  We evaluate the exposure during nadir observations with JEM-EUSO, the Extreme
Universe Space Observatory, on-board the Japanese Experiment Module of the
International Space Station. Designed as a mission to explore the extreme
energy Universe from space, JEM-EUSO will monitor the Earth's nighttime
atmosphere to record the ultraviolet light from tracks generated by extensive
air showers initiated by ultra-high energy cosmic rays. In the present work, we
discuss the particularities of space-based observation and we compute the
annual exposure in nadir observation. The results are based on studies of the
expected trigger aperture and observational duty cycle, as well as, on the
investigations of the effects of clouds and different types of background
light. We show that the annual exposure is about one order of magnitude higher
than those of the presently operating ground-based observatories.
