A Mars expedition would, of necessity, be an “Earth-independent” mission, different from the ISS mission, which relies on resupply for basic resources such as food, water, fuel, and even breathable air. Just seven years into its utilization phase, the ISS must continue to serve as a testbed for research into how people can stay healthy and serve capably on a mission that will take them no less than 78 million miles, round-trip, for a duration of two years or more.

In recent years, ISS researchers have discovered several challenges associated with long-term human habitation of space, including bone loss and, more recently, vision impairment. Resistance exercise, diet, and vitamin D supplements have proved a relatively effective counter to bone loss aboard the station, for long expeditions, but Montalbano said the size of a deep-space exploration vehicle and the duration of a Mars mission will make it more difficult. “Today, we have a treadmill onboard,” he said. “We have a bicycle, and we have a resistive exercise machine. And we’re going to have to figure out a plan because of the real estate requirements. As we go to the Moon, and definitely to Mars, we’re going to have to shrink those three pieces of hardware to one – and it’s not even going to be the size of one. It’s going to have to be smaller than the smallest one we have on orbit now. We’ll need the space station to test and wring out the new device.”

Life support technologies have improved remarkably over the past 20 years, but several haven’t yet reached Earth-independence levels. ISS oxygen generators work well, but, like the exercise equipment, will need to be downsized. The ability of ISS systems to scrub carbon dioxide from the interior air and to recover water will need to be improved, said Montalbano, before they’re ready for a Mars mission. “We’re not into a solid 90 percent yet on water recovery on the space station, and if you’re going to leave the Earth system, you’re going to want to be better than 90 percent on that.”

First, regardless of the degree to which operations aboard the U.S. Orbital Segment become commercialized in the coming years, it will be important that this research remains.

In the summer of 2018, a report from NASA’s inspector general pointed to the possibility that the research necessary for enabling long-term spaceflight is unlikely to be completed by 2024:

As of February 2018, research for at least 6 of 20 human health risks that require the ISS for testing and 4 of 40 technology gaps will not be completed by the end of FY 2024 when funding for the station’s operation is scheduled to end. In addition, research into 2 human health risks and 17 technology gaps is not scheduled to be completed until sometime during 2024, which increases the risk that even minor schedule slippage could push completion past the end of that fiscal year. This is due in part to difficulties with characterizing and mitigating the health risks and, for technology demonstrations, obtaining the required funding and on-orbit research time.

While the future of the ISS beyond 2024 isn’t clear, the unfinished business of deep-space research has clarified a few things for NASA and its partners. First, regardless of the degree to which operations aboard the U.S. Orbital Segment become commercialized in the coming years, it will be important that this research remains. Studies must be conducted in parallel to preparations for international collaboration on the Gateway, an outpost envisioned as a staging point for explorations of the Moon and eventually Mars.

“We want to continue flying space station until we have our sustained presence in cislunar orbit,” said Montalbano, though he says the future of the ISS may not necessarily involve a continuous human presence. “We have too many examples in our past where you end one program, and then you’re idle for a while before the next program, and that’s really not a good path for human spaceflight.” A hiatus will mean losing not only the sharpness of decision-making both on orbit and on the ground, but also the collective strength cultivated over the last 20 years aboard the ISS. “To me, that unites a world community,” Montalbano said, “and if you’re not flying, you lose that.”

In March 2017, the House Subcommittee on Space hosted a hearing on possibilities for the ISS after 2024. In his testimony, William Gerstenmaier, NASA’s associate administrator for Human Exploration and Operations, said it would be critical to smoothly transition human spaceflight capabilities from low-Earth orbit aboard the ISS, to regular missions of the Orion spacecraft and its Space Launch System into cislunar space. “I think the station plays a pretty critical role,” he said. “We’re going to need some facility in space as we break the tie of the planet and move human presence farther into the solar system.”

Meanwhile, NASA’s ISS program is focused on how to do the work necessary for completing planned space-related research while accelerating the involvement of private-sector partners. Given the reaction of many legislators to the White House’s 2019 budget proposal, complete defunding of the station by 2025 doesn’t seem a sure thing. Many public and private ISS partners think it may take longer than that to create a viable commercial marketplace in low-Earth orbit, which is one of the issues being examined in the studies recently commissioned by NASA.

“One of the responses could be that for the space station to be a viable investment, companies will want to have it in place late into the 2020s,” said Montalbano, “which I think we can do. Right now the work being done easily gets us to 2028. I think we can get to 2030. We’re taking steps today to make sure we remain a platform that’s operational. The goal is to be there for as long as there is a need.”