In January 2004, President George W. Bush announced the Vision for Space Exploration (VSE), which instructed NASA to "Extend human presence across the solar system, starting with a human return to the Moon by the year 2020, in preparation for human exploration of Mars and other destinations," among other objectives. As acknowledged in the VSE, significant technology development will be necessary to accomplish the goals it articulates. NASA's Exploration Technology Development Program (ETDP) is designed to support, develop, and ultimately provide the necessary technologies to meet the goals of the VSE. This book, a review of the ETDP, is broadly supportive of the intent and goals of the VSE, and finds the ETDP is making progress towards the stated goals of technology development. However, the ETDP is operating within significant constraints which limit its ability to successfully accomplish those goals-the still dynamic nature of the Constellation Program requirements, the constraints imposed by a limited budget, the aggressive time scale of early technology deliverables, and the desire to fully employ the NASA workforce.

In spring 2011 the National Academies of Sciences, Engineering, and Medicine produced a report outlining the next decade in planetary sciences. That report, titled Vision and Voyages for Planetary Science in the Decade 2013-2022, and popularly referred to as the "decadal survey," has provided high-level prioritization and guidance for NASA's Planetary Science Division. Other considerations, such as budget realities, congressional language in authorization and appropriations bills, administration requirements, and cross-division and cross-directorate requirements (notably in retiring risk or providing needed information for the human program) are also necessary inputs to how NASA develops its planetary science program. In 2016 NASA asked the National Academies to undertake a study assessing NASA's progress at meeting the objectives of the decadal survey. After the study was underway, Congress passed the National Aeronautics and Space Administration Transition Authorization Act of 2017 which called for NASA to engage the National Academies in a review of NASA's Mars Exploration Program. NASA and the Academies agreed to incorporate that review into the midterm study. That study has produced this report, which serves as a midterm assessment and provides guidance on achieving the goals in the remaining years covered by the decadal survey as well as preparing for the next decadal survey, currently scheduled to begin in 2020.