Alumina supported iridium catalysts used in satellite thrusters are prepared several years before satellite launch and the ageing of such catalysts is an important parameter for the lifetime of the satellite. The ageing has been studied using two procedures and different characterization methods: metal dispersion through hydrogen chemisorption followed by oxygen and hydrogen titrations, powder X-ray diffraction, transmission electron microscopy and catalytic activity in ammonia decomposition. Long term storage during six months and one year under static argon, air or hydrogen atmosphere led to the same slight decrease of the metallic dispersion. After accelerated ageing using dynamic oxygen or water vapor flow, the samples display the same dispersion drop and the same evolution of the crystallite size distribution with a drop of the smallest crystallites due to sintering effects or to formation of oxichloride surface species. The catalytic activity discloses a difference between the ageing treatments: the treatment with oxygen leads to an increase of the measured activity while a treatment with water vapor gives the opposite behavior. The removal of surface chloride ions stabilizes the catalyst against water vapor ageing effects without any modification of dispersion, size distribution and catalytic activity.