approximately 500 kW and millimeter-precision lifting systems that allow them to attach the antennas, lift them, and place them with extraordinary accuracy onto the concrete platforms prepared at the AOS.
The driver of these engineering monsters It has a station with an oxygen supply, because the ascent to AOS involves facing low pressure and physical exertion at high altitude. The first of these machines passed the tests in 2007, and by 2008 it was already transporting antennas from the assembly hangars to the test platforms.
The ALMA construction schedule It was accomplished in stages: in 2002 prototype tests were carried out at the VLA facilities in New Mexico; in 2003 the Chilean site was officially inaugurated; in 2007 the first antenna arrived in Chile; in 2008 the first operational antenna was approved, and in 2009 the signals of three antennas were combined for the first time in the AOS, marking a technical milestone that allowed the definitive leap towards scientific observations.
The antennas were supplied by various industrial consortiums. In North America, Europe, and Japan, for both technical and political reasons. In North America, General Dynamics C4 Systems was contracted to manufacture a batch of 25 twelve-meter antennas, while in Europe, Thales Alenia Space undertook the production of another 25, in what was one of the most significant industrial contracts signed on the continent in the field of astronomy. East Asia supplied the ACA antennas, thus completing the final assembly.
ALMA Chronology: From Idea to First Light
The development of ALMA formally began in the 1990s.Although the idea of having a large millimeter-wave interferometer in the Southern Hemisphere had been under consideration for some time, site tests began in 1995 in collaboration with Chile, and the design and development phase commenced in 1998. In 1999, a US-European memorandum of understanding was signed, formalizing the collaboration for the project.
The final North American-European agreement was reached in 2003.establishing an equal funding share between ESO and the US-Canada bloc. That same year, testing began with the first antenna prototype at the ALMA Test Facility in Socorro, New Mexico, and the site's inauguration ceremony was held in Chile. In 2004, the joint ALMA office opened in Santiago, and Japan joined as a partner through the agreement with NAOJ.
In 2005 Taiwan joined the project through collaboration with JapanAnd in 2006 the agreements were amended to accommodate the expanded ALMA. From 2007 onwards, with the arrival of antennas in Chile, the pace of construction accelerated: in 2008 the first antenna was approved, in 2009 the first unit was moved to Chajnantor and the first interferometry was achieved with several antennas at the array's operations site.
The “Initial Science” phase began in 2011 With the so-called Cycle 0, some 16 twelve-meter antennas were already operational in the main array. In 2013, Cycle 1 began with more than 30 antennas available, and on March 13 of that same year, ALMA was officially inaugurated. In 2014, Cycle 2 began, with more antennas both in the main array and in the ACA, consolidating the observatory's scientific capacity.
Since its first observation cycles, ALMA has operated through competitive calls for proposals.These calls for proposals are submitted by scientific teams from around the world. An international committee of experts evaluates the applications and allocates observation time to the highest-rated projects. The community's response has been overwhelming, with thousands of proposals received in each cycle and a relatively low acceptance rate due to the enormous demand.
In parallel with the launch of the observatoryA network of ALMA Regional Centers (ARCs) was deployed across Europe, North America, and East Asia. These centers act as an interface between the JAO and user astronomers, helping to prepare proposals, design observational experiments, process data, and provide scientific and technical support.
Technical capabilities and scientific performance
ALMA is designed to cover virtually all atmospheric “windows” between 350 microns and 10 millimeters, with an array of band receivers that have been installed in phases. In 2016, for example, band 5 (1,4-1,8 mm) was incorporated, especially useful for studying water lines in the interstellar medium and in protoplanetary disks, which opened up new possibilities for investigating the presence of water vapor in different cosmic environments.
The observatory offers angular resolutions of up to about 10 milliarcsecondsThese resolutions are about ten times better than those achieved by the Very Large Array (VLA) in radio and several times better than Hubble in visible light. In combination with other radio telescopes such as LLAMA, using very long baseline interferometry, these resolutions can even reach the microarcsecond scale at certain frequencies.
In terms of sensitivity, ALMA marks a before and after in the millimeter and submillimeter range. It is capable of detecting sources about 20 times fainter than those observable with previous installations, and its speed in generating detailed sky maps makes it a much faster and more versatile instrument than the VLA itself in these bands.
The computational heart of ALMA is the correlatorA massive data processing system with approximately 134 million chips, affectionately nicknamed "Don Corleone," this machine ingests around 120 gigabits of data per second, 24 hours a day, cross-references the signals from each pair of antennas, and produces data products that, after further complex processing, are transformed into extremely high-resolution astronomical images.
The observatory's scientific output is overwhelming.Even before reaching full capacity, ALMA had already generated hundreds of articles in top-tier international journals. Within a few years of achieving full capacity, the annual number of publications associated with ALMA data was in the hundreds, covering topics ranging from planet formation to the study of active galaxies and the interstellar medium.
What ALMA observes: the cold and dusty Universe
ALMA's specialty is the so-called "Cold Universe"These are dense molecular clouds, disks of gas and dust, star-forming regions, and very distant, young galaxies whose original infrared light has been stretched by cosmic expansion to millimeter wavelengths. In visible light, these objects can appear very faint or even completely hidden behind dense curtains of dust.
The enormous molecular clouds where stars are born They emit very intensely in the millimeter range thanks to the presence of molecules such as carbon monoxide, methyl cyanide, and many other organic species. ALMA allows scientists to map the distribution of the gas with unprecedented detail, measure its temperatures, densities, and velocities, and follow the gravitational collapse process that leads to the birth of new stars.
