The experimental search for ultra high energy cosmic messengers , from E \sim 10 ^ { 19 } Â eV to beyond E \sim 10 ^ { 20 } Â eV , at the very end of the known energy spectrum , constitutes an extraordinary opportunity to explore a largely unknown aspect of our universe .
Key scientific goals are the identification of the sources of ultra high energy particles , the measurement of their spectra and the study of galactic and local intergalactic magnetic fields .
Ultra high energy particles might , also , carry evidence of unknown physics or of exotic particles relics of the early universe .

To meet this challenge a significant increase in the integrated exposure is required .
This implies a new class of experiments with larger acceptances and good understanding of the systematic uncertainties .
Space based observatories can reach the instantaneous aperture and the integrated exposure necessary to systematically explore the ultra high energy universe .

In this paper we focus on the Super Extreme Universe Space Observatory – { \mathcal { S } } \mathrm { - \textit { EUSO } } , a mission concept developed in the framework of the first Announcement of Opportunity of the ” Cosmic Vision 2015-2025 ” program , the long term science plan of the European Space Agency . { \mathcal { S } } \mathrm { - \textit { EUSO } } will observe from space , in a free flyer configuration , the extensive air showers produced by ultra high energy primaries which traverse the Earth atmosphere .
From a variable altitude orbit of 800 \div 1100 \mathrm { km } , { \mathcal { S } } \mathrm { - \textit { EUSO } } will have an instantaneous geometrical aperture of { A } _ { geo } \geq 2 \times 10 ^ { 6 } Â km ^ { 2 } Â sr with an estimated duty cycle in the range 10 \div 20 % .

In this paper , after briefly summarising the science case of the mission , we describe the scientific goals and requirements of the { \mathcal { S } } \mathrm { - \textit { EUSO } } concept .
We then introduce the { \mathcal { S } } \mathrm { - \textit { EUSO } } observational approach and describe the main instrument and mission features .
We conclude discussing the expected performance of the mission .