In-situ resource utilization (ISRU) is the practice of harvesting and processing materials found at an extraterrestrial destination rather than bringing everything from Earth. The most near-term ISRU application is extracting water ice from permanently shadowed craters at the lunar poles, which can be electrolyzed into hydrogen and oxygen for rocket propellant and life support. NASA's VIPER rover was designed to prospect for lunar water ice.
ISRU is considered essential for sustainable human presence beyond Earth. On Mars, the Sabatier reaction can convert atmospheric CO2 and water into methane and oxygen propellant (the basis for SpaceX's Mars architecture). NASA's MOXIE experiment on the Perseverance rover demonstrated oxygen extraction from the Martian atmosphere. At scale, ISRU could reduce the mass that must be launched from Earth by 60-80% for crewed missions, fundamentally changing the economics of deep-space exploration.