The Extreme Universe Space Observatory (EUSO) is the first Space mission concept devoted to the investigation of cosmic rays and neutrinos of extreme energy (E > 5×1019eV). Using the Earth's atmosphere as a giant detector, the detection is performed by looking at the streak of fluorescence produced when such a particle interacts with the Earth's atmosphere.
EUSO was a mission of the European Space Agency, designed to be hosted on the International Space Station as an external payload of the Columbus. 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 mission was then renamed JEM-EUSO.
JEM-EUSO is currently studied by RIKEN and JAXA, in collaboration with 60 other institutions from 12 countries, aiming for a flight in 2017. 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 metres 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 48x48 array of photomultipliers. The imaging takes place in the 300nm-450nm band (low-energy UV through deep-blue), and photons are time-tagged with 2.5-microsecond precision.