Caption: A cartoon of a near-Earth asteroid (NEA) on a apsidally precessing orbit in the inner Solar System.
Where do NEAs come from?
Gravitational perturbations (which often can be called natural gravitational assists) knock an asteroid from a stable asteroid reservoir (usually the asteroid belt or some Trojan zone ) into the inner Solar System.
Probably some asteroids in the inner Solar System are extinct comets.
By being in the inner Solar System, an asteroid is ipso facto a NEA.
What happens to NEAs?
Astronomical perturbations (mostly gravitational perturbations) will eventually cause it to impact a large inner Solar System object (Sun ☉, Mercury ☿, Venus ♀, Earth ⊕, Moon ☽, Mars ♂) or be ejected (often by what can be called natural gravitational assists) out of the inner Solar System.
The motion of a NEA is often an apsidally precessing orbit which causes the NEA to eventually come close enough to a large inner Solar System object to impact it or be ejected from inner Solar System by it.
NEAs typically have lifetimes of only a few million years (see Wikipedia: Near-Earth object: Near-Earth asteroids (NEAs)).
Credit/Permission: ©
David Jeffery,
2003 / Own work.
Image link: Itself.
Local file: local link: nea_apsidal_precession.html.
File: Asteroid file:
nea_apsidal_precession.html.