Near-Earth asteroids and comets, collectively the near-Earth objects (NEOs), represent a large population of minor planetary bodies whose orbits lie mostly within the zone between Venus and Mars. Many of these objects cross Earth's orbit, providing relatively easy access from Earth for manned or robotic sampling and exploration missions with fewer propulsion requirements than trips to the Moon or to Mars. This chapter provides a review of NEOs in the context of supporting, through in-situ resource utilization, an active and expanding space exploration and resource development program capable of becoming self-funding and supporting a solar systemwide expansion program. The NEO compositions range from highly metallic asteroids composed predominantly of iron, nickel, and cobalt to cometlike objects composed of frozen water and gases of various compositions. The NEOs are the most easily accessible objects in near-Earth space, and they are numerous. As of January 2011, a total of 7872 NEOs had been identified. The number of NEOs with diameters greater than 1 km (0.6 mi) reached 1269 by June 2012. Moreover, 1176 have been identified as potentially hazardous Earth impactors by the National Aeronautics and Space Administration's Near-Earth Object Program, approaching Earth to within 0.05 astronomical units or approximately 7,480,000 km (4,647,860 mi).

The value of NEOs for space exploration may far exceed the immediate scientific information that they provide on the origin of the solar system: NEOs have the potential to provide fuel for rockets; oxygen and life support materials for explorers; valuable materials and metals for construction in space; and critical, strategic, and highly valuable materials for Earth. Water ice derived from extinct NEO comets or water-rich asteroids can be refined to provide liquid oxygen and liquid hydrogen for rocket fuel and the oxygen necessary for life support. Carbonaceous chondrites contain kerogenlike compounds that can support the immense carbon chemistry developed for our petroleum industry, and metallic asteroids contain platinum-group and rare-earth elements that have been conservatively valued in the hundreds of billions to trillions of dollars if they were made available in Earth markets. These resources are accessible using existing rockets and boosters, but these existing systems and technologies are nearly 50 years out-of-date. Active space exploration and development programs require highly efficient nuclear rockets and space-based nuclear power systems to reduce launch costs to economically tolerable numbers and to provide the heavy-lift capacity and highly efficient rocket engines for crew health and safety and minimum duration missions. Once flight launches are outside Earth's atmosphere, the NEOs can provide nearly unlimited resources for further exploration.