Echo 2

Echo 2 was a 41.1-meter-diameter (135 ft) balloon satellite, the last launched by Project Echo. A revised inflation system was used for the balloon, to improve its smoothness and sphericity. Echo 2's skin was rigidizable, unlike that of Echo 1A. Therefore, the balloon was capable of maintaining its shape without a constant internal pressure; a long-term supply of inflation gas was not needed, and it could easily survive strikes from micrometeoroids. The balloon was constructed from "a 0.35 mil (9 µm) thick mylar film sandwiched between two layers of 0.18 mil (4.5 µm) thick aluminum foil and bonded together."