In the case of protoplanetary disks —structures of gas and dust surrounding young stars—, ALMA has obtained iconic images, such as the famous image of the HL Tauri disk, in which rings and grooves can be seen indicating the presence of planets in formationThis type of observation has revolutionized the way we understand the construction of planetary systems similar to our own.
ALMA also plays a key role in the study of complex molecules associated with the origin of life. A prime example is the detection of large quantities of methyl cyanide (CH3CN) in a protoplanetary disk around the young star MWC 480. The abundance of this type of molecule suggests that the chemical building blocks necessary for life may be quite common in environments where new planets form.
In the realm of distant galaxiesALMA allows us to trace the cold gas that fuels star formation in the early Universe, and to study how stars arose. first massive galaxies and how their gas and dust contents evolved. The radiation they emitted more than ten billion years ago reaches us today shifted towards millimeter wavelengths, making ALMA an irreplaceable instrument for investigating that remote era.
Life and work at the world's largest observatory
Behind the spectacular images of ALMA A diverse community of people lives and works between the base camp and the Chajnantor Plateau. Astronomers, engineers, technicians, support staff, and IT specialists from over 60 countries coordinate intensive shifts to keep the observatory running virtually every day of the year.
At base camp, at about 2600-2900 metersThe living quarters are concentrated there: dormitories, a dining hall, leisure areas, and, above all, the control room from which the operation of the antenna array is monitored in real time. It is there that astronomers like the Spaniard Itziar de Gregorio or the American Adele Plunkett have spent years coordinating observations, dealing with extreme weather, and managing hundreds of different scientific projects.
Above 5000 meters, at the Operations Site of the Joint Task ForceThe work is more technical and specialized. Teams like the one formed by Chileans Sebastián Castillo and Luis Titichuca are responsible for maintaining the electrical and electronic systems inside the antennas, the "heart" where the cryogenic receivers and signal conversion systems are housed. Safety is paramount, and that's why they always work in pairs, at least, alert to any signs of altitude sickness.
Daily life at ALMA combines routine with the extraordinaryIn a single shift, between 10 and 20 different observations can be performed, depending on the duration of each program. Each project has been pre-selected by a committee of experts and receives a specific allocation of antenna hours. Support astronomers are responsible for translating the scientific requirements (what to observe, at what resolution, at what sensitivity) into specific interferometer configurations.
The data obtained by ALMA is stored and distributed The data is shared with the teams responsible for each proposal through the ALMA Regional Centers. These centers help researchers calibrate observations, generate scientific images, and extract physical results. Many users simply connect from their own institutions, but they know that behind the web interface lies a vast global infrastructure supporting the flow of data.
Visit ALMA from San Pedro de Atacama
The curious thing is that many people who travel to San Pedro de Atacama They are unaware that just a few kilometers away lies the world's largest astronomical project. To bring the observatory closer to the general public, ALMA organizes free weekend visits to the Operations Support Site (OSF), always with prior reservation and subject to availability.
The visits are guided and free of chargeThese tours usually take place on Saturday and Sunday mornings. A bus departs from a parking lot near the San Pedro Museum, typically around 9:00 a.m., and returns visitors to the same spot around 13:00 p.m. Visitors are required to carry identification during the tour, and reservations are personal and non-transferable, including for minors.
During the tour you can see the control room, the laboratories and some of the antennas undergoing maintenance at the OSF, as well as information panels and scientific images obtained by ALMA. Currently, for safety and health reasons, climbing to the Chajnantor Plateau above 5000 meters is not permitted, so visitors cannot access the operational antenna array.
The demand for places is very highTherefore, it's advisable to register well in advance through the official booking system. Even so, those initially placed on the waiting list sometimes manage to move up if they show up at the start time, as there's often a last-minute opening when someone doesn't appear.
For those who cannot visit ALMA directlyThere are alternatives in the area surrounding San Pedro de Atacama, such as nighttime astronomical tours in private observatories where optical telescopes are used to contemplate the desert sky. Although they have nothing to do with the scale of ALMA, they are a magnificent way to appreciate the quality of the Atacama sky and learn about constellations, nebulae, and star clusters.
The role of SOUL in our understanding of the cosmos
Beyond the technical data and the epic engineeringALMA is helping to answer some of the oldest questions we ask ourselves as a species: how planetary systems like our own form, how common the precursor molecules of life are, and how the first galaxies arose in the early Universe. In other words, what place do we occupy in the cosmos, and could life exist in other corners of the galaxy?
ALMA's most publicized results —such as the ringed image of the HL Tauri disk or the detection of complex organic molecules in planet-forming environments— are just the tip of the iceberg. Every year, dozens and dozens of studies are published exploring everything from the interiors of active galaxies with supermassive black holes to the behavior of gas in regions where massive star clusters are born.
The observatory is also complemented by other large facilitiesBoth on Earth and in space, these telescopes provide information in a previously poorly explored region of the spectrum. The combined images across different ranges—radio, infrared, visible, and X-ray—reveal a universe far more dynamic and complex than we could have imagined with just one type of telescope.
Taken as a whole, ALMA is much more than a scientific instrumentIt is a symbol of what international collaboration can achieve when resources, knowledge, and political will come together. In an extreme environment where almost no one would want to live permanently, a community of specialists has developed a tool capable of tracing our cosmic origins, studying how stars and planets form, and, in the process, offering us a new perspective on our own planet and on ourselves.
