Why doesnò??t NASA reuse space probe designs from successful missions like Cassini, New Horizons, and the Mars rovers? Using the same design and tweaking for landing on different planets or orbiting them would seemingly make the entire endeavor cheaper.

This is a great question that I have asked myself many times! The simplest answer is that NASA does (or at least tries to). For example, the upcoming Mars2020 rover uses the same spacecraft bus — payload infrastructure — as the current Mars Science Laboratory rover Curiosity. Similarly, Cassini was originally conceived as the first in a multiple spacecraft build named Mariner Mark II. Congress approved the second, Comet Rendezvous Asteroid Flyby (CRAF), in 1990 but it was ultimately canceled due to cost overruns. In many cases, however, reusing spacecraft designs is impractical.

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