Menda was an Earth-like planet located in Janus GM2797's orbit. Its orbit directly opposed that of Janus Prime. It had a temperate climate, indigenous fruits and vegetables, and clean drinkable water, making it an ideal human colony.
The original inhabitants
The original inhabitants of Menda were very advanced, with great artistic and scientific achievement. They went to war with the Janusians, the inhabitants of Janus Prime. After a long and bloody conflict, the two races overcame their differences and achieved peace.
To ensure that another war never started, they used their advanced technological skills to turn Janus GM2797's solar system into a gigantic bomb. The Eighth Doctor explained this process to Sam Jones:
"It was a terrific feat of planetary engineering. Firstly, they designed two special moons which each had the equivalent of one solar mass. The moons exist in normal space, whereas the masses coexist in hyperspace. By aligning the artificial moons with their sun and linking them via hyperspace, the sun's critical mass would be increased to the point where the nuclear reactions at its centre become unstable and it explodes in a supernova.'"
Another war broke out, but the doomsday weapon was not fully used. The inhabitants of Menda were slaughtered, although they managed to seed Janus Prime with isotope decay bombs.
A few million years later, humanity learned of Menda's existence from Dalek star charts left over from the 22nd century Dalek invasion. Using a large spaceship created from captured alien technology, over a thousand hopeful pioneers left Earth to establish a colony in the Janus GM2797's solar system. Unfortunately, the ship was hit by a rogue meteor near the end of their voyage, forcing them to crash into Menda and preventing them from establishing a link with Earth. Only two colonists were killed in the crash.
Spreading out from the remains of the spaceship, the colonists created the city of Newtown. They discovered the ruins of several buildings built by Menda's original inhabitants and integrated them into Newtown. Inside the ruins they discovered the remnants of the superweapon: a spacial link between Menda and Janus Prime.
Hoping to find a way to contact Earth, Gustav Zemler, a mercenary hired by the colonists to give them protection, travelled through the Link and set up a base on Janus Prime. He was followed by several other mercenaries. Eventually, Zemler and his men discovered that they were dying of radiation poisoning, leading to hostile relations between them and the colonists.
In 2211, the colonists bore five babies: the first of a new generation of Mendans.
On 15 September 2211, the Eighth Doctor and Sam Jones landed on Janus Prime. Coincidentally, 15 September was also the day that Zemler decided to destroy Menda using its moon. However, after millions of years, Janus GM2797 was a much larger star than when the doomsday weapon was designed, meaning that if the weapon was used the star would become a black hole and wipe out a large portion of the nearby galaxy. The Doctor attempted to reach Zemler before he could activate the weapon, but was unsuccessful. He was forced to activate Menda's weapon and destroy Janus Prime before its weapon could destroy Menda. However, the spacial link between Menda and Janus Prime could have transferred some of the destruction onto Menda. Mere seconds before Janus Prime's destruction, Zemler sacrificed himself by jumping into the Link, his bodymass destroying it before it could destroy Menda. (PROSE: The Janus Conjunction)