EUSO: a Mission from the International Space Station

The mission "Extreme Universe Space Observatory EUSO" is devoted to the exploration from space of the highest energy processes present and accessible in the Universe, which are directly related to the extreme boundaries of the physical world. The observational goal of the mission can be outlined as:- Extension of the measurement of the energy spectrum of the CosmicRadiation beyond the GZK conventional limit (GZK = 5x101 9 eV) toidentify possible structural features in the spectral shape.- Increase of the available data-set for E>102 0 eV from the few eventsavailable today and the hundreds from the major ground experiments nowplanned or under construction (Auger) to the several thousands expectedfrom the operational three year lifetime foreseen for EUSO on the ISS.- Detailed map of the arrival distribution for the EECRs extended to entiresky (North and South hemispheres).- Observation of a possible flux of High Energy Cosmic Neutrinos andopening of the field of high-energy Neutrino Astronomy.- Detection and detailed investigation of "Atmospheric Phenomena" such asmeteoroids and electrical discharges. The corresponding specific goals for science:- Nature and source distribution of EECRs at universal scale;- Information on dark matter distribution;- Probing of the far/extreme regions of the Universe;- Information about the validity of the Theory of Relativity at values of theLorentz factor >101 1 , far beyond the domain reached by the particle man-made accelerators. EUSO will detect the Extensive Air Showeers induced by the Extreme Energy Cosmic Rays and the High Energy Cosmic Neutrino flux looking at the streak of fluorescence light produced within the Earth's atmosphere, and the Cerenkov signal diffused when the shower hits the ground or the top of a cloud. Initially EUSO originally submitted to ESA as a Free-Flyer mission in answer to the Announcement of Opportunity for the F2/F3 missions on January 2000, has been approved in March 2001 for a Phase A Study for accommodation on the CEPF (Columbus External Payload Facility) of the ISS. EUSO will observe the EAS fluorescence signal looking to Nadir at the dark Earth's atmosphere from its location on the CEPF under a 60° full field-o f-view. Fluorescence light will be imaged by a large Fresnel lens optics onto a finely segmented focal plane detector. The segmentation and the time resolution adopted will enable the reconstruction with acceptable precision of the shower energy ( < about 30% ) and arrival direction ( from a fraction of a degree to a few degrees ) depending on the energy and on the inclination to the vertical.