  Gamma-ray astronomy is a branch whose potential has not yet been fully
exploited. The observations of elemental and isotopic abundances in supernova
(SN) explosions are key probes not only of the stellar structure and evolution
but also for understanding the physics that makes Type-Ia SNe as standard
candles for the study of the Universe expansion properties. In spite of its
crucial role, nuclear astrophysics remains a poorly explored field mainly for
the typical emission lines intensity which are vanishing small and requires
very high sensitivities of the telescopes. Furthermore, in spite that the
Galactic bulge-dominated intensity of positron annihilation line at 511 keV has
been measured, its origin is still a mystery due to the poor angular resolution
and insufficient sensitivity of the commonly employed instrumentation in the
sub-MeV energy domain. To answer these scientific issues a jump in sensitivity
and angular resolution with respect to the present instrumentation is required.
Conceived within the EU project AHEAD, a new high energy mission, capable of
tackling the previously mentioned topics, has been proposed. This concept of
mission named ASTENA (Advanced Surveyor of Transient Events and Nuclear
Astrophysics), includes two instruments: a Wide Field Monitor with Imaging and
Spectroscopic (WFM-IS, 2 keV - 20 MeV) capabilities and a Narrow Field
Telescope (NFT, 50 - 700 keV). Thanks to the combination of angular resolution,
sensitivity and large FoV, ASTENA will be a breakthrough in the hard X and soft
gamma--ray energy band, also enabling polarimetry in this energy band. In this
talk the science goals of the mission are discussed, the payload configuration
is described and expected performances in observing key targets are shown.
