Abstract: A spacecraft comprising a spacecraft body and a deployable structure coupled to the spacecraft body. The deployable structure is operable between a stowed state and a deployed state. The deployable structure is held in the stowed state during a first operative phase by a holding device. The holding device is responsive to a preselected environmental circumstance and is operative to release the deployable structure after exposure to the preselected circumstance for a preselected criterion occurring in a second operative phase. A deployment mechanism is operative to deploy the deployable structure after the holding device releases the deployable structure. The preselected environmental criterion can be atomic oxygen, radiation, thermal cycling or any other environmental circumstance known to exist in space, and, the preselected criterion can be total dosage, number of thermal cycles or the like.