Phase 2: The Deep Space Transport

Once phase 1 is complete, NASA will move onto phase 2, during which the DSG becomes a lunar port of call for a large crew vehicle called the Deep Space Transport, or DST.

The DST supports a crew of four for 1,000 days at a time. Between each trip, it will be resupplied, refurbished and refueled at the DSG. NASA expects it to be long-lived enough for a total of three round trips to Mars.

The vehicle could be Skylab-esque in volume. International Space Station modules like Unity have a diameter of 4.3 meters. Skylab, essentially the size of a Saturn V rocket's upper stage, was 6.6 meters wide—wide enough to allow astronauts to float in the center without touching the walls. SLS has a fairing diameter of 8.4 meters, and NASA says the DST will take "full advantage of the large volumes and mass that can be launched by the SLS rocket."

The DST is heavy, too—NASA predicts a mass of 41 metric tons, without supplies. A single SLS cargo flight would carry it to lunar orbit in 2027.

"There's really no (other) vehicle today, or even planned, that can launch 41 metric tons (to the Moon) in one piece," Gerstenmaier said. "We think that that is the minimum size for this Mars-class transport."

The closest contender is SpaceX's upcoming Falcon Heavy launcher. The rocket is advertised as being able to heave 64 metric tons into low-Earth orbit, but there is no reliable public estimate on how much mass it can send to the Moon. (An article by Universe Today last year suggested 13 metric tons, which is the same capability SpaceX is projecting for Mars.)

All or nothing

NASA's all-in-one-launch approach for the Deep Space Transport actually resembles SpaceX's far-flung plan to send a million people to Mars.

That plan envisions launching 100 people into Earth orbit atop a behemoth rocket powered by 42 engines. SpaceX, having no on-orbit assembly experience, doesn't seem to be interested in smaller rockets and numerous launches. NASA, despite having almost 20 years of experience with the ISS, isn't favoring that approach either.

In NASA's case, Gerstenmaier said the single-launch approach saves weight, since assembling multiple components requires extra berthing ports and internal hatches. The agency also has the Space Launch System at its disposal, and likely wants to avoid the cost of a multi-mission assembly project.

After receiving supplies and equipment at the DSG, the DST would be ready to fly. In 2029, a crew would climb aboard for a one-year shakedown cruise, during which the DST would fly on its own in lunar orbit. Assuming everything goes smoothly, the DST would return to the DSG (still following the acronyms?) for refueling and supplies, before embarking on its first crewed flight to Mars orbit in 2033.

The journey might involve a Venus gravity assist, in which case the chosen crewmembers would get to see the clouds of Venus and volcanoes of Mars all in the same trip—with their own eyes.

Partners or competitors?

Shortly after President Trump signed the NASA Transition Authorization Act of 2017, SpaceX CEO Elon Musk let it be known on Twitter he wasn't happy: