  HERMES is a scientific mission composed of 3U nano-satellites dedicated to
the detection and localization of high-energy astrophysical transients, with a
distributed space architecture to form a constellation in Earth orbits. The
space segment hosts novel miniaturized detectors to probe the X-ray temporal
emission of bright events, such as Gamma-Ray Bursts (GRBs), and the
electromagnetic counterparts of Gravitational Wave Events (GWEs), playing a
crucial role in future multi-messenger astrophysics. During operations, at
least three instruments, separated by a minimum distance shall observe a common
area of the sky to perform a triangulation of the observed event. An effective
detection by the nano-satellite payload is achieved by guaranteeing a
beneficial orbital and pointing configuration of the constellation. The design
has to cope with the limitations imposed by small space systems, such as the
lack of on-board propulsion and the reduced systems budgets. The paper
describes the methodologies and the proposed strategies to overcome the mission
limitations, while achieving a satisfactory constellation visibility of the sky
throughout the mission duration. The mission design makes use of a
high-fidelity orbit propagator, combined with an innovative mission analysis
tool that estimates the scientific performances of the constellation. The
influence of the natural relative motion, which is crucial to achieve an
effective constellation configuration without on-board orbit control, is
assessed. The presented methodology can be easily extended to any kind of
distributed scientific space applications, as well as to constellations
dedicated to Earth and planetary observation. In addition, the visibility tool
is applicable in the context of the constellation flight dynamics operations,
yielding optimized results and pointing plans based on actual satellite orbital
positions.
