The Service Module had more reaction control gas and less main engine propellant. The old Service Modules had vented excess water, produced by the onboard batteries; the Ares model stored its water in tanks, to avoid having frozen ice particles drifting around near the cluster. The whole configuration had more batteries, and there was more stowage area and locker space in the Command Module. There was an atmosphere interchange duct in the upper docking assembly, to cycle air from the Mission Module into the Command Module. And so on.

Reliability was essential on long-duration missions. Many of Apollo’s systems had redundant backups — straightforward copies, to be substituted in case of a failure — but the old triple-redundancy design paradigm they’d used to get to the Moon wouldn’t work, it had been found, on long-duration missions. Enough redundancy to achieve an acceptably low level of risk over such a span of time would have resulted in a spacecraft of immense weight and complexity.

So the designers had gotten smarter. In addition to simple redundancy, some functions could be performed by dissimilar components, or by components from different subsystems, to reduce the chance of a single failure mode knocking out many functions altogether — as had happened in Apollo 13. And the maintenance capabilities of the crew weren’t ignored, either. The whole ship was more modular and accessible than in its first design, so that components could be reached, and repaired or replaced comparatively easily. There were also isolation valves, switches, test equipment, and fault diagnosis tools. Some of the components contained their own BITEs, microelectronic built-in self-test units.

Hauling an Apollo all the way to Mars also provided some abort options. On return to Earth the Apollo, with the Mission Module, was due to be inserted into a highly elliptical orbit around the planet: two hundred by a hundred thousand miles, a swooping curve that would take the stack halfway out to the Moon and back, an orbit accessible to Ares at a relatively low expenditure of fuel. The Command Module would be able to take them down to the surface of Earth from such a trajectory; the reentry heating would be less than a return from the Moon. And if Apollo were to fail, the crew could survive in its high orbit until rescue came, in the form of another five-man stretched Apollo.

If they couldn’t make Earth orbit at all — for instance if the J-2S, the single engine of the final MS-IVB booster stage, were to fail — they could attempt a direct entry from the interplanetary coast. The heat shield on the Command Module’s underside had been thickened and toughened up, so that it would at least give them a fighting chance of surviving a direct reentry into Earth’s atmosphere. The velocity would only be around 15 percent faster than a lunar return.