Space missions to worlds that might support life are sterilized to reduce the chance that terrestrial microbes will contaminate scientific measurements or alter an extraterrestrial environment. Stricter sterilization increases cost and can damage sensitive instruments, while absolute cleanliness is unattainable. Some researchers argue that missions should be delayed until contamination risk is extremely low; others contend that delay also has costs, including lost knowledge that could guide later protection. Different destinations and mission types pose different risks, making a single threshold difficult to defend. The controversy concerns how precaution should be calibrated when both action and delay can affect future science.