  GrailQuest (Gamma Ray Astronomy International Laboratory for QUantum
Exploration of Space-Time) is a mission concept based on a constellation
(hundreds/thousands) of nano/micro/small-satellites in low (or near) Earth
orbits. Each satellite hosts a non-collimated array of scintillator crystals
coupled with Silicon Drift Detectors with broad energy band coverage (keV-MeV
range) and excellent temporal resolution ( below or equal 100 nanoseconds) each
with effective area around 100 cm2. This simple and robust design allows for
mass-production of the satellites of the fleet. This revolutionary approach
implies a huge reduction of costs, flexibility in the segmented launching
strategy, and an incremental long-term plan to increase the number of detectors
and their performance: a living observatory for next-generation, space-based
astronomical facilities. GrailQuest is conceived as an all-sky monitor for fast
localisation of high signal-to-noise ratio transients in the X/gamma-ray band,
e.g. the elusive electromagnetic counterparts of gravitational wave events.
Robust temporal triangulation techniques will allow unprecedented localisation
capabilities, in the keV-MeV band, of a few arcseconds or below, depending on
the temporal structure of the transient event. The ambitious ultimate goal of
this mission is to perform the first experiment, in quantum gravity, to
directly probe space-time structure down to the minuscule Planck scale, by
constraining or measuring a first order dispersion relation for light in vacuo.
This is obtained by detecting delays between photons of different energies in
the prompt emission of Gamma-ray Bursts.
