Situated within a three-year design-based research project, this case study investigates how families learned about astroengineering. Eleven families (11 adults, 12 children) engaged in activities related to making a lunar rover in four library programs. To support their engagement in design tasks and engineering thinking, think-pair-share discussion prompts were employed between two-and four-times during workshops. Analyses of the implementation of the prompts by the astronomer leading the program and the family talk that resulted from the prompts found that parents were integral to supporting participation in the engineering activities. Youths often did not answer the astronomer's questions directly, instead, the parents re-voiced the prompts prior to the youths' engagement. The family prompts supported reUlecting upon prior experiences, deUining the design problem, and maintaining the activity Ulow.