The necessity of developing atomic clocks with high long-term stability for space exploration and improving the operation of satellite systems is substantiated. The photon detection system for the design of atomic clocks on  mercury-199 ions for space applications is developed. A comparison is made of various data on the Allan deviation for various models of atomic clocks based on mercury-199 ions. For comparison, we used the results obtained by us and other scientists. The directions for further research are determined.