On October 4, 1957, the Soviet Union launched Sputnik 1, the first human-made object to orbit the Earth. Six decades later, space-faring nations face a much different space environment, one that's more diverse, disruptive, disordered, and dangerous. Today's space domain presents a number of asymmetries that differ from other domains, creating a deterrence environment with unique policy implications. Escalation and Deterrence in the Second Space Age, a report from the CSIS Aerospace Security Project, discusses the evolution of space as a contested domain, the changing threats to U.S. space systems, deterrence theory and its applications to the space domain, and findings from a space crisis exercise administered by CSIS in late 2016.