  We present the first detection in space of thionylimide (HNSO) toward the
Galactic Center molecular cloud G+0.693-0.027, thanks to the superb sensitivity
of an ultradeep molecular line survey carried out with the Yebes 40$\,$m and
IRAM 30$\,$m telescopes. This molecule is the first species detected in the
interstellar medium containing, simultaneously, N, S and O. We have identified
numerous $K$$_a$ = 0, 1 and 2 transitions belonging to HNSO covering from
$J$$_{\rm up}$ = 2 to $J$$_{\rm up}$ = 10, including several completely
unblended features. We derive a molecular column density of $N$ = (8 $\pm$
1)$\times$10$^{13}$ cm$^{-2}$, yielding a fractional abundance relative to
H$_2$ of $\sim$6$\times$10$^{-10}$, which is about $\sim$37 and $\sim$4.8 times
less abundant than SO and SO2, respectively. Although there are still many
unknowns in the interstellar chemistry of NSO-bearing molecules, we propose
that HNSO is likely formed through the reaction of the NSO radical and atomic H
on the surface of icy grains, with alternative routes also deserving
exploration. Finally, HNSO appears as a promising link between N- , S- and O-
interstellar chemistry and its discovery paves the route to the detection of a
new family of molecules in space.
