Destruction of Mondas

The destruction of Mondas occurred when the planet returned to the solar system in an attempt to steal energy from its twin planet Earth. (TV: The Tenth Planet)

Dating
Most accounts dated the event to December 1986, as the arrival of Mondas was covered by media, (TV: The Tenth Planet, PROSE: Iceberg, AUDIO: The Reaping) although two other accounts dated Mondas's arrival to the 1990s (PROSE: The Power of the Daleks) or the year 2000. (PROSE: Doctor Who and the Tenth Planet, Doctor Who and the Tomb of the Cybermen)

The End of Mondas
Needing to draw power from Earth, the First Doctor warned the Cyber-Leader Regos Krang that Mondas was finished and would disintegrate. Hours later, having finally exhausted its own energy, the planet appeared to "melt." The invading CyberMondans, completely reliant on power from Mondas, disintegrated along with it. (TV: The Tenth Planet)

Aftermath
Despite the media coverage, an older Ben Jackson believed Mondas was covered up when he and Polly Wright celebrated New Year's Eve, puzzled over the lack of coverage. (PROSE: Mondas Passing)

Because Mondas was destroyed, the Cybermen colonised Telos, (TV: Attack of the Cybermen) although another account claimed it was settled by the first space travellers from Mondas before 1986. (PROSE: Doctor Who and the Cybermen) With their first planet destroyed and facing extinction, the CyberTelosians sealed themselves in their tomb on Telos. With their machinery stopped and supply of replacements depleted, (TV: The Tomb of the Cybermen) they later launched an assault on the Moonbase in 2070, also planning to use the Gravitron to destroy Earth and remove the threat it posed. (TV: The Moonbase)

Legacy
By 2096, records detailed the "Mondas Asteroid" as being a massive asteroid with interesting topography that either burned up in the Earth's atmosphere or went supernova. (PROSE: The Indestructible Man)

Later, CyberNeomorphs operating from Telos attempted to divert Halley's Comet to destroy Earth in 1985 and prevent Mondas's destruction. Gustave Lytton commented that Mondas being destroyed had "now become part" of the Web of Time, implying it wasn't initially. (TV: Attack of the Cybermen) As part of the Web, however, the Sixth Doctor reflected that saving Mondas could cause disastrous repercussions in every corner of the universe, killing billions, instantly disappearing major civilisations, and threatening the fabric of the universe itself. (PROSE: Attack of the Cybermen)