Asteroid

An asteroid was a chunk of rock in space, large compared to many things but small compared to planets and stars. They were similar to, but not to be confused with, comets and planetoids, and were usually located in asteroid belts.

Asteroid Impacts
Sufficiently large asteroids could create massive amounts of damage when they hit a planet, causing mass extinctions and societal collapse. Wherever they landed, all the surroundings would be incinerated within seconds. The impact would kick up a cloud of dust which could cover the planet and cause an "Impact Winter" for months or years. Bburning debris and acid would happen. Following the "Impact Winter" was the "Ultraviolet Spring", which would cause mutations and cancer. (AUDIO: Blood of the Daleks)

The extinction of the dinosaurs was believed to be caused by an asteroid impact (TV: Time and the Rani, TV: Fragments) but it was actually caused by a spacecraft (TV: Earthshock).

Red Rocket Rising was hit by an asteroid after the manipulations by the Daleks. (AUDIO: Blood of the Daleks)

Liveable Asteroids
While many asteroids were barren and lifeless, some supported life, either naturally or due to design.

The Sycorax were native to asteroids and fashioned starships out of other, smaller asteroids. (TV: The Christmas Invasion)

A wide variety of buildings and structures were built into asteroids, including the Bi-Al Foundation, (TV: The Invisible Enemy) Braxiatel Collection, (PROSE: Tears of the Oracle) The Delirium Archive, (TV: The Time of Angels) Demon's Run, (TV: A Good Man Goes to War) Lavonia, (AUDIO: Fear of the Daleks) the Shadow Proclamation headquarters (TV: The Stolen Earth) and the Stormlight House (COMIC: Mortal Beloved). Other asteroids were centres of trade, such as one on which the rare metal Baloovium was sold. (TV: Army of Ghosts) They generally needed special features to bring the gravity to acceptable levels. (PROSE: Tears of the Oracle)

Usage of Asteroids
The Ice Warriors dropped an asteroid on Paris in 2086; this was the beginning of the Thousand Day War. (PROSE: GodEngine, PROSE: Beige Planet Mars)

The Daleks disguised their saucers as asteroids to fool their opponents upon their approach. (COMIC: The Only Good Dalek)

Other References

 * Upon taking Rose Tyler to see the Earth's destruction in the year five billion, the Ninth Doctor mentioned asteroids as one of the things humanity thought would cause their extinction. (TV: The End of the World)
 * While on their honeymoon, Amy Pond and Rory Williams visited a planet and an asteroid who were also on their honeymoon. (TV: Death of the Doctor, WC: The Night After Hallowe'en)
 * On By the Light of the Asteroid the characters lived on an asteroid. (TV: Voyage of the Damned)