  Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was
adopted as the fourth medium-class mission in ESA's Cosmic Vision programme to
be launched in 2029. During its 4-year mission, Ariel will study what
exoplanets are made of, how they formed and how they evolve, by surveying a
diverse sample of about 1000 extrasolar planets, simultaneously in visible and
infrared wavelengths. It is the first mission dedicated to measuring the
chemical composition and thermal structures of hundreds of transiting
exoplanets, enabling planetary science far beyond the boundaries of the Solar
System. The payload consists of an off-axis Cassegrain telescope (primary
mirror 1100 mm x 730 mm ellipse) and two separate instruments (FGS and AIRS)
covering simultaneously 0.5-7.8 micron spectral range. The satellite is best
placed into an L2 orbit to maximise the thermal stability and the field of
regard. The payload module is passively cooled via a series of V-Groove
radiators; the detectors for the AIRS are the only items that require active
cooling via an active Ne JT cooler. The Ariel payload is developed by a
consortium of more than 50 institutes from 16 ESA countries, which include the
UK, France, Italy, Belgium, Poland, Spain, Austria, Denmark, Ireland, Portugal,
Czech Republic, Hungary, the Netherlands, Sweden, Norway, Estonia, and a NASA
contribution.
