Thanks to giant extensive air-showers observatories, such as the Pierre Auger Observatory and the Telescope Array, we know that the sources of ultrhigh energy cosmic rays (UHECRs) are extragalactic. The spectrum shows either the interaction with cosmic microwave background (CMB), as predicted, or sources run out of energy at the energy that triggers pion production on the CMB! Their composition is either surprising (dominated by heavier nuclei at the highest energies) or hadronic interactions at 100 TeV are not a simple extrapolation of LHC interaction energies. Hints of anisotropies begin to appear at energies above ~60 EeV, just when statistics become very limited.
Basic questions remain unanswered: What cosmic objects generate such extremely energetic particles that reach above 10^20 eV (100 EeV)? What is the extreme acceleration mechanism? What is the composition of these particles? What is the corresponding neutrino flux? How do particles interact at extreme energies?
To resolve this mystery, we need to increase the statistics of UHECRs observations above 60 EeV. In addition, neutrino and gamma-ray observations at ultra-high energies will be decisive. An international collaboration built the Extreme Universe Space Observatory (EUSO) on a super pressure balloon (SPB) to detect UHECR fluorescence from above. EUSO-SPB1 flew in the spring of 2017. We are now planning EUSO-SPB2 to also observe Cherenkov from UHECRs. EUSO-SPB2 will inform the design of the POEMMA (Probe of Extreme Multi-Messenger Astrophysics) space mission yo discover the sources of UHECRs named and EeV neutrinos.