  HERMES (High Energy Rapid Modular Ensemble of Satellites) Technological and
Scientific pathfinder is a space borne mission based on a LEO constellation of
nano-satellites. The 3U CubeSat buses host new miniaturized detectors to probe
the temporal emission of bright high-energy transients such as Gamma-Ray Bursts
(GRBs). Fast transient localization, in a field of view of several steradians
and with arcmin-level accuracy, is gained by comparing time delays among the
same event detection epochs occurred on at least 3 nano-satellites. With a
launch date in 2022, HERMES transient monitoring represents a keystone
capability to complement the next generation of gravitational wave experiments.
In this paper we will illustrate the HERMES payload design, highlighting the
technical solutions adopted to allow a wide-energy-band and sensitive X-ray and
gamma-ray detector to be accommodated in a CubeSat 1U volume together with its
complete control electronics and data handling system.
