  We present multiwavelength (X-ray/optical/near-infrared/millimetre)
observations of GRB 051022 between 2.5 hours and ~1.15 yr after the event. It
is the most intense gamma-ray burst (~ 10^-4 erg cm^-2) detected by HETE-2,
with the exception of the nearby GRB 030329. Optical and near infrared
observations did not detect the afterglow despite a strong afterglow at X-ray
wavelengths. Millimetre observations at Plateau de Bure (PdB) detected a source
and a flare, confirming the association of this event with a moderately bright
(R = 21.5) galaxy. Spectroscopic observations of this galaxy show strong [O
II], Hbeta and [O III] emission lines at a redshift of 0.809. The spectral
energy distribution of the galaxy implies Av (rest frame) = 1.0 and a starburst
occuring ~ 25 Myr ago, during which the star-forming-rate reached >= 25
Msun/yr. In conjunction with the spatial extent (~ 1'') it suggests a very
luminous (Mv = - 21.8) blue compact galaxy, for which we also find with Z Zsun.
The X-ray spectrum shows evidence of considerable absorption by neutral gas
with NH, X-ray = 3.47(+0.48/-0.47) x 10^22 cm^-2 (rest frame). Absorption by
dust in the host galaxy at z = 0.809 certainly cannot account for the
non-detection of the optical afterglow, unless the dust-to-gas ratio is quite
different than that seen in our Galaxy (i.e. large dust grains). It is likely
that the afterglow of the dark GRB 051022 was extinguished along the line of
sight by an obscured, dense star forming region in a molecular cloud within the
parent host galaxy. This galaxy is different from most GRB hosts being brighter
than L* by a factor of 3. We have also derived a SFR ~ 50 Msun/yr and predict
that this host galaxy will be detected at sub-mm wavelengths.
