  We present a satellite mission concept to measure the dark energy equation of
state parameter w with percent-level precision. The Very Ambitious Dark Energy
Research satellite (VADER) is a multi-wavelength survey mission joining X-ray,
optical, and IR instruments for a simultaneous spectral coverage from 4microns
(0.3eV) to 10keV over a field of view (FoV) of 1 square degree. VADER combines
several clean methods for dark energy studies, the baryonic acoustic
oscillations in the galaxy and galaxy cluster power spectrum and weak lensing,
for a joint analysis over an unrivalled survey volume. The payload consists of
two XMM-like X-ray telescopes with an effective area of 2,800cm^2 at 1.5keV and
state-of-the-art wide field DEPFET pixel detectors (0.1-10keV) in a curved
focal plane configuration to extend the FoV. The X-ray telescopes are
complemented by a 1.5m optical/IR telescope with 8 instruments for simultaneous
coverage of the same FoV from 0.3 to 4 microns. The 8 dichroic-separated bands
(u,g,r,z,J,H,K,L) provide accurate photometric galaxy redshifts, whereas the
diffraction-limited resolution of the central z-band allows precise shape
measurements for cosmic shear analysis.
  The 5 year VADER survey will cover a contiguous sky area of 3,500 square
degrees to a depth of z~2 and will yield accurate photometric redshifts and
multi-wavelength object parameters for about 175,000 galaxy clusters, one
billion galaxies, and 5 million AGN. VADER will not only provide unprecedented
constraints on the nature of dark energy, but will additionally extend and
trigger a multitude of cosmic evolution studies to very large (>10 Gyrs)
look-back times.
