Extremely-high-energy cosmic rays: Prospects for research with space-based detectors

The experimental study of high-energy cosmic rays at SINP is based on the legacy left by S.N. Vernov. An abrupt change (cutoff) of the cosmic-ray energy spectrum in the range of ultrahigh energies, ∼5 × 10$^{19}$ eV, was predicted in the works of Greisen-Zatsepin-Kuzmin (GZK). In these and in recent works, it was hypothesized that the shape of the spectrum beyond the GZK limit is related to the evolution of the Universe at the earliest stage of its development, which arouses special interest in studying such extremely high energies. At the same time, experimental study of cosmic-ray particles with such high energies encounters difficulties in connection with their extremely low intensity. To develop a method of particle detection on a maximum area of the order of the Earth’s disk area, in recent years it has been suggested to move from ground-based to space-based detectors. The program of space experiments to study extremely-high-energy cosmic-ray particles is discussed.