Though school is out for the summer, ninth- and tenth-grade students at Union Intermediate High School are burning off energy playing a game of tag on the soccer field. But that is not all they are doing. They are also synthesizing and applying key chemistry concepts they have just learned related to the conditions of the early solar system. They are acting out concepts including the fundamentals of matter--atoms, ions, and isotopes--while deepening their understanding of how the solar nebula transformed into our present solar system about 4.6 billion years ago. These students are engaged in Cosmic Chemistry--a two-week summer learning program focused on chemistry with an astronomical twist. Why do districts, including Union Public Schools in Tulsa, Oklahoma, choose to implement summer programs that, like Cosmic Chemistry, address science concepts? Some districts use summertime to help reinforce or remediate learning from the previous year. Others, like Union, hope to accelerate future learning by providing young people with engaging, high-quality learning that builds a foundation of essential knowledge and skills for next year's science content. Cosmic Chemistry engages ninth and tenth graders to prepare them to take chemistry in school the following year. This proactive approach to summer learning provides rigorous, relevant science experiences to help the students who need it most "before" they fall behind. Cosmic Chemistry was developed by educators at McREL International and Education Northwest. In this article, the authors describe their process of designing, implementing, and evaluating Cosmic Chemistry and offer lessons that may be useful to designers of other academically oriented out-of-school time (OST) programs.