Ultra High Energy Neutrinos and Cosmic Rays: a 'Vision' for the next decade

Ultra High Energy Neutrinos, with energies from a few 1018 eV to beyond the decade of 1020 eV, and Cosmic Rays with E≥5x1019 eV appear to be the only suitable messengers to explore the Universe at frontier energies, where radiation is expected to be produced under the most extreme physical conditions. Observations of these UHE particles will certainly provide new information on the sources and on the physical mechanisms able to accelerate these extreme messengers to macroscopic energies. Moreover, they might, also, provide evidence of yet unknown physics or of exotic particles, relics of the early Universe. To reach these goals, innovative experiments with larger effective aperture (Aeff≥105 km2sr) and good understanding of systematic uncertainties (less than ∼ 10%) must be developed. The ground-based Pierre Auger Observatory, whose southern site is expected to be completed in Malargue, Argentina by the end of 2006, will surely provide, in the near future, a more solid observational scenario (Flux, Spectral shape, Composition) for UHE Cosmic Rays (UHECR). However, only space-based observatories can reach the effective area necessary to systematically explore the UHE Universe. In the present paper 1.) we present the Science Rationale behind UHE studies; 2.) we review the status of current experimental efforts, with main emphasis on the actual generation of space-based observatories; 3.) we briefly discuss the science goals, requirements, and R and D of a 'next-generation' space-based mission for UHE observations. To develop such a challenging and innovative observatory for UHE particles, the ESA 'Cosmic Vision 2015-2025' long term plan provides certainly an unique opportunity