Reflecting upon the history of development and research accomplishments with Explorer, it is evident that the hopes and visions of the early advocates of the use of Explorer for cloud physics research were well founded. The technical development of improved and new instruments for airborne cloud physics research and the unique capabilities of Explorer to ascend in updrafts of growing cumulus clouds led to significant scientific accomplishments. These advancements were made possible by the availability of sustained funding and a crew of dedicated scientists, technical and support staff, and skilled, adventuresome pilots. Even with significant scientific advances by Explorer and other scientific endeavors, questions remain in the areas of precipitation development and cloud electrification. The spread and multiplication of ice particles inside cumulus clouds and quantification of the non-inductive ice-graupel collision mechanism through charge measurements remain as challenging areas of research. One wonders what the capabilities of a platform such as Explorer combined with modern miniaturized instrumentation might be able to achieve.

In conclusion we want to acknowledge the efforts of the many, many people who contributed to Explorer success. We also wish to thank Charlie Knight and Brian Ridley for comments on the manuscript and especially thank Steve Williams for encouraging and fostering this effort on documenting the history and accomplishments of Explorer – the NCAR sailplane.