The Cosmic Background Explorer (COBE) was one of the most successful scientific packages ever launched by NASA; here, the chief project scientist tells its story. COBE was designed to make three delicate measurements of the microwave energy that cosmologists believe to be the last remnant of the Big Bang. Mather and Boslough (a freelance journalist) begin with a quick look at the launching of the satellite, then flash back to cover the scientific background. The Big Bang theory arose from the discovery, early in this century, that distant galaxies are moving away from Earth at high speeds. By the 1940s, scientists had concluded that the universe must have originated in a gigantic explosion, the Big Bang. The greatly cooled energy of that explosion--the cosmic background--was detected in 1964. But the microwave frequencies of the energy were difficult to measure precisely from the surface of the Earth. When NASA put out a call for experiments to be conducted on future satellite launches, the cosmic background measurement was an obvious candidate; NASA eventually combined three proposals into a single project, the COBE. Some 1,600 scientists and technicians would eventually be involved in the project. The design team faced great challenges: Instruments had to be fitted into a severely restricted space, and, while the instrumentation had to be sufficiently precise to do the job, it also had to be tough enough to withstand the rigors of a space shuttle launch. Meanwhile, the team had to keep the project moving forward despite budget cuts, changes in NASA administrators, and rivalry among engineers, scientists, and bureaucrats. In the end, the experiments were spectacularly successful; the instruments sent back data that confirmed the Big Bang theory. Mather concludes by considering the larger cosmological questions yet to be answered, and reflecting on the place of human beings in the cosmos. A useful look behind the scenes of modern science, as told by one of the key players.