  The TUS observatory was the first orbital detector aimed at the detection of
ultra-high energy cosmic rays (UHECRs). It was launched on April 28, 2016, from
the Vostochny cosmodrome in Russia and operated until December 2017. It
collected $\sim80,000$ events with a time resolution of 0.8~$\mu$s. A
fundamental parameter to be determined for cosmic ray studies is the exposure
of an experiment. This parameter is important to estimate the average expected
event rate as a function of energy and to calculate the absolute flux in case
of event detection. Here we present results of a study aimed to calculate the
exposure that TUS accumulated during its mission. The role of clouds, detector
dead time, artificial sources, storms, lightning discharges, airglow and moon
phases is studied in detail. An exposure estimate with its geographical
distribution is presented. We report on the applied technique and on the
perspectives of this study in view of the future missions of the JEM-EUSO
program.
