Between March 2015 and March 2016, a man spent a total of 357 hours being poked and prodded: sacrificing blood and urine, sweating through sprints and letting others read his journals — oh, and living on the International Space Station.

The man was Scott Kelly, and with his Russian counterpart, Mikhail Kornienko, he spent a total of 340 days living in space as part of NASA's first attempt to understand how such long stints without gravity affect the human body. But while the men returned to Earth nearly three years ago, scientists are still trying to figure out what they've learned. A new paper lays out the tests built into the "One-Year Mission" of Kelly and Kornienko and contextualizes it in the context of longer spaceflight — like that which would required by a mission to Mars, lasting at least two years.

What stands out most is how little NASA knows about how the body might respond to such conditions: Before Kelly's launch, no U.S. astronaut had ever spent more than six consecutive months in space, so the agency was desperate for data. (Russia had sent six cosmonauts to live on the Mir space station for 300 days or more, but doesn't make that data public.) ['Infinite Wonder': Photos from Scott Kelly's Year in Space Mission]

NASA astronaut Scott Kelly and Russian cosmonaut Mikhail Kornienko spent 11 months in space to help scientists understand how long-duration space flights may affect the human body. (Image credit: NASA)

So before the flight, scientists came up with a suite of 17 different investigations the pair of astronauts would participate in — tackling unknowns like how fluids move through the body in microgravity, how they slept and how the communities of microbes living on and inside them changed. By running tests on the two astronauts before, during and after their 11-month stay, they could see how the astronauts' bodies responded to a long spaceflight. They could also compare the measurement with data gathered in conjunction with previous, shorter spaceflights. And in a separate analysis, Kelly's data is also being compared to data about his twin brother, NASA astronaut Mark Kelly, who stayed on Earth as a control.

But even the One-Year Mission, the best study scientists have to date on the effects of long-term spaceflight only includes two test studies — and those are both men, both Caucasian, both in their early-to-mid 50s. And the 11-month flight doesn't match the timeline needed to get to Mars.

But the new paper argues that the study acts as a crucial foundation for future, larger studies, and that comparing six-month and one-year results will help scientists better extrapolate out to longer flights.

The next step, the researchers argue, is to build up a larger study. That would use the same tests and procedures developed for Kelly and Kornienko, but apply them more broadly: to 10 astronauts on year-long missions, 10 on six-month missions and 10 on two-month missions. Doing so, the authors write, will help space agencies further bridge the gaps in research; NASA has already expressed interest in proposals along these lines.

The overview of the research is included in a paper published Jan. 1 in the journal Aerospace Medicine and Human Performance. The results of individual studies conducted as part of the One-Year Mission will be published separately.

Email Meghan Bartels at mbartels@space.com or follow her @meghanbartels. Follow us @Spacedotcom and Facebook. Original article on Space.com.