  Athena is an X-ray observatory-class mission concept, developed from April to
December 2011 as a result of the reformulation exercise for L-class mission
proposals in the framework of ESA's Cosmic Vision 2015-2025. Athena's science
case is that of the Universe of extremes, from Black Holes to Large-scale
structure. The specific science goals are structured around three main pillars:
"Black Holes and accretion physics", "Cosmic feedback" and "Large-scale
structure of the Universe". Underpinning these pillars, the study of hot
astrophysical plasmas offered by Athena broadens its scope to virtually all
corners of Astronomy. The Athena concept consists of two co-aligned X-ray
telescopes, with focal length 12 m, angular resolution of 10" or better, and
totalling an effective area of 1 m2 at 1 keV (0.5 m2 at 6 keV). At the focus of
one of the telescopes there is a Wide Field Imager (WFI) providing a field of
view of 24'\times 24', 150 eV spectral resolution at 6 keV, and high count rate
capability. At the focus of the other telescope there is the X-ray
Microcalorimeter Spectrometer (XMS), a cryogenic instrument offering a spectral
resolution of 3 eV over a field of view of 2.3' \times 2.3'. Although Athena
has not been selected as ESA's Cosmic Vision 2015-2025 L1 mission, its science
goals and concept conform the basis of what should become ESA's X-ray astronomy
flagship.
