Reliability study for LEO satellites to assist the selection of end of life disposal methods

Following a business as usual scenario, some Low Earth Orbit (LEO) regions could be unusable for many decades because of the space debris growth. In order to reduce that trend, the current probability of success of the chosen End of Mission (EOM) disposal method shall ensure a target value of 90% [1]. Understanding reliability of satellites and their subsystems for different spacecraft classes allows determining which disposal solution could better fit with a particular space mission. However, spacecraft are quite often different from each other, so a statistical approach is required. An in depth study has been performed on 1086 spacecraft launched between January 2000 and December 2014 using data from the SpaceTrak™ database. Spacecrafts have been separated by mass and by the presence/absence of the propulsion subsystem. The non-parametric Kaplan-Meier survival analysis has been used because the dataset presented censored events, namely the observed variable value is partially known. Empirical reliabilities obtained have been fitted using the Weibull distribution. Because each disposal method needs a combination of subsystems in order to operate, the reliabilities of the different subsystems have been combined by means of the System Reliability Theory. General spacecraft reliability was found to be about 92% after 4 years. The presence of the propulsion subsystem results in a better reliability trend. Furthermore, the propulsion presence/absence classification being equal, the heavier the mass the worse the reliability. Disposal solutions that use communication and power subsystems can count on reliabilities above 90% up to 7 years, whereas those ones that need also the attitude control can rely on only an 85% reliability after 4 years. A trade-off was performed and it showed that the film aerobrake and the propulsive D-Orbit decommissioning device can be key resources as disposal methods for future missions. The results presented could be useful to the space industry, to better address its efforts in improving spacecraft reliability and to design more reliable EOM disposal methods in order to reduce space debris growth.