  Measurements in the infrared wavelength domain allow us to assess directly
the physical state and energy balance of cool matter in space, thus enabling
the detailed study of the various processes that govern the formation and early
evolution of stars and planetary systems in galaxies over cosmic time. Previous
infrared missions, from IRAS to Herschel, have revealed a great deal about the
obscured Universe, but sensitivity has been limited because up to now it has
not been possible to fly a telescope that is both large and cold.
  SPICA is a mission concept aimed at taking the next step in mid- and
far-infrared observational capability by combining a large and cold telescope
with instruments employing state-of-the-art ultra-sensitive detectors. The
mission concept foresees a 2.5-meter diameter telescope cooled to below 8 K.
With cooling provided by mechanical coolers instead of depending on a limited
cryogen supply, the mission lifetime can extend significantly beyond the
required three years.
  SPICA offers instrumentation with spectral resolving powers ranging from R
~50 through 11000 in the 17-230 $\mu$m domain as well as R~28.000 spectroscopy
between 12 and 18 $\mu$m. Additionally SPICA will provide efficient 30-37
$\mu$m broad band mapping, and polarimetric imaging in the 100-350 $\mu$m
range. SPICA will provide unprecedented spectroscopic sensitivity of ~5 x
$10^{-20}$ W/m$^2$ (5$\sigma$/1hr) - at least two orders of magnitude
improvement over what has been attained to date.
  With this exceptional leap in performance, new domains in infrared astronomy
will become accessible, allowing us, for example, to unravel definitively
galaxy evolution and metal production over cosmic time, to study dust formation
and evolution from very early epochs onwards, and to trace the formation
history of planetary systems.
