EUSO: using high energy cosmic rays and neutrinos as messengers from the unknown universe

Exploiting the earth atmosphere as a giant detector for the incoming extraterrestrial fl ux of high energy cosmic rays and cosmic neutrinos, the mission EUSO-Extreme Universe Space Observatory is devoted to the exploration of the domain of the highest energy processes occurring in the universe up to its accessible boundaries. The observable is provided by the air nitrogen fl uorescence light emitted in the UV band 300-400 nm by the extensive air showers produced by the cascading processes of the primary cosmic radiation particles interacting with the atmosphere. The EUSO telescope is based on a double Fresnel lens optics (diameter 2.5 m) coupled to a highly pixelized focal surface composed multianode PMTs; the image at the earth surface is detailed at 1 km2 over a total of several hundred thousand of km2. EUSO will fl y on the International Space Station accommodated as external payload of the European Space Agency Columbus module. The mission is scheduled to last 3 years, with the start of operations foreseen for 2007/8. The expectations are of a collection rate of a thousand events/year for cosmic rays at E > 1020 eV together with tens/hundreds of cosmic neutrinos at energy above about 4 ￥ 1019 eV. EUSO is the result of the collaborative effort of several institutions in Europe, Japan and USA and it is conceived within the science program sponsored by various space agencies coordinated by ESA.