The Nibiru cataclysm is a supposed disastrous encounter between Earth and a large planetary object that certain groups believed would take place in the early 21st century. Believers in this doomsday event usually refer to this object as Nibiru or Planet X. The idea was first put forward in 1995 by Nancy Lieder, founder of the website ZetaTalk. Lieder claims she is a contactee with the ability to receive messages from extraterrestrials from the Zeta Reticuli star system through an implant in her brain. She states that she was chosen to warn mankind that the object would sweep through the inner Solar System in May 2003 causing Earth to undergo a physical pole shift that would destroy most of humanity.

A near-Earth object (NEO) is any small Solar System body orbiting the Sun whose closest approach to the Sun (perihelion) is less than 1.3 times the Earth–Sun distance. This definition applies to the object's orbit around the Sun, rather than its current position, thus an object with such an orbit is considered an NEO even at times when it is far from making a close approach of Earth. If an NEO's orbit crosses the Earth's orbit, and the object is larger than 140 meters (460 ft) across, it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids, but about 0.35% are comets.