There are many, many, many obstacles to establishing a human colony on Mars. One huge challenge is the cost of ferrying people and the supplies they'll need between the two planets. Another challenge is the length of the trip to Mars. It took four days for Dr. Aldrin to travel from the Earth to the moon; it took the Curiosity Rover nine months to travel from the Earth to Mars.
|Mars Science Laboratory, on its way to the red planet.|
These cyclers would essentially be space stations orbiting a path that would take them between Earth and Mars every few months. They'd be similar to the international space station, but with heavy-duty rockets attached, more radiation shielding, and maybe a big centrifuge creating artificial gravity. You could have two of these cyclers, with one always going towards Earth and one away. Or you could launch even more cyclers, allowing for more frequent trips between the two planets.
|A Mars cycler approaches Mars.|
Source: Scientific American, March 2000.
|Voyager 1 gained the momentum needed to escape the Sun's gravity via a gravity assist from Jupiter and Saturn.|
|The little bitty command module, that I've circled in red, is the only bit of Apollo 11 that made it home to Earth.|
Source: Universe Today.
Another problem is actually reaching the cycler from vehicles launching from Earth or Mars. They must catch up as the cyclers make their once-every-five-months pass by the Earth or Mars. The cycler could be travelling as fast as 27,000 miles per hour as it encounters Mars. That's close to the fastest speeds that the Apollo spacecraft ever traveled. So, a rocket leaving Mars attempting to rendezvous with the cycler would expend a great deal of energy. Or, alternatively, you could significantly slow down the cycler when it reaches Mars (by aerobraking- dipping into and out of the Martian atmosphere, with the friction of Martian air slowing the craft down). Then, it would be easy for a spaceship leaving Mars to rendezvous with the slowed cycler... though the cycler would need a big rocket boost to speed up and travel back to Earth.
|An illustration of Mars Reconnaissance Orbiter aerobraking on arrival to Mars.|
Sources: Next Big Future; March 2000 Scientific American; buzzaldrin.com; NASA; D.V. Burnes, J.M. Longuski; B. Aldrin, Cycler Orbit Between Earth and Mars, Journal of Spacecraft and Rockets (1993); Buzz Aldrin et al.; Evolutionary Space Transportation Plan for Mars Cycling Concepts.