Near-Earth objects are asteroids and comets whose orbits approach or intersect the Earth’s orbit. NEOs range in size from a few meters across to as large as 30 kilometers (km) across. Scientific interest in comets and asteroids is due largely to their status as the relatively unchanged remnant debris from the formation of the inner solar system some 4.6 billion years ago. They are also potential sources of raw materials for future space exploration and colonization.
The potential danger from Earth impacts has stimulated interest in surveying known NEOs and predicting their trajectories. NASA’s current survey program (HORIZONS) was mandated by Congress in 1998. It covers NEOs greater than1 km in diameter—about 3% of the total NEO population known to date. As of December 2007, 5065 NEOs have been discovered, and over 900 of these were identified as potentially hazardous objects, or PHOs.
A proposed next-generation survey, using newer, more powerful telescopes, is expected to survey 80% of known NEOs by 2018 and find ~20,000 PHOs 140 m and larger by 2021. Data from the expanded survey will be essential for planning a crewed NEO mission, since they will identify the NEOs coming in close proximity to Earth several years in advance.
NEOs that are good targets of opportunity for initial piloted missions are those with the following characteristics:
By the middle of the next decade there should be hundreds of possible new candidate NEOs accessible for a CEV mission.