Since the 1950s, the U.S. government has developed and launched a number of space-based nuclear systems based on both decay of radioisotopes and fission. While private entities have supported the development and launch of nuclear payloads as contractors, the federal government funded and drove the development and operation of such systems. In recent years, the private sector has developed interest in leading the development, launch, and use of nuclear technologies for space applications. This growth mirrors similar trends toward commercialization in the space sector as a whole. This paper investigates that private sector interest in space nuclear systems based on interviews with over a dozen companies in the space and nuclear industries. It presents a definition of commercial space activities, develops a model for the commercial use of space nuclear systems, and explores the status of commercial space nuclear activities in the United States. Our research finds that private sector capabilities in developing, testing, and operating space nuclear systems are growing but require the development of advanced nuclear technology, growth and diversification of the space economy, and government regulatory action.