Eight months have passed since NASA astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko returned to Earth from a nearly year-long mission aboard the International Space Station. During that time, the long-duration fliers completed a battery of follow-up tests, and US and Russian scientists have busily crunched away at data collected before, during, and after the extended space mission. Researchers plan to present preliminary results at a scientific meeting in January.

The one-year mission was just the beginning, however. NASA’s Human Research Program, which supports safe and productive space travel, has begun devising follow-up missions to ensure it knows enough about prolonged stays in microgravity before astronauts venture into deep space for extended periods of time. And as important as Kelly's and Kornienko’s data is, a study with just two participants doesn’t allow scientists to draw meaningful conclusions.

“It’s just not enough,” said William Paloski, the director of the Human Research Program at NASA’s Johnson Space Center. “To extrapolate we need to have more time in space, and more observations. We started working on additional missions two years ago.” The question is how best to collect that additional data.

Five more missions

Paloski told Ars that the space agency wants to fly five additional one-year missions on board the International Space Station with a similar setup to the Kelly-Kornienko flight. Each mission will include one Russian and one US or international partner astronaut. NASA hopes to embellish the one-year missions by flying concurrent six-month and six-week missions to directly compare the health effects among the astronauts. The candidate pool for these one-year missions will be broadened to include female and rookie astronauts.

NASA hasn’t finalized these plans yet, Paloski said, because the agency is still working out some logistics issues with Russia and its international partners. But the current plan calls for the next one-year mission to begin no earlier than September 2018. The five additional one-year missions would be completed before 2024, the nominal end date of the space station’s lifetime. One or more of them may overlap.

In discussions with NASA, the Russian space agency has proposed still more dramatic missions, Paloski said. It has suggested a simulated mission to Mars in which a crew goes to the station for six months to mimic the travel time to Mars, lands on Earth for three to six months in isolation, and then launches for another six months in space to simulate the flight “back” to Earth from Mars. From a scientific and pragmatic standpoint, however, NASA prefers the one-year mission setup.

Is one year long enough?

During a subcommittee meeting of NASA’s Advisory Committee earlier this month, former Space Shuttle Program Manager Wayne Hale asked Paloski why the space agency wasn’t considering missions longer than a year to truly reflect the time astronauts would have to spend away from Earth were they to go to Mars or other destinations beyond cislunar orbit. “It seems to me that you’re not there yet in determining the health factors for a 30-month voyage,” Hale said.

In a follow-up interview, the Human Research Program’s chief scientist, John Charles, explained to Ars that from a logistic and scientific standpoint, the one-year missions offered a reasonable compromise. The station probably has seven years left in its lifetime, and because of advanced planning requirements, there would be the capability to fly, at most, just a single two- or three-year mission during that time. Not only would this adversely affect crew rotations, there’s also the question of statistical significance from just two data points.

“Darn it, we biologists like to have statistical validity,” Charles explained. “We have discussed it internally and really think we’re going to be pushing our luck to get five more one-year missions during the station’s lifetime, to get a statistically significant database.”

Paloski also said he believes that a one-year mission should be adequate to capture most of the physiological and psychological risks of long-duration missions. Moreover, the station isn’t the best testbed for some spaceflight hazards, such as radiation and communication delays. For those threats NASA will probably have to conduct long-duration missions in cislunar space, near the Moon, which is far enough away to be exposed to those hazards but still just a few days away from Earth in case of an emergency.

To that end, NASA has begun working with six different contractors on concepts for deep space “habitats,” one of which could be delivered to cislunar space in the 2020s. Tentatively, NASA has planned to fly a one-year mission aboard this deep-space habitat by 2030.