Travel to Chajnantor — APEX and ALMA

Please note that visitors do not have access to ALMA AOS. 
Information for weekend visits to ALMA OSF is available here
Media visitors please consult this page.

ALMA (the Atacama Large Millimeter/submillimeter Array) is the major new facility for observations in the millimetre/submillimetre regime. ALMA is comprised of a giant array of 54 12-m and 12 7-m antennas. ALMA is at one of the highest observatory sites on Earth, at an elevation of 5,000 metres on the Chilean Chajnantor plateau. The ALMA project is a partnership between Europe, Japan and North America in cooperation with the Republic of Chile.

The Chajnantor plateau is also the site of APEX, the Atacama Pathfinder Experiment, a 12-metre diameter antenna observing the Universe at millimetre and submillimetre wavelengths. APEX is a Max-Planck Institute for Radioastronomy (MPIfR) facility, hosted by ESO and operated by ESO on behalf of MPIfR.

How to reach Chajnantor

If you arrive by own means, the ALMA Operations Support Facility (OSF), at 2900 meters above sea level, is located 40 driving minutes from San Pedro de Atacama. Note that transportation for the public visits is arranged (see here). The drive from the OSF to the Array Operations Site (AOS) and APEX, located at 5000 m altitude, takes about 50 minutes (~25 km). At the AOS are the antennas and the Correlator. Only ALMA certified vehicles and drives with an ALMA “driver’s license” are allowed to get to the AOS. Children under 16 years are not allowed to go up.

Arriving by plane

Please note that airfares to and from Chile can get quite pricey when booking shorter than 3 months in advance. The closest airport to Chajnantor is the El Loa International Airport located in Calama, about 120 kilometres west of the observatory. There is no public transport to Chajnantor, so you will need to rent a car.

ALMA roadmap
ALMA roadmap. Credit: ESO

Arriving by car

There are several car rental companies available at the airport, including AlamoBudgetEconorent or Hertz. The average daily cost of a car in Calama rises to CLP 65 000 (about EUR 100) and it takes about two and a half hours to drive to OSF from the airport. Important: Only certified ALMA drivers are allowed to drive to the ALMA Array Operations Site (AOS), on Chajnantor.

To drive from a different city, take the Panamerican Highway as far as the turn for route 25 to Calama. Pass the El Loa Airport, and take route 23 to San Pedro de Atacama. After San Pedro take the route to Toconao until you reach the ALMA/APEX main gate (before Toconao). It takes about four hours to reach APEX from the Panamerican Highway. Please be aware that the road from the ALMA/APEX main gate is private. Only authorised visits are allowed to enter.

Facilities at Chajnantor

The ALMA Operations Support Facility (OSF) is the base camp for the every-day, routine operation of the observatory. It is located at an altitude of about 2900 meters, quite high compared to standard living conditions, but still quite acceptable for scientific projects in astronomy of similar scope. At the OSF are the camp, cantina, antennas assembly sites, labs, control room, etc.

Accommodation and Food

Only astronomers, engineers and support staff have access to accommodation and food on site. ALMA does not provide accommodation for visitors. Meals can be provided if requested in advance. For further information on media visits please check here. Further information on restaurants in San Pedro de Atacama is available here.


Wi-Fi Internet connection is available at the ALMA OSF.
Cell-phone signal is typically good in San Pedro de Atacama and at ALMA OSF. The quality of the signal worsens at ALMA AOS.

Health & Safety

Weather and Clothing

At the OSF the days are generally warm, but the nights can be cold. At the AOS, at 5,000 m, it can be very cold with strong winds and one needs a ski parka or a down jacket, gloves, a hat with ear protection, and wind trousers. Sun glasses, hats and strong sun blocking cream are needed at the OSF and AOS. Here you can find climatic information about Calama and San Pedro de Atacama. Also find the current weather forecast for San Pedro de Atacama here.

High Altitude Health Issues

There are inherent risks in traveling at high altitude. The information below is designed for general information, and is not a substitute for specific training or experience and does not constitute medical advice. If you have any questions or concerns please consult to your doctor.

