The Extreme Energy Cosmic Radiation (EECR) with energy >1020 eV can be considered as the "Particle channel complementing the Electromagnetic" channel, specific of conventional Astronomy. EECRs present us with the challenge of understanding their origin in connection with problems in Fundamental Physics, Cosmology and Astrophysics. Focal points are represented by :
- the change in the spectral index at the "Ankle" (" 5 x 1018 eV). This could correspond to : a change in the production mechanism in the original sources; a change in the primary elemental composition connected with a different confinement region ; a change in the interaction process in the first collision inducing the extensive showers in the atmosphere.
- Evidence of the existence of Cosmic Rays (CRs) with energy > 1020 eV.
- From the Astroparticle Physics point of view, the EECRs have energies only a few decades below the Grand Unification Energy (1024 -1025 eV), although still rather far from the Planck Mass of 1028 eV.

Cosmic neutrinos with high enough energy produce detectable Extensive Air Showers (EAS) Not suffering of the Greisen-Zatsepin-Kuzmin (GZK) effect and being immune from magnetic field deflection or from an apreciable time delay caused by Lorentz factors. These particles are ideal for disentangling source related mechanisms from propagation induced effects.
The opening of the High Energy Neutrino Astronomy as a new branch of Science will allow to probe the extreme boundaries of the Universe.
Astronomy at the highest energies must be performed by neutrinos rather than by photons because the Universe is opaque to photons at these energies.
Astrophysical neutrinos, however demand a very large detector for observation. The orbiting night-sky watcher, EUSO, will observe a large area of Earth's atmosphere providing several thousands of nucleonic events above 1020 eV and possibly allowing an exploration of the neutrinos flux. Some theories predict abundant neutrinos above 1021 eV. If so, a further exploration of the Big Bang relic neutrinos in the Cluster of Galaxies can be envisaged, since they should become observable by EUSO due to the ZO-resonance by neutrinos above 1021 eV.

The Earth atmosphere viewed from space with an acceptance area of 106 km2 sr and target mass of the order of 1013 tons, constitutes an ideal absorber/detector for the EECRs and for Cosmic Neutrinos.
EECRs and EE Gamma Rays and Neutrinos, colliding with air nuclei, produce secondaries that in turn collide with the air molecules giving rise to a propagating cascade of particles (Extensive Air Showers, EAS).
EAS electrons moving through the atmosphere ionise the air and excite metastable energy levels in its atoms and molecules emitting a characteristic isotropic fluorescence light with peaks at wavelenghts from 330 nm to 450 nm.
Observation of the fluorescence light with a detector at distance from the shower axis is the best way to control the cascade profile of the EAS. When viewed continously the object moves on a straight path with the speed of light.

What is the maximum Cosmic Ray energy, if there is any limit?
Two general prodution mechanisms have been proposed for the EECR:
- "bottom-up", with acceleration in rapidly evolving processes occuring in Astrophysical Objects with an extreme case in this class being represented by the Gamma Ray Bursts (GRBs). The observation of "direction of arrival and time coincidences between the optical-radio transient and Extreme Energy Neutrinos could provide a crucial identification of the EECR sources, together with a unique test of the Relativity Principle.
- "top-down" processes with the cascading of ultrahigh energy particles from the decay of Topological Defects (TD) ; these are predicted to be the fossil remnants of the Grand Unification phase in the vacuum of space.
They go by designations such as cosmic strings, monopoles, walls, necklaces and textures.
Inside a topological defect the vestiges of the early Universe may be preserved to the present day. Topogical defects are expected to produce very heavy particles (X-particles) that decay with production of ultrahigh-energy particles.
Relics of an early inflationary phase in the history of the Universe can also lead to the production of extreme energy (EE) particles. These particles may survive to the present as a part of dark matter. Their decays can give origin to the highest-energy cosmic rays either by emission of hadrons and photons or through production of EE neutrinos. Observations of these neutrinos may teach us about the dark matter of the Universe as well as its inflationary history.

The resulting event seen by the detector looks like a narrow track in which the recorded amount of light is proportional to the shower size at the various penetration depth in the atmosphere.
Showers intiated very deep in the atmosphere indicate an origin by neutrinos because the neutrino-air nuclei interaction cross section is several orders of magnitude lower than the cross section for hadrons or photons.
From a Low Earth Orbit (LEO) space platform the UV fluorescence induced in the atmospheric nitrogen by the incoming radiation can be monitored and studied; the luminescence coming from EAS produced by the Cosmic Ray quanta (protons, nuclei, gamma rays, neutrinos....) can be disentangled from the general blackground and measured. Other phenomena such as GRBs meteors, space debris, lightning, atmopheric flashes, distribution of minor components in the atmosphere, can also be observed.