Buzz Aldrin, one of the Apollo 11 astronauts, earned an Sc.D.. in Astronautics from MIT. He proposed use of a “Mars Cycler” to minimize transit costs between Mars and Earth. A cycler trajectory encounters two or more bodies on a periodic basis. A Mars cycler is an elliptical orbit that crosses the orbits of Earth and Mars, and encounters both planets at the points where it crosses their orbits, although not necessarily on every orbit.
Aldrin identified a Mars cycler corresponding to a single synodic period. The Aldrin cycler (as it is now known) makes a single eccentric loop around the Sun. It travels from Earth to Mars in 146 days, spends the next 16 months beyond the orbit of Mars, and takes another 146 days going from Mars back to Earth. (Wikipedia) Since a cycler is in a (relatively) stable orbit, it needs (relatively) little fuel to stay on track. It can serve as a transport for humans and/or materials from Earth to Mars and vice versa.
In my sci fi future Worlds of Sol the United Nations of Sol have moved mining and manufacturing off Earth into space for climate remediation. Mining extracts raw materials from asteroids. The raw materials are then transferred to factories in space. The UNS has mining/manufacturing complexes in both L4 and L5 Earth-Moon Lagrangian points. The UNS uses repurposed nuclear weapons from the 20th century Cold War to move asteroids between the Belt and either Earth or Mars on Hohmann trajectories. Mars is a waypoint to support asteroid mining as well as a terraforming project to give all humans on Earth a stake in a future in space. This world needs a lot of infrastructure to be viable.
Asteroid mining and Lagrangian point manufacturing allows the UNS to create a closed cycle with minimal requirement to boost mass out of Earth’s gravity. Raw materials come from space. Manufactured goods are sent to Earth in cargo pods by decelerating the pods with electromagnetic accelerators at the factories. The pods reenter, bleed off significant speed (and energy) in the atmosphere and at the surface fly into electromagnetic linear decelerators. Only the empty pod need be sent aloft again. Or, the pod itself could be used as a cargo container for ships to transport goods from an oceanic landing complex around the world.
To send manufactured goods or humans to Mars, the UNS must sent them in pods to rendezvous with the cycler.
One of the major problems for humans going from Earth to Mars is radiation. Radiation shielding is heavy and thus fuel-consuming for a conventional spacecraft. There’s a lot of speculation on the Internet about building a Mars cycler and using it to allow two-way travel between Earth and Mars. It occurred to me that using an asteroid as the Mars cycler would allow (1) fantastic shielding for a crew inside, (2) the possibility of creating artificial gravity inside an asteroid by hollowing it out and spinning it and (3) the ability to send enormous amounts of bulk and weight between the two planets. The only problem is, how do you get an asteroid into a cycler orbit?
My idea is to use the nuclear weapon propulsion mechanism to change the orbit of an Earth-crossing asteroid to a Mars cycler orbit. Once the asteroid is in orbit, people and/or cargo can be loaded aboard at the embarkation point and offloaded at the debarkation point.
It’s necessary to accelerate the payload to the cycler orbital velocity prior to departure, join in formation on the cycler and transfer the payload to the asteroid. It’s also necessary to decelerate the payload at the destination. If this is done with fuel aboard the rendezvous vehicle it’s necessary to bring fuel from Earth for both of these purposes.
A lot of people complain that a cycler orbit takes so much delta-V to reach that the carrier vehicle might as well continue all the way to Mars. But imagine something like a 747, with people crowded in and minimal shielding, to take people or cargo to the cycler. People and cargo can be transferred inside where the major storage and living facilities are carved out of the asteroid.The transfer vehicle need not provide a lot of life support, artificial gravity, radiation shielding or much else. It can go back to Earth or ride along and transfer goods to Mars at the other end.
The Mars cycler figures in my WiP novel Fire in the Sky, which uses a relatively flimsy cycler to go to Mars, then to the Belt, to find an asteroid for the Solar Express.