  Two types of orbital detectors of extreme energy cosmic rays are being
developed nowadays: (i) TUS and KLYPVE with reflecting optical systems
(mirrors) and (ii) JEM-EUSO with high-transmittance Fresnel lenses. They will
cover much larger areas than existing ground-based arrays and almost uniformly
monitor the celestial sphere. The TUS detector is the pioneering mission
developed in SINP MSU in cooperation with several Russian and foreign
institutions. It has relatively small field of view (+/-4.5 deg), which
corresponds to a ground area of 6.4x10^3 sq.km. The telescope consists of a
Fresnel-type mirror-concentrator (~2 sq.m) and a photo receiver (a matrix of
16x16 photomultiplier tubes). It is to be deployed on the Lomonosov satellite,
and is currently at the final stage of preflight tests. Recently, SINP MSU
began the KLYPVE project to be installed on board of the Russian segment of the
ISS. The optical system of this detector contains a larger primary mirror (10
sq.m), which allows decreasing the energy threshold. The total effective field
of view will be at least +/-14 degrees to exceed the annual exposure of the
existing ground-based experiments. Several configurations of the detector are
being currently considered. Finally, JEM-EUSO is a wide field of view (+/-30
deg) detector. The optics is composed of two curved double-sided Fresnel lenses
with 2.65 m external diameter, a precision diffractive middle lens and a pupil.
The ultraviolet photons are focused onto the focal surface, which consists of
nearly 5000 multi-anode photomultipliers. It is developed by a large
international collaboration. All three orbital detectors have multi-purpose
character due to continuous monitoring of various atmospheric phenomena. The
present status of development of the TUS and KLYPVE missions is reported, and a
brief comparison of the projects with JEM-EUSO is given.
