Engineering places: Atacama Large Millimetre Array (ALMA)
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In celebration of IET@150 we look at feats of engineering from around the world. Here, we explore an astronomical observatory built in the middle of the desert.
At the heart of the Chajnantor plateau in Chile lies the Atacama Large Millimetre/sub-millimetre Array (ALMA). It is the most complex astronomical observatory ever built on Earth and is used to observe light from space at millimetric and submillimetric wavelengths.
The facility can study cosmic light that straddles the boundary between radio and infrared – most objects in the universe emit this kind of energy, so the ability to detect it has been a driver for astronomers for decades.
ALMA uses a system called an ‘interferometer’ that arrays many small antennas across a wide area and links them together to operate as one huge telescope. By combining 54 parabolic antennas with 12m diameters and 12 parabolic antennas with diameters of 7m, it creates one huge radio telescope comprising 66 antennas in total.
The astronomy facility is the brainchild of an international partnership between the European Southern Observatory (ESO), the National Science Foundation (NSF) of the US, and the National Institutes of Natural Sciences of Japan in collaboration with the Republic of Chile.
The construction of the observatory dates to 2004. However, two decades prior to this, the global scientific community had already identified the need for a radio telescope with the characteristics of ALMA.
In 1983, eminent radio astronomers in the US met to discuss priorities for an array of millimetre-wavelength telescopes. By 1990, the National Radio Astronomy Observatory in the US proposed a project called Millimetre Array (MMA), which considered the construction of 40 antennas with 8m diameters, reaching an atmospheric window of 30 to 350GHz to receive millimetre wavelengths. The NSF approved further MMA planning for this in 1994.
In parallel to this, ESO planned the Large Southern Array, a proposal that considered the installation of 50 antennas with 16m diameters, operating with frequencies below 350GHz and receiving millimetre wavelengths.
Yet it wasn’t until April 2001 when the National Astronomical Observatory of Japan proposed the construction of a Large millimetre/Sub-millimetre Array, which considered the same number of antennas as ESO, but with a 10m diameter, and unlike the others, was also planned to receive sub-millimetre wavelengths.
However, it soon became clear that such ambitious projects could not be developed by a single community, so all three parties joined forces. After its design, the team had to establish a location for this type of radio-astronomy project. Why did the team choose Chile?
Low-frequency waves are better received at high altitudes and in dry climates, as this reduces the amount of noise. In addition, the surface area should be large and flat enough for distribution of the antennas. Several areas in New Mexico, Hawaii (Mauna-Kea), France (Plateau de Bure) and northern Africa were explored. However, none of these sites measured up to the conditions in an arid plateau between the Andes and the Chilean Coast mountains.
In 1995, the three partners ran tests in the Chilean plains, with positive results. In 1999, Europe and North America signed a Memorandum of Understanding, and two years later in Tokyo a resolution was signed to support the joint intent to construct ALMA between Europe, the US, and Japan. By the end of 2003, after a series of tests, the first stone was placed at the site high up in the Atacama Desert, in what was to become the most ambitious radio observatory on Earth.
Indeed, ALMA is still used to this day, and has helped to discover the most spectacular sightings within our universe. In 2019, astronomers using the ALMA Observatory found counter-rotating discs of gas around a supermassive black hole, with observations providing an unprecedented close-up view of a swirling disc of cold interstellar gas rotating around a black hole.
Meanwhile, in the same year, astronomers witnessed 3D motions of gas in a planet-forming disc using ALMA for the first time. At three locations in the disc around a young star called HD 163296, gas was seen flowing like a waterfall into gaps that are believed to be caused by planets in formation.
The overall scientific goals of ALMA include observations of star formation, molecular clouds, and signs of our early universe – and the facility has already produced a series of unique images and data that achieve this. It has provided images of proto-planetary discs such as HL Tauri, which transformed the previously accepted theories about the planetary formation.
When the HL Tauri image was released in 2014, it was the sharpest picture ever made at submillimetre wavelengths, and could only be achieved with ALMA’s long baseline capabilities. Tim de Zeeuw, then director general of ESO, said at the time that such high-resolution images started “a new era in our exploration of the formation of stars and planets”.
ALMA has also detected complex organic molecules – carbon-based, pre-biotic structures, necessary for building life – in distant proto-planetary discs, confirming that our solar system is not unique in potentially fostering life. Scientific results from ALMA are helping exoplanet researchers determine the types and locations of systems that could support habitable planets. It has also provided valuable information about worlds closer to home, including Saturn’s moon, Titan.
According to ESO, ALMA is “the most powerful telescope for observing the cool Universe - molecular gas and dust”. Its detailed images of the birth of stars and planetary systems let scientists address “some of the deepest questions about our cosmic origins”.
1997: National Radio Astronomy Observatory and the European Southern Observatory (ESO) agree to a common project that would merge the Millimetre Array of the US, and the Large Southern Array of Europe.
1999: Europe and North America sign a Memorandum of Understanding on the project.
April 2001: A resolution is signed between Europe, North America and Japan.
14 Sept 2004: Japan officially joins consortium.
17 Nov 2009: ALMA makes first measurements using just two of its antennas.
4 Jan 2010: Three antennas work in unison.
Sept 2011: Early Science observation starts with 16 antennas.
3 Oct 2011: ALMA opens for astronomers – using the partially constructed antenna array.
13 March 2013: ALMA is inaugurated in an official ceremony marking the completion of all the major systems of the giant telescope.
13 June 2014: The 66th ALMA antenna is transported to the Array Operations Site. The 12m-diameter dish is the 25th European antenna.
March 2015: ALMA combines its collecting area and sensitivity with that of the APEX (Atacama Pathfinder Experiment) Telescope to create a single instrument through a process known as VLBI, where data from two telescopes are combined to form a virtual telescope, yielding magnifying power.
July 2015: ALMA successfully opens on another frequency range after obtaining the first fringes with a Band 5 receiver, specifically designed to detect water in our universe.
4 Nov 2015: A new instrument is attached to the 12m APEX telescope at 5,000m above sea level. The Swedish-ESO Pi receiver for APEX detects faint signals from water and other molecules within the Milky Way, other nearby galaxies, and the early universe.
12 July 2018: To maintain the leading-edge capabilities of the observatory, the ALMA Board designates a Working Group to prioritise recommendations from the ALMA Science Advisory Committee on developments for the observatory until 2030.
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