Overview

By the end of the twentieth century, space probes—some of them conducting long-term studies of planets and their satellite systems—had visited every planet in the solar system except Pluto. In addition to these exploratory missions, a number of large orbital observatories were launched to better study the cosmos. Most of these missions were expensive and took many years of preparation and construction. Their cost siphoned money and time away from other projects and, in the event of a failure, left no backup. In the 1990s NASA began to replace large, expensive missions that tried to do everything with smaller, cheaper, more limited missions that could be built and launched more quickly. This philosophy promises to return a...