  A detailed study of comets active at large heliocentric distances (greater
than 4 au) which enter the Solar System for the first time and are composed of
matter in its elementary, unprocessed state, would help in our understanding of
the history and evolution of the Solar System. In particular, contemporary
giant planet formation models require the presence of accretion of volatile
elements such as neon, argon, krypton, xenon and others, which initially could
not survive at the distances where giant planets were formed. Nevertheless, the
volatile components could be effectively delivered by the Kuiper-belt and
Oort-cloud bodies, which were formed at temperatures below 30 K. This review is
dedicated to the results of a multi-year comprehensive study of small bodies of
the Solar System showing a comet-like activity at large heliocentric distances.
The data were obtained from observations with the 6-meter telescope of SAO RAS
equipped with multi-mode focal reducers SCORPIO and SCORPIO-2.
