Orbital Decay Simulation

Atmospheric drag is an important source of orbital-energy loss for satellites operating in low Earth orbit. Although the atmosphere is extremely thin at these altitudes, it still contains enough particles to gradually slow spacecraft and reduce their orbital altitude. This study develops a numerical model for investigating atmospheric-drag-driven orbital decay. The model combines Earth's gravitational field with atmospheric drag and atmospheric-density values obtained using the NRLMSISE-00 empirical atmospheric model through PyMSIS. The model was first validated using a zero-drag orbital test and an independent atmospheric-density comparison with NASA NRLMSISE-00 reference results. The density comparison covered altitudes from 250 km to 600 km and showed agreement within approximately 1 percent across the tested points. Three main experiments were then performed. The first investigated the effect of initial altitude on orbital decay. For an identical 100 kg satellite, simulated 24-hour semi-major-axis decay decreased from 1,405.694 m at 300 km to 6.130 m at 600 km. The second experiment investigated space-weather sensitivity. At 400 km, simulated decay increased from 53.864 m under low atmospheric activity to 680.847 m under the simulated geomagnetic-storm scenario. The third experiment investigated long-term decay, producing 7.618 km of decay after 30 days, 32.254 km after 90 days, and 109.200 km after 180 days under the specified moderate-activity conditions. The results demonstrate that orbital altitude and atmospheric conditions have strong effects on orbital decay. They also show that decay is nonlinear over long periods because a descending spacecraft encounters progressively denser atmospheric regions.