Online courses have become increasingly popular in education and professional development because of their user-friendliness, accessibility, and ability to provide specialized training beyond formal education for workforce preparation. We demonstrate the efficacy of using a Massive Open Online Course (MOOC) to teach specialized engineering concepts through the case study of a multi-body gravitational dynamics course; this topic is not typically taught in undergraduate courses but essential for specialized roles in aerospace engineering. Our use of MOOCs to bridge the knowledge gap between engineering education and practice represents a critical advance in tools for workforce preparation. Our MOOC, Designing the Moonshot, was released to the public in June 2021 and again in June 2022 as part of an international virtual event entitled OrbitCamp (originally called AstroCamp) where we collected data from a sample of 840 participants to study learning gains through pre-post testing and retention. Participant drop-off rates were steep (66% of participants did not complete the first module) but lower among men and those with higher degrees. Statistical analysis demonstrated that participants gained significant knowledge on the 12 (of 14 total) learning outcomes for which sample sizes were sufficient. Some of this knowledge was retained three months after OrbitCamp. These results support the use of MOOCs as a tool for furthering the expertise of the engineering workforce in highly specific domains.