Near-Earth object focused on asteroids and short-period comets. CNEOS even restricts its definition of NECs to short-period comets—, 107 such objects have been discovered. , only 20 comets have been observed to pass within of Earth, including 10 which are or have been short-period comets. Two of these comets, Halley's Comet and 73P/Schwassmann–Wachmann, have been observed during multiple close approaches. The closest observed approach was 0.0151 AU (5.88 LD) for Lexell's Comet on July 1, 1770. After an orbit change due to a close approach of Jupiter in 1779, this object is no longer a NEC. The closest approach ever observed for
Earth-centered inertial specifying the direction toward celestial objects. The extent to which an ECI frame is actually inertial is limited by the non-uniformity of the surrounding gravitational field. For example, the Moon's gravitational influence on a high-Earth orbiting satellite is significantly different than its influence on Earth, so observers in an ECI frame would have to account for this acceleration difference in their laws of motion. The closer the observed object is to the ECI-origin, the less significant the effect of the gravitational disparity is. It is convenient to define the orientation of an ECI frame using the Earth's orbit plane and