S-45A was an American satellite, which was lost in a launch failure in 1961. The satellite was intended to operate in a highly elliptical orbit, from which it was to have provided data on the shape of the ionosphere, and on the Earth's magnetic field. It was part of the Explorer programme, and would have been designated Explorer 12 had it reached orbit. It was the second of two identical satellites to be launched; the first, S-45, had also been lost in a launch failure, earlier in the year.