Natural reentry predictions of a close earth satellite illustration by Cosmos 1402

Computational considerations for predicting the reentry of an artificial satellite whose size and mass are unknown are discussed. The methods were explored at CNES in order to track the reentry path of the Cosmos 1402 satellite. The Keplerian orbital phase above 120 km altitude has for controlling forces the earth's gravitation field up to order 8 zonal harmonics and the atmospheric drag force. The atmospheric phase, below 120 km, features the Newtonian attraction with J2 perturbation and the drag force, which varies with the Mach number. The satellite shape also influences the parameters, and can be partially determined using sighting data for orbit restitution. An expression is defined for semimajor axis decay, noting that the accuracy will depend on the atmospheric model selected, the presence of magnetic storms, and solar activity. Comparison with actual reentry trajectory demonstrates that the reentry models are statistical and do not actually represent the physical events.