On July 4, 2005, NASA hit a comet moving at roughly 23,000 miles per hour with an 820-pound autonomous projectile carrying 113 kilograms of solid copper. This episode unpacks the Deep Impact mission, built by Ball Aerospace for $330 million as a two-part system: a minivan-sized flyby spacecraft loaded with imagers and spectrometers, and a Smart Impactor guided by 20,000 lines of code. We explain why kinetic energy at 10.2 kilometers per second made explosives unnecessary and why copper was chosen because comets lack it, letting scientists subtract its spectral signature from the debris.

We cover the early safe-mode scare, the seven-and-a-half-minute signal delay that forced full autonomy, the impact that made Comet Tempel 1 six times brighter, and the ejecta plume so large the flyby craft could not see the crater. The results overturned the ...