The Extreme Universe Space Observatory (EUSO) is a consortium of 250 researchers
drawn from 14 countries, working to put a 2.5 meter, down-looking Fresnel lens on
the International Space Station (ISS). The purpose is to gather a hundred extreme
energy (above 10^20 eV) cosmic-ray events (and 1000 events above 50 EeV) to finally
reveal the cosmic origin and dynamics of such events. The instantaneous aperture
of JEM-EUSO is 50 times that of Auger; the time-averaged aperture is 9 times that
of Auger. The ISS circles the Earth every 90 minutes, thereby providing EUSO with
all-sky coverage at a nearly constant efficiency. A launch date of 2018 is a
possibility. Open issues to be studied with the EUSO event sample include
(i) showering details (Xmax, muon abundance, variance in fluctuations, ~E) -- do we
understand QCD at such energies?
(i) composition - e.g., are the primaries dominantly protons or nuclei? Are there
photons, as might be expected from super-massive DM decay? Any detectable
neutrinos?
(ii) source type - Galactic versus extragalactic origins, inference of massive
black holes;
(iii) GZK suppression of the spectrum versus energy-bounds on cosmic accelerator
capabilities;
(iv) the crossover point in the spectrum, from a Galactic flux to an extragalactic
flux.
Speculative goals include observations of, or bounds on, extreme-energy photons,
neutrinos, magnetic monopoles,
dark matter particles, etc.
