A near-Earth object (NEO) is any small solar system body whose orbit brings it into proximity with Earth. By definition, a solar system body is a NEO if its closest approach to the Sun (perihelion) is less than 1.3 astronomical unit (AU). 1 AU is the mean distance from Earth to the Sun which is about 150 million kilometers (or 93 million miles, or 389 times the distance from Earth to Moon).
NEOs include more than fifteen thousand near-Earth asteroids (NEAs) and more than one hundred near-Earth comets (NECs). In the most general terms, one refers to this population as NEOs rather than NEAs to recognize that both asteroid and comet origins are possible. NEOs have orbits that lie partly between 0.983 and 1.3 AU. This means the orbit of the NEO allows it to approach closer (and much closer) than about 45 million kilometers to the Earth orbit. A distance of 0.3 AU or 117 times the Earth-Moon distance is indeed considered near compared to usual astronomical scales.
The NEO population is especially interesting because :
- These are the objects that (by definition) are capable of impacting Earth, thereby delivering meteorites (many times annually) and civilization-threating impacts (centuries to geologic timescales)
- These are the most accessible spaceflight destinations for both robotic and human missions
- The population is dynamic in terms of its ongoing resupply from both main-belt asteroid and comet sources, where such resupply must occur because planetary encounters have long ago depleted the initial population
- Individual objects, ranging from meter-scale to tens of kilometres undergo a diverse range of physical processes involving their response to external factors such as solar flux and impacts
- Their proximity to Earth observers allows a variety of observational techniques through which their resolved shapes, surface properties, and diverse physical configurations can be revealed.
NEOs are classified as meteoroids, asteroids, or comets depending on size and composition. Asteroids can also be members of an asteroid family (= population of asteroids that share similar proper orbital elements), and comets create meteoroid streams that can generate meteor showers.
NEOs are also divided into groups based on their semi-major axis, perihelion distance, and aphelion distance. Of those there are 1,105 Aten asteroids (semi-major axis of less than 1 AU and cross Earth’s orbit), 5,835 Amor asteroids (orbits strictly outside Earth’s orbit), and 8,144 Apollo asteroids (semi-major axis of more than 1 AU and cross Earth’s orbit). There are currently 1,743 NEOs that are classified as potentially hazardous asteroids (PHAs). Near-Earth asteroids are objects in a near-Earth orbit without the tail or coma of a comet.
Whenever an NEO is detected it is submitted to the International Astronomical Union’s (IAU) Minor Planet Center for cataloguing. The IAU is an international association of professional astronomers.
The ESA SSA-NEO Coordination Centre has started to publish a monthly newsletter summarising the most relevant data and events on asteroids and comets approaching the orbit of the Earth. The newsletter provides information useful to experts, journalists and the public at large.
 Binzel, R. P. et. al.: “The Near-Earth Object Population: Connections to Comets, Main Belt Asteroids, and Meteorites”, Asteroids IV, Univ. Arizona Press, 2015