In the distant future our children had no other choice. Nearly all life-sustaining resources had been depleted. A terrible accident occurred in an attempt to relocate to another suitable planetoid. The human race witnessed, on world broadcast television, their own astronauts being crushed and ripped apart at the same time within a wormhole. They deemed that warping space and time was unsuitable for human traversal. Worm holes were simply too dangerous and unpredictable. Energy harnessed from the Sun and the Earth's core was pure and bountiful, but food and other resources were running out. They were left only with Earth as their home and as their hope for future generations. But hope was running out.
Unbelievably someone uncovered a technology which would allow them to shrink the entire human race; And that, is exactly what they did. The processing removed unnecessary molecules from their bodies. In essence, purifying them, leaving them only with the most pristine and structurally sound cellular make up. This procedure was widely accepted.
Most of them, now nanoscopic in size, relocated for safety, to multiple colony satellites anchored to the earth... As a result the Earth was able to begin replenishing itself.
The satellites were more than enough "world" to expand the human race to over 100 billion individuals. Some of them even considered this "heaven". But for others it wasn't as perfect... They developed the NanoMechs, bio-mechanical militaristic mobile suits, to protect their nanoscopic world from impending doom.