Near-Earth asteroids (NEAs) are asteroids whose orbits are close to Earth's orbit.
Some NEAs' orbits intersect Earth's so they pose a collision danger.
On the other hand, NEAs are most easily accessible for spacecraft from Earth; in fact, some can be reached with much less delta-v than it takes to reach the Moon.
Two NEAs have been visited by spacecraft: 433 Eros, by NASA's Near Earth Asteroid Rendezvous probe, and 25143 Itokawa, by the JAXA Hayabusa mission.
Roughly 1000 near-Earth asteroids are known, ranging in size up to approximately 32 kilometres (1036 Ganymed).
Tens of thousands probably exist, with estimates placing the number of NEAs larger than one kilometer in diameter at up to 2,000.
NEAs only survive in their orbits for 10 million to 100 million years.
They are eventually eliminated by orbital decay and accretion by the Sun, collisions with the inner planets, or by being ejected from the solar system by near misses with the planets.
Such processes should have eliminated them all long ago, so it is assumed they are resupplied on a regular basis by orbital migration of objects from the asteroid belt.