The use of low earth orbit microsatellite to provide messageing service is discussed. To keep the system overall cost very low, the satellite payload is kept simple whereas all the complexity is brought to the ground terminals. In this article, we will be interested in the evaluation of the capacity of Slotted Aloha in the messageing system by micosatellite in low earth orbit. In such a system, an appropriate measure of the performance is the capacity which is the maximum number of terminals that the system supports.