The High Energy Astronomy Observatory (HEAO A) satellite, 1977.
© National Aeronautics & Space Administration / Science & Society
The satellite, seen here undergoing pre-flight check out, was launched on 12th August 1977. It mapped the sky at X-ray wavelengths, increasing the number of known X-ray sources from 500 to 1500. Two more HEAO satellites were launched in November 1978 and September 1979.