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 dirty snowball model: Tempel 1 was about 75 percent empty space with talcum-powder dust containing clays and carbonates that require liquid water. We also cover the Russian astrologer’s $300 million lawsuit, the Bill Haley concert at JPL, the EPOXI extension to Hartley 2 after Comet Boethin vanished, Stardust’s 2011 look at the 150-meter crater, and the 32-bit time counter overflow that killed the spacecraft in August 2013.

• Why copper works like a green screen for reading a comet’s chemistry
• How the impactor targeted a comet on its own and sent images until 3.7 seconds before destruction
• What a porous, powdery interior with water-formed clays means for the early solar system
• How the impact shifted a mountain of ice by about 10 centimeters and still drew a lawsuit
• How an unsigned 32-bit integer counting tenths of a second trapped the probe in an endless reboot