Travel from Earth to Mars is demanding. A vehicle from Earth needs to accelerate to get to Mars and then decelerate to enter Mars orbit. It needs even more fuel to land and take off and the fuel requirements to repeat the process to get to Earth orbit are daunting.
A single spaceship carrying humans to Mars from Earth needs to carry enough fuel for the entire trip or refuel at Mars from a base that was established there first. During the entire journey the ship has to protect the crew from solar radiation, galactic cosmic radiation, and the detrimental effects of microgravity. The challenge is enormous.
In the 1970’s astronaut Buzz Aldrin showed that a vessel on a trajectory called a Mars cycler can pass both Earth and Mars on a regular basis with little to no requirement for fuel once established on he trajectory. The most famous of these is the Aldrin Cycler, first postulated by the Apollo astronaut Buzz Aldrin. The Aldrin Cycler allows travel from Earth to Mars in about five months. once established on a cycler orbit.
A ship launched from Earth would have to use a lot of fuel to establish itself on the Aldrin Cycler and a lot more to decelerate and land on Mars.
After a ship on the Aldrin cycler passes Mars, it takes another twenty-one months to return to Earth. An Aldrin cycler orbit that takes five months to travel from Mars to Earth requires twenty-one months to get back to Mars.
A ship that wants to use a cycler trajectory must accelerate to match the speed of the trajectory as it passes Earth and decelerate as it passes Mars. With the demands for propulsion fuel, radiation shielding for the crew, and microgravity, this is a difficult requirement.
Aldrin suggested use of a “castle” on a Mars cycler orbit. The castle would make a continuous round trip from Earth to Mars and from Mars to Earth. It could provide substantial cargo capacity, radiation shielding and spin gravity. A lightweight shuttle could bring cargo and astronauts from Earth to the castle as it passes Earth and from the castle to Mars as it passes Mars. The shuttle can have limited radiation shielding and no spin gravity, reducing the fuel requirements to a minimum at both ends.
An asteroid on an Earth-Mars cycler trajectory makes an ideal castle. A solid enough asteroid can rotate for spin gravity and houses personnel, equipment, and supplies in rooms hollowed out of the asteroid beneath the surface. In my Worlds of Sol future, the United Nations of Sol use such asteroids to bring people and supplies to and from Earth.
Humans being humans, nothing ever happens without conflict. My WiP short story Red Queen’s Gambit tells the tale of pirates who intercept Sleipner, an asteroid ship on a cycler orbit carrying a cargo of nuclear warheads to Ceres for asteroid redirection. Jill and Jimmy, the crew of Sleipner, realize the pirates will leave no one alive to tell the UNS authorities what happened. They need to outmaneuver the pirates if they hope to survive.