The Cosmic Background Explorer (COBE) satellite under construction at Goddard Space Flight Center. The satellite will study the structure & spectrum of the microwave background (the cold, faint echo of the radiation emitted during the Big Bang). The radiation was emitted when the universe was a hot fireball and initially had a spectrum characteristic of this temperature. As the universe expanded, the radiation cooled and now has a temperature of only 2. 7 degrees Kelvin. Any fine structure found in the microwave background would indicate inhomogeneities in the primaeval fireball. These would have profoundly influenced the large-scale structure of the present universe.