Cambodia's first CubeSat Apsara-1 is being developed through an ongoing collaboration between the Institute of Technology of Cambodia and the University of Tokyo. In the early project phase, simulation methods are required for mission design. One important component, needed to predict the mission lifetime, is an orbital simulator. For example, Apsara1 is planned to be launched into orbit in 2024 when solar activity is near its peak, leading to higher than usual atmospheric density in low Earth orbit (LEO) which results in the reduction of the flight time. It is important to predict how long the satellite will remain in orbit, both for effective operation planning and to meet international space regulations. In this paper, an inhouse orbital simulator is developed for this purpose. The simulator includes all the dominant forces needed to model the orbital dynamics at Apsara-1’s planned altitude in LEO: gravitational forces due to Earth’s point mass and the J2 perturbation, as well as aerodynamic drag which determines the orbital lifetime of the satellite. The simulator is verified against a publicly available orbital simulation tool. Differences in the predicted orbital decay time between the in-house simulator and the existing tool are found to be less than 8%. The results show that Apsara1 will remain in orbit for between 75 and 255 days, depending on the solar activity and the altitude at release from the ISS. In the nominal case, the mission will last for around 140 days. Beyond orbital prediction, it is shown that the in-house modular simulator can easily be expanded to perform evaluation of various satellite subsystems, such as the onboard electrical power and attitude control systems, which depend on the orbital motion. Overall, this numerical tool is expected to be valuable not only for Apsara-1 but for the future Cambodian satellite missions in LEO.