High Energy Astronomy Observatory (HEAO 2) satellite, 1977.
© National Aeronautics & Space Administration / Science & Society
This satellite, seen here undergoing pre-flight check out, was launched on 13th November 1978. It mapped the sky at X-ray wavelengths, increasing the number of known X-ray sources from 500 to 1500. HEAO 2, also known as the Einstein Observatory, was the second of three HEAO satellites launched between August 1977 and September 1979.