Located on the Chajnantor plateau in the Atacama desert of northern Chile, the Atacama Large Millimeter/submillimeter Array (ALMA) is a story of superlatives. The largest radiotelescope ever built, ALMA is no ordinary ground-based astronomical system. The array consists of 66 giant dish antennas, 54 with a 12-meter diameter and 12 with a 7-meter diameter, observing at millimeter and submillimeter wavelengths. This colossal facility is a global partnership between Europe, the United States and Japan.
A new view of space
ALMA will observe the “cool” Universe, the molecular gas and dust between the stars, providing astronomers and scientists with new insights into how stars and planets form and detecting nascent galaxies, as they may have existed more than 10 billion years ago. This means the ALMA observatory will provide fresh clues to the existential questions about our “cosmic” origins.
A design challenge
As lead company in the European AEM consortium, Thales Alenia Space acted as prime contractor for the 25 European antennas for the ALMA project. It took 10 years to build the subassemblies in Europe, transport them to Chile, install them at the site in the Atacama desert, 5,000 meters above sea level, then calibrate them and demonstrate compliance with technical specifications.
Achieving the requisite levels of performance for these huge antennas — including a reflector surface engineered to be accurate to 11 microns, or one-fifth of the thickness of a human hair, combined with extreme pointing precision on the furthest objects — called for an unprecedented feat of engineering. All these machines need to point at exactly the same location in space at exactly the same time, despite the extreme conditions of the high-altitude desert environment. The main challenges were the effect of Earth’s gravity, exposure to wind gusts and temperature fluctuations of almost 40°C in the space of just a few hours. Even the slightest design weakness could cause pointing accuracy to be compromised. In operation for several years now, the ALMA antennas have not once failed in their task!
On the top of the world
To transport the huge antennas from the ALMA base camp at 3,000 meters above sea level to the Chajnantor plateau at 5,000 meters, ALMA and the European Southern Observatory (ESO) built a 47-kilometer access road and used two heavy haulers, each designed to carry one antenna at a time. These custom-built trucks were 20 meters long and 10 meters wide, weighed 130 metric tons and were powered by twin diesel engines, each developing over 1,000 horsepower. A trip up the mountainside took three to four hours. How can an observatory on such a colossal scale, with its giant 100-ton antenna installations, achieve such extraordinary levels of pointing accuracy? This is all part of the magic of the ALMA Observatory!
ALMA image of a ring around the bright star Fomalhaut
In September 2013, after successful acceptance tests, Thales Alenia Space handed ownership of the 25th and final European antenna to its customer ESO. In the last few weeks, with the warranty period now at an end, ESO and the AEM consortium signed a close-out agreement, marking the effective completion of the 10-year-plus contract. The antennas are working perfectly, with their remarkable levels of performance recognized by users and attested by ESO. Hardly a week goes by without the ALMA Observatory announcing new scientific findings, some complete with spectacular images of space – just a click away on the ESO website!
Photos copyrights: ESO