NWNH, p. 208:
Priority 2 (Large, Space). Explorer Program
The Explorer program’s Small Explorer (SMEX) and Medium-scale Explorer (MIDEX) missions, developed and launched on few-year timescales, enable rapid response to new discoveries and provide platforms for targeted investigations essential to the breadth of NASA’s astrophysics program. From the WMAP MIDEX measurements of the age and content of the universe accomplished through its mapping of the cosmic microwave background (see Figures 2.4 and 2.5 in Chapter 2), to the GALEX SMEX contributions to understanding of the evolution of galaxies, Explorers are on the forefront of scientific discovery (Figure 7.4). With multiple missions launched per decade for a cost substantially less than that of a single flagship mission, the Explorer program is unique in the world for its versatility and scientific return for the investment. The Explorer program also offers highly leveraged Missions of Opportunity (MoOs), which enable U.S. scientists to make scientific and hardware contributions to non-NASA missions, and which provide a mechanism to develop large suborbital experiments. The frequent opportunity to deploy SMEX (currently $160 million) and MIDEX (currently $300 million) experiments on timescales significantly less than a decade has enabled the United States to seize scientific opportunities, exploit new technologies and techniques, and involve university groups, including students and postdoctoral scholars, in significant development roles.
The Midterm Assessment Report recommended the following:
NASA’s Astrophysics Division should execute its current plan, as presented to the committee, of at least four Explorer Announcements of Opportunity during the 2012-2021 decade, each with a Mission of Opportunity call, and each followed by mission selection.4
This plan included the SMEX round that is the subject of this report.
Table 1.1, provided to the CAA by NASA, indicates that the response to Astrophysics SMEX solicitations is still very high, both in number and quality. SMEX proposals are placed into one of four categories.5 Of the 23 selectable proposals (Categories I-III) in the most recent three rounds, 9 were in Category I, 12 were in Category II, and 2 were in Category III. Thus, there were many more Category I and II proposals than there were launch opportunities, suggesting there is ample compelling science remaining in this mission class.
4 National Academies of Sciences, Engineering, and Medicine, New Worlds, New Horizons: A Midterm Assessment, The National Academies Press, Washington, D.C., 2016, p. 10.
5 Category I proposals are judged by the reviews to be well conceived, scientifically and technically sound, and recommended for acceptance with the highest priority. Category II proposals are like those in Category I, but recommended at lower priority. Category III proposals are judged to be scientifically or technically sound, but require further technology development. Category IV proposals are not recommended for selection by the reviews, with the statistics of the reasons for Category IV designation given in the right-most three columns of Table 1.1.