CAPE CANAVERAL, Fla. — An American astronaut and Russian cosmonaut are just two months shy of launching to the International Space Station for an entire year. Already, though, scientists are clamoring for additional long-term subjects.

Two people are not enough from a scientific perspective, NASA's space station program scientist, Julie Robinson, said Thursday. The space agency wants to start collecting data from Scott Kelly and Mikhail Kornienko before making any firm decisions on further one-year missions, she said.

NASA and its partners — Russia, Europe, Japan and Canada — are considering as many as 12 one-year test subjects at the space station. All but Russia are new to such long orbital hauls.

Space veterans in their 50s, Kelly and Kornienko will rocket into orbit in late March from Kazakhstan and remain aboard the space station until the following March.

It will be the first time NASA sends someone into space for 12 months; station stints typically last six months. The Russians are old pros at this, but medical and technological breakthroughs since Russia's yearlong missions from the 1980s and 1990s means even more will be learned this time around, according to Robinson. The two sides will collaborate on many of the experiments.

NASA wants to learn how the body fares after a year in space, before committing to lengthy trips to Mars and elsewhere.

Right now, it's a big question mark as to what happens beyond six months in orbit.

"What we don't know right now is what that six- to 12-month period looks like," Robinson told reporters. "We're talking about it scientifically, but we're not really having deep discussions about it until we have the first information from the first two. If we see something dramatic, that's going to change how everybody looks at having additional one-year missions."

Kelly will provide an especially unique set of data.

His blood and urine samples, as well as other measurements, will be compared with those from his identical twin brother, Mark, a retired astronaut.

Last week, space station program manager Mike Suffredini said NASA may wait until commercially developed crew capsules are ready to launch astronauts from U.S. soil, before building on Kelly and Kornienko's flight. That won't happen before 2017 or 2018. SpaceX and Boeing are the two chosen contractors.

Since the retirement of the U.S. space shuttle fleet in 2011, Russian Soyuz spacecraft have served as the only means to ferry crew to and from the space station.