99942

99942 Apophis is a near-Earth asteroid of the Apollo group. It was discovered on December 19, 2004 by the LINEAR program at Lincoln Laboratory's Experimental Test Site and designated 2004 MN4 before being numbered 99942. The name Apophis derives from the Egyptian god Apep (Apophis), a figure of chaos.

The asteroid is estimated to be about 340 meters in diameter, with a composition and surface properties broadly consistent with stony S-type asteroids, though exact composition remains uncertain. As a member of the Apollo group, Apophis's orbit crosses that of Earth, and its motion is governed by the Sun-Earth gravitational system. Its orbital period is about 0.9 years, with a distance from the Sun ranging roughly from 0.75 to 1.10 astronomical units.

Apophis is notable for its close approach history and monitoring. It will pass by Earth on a highly anticipated flyby in 2029, at an altitude of about 31,000 kilometers (19,000 miles) above the planet’s surface, closer than many geostationary satellites. This encounter will provide an opportunity for detailed observations and data collection. After its initial discovery, early calculations suggested a nonzero probability of impact in 2036, but subsequent observations significantly reduced and eventually eliminated the collision risk for that year and for the following century. Current assessments indicate no meaningful chance of impact for at least the next 100 years, though Apophis continues to be observed to refine its future orbit.