ALMA — the Atacama Large Millimeter/submillimeter Array — is the most expensive ground-based telescope ever built. Sixty-six radio dishes spread across the Chajnantor Plateau at 5,050 meters in the Atacama Desert, working together as a single instrument to observe the coldest, most distant objects in the universe. It peers through cosmic dust to see star formation, protoplanetary disks, and the structure of galaxies billions of light-years away.

Visiting ALMA

The actual telescope array sits at 5,050 meters — too high for casual visitors. Instead, free public visits run at the ALMA Operations Support Facility (OSF) at 2,900 meters, about 30 kilometers east of San Pedro de Atacama.

Tours run on Saturday and Sunday mornings, departing from San Pedro. Registration is required through the ALMA website and opens roughly two months in advance. Slots fill fast — book as early as possible. The tour lasts about two hours and includes a bus ride to the facility, a presentation by ALMA staff, and a walk through the control room where astronomers operate the array in real time.

You do not see the dishes themselves (they are 2,100 meters higher), but scale models, videos, and the control room give a sense of the project's ambition. The staff are genuinely enthusiastic — many are astronomers themselves — and the Q&A sessions can be fascinating.

What ALMA Does

Unlike optical telescopes that collect visible light, ALMA detects millimeter and submillimeter wavelengths — radiation emitted by cold gas and dust in space. This lets it see things optical telescopes cannot: the formation of stars inside dense dust clouds, the chemical composition of protoplanetary disks where planets are forming, and the structure of galaxies in the early universe.

ALMA's 66 dishes can be spread across distances of up to 16 kilometers, acting as a single telescope with extraordinary resolution. When combined, they produce images sharper than the Hubble Space Telescope at their target wavelengths. The first-ever image of a black hole (in galaxy M87) used ALMA as part of the Event Horizon Telescope network.

Why the Atacama

The Chajnantor Plateau was chosen for three reasons: extreme altitude (above most of the atmosphere's water vapor, which absorbs millimeter waves), extreme dryness (the Atacama's near-zero humidity), and distance from radio interference (no cities, no airports, minimal electronics). The site is operated jointly by the European Southern Observatory, the US National Science Foundation, and Japan's National Institutes of Natural Sciences.

Other Observatories

The Atacama region hosts several other major observatories open to visitors:

  • Paranal (VLT): ESO's Very Large Telescope, 130 km south of Antofagasta. Free Saturday tours. The building appeared in a James Bond film.
  • La Silla: ESO's oldest Chilean site, east of La Serena. Free Saturday tours.
  • Cerro Tololo: Near La Serena. Free Saturday tours with advance registration.

For tourist stargazing tours with telescopes (no advance booking months ahead), SPACE in San Pedro and Mamalluca near Vicuna are the best options.

Practical Information

Booking: Free. Register at almaobservatory.org/en/visit-alma. Opens ~2 months ahead. Saturdays and Sundays only.

Altitude: The visitor facility is at 2,900 meters — manageable if you have acclimatized in San Pedro (2,400m). The actual array at 5,050m is not accessible to visitors.

Getting there: ALMA provides bus transport from San Pedro as part of the tour. No independent access to the facility.

What to bring: Warm layers (it is cold and windy at 2,900m even in summer), sunscreen, water, sunglasses. No drones, no professional photography equipment without prior permission.