In 2009, the Herschel telescope was successfully launched in space. It is a scientific space mission developped by the European Space Agency (ESA) dedicated to the observation of the Universe in the infrared and sub-millimetre ranges (wavelengths between 60 et 670 μm), a window of the electromagnetic spectrum that is still largely unexplored. 


The main objectives of the mission are based on two approaches related to the question of Origins. Close to Earth, Herschel will probe the molecular clouds, which are true breeding grounds for young stars, with a view to understand the first stages in star formation. Further away, it will map out the heavens to discern galaxies at the time they were formed and thus enrich our attempts to explain the evolution of the Universe, from the Big Bang to the present time.
An other facility will soon operate : the interferometer ALMA on the Chajnantor plateau in Chile. ALMA will allow astrophysicists to make ultra deep searches for primordial galaxies (Blain 2001), as well as detailed kinematical investigations of individual protostars (Evans 2001). 

Herschel Telescope: http://www.herschel.fr ©eso

However both instruments will suffer limitations, moderate angular resolution for Herschel which will prevent the study of individual protostars in all but the nearest star-forming clusters of our Galaxy, small field of view (10”) and limited observable conditions for ALMA which prevent extensive wide-field mapping because of the amount of time necessary to cover large star-forming complexes and fields of primordial galaxies.


To complete these studies, a large telescope on Earth is needed. Indeed, the sensibility of Herschel telescope has to be completed by a high-resolution telescope  large in diameter. This raises the question of which site is the best for sub-millimetre observations on Earth. 


The most important limitation for ground-based sub-millimetre astronomy is the broad-band absorption of the total water vapour in the atmosphere above an observation site, often expressed as the Precipitable Water Vapour (PWV). A long-term statistic on the PWV is thus mandatory to characterize the quality of an existing or potential site for observational submm-astronomy. Data from IASI (Infrared Atmospheric Sounding Interferometer) on the Metop-A satellite can be used to retrieve water vapour profiles for any site on Earth. The atmospheric model  MOLIERE (Microwave Observation and LIne Estimation and REtrieval) can compute the corresponding atmospheric absorption for wavelengths between 150 μm and 3 mm that gives a PWV-to-Transmission calculator for the best known and potential sites for submm astronomy on Earth. One of the most promising site is Dome C in Antarctica on which an important site testing campaign is performed.