EUSO successfully completed the "Phase A" study, however in 2004, ESA decided not to proceed with the mission because of programmatic and financial constraints.
The mission was then re-oriented as a payload to be hosted on board the JEM module of the Japanese KIBO facility of the ISS.
The proposed instrument consists of a set of three large Fresnel lenses of 2.65-metre diameter (with top and bottom cut off to reduce the minimum diameter to 1.9-metre so that they fit in the HTV resupply vehicle in which the instrument is to be launched) feeding a detector consisting of 137 modules each a 48 x 48 array of photomultipliers.
The imaging takes place in the 300 nm-450 nm band (low-energy UV through deep-blue), and photons are time-tagged with 2.5-microsecond precision.
[2] In addition to its main, science mission, EUSO might also be used to detect orbiting space junk that could pose a threat to ISS, that is too small to be spotted by astronomers (1 to 10 cm).