  GrailQuest (Gamma-ray Astronomy International Laboratory for Quantum
Exploration of Space-Time) is an ambitious astrophysical mission concept that
uses a fleet of small satellites whose main objective is to search for a
dispersion law for light propagation in vacuo. Within Quantum Gravity theories,
different models for space-time quantization predict relative discrepancies of
the speed of photons w.r.t. the speed of light that depend on the ratio of the
photon energy to the Planck energy. This ratio is as small as 1E-23 for photons
in the gamma-ray band (100 keV). Therefore, to detect this effect, light must
propagate over enormous distances and the experiment must have extraordinary
sensitivity. Gamma-Ray Bursts, occurring at cosmological distances, could be
used to detect this tiny signature of space-time granularity. This can be
obtained by coherently combine a huge number of small instruments distributed
in space to act as a single detector of unprecedented effective area. This is
the first example of high-energy distributed astronomy: a new concept of
modular observatory of huge overall collecting area consisting in a fleet of
small satellites in low orbits, with sub-microsecond time resolution and wide
energy band (keV-MeV). The enormous number of collected photons will allow to
effectively search these energy dependent delays. Moreover, GrailQuest will
allow to perform temporal triangulation of impulsive events with arc-second
positional accuracies: an extraordinary sensitive X-ray/Gamma all-sky monitor
crucial for hunting the elusive electromagnetic counterparts of Gravitational
Waves, that will play a paramount role in the future of Multi-messenger
Astronomy. A pathfinder of GrailQuest is already under development through the
HERMES (High Energy Rapid Modular Ensemble of Satellites) project: a fleet of
six 3U cube-sats to be launched by the end of 2022.
