It could be a while before another Canadian astronaut visits the giant orbiting space laboratory — unless Canada makes a lot more contributions to the International Space Station.

What's clear for now is that no Canadians will be heading up to the space station before 2017 — at the earliest.

"We've kind of booked up the flights through the end of 2016," NASA's chief astronaut Bob Behnken said in an interview from Houston.

Right now, Canada is not even in a position to get a spot for one of its two active astronauts to take part in a mission.

Under a bartering system, countries collect "credits" based on their contributions to the development of the space station, with the credits traded in for trips by astronauts.

Behnken says Canada used up most of them for Chris Hadfield's five-month visit, which ended in May 2013.

"They (Canadians) have another opportunity that's projected, but not out until the 2019-2020 time frame, just depending on how the balance of contributions works out." Behnken said.

"So if they wanted to fly more often, they unfortunately would have to contribute more to the space station."

Canadian Space Agency astronauts Jeremy Hansen, left, and David St-Jacques have been training since 2009 but have never flown in space. With Tuesday's announcement, they should both have a chance to blast off before 2024. (Sean Kilpatrick/Canadian Press)

The NASA astronaut was involved in two space station assembly missions in 2008 and 2010 and he made the trips on the now-retired U.S. Space Shuttle Endeavour.

Behnken, who has also served as chief of NASA's space station operations branch, operated Canadarm2 and the Canadian-built DEXTRE robot on the space lab.

When the Canadian Space Agency was contacted recently, it had nothing new to add to what CSA President Walt Natynczyk said in April about the next space trip by a Canadian.

"We're working with the international community or the partnership with the International Space Station to see when is the next opportunity that we can get one or both of our astronauts into space," Natynczyk said on April 20.

"I wouldn't want to speculate on any time, but rather work bilaterally with NASA and then multilaterally with the other partners," he told The Canadian Press.

Meantime, Canada's two astronauts, David Saint-Jacques and Jeremy Hansen, continue their training.

In September, Hansen will take part in NEEMO 19, an underwater mission that closely resembles a space environment. He 'll be on a team of engineers, scientists and astronauts who will spend seven days under 19 metres of water in "Aquarius", the world's only undersea research station.

NEEMO stands for NASA Extreme Environment Mission Operations.

Saint-Jacques took part in a similar NEEMO mission off Key Largo in the Florida Keys in 2011.

More training required

Even if a Canadian astronaut is eventually chosen for a space station visit in the coming years, more training will be required.

Astronauts need to be trained to make sure they can handle the different modules which are built by different countries.

"We need to identify the crewmembers at least two-and-a-half-years in advance right now in order to make sure (they do) the things that need to be done to get them trained," Behnken said.

"(And) we need another three months or so to kind of make the decisions on who is going to have what role when they actually fly on that mission."

Former Canadian astronaut Marc Garneau points out that since the retirement of the U.S. space shuttle fleet in 2011, trips to the giant laboratory in the sky are less frequent.

"That was a source of flights for Canadians and because we did very well, we were quite often chosen to go on flights of the shuttle," he said in a recent interview.

"We now have to wait beyond the Americans, Europeans, Japanese, Russians and wait until our next turn occurs," the astronaut-turned Liberal MP said in a recent interview.

"And, as a result of that, it's going to be a while before we see another Canadian go up to the International Space Station."

Canada could partner with other agencies

But Garneau remained hopeful that Canada could partner with other programs from other space agencies to get its astronauts "back up there."

"After a big high like Chris Hadfield's flight, people are saying: 'well, what's next?' and it's a little hard for us to wait, but if people are patient, I think they'll see that we're going to get back there — eventually."

Right now, the only way for any international astronaut to get to the space station is by hitching a ride on a Russian Soyuz spacecraft.

But that could change as private U.S.-based companies continue to develop space capsules to ferry astronauts up to the space station.

Canada's two astronauts may even have a choice of their space bus when their turn finally comes.

"Our current plan is to continue to use the Soyuz until Boeing, Sierra Nevada or SpaceX — one of the three companies that we have on the U.S. side that are developing vehicles to visit the space station — are kind of ready to take us there instead," Behnken said.

"We don't yet have the contracts in place to specify what dates those vehicles would be out there, but it's in the 2017 time frame."