  The Molecular Hydrogen Explorer, H2EX, was proposed in response to the ESA
2015 - 2025 Cosmic Vision Call as a medium class space mission with NASA and
CSA participations. The mission, conceived to understand the formation of
galaxies, stars and planets from molecular hydrogen, is designed to observe the
first rotational lines of the H2 molecule (28.2, 17.0, 12.3 and 9.7 micron)
over a wide field, and at high spectral resolution. H2EX can provide an
inventory of warm (> 100 K) molecular gas in a broad variety of objects,
including nearby young star clusters, galactic molecular clouds, active
galactic nuclei, local and distant galaxies. The rich array of molecular,
atomic and ionic lines, as well as solid state features available in the 8 to
29 micron spectral range brings additional science dimensions to H2EX. We
present the optical and mechanical design of the H2EX payload based on an
innovative Imaging Fourier Transform Spectrometer (IFTS) fed by a 1.2m
telescope. The 20'x20' field of view is imaged on two 1024x1024 Si:As
detectors. The maximum resolution of 0.032 cm^-1 (FWHM) means a velocity
resolution of 10 km s^-1 for the 0-0 S(3) line at 9.7 micron. This instrument
offers the large field of view necessary to survey extended emission in the
Galaxy and local Universe galaxies as well as to perform unbiased extragalactic
and circumstellar disks surveys. The high spectral resolution makes H2EX
uniquely suited to study the dynamics of H2 in all these environments. The
mission plan is made of seven wide-field spectro-imaging legacy programs, from
the cosmic web to galactic young star clusters, within a nominal two years
mission. The payload has been designed to re-use the Planck platform and
passive cooling design.
