n this paper we describe the main characteristics of the JEM-EUSO instrument. The Extreme Universe Space Observatory on the Japanese Experiment Module (JEM-EUSO) of the International Space Station (ISS) will observe Ultra High-Energy Cosmic Rays (UHECR) from space. It will detect UV-light of Extensive Air Showers (EAS) produced by UHECRs traversing the Earth's atmosphere. For each event, the detector will determine the energy, arrival direction and the type of the primary particle. The advantage of a space-borne detector resides in the large field of view, using a target volume of about 10(12) tons of atmosphere, far greater than what is achievable from ground. Another advantage is a nearly uniform sampling of the whole celestial sphere. The corresponding increase in statistics will help to clarify the origin and sources of UHECRs and characterize the environment traversed during their production and propagation. JEM-EUSO is a 1.1 ton refractor telescope using an optics of 2.5 m diameter Fresnel lenses to focus the UV-light from EAS on a focal surface composed of about 5,000 multi-anode photomultipliers, for a total of a parts per thousand integral 3a <...10(5) channels. A multi-layer parallel architecture handles front-end acquisition, selecting and storing valid triggers. Each processing level filters the events with increasingly complex algorithms using FPGAs and DSPs to reject spurious events and reduce the data rate to a value compatible with downlink constraints

Meteor and fireball observations are important to derive the inventory and physical characterization of the population of small solar system bodies orbiting in the vicinity of the Earth. After decades of ground-based activities, the proposed JEM-EUSO mission has some chances to become the first operational space-based platform having among its scientific objectives the observation of fireball and meteor events. The observing strategy developed to detect these phenomena, which are eminently "slow" events with respect to the extremely energetic cosmic ray events which are the primary objective of the mission, can prove to be very suitable also for the possible detection of nuclearites, an exciting possibility which enhances the overall scientific rationale of JEM-EUSO, and suggests that the planned observation of slow events may be very interesting in many respects.

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In this paper we describe the observational principle and the expected performances of JEM-EUSO. Designed as the first mission to explore the ultra-high energy universe from space, JEM-EUSO monitors the Earth's atmosphere at night to record the UV (300-430 nm) tracks generated by the Extensive Air Showers. We present the expected geometrical aperture and annual exposure in nadir and tilt modes for Ultra-High Energy Cosmic Rays as a function of the ISS altitude.