ALMA Observatory

ALMA is an array of sixty-six radio antennas on the Chajnantor plateau at about 5,000 metres, built and operated jointly by Europe, North America and East Asia with Chile - the largest ground-based astronomical project ever undertaken. It works at millimetre and submillimetre wavelengths, which water vapour absorbs, so it needed the driest high ground available on earth; Chajnantor supplies that. The antennas are moved around the plateau by two custom transporters to change the array's resolution. Public visits go to the operations support facility at 2,900 metres, not to the array itself - the altitude at the high site requires medical clearance and oxygen.

What you actually see

• Sixty-six antennas at 5,000 metres - the largest ground-based astronomy project.
• Built for millimetre wavelengths - which need extremely dry air.
• Antennas moved by transporters - to change the array's resolution.
• Visits to the base, not the plateau - the high site needs medical clearance.

When to go

Year round on scheduled weekend visits, booked well in advance. Half a day.

Honest expectations

Visits are limited, free and book out weeks ahead; the high array is not part of them. This is a radio observatory, so there is nothing to look through. Visits are suspended at times for operational reasons.

Nearby

San Pedro de Atacama is west; the Jama pass road runs past.