Terraforming Mars. Scientists believe we will one day colonize Mars by first drastically changing it. By the middle of this millenium, the Red Planet will hopefully have turned from a cold, dead world into a lush, Eden-like paradise.
The project will likely be initiated in the early 22nd century. From the outset, it will face major hurdles. Aside from the sheer scale of geo-engineering an entire world, there will be political, social and cultural issues too. Giant solar mirrors may be placed in orbit.
Measuring tens of kilometres across, these will reflect and focus the Sun's rays onto the poles. Other projects will include the seeding of artificial extremophile bacteria. These will begin converting CO2 into oxygen. Vast swarms of nanobots are introduced, accelerating this process and offering a greater degree of control, since they are fully programmable. They also help to produce nitrogen, as well as regulating the overall composition of the atmosphere.
A series of enormous "heat factories" are constructed. These belch out huge amounts of CO2 - which is converted by the bacteria and nanobots - but also has the effect of raising air pressure.
To increase the volume of water on Mars, comets and ice-rich asteroids are manoeuvred into orbit. These are made to slowly burn up as they descend, without impacting the ground and causing damage.
This still leaves the problem of Mars' lack of a magnetosphere, which exposes the surface to harsh ultraviolet radiation. Giant superconducting rings are placed around the latitude lines - focusing mainly on the equator - and buried deep below ground. These are thousands of miles in length and take many decades to construct, but are sufficient to create an artificial magnetic field.
All this may sound like a science fiction story, but I guess all great endeavors start out that way. Our destiny lies above us...