Terraforming

Terraforming is the process of converting a uninhabitable planet such as Mars into one that is more similar to Earth.  To use Mars as an example, this would involve creating oceans of some sort, as well as producing a sustainable oxygen-nitrogen atmosphere.  After a water cycle and atmosphere are established, the planet can be seeded with life forms such as oxygen-producing plants, microbes, and small animals.  Oceans can be created using comets captured from the outer edges of the solar system, and microbes can be seeded into the planet that produce oxygen from the soil until photosynthesis can sustain the atmosphere.

Space Vessel construction

In order to be able to send large amounts of people even a “short” (astronomically speaking) distance to Mars, humanity will need to develop a way to create larger space ships, ones that can support large amounts of people and store everything that the new colony will need to begin, such as foodstuffs, prefabricated buildings, and other equipment.  They might also carry the necessary elements for terraforming the planet.  In any case, the ship will need to be much larger than today’s space shuttles and small capsules that carry only a few humans each.

Efficient Power Generation

Such a large spacecraft would require enormous amounts of power to provide heat, light, atmosphere recycling, as well as thrust to propel the ship towards its destination.  A large colony ship would require a larger, more powerful engine in order to reach its destination in a reasonable amount of time.  Solar power currently used on satellites orbiting the Earth would not be sufficient for these needs.  One solution is to effectively harness nuclear power, specifically nuclear fusion.  Fusion is the process that powers the sun, in which two hydrogen atoms combine into a helium atom, releasing large amounts of heat and light.  It leaves almost no waste and is much cleaner than nuclear fission, which is the current form used in nuclear power plants.  While this is not the only solution, it is the only viable one for the near future. 

Propulsion

As said before, a large ship needs a more powerful form of propulsion.  While any colony ship that large would start its journey parked in orbit, it would still need to be able to break orbit and maneuver towards the end point of the journey.  Current chemical rockets that are used in every vehicle launch today are not efficient enough to boost a large mass out of orbit, and carrying enough fuel to do so would make such a venture too costly to undertake.  Ion engines such as those used on deep space probes are much less powerful (their thrust is equal to the force of a sheet of paper pressing down on someone’s hand), but they provide a constant acceleration that over time can make the craft achieve amazing speeds.  They also possess a much higher propellant efficiency than chemical rockets.  The chief benefit of this propulsion is not just the constant acceleration, but also the elimination of the need to carry a large amount of potentially explosive fuel.  Chemical rockets could be used to boost the ship out of orbit, and then the ion engines could be used to propel the ship the rest of the way.  If higher accelerations could be achieved with ion propulsion, then chemical rockets may not even be necessary.  For more information on ion thrusters, I suggest the Wikipedia article, found at http://en.wikipedia.org/wiki/Ion_thruster

Food Production

In order to support a large population on a potentially hostile environment or one that does not have soil or arable land, new methods of food production have to be developed.  Two methods are hydroponics and synthetic/artificial food.  Hydroponics is currently used on the ISS (International Space Station), it involves growing crops in a rotating environment that provides gravity through “centrifugal” force.  Little to no soil is required, and more crops can be grown in a smaller area.  Synthetic food would require the ability to manipulate molecules and atoms to form edible proteins, carbohydrates, and fats, as well as add flavors.  The chief benefit of artificial food production is that much less space is required than if crops were to be grown, since the only space used would be that which is used to house the machines, as well as the stores of molecules and atoms used in the synthesis of the food.