The Extreme Universe Space Observatory on board the Japanese Experiment\ud
Module of the International Space Station, JEM-EUSO, is being designed\ud
to search from space ultra-high energy cosmic rays. These are charged\ud
particles with energies from a few 10(19) eV to beyond 10(20) eV, at the\ud
very end of the known cosmic ray energy spectrum. JEM-EUSO will also\ud
search for extreme energy neutrinos, photons, and exotic particles,\ud
providing a unique opportunity to explore largely unknown phenomena in\ud
our Universe. The mission, principally based on a wide field of view (60\ud
degrees) near-UV telescope with a diameter of similar to 2.5 m, will\ud
monitor the earth's atmosphere at night, pioneering the observation from\ud
space of the ultraviolet tracks (290-430 nm) associated with giant\ud
extensive air showers produced by ultra-high energy primaries\ud
propagating in the earth's atmosphere. Observing from an orbital\ud
altitude of similar to 400 km, the mission is expected to reach an\ud
instantaneous geometrical aperture of A (g e o) a parts per thousand yen\ud
2 x 10(5) km(2) sr with an estimated duty cycle of similar to 20 \%.\ud
Such a geometrical aperture allows unprecedented exposures,\ud
significantly larger than can be obtained with ground-based experiments.\ud
In this paper we briefly review the history of space-based search for\ud
ultra-high energy cosmic rays. We then introduce the special issue of\ud
Experimental Astronomy devoted to the various aspects of such a\ud
challenging enterprise. We also summarise the activities of the on-going\ud
JEM-EUSO program