The experimental search for ultra-high-energy cosmic messengers, from E~10¹⁹ eV to beyond E~10²⁰ 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 that are 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 (-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. -EUSO will observe from space, in a free flyer configuration, the extensive air showers produced by ultra-high-energy primaries that traverse the Earth atmosphere. From a variable altitude orbit of 800–1100 km, -EUSO will have an instantaneous geometrical aperture of A_geo≥2×10⁶ km² sr with an estimated duty cycle in the range 10–20%. In this paper, after briefly summarizing the science case of the mission, we describe the scientific goals and requirements of the -EUSO concept. We then introduce the -EUSO observational approach and describe the main instrument and mission features. We conclude by discussing the expected performance of the mission.