When visiting the APEX/ALMA observatory site, you should be prepared to recognize and respond to the symptoms of altitude illness caused by the lower level of oxygen available at high elevations. The human body can adjust to changes in altitude, by the process called acclimatization.

Symptoms of Altitude Illness

  • Headache
  • Insomnia
  • Irritability
  • Dizziness
  • Muscle aches
  • Fatigue
  • Loss of appetite
  • Nausea or vomiting
  • Swelling of the face, hands and feet

Danger signs include severe headache, extreme fatigue or breathlessness (especially while resting), and any neurological problems such as stumbling, confusion, poor judgment or changes in consciousness. It is crucial to descend until symptoms begin to diminish if these signs are present.

Consult your health care provider or travel medicine specialist for specific recommendations about prevention and treatment.

If you are interested in reading more:


  • Although it is no guarantee against problems at high altitude, having a generally good fitness may help you adapt.
  • Avoid alcohol.
  • Avoid any unnecessary medication (since their effects may be increased at high altitudes). Sleeping pills, tranquilizers and narcotic-based pain relievers, in particular, can cause serious problems at high altitudes because they can decrease breathing rate. Consult with your health care provider about any medications you plan to bring with you.
  • Avoid smoking.
  • Avoid being very active after you arrive, and get plenty of rest.
  • Drink plenty of water.

Special Health and Safety advisory

A visit to ALMA or APEX at the Array Operations Site (AOS) at 5000 metres altitude at the Chajnantor Plateau, has some significant safety, security and health constraints. For this reason:

  • Access to Array Operations Site (AOS) is granted only to staff and authorized visits, under the conditions stated below.
  • Upon arrival at the ALMA Operations Support Facility (OSF), the visitor will be given a safety talk explaining the rules and procedures that have to be duly respected.
  • The ALMA Safety Manual is the authoritative text and must be consulted for full details.
  • Media visitors or others visiting as part of a job assignment may wish to consult with their supervisor or Human Resources department.
  • The High Altitude Medical Examination (HAME) is a medical examination that certifies that the individual is fit to perform work at 5000 metres. The relevant extract to show to the occupational physician is on p. 93 of the ALMA Safety Manual. A certificate is valid during 1 year from the date of release, unless a specific lesser term is indicated by the evaluating doctor.

In order to be granted access to ALMA AOS, all AOS visitors are required to:

  1. Have slept at least one night in San Pedro de Atacama or Calama for acclimatization before they can go up to the AOS.
  2. Undergo a paramedic medical check-up (blood pressure, blood oxygen level, altitude sickness symptoms, diabetes) by the OSF paramedic. Following the “OSF Clinic Exam Protocol” described on p. 94 of the ALMA Safety Manual, those who do not pass the test will not be allowed to continue to the AOS. The medical check-up is also repeated at AOS.
  3. Must meet the requirements given in the ALMA Safety Manual p. 91-93 in the version of 14 Feb. 2012. In particular:
    1. External Visitors (defined as any person not in a contractual or working relationship with any of the executives or JAO, e.g. journalists) staying at AOS less than 2 hours: need to sign a waiver form before every visit.
    2. External Visitors (defined as above) staying at AOS more than 2 hours: need to present an High Altitude Medical Examination, HAME.
    3. All Contractors (defined as any person in a contractual or working relationship with any of the ALMA executives or JAO, including so-called “freelancers”) need to present either:
      1. A statement from their Human Resources department which asserts that the person in question is covered by their work accident insurance to work at 5000 metres, or
      2. An High Altitude Medical Examination (HAME)
      3. In the special case of freelancers without work accident insurance, you may sign a waiver form although it is recommended that you also consult your physician.
    4. Members of staff of the ALMA executives or JAO: please consult your Human Resources department for full details, but a High Altitude Medical Examination (HAME) is required.

Virtual tour

Experience ALMA and APEX with ESO's Virtual Tours.

View Larger Map

APEX Chajnantor Site
II Región