When I spoke with international space station commander Dan Burbank and flight engineer Don Pettit on Friday, they should have been talking a lot about the upcoming arrival of the first ever commercial space craft called Dragon. But the date of that arrival is now uncertain. “I suppose on a personal level it’s maybe a little bit disappointing,” Burbank told me during our interview.

The Dragon vehicle built by SpaceX was to launch in early February. That won’t happen. The unmanned craft would have rendezvoused with the space station very much like the Japanese HTV cargo vessel does. “One of the unique things about SpaceX,” says Pettit, “is it flies up like HTV and just gets close to the space station and we kind of lasso it with the robotic arm and bring it in.”

But the launch of Dragon has run into problems. CNN has learned the launch will be in late March at the earliest. SpaceX spokeswoman Kirsten Grantham says, “we need more time to check out and test the systems.” Grantham added, “Its an incredibly challenging mission.”

“Space flight is tough,” Burbank told me. “It’s really, really hard, and to think that anybody could just roll into this and very quickly field a system that’s gonna be ready to go on the first day planned, and be absolutely reliable, is a little bit unrealistic,” he said.

Space flight can be tough on the body as well as machinery. Ten astronauts have come back from Space Station missions with changes to their eyesight, which are sometimes permanent. Burbank says, “The kinds of problems that have happened in a fairly small subsection of people that have flown here before seem to isolated. Some people have it, many people don’t have any at all.”

After a couple months into flight, some astronauts start getting far-sighted. The problem only shows up in male astronauts. NASA flight doctors say it is right now their top priority. There’s little chance that a two- or three-year Mars mission could take place without resolving the issue.

The six-member crew of the station is participating in studies to help doctors and researchers on the ground figure out the problem. They have imaged the backs of their eyes and the optic nerve. They are taking high-resolution images of the retina and measuring the pressure in their eyes. All the data is sent back to doctors on the ground. “I think we came through, and our eyes look very good,” Burbank said.

During the 10-minute interview, Burbank and Pettit discussed all the space junk flying around up there. They view it as a major concern. Burbank said the number of avoidance maneuvers they have to perform is a measure of two things: the amount of debris up there and the increased technological ability to see it.

But the commander said more needs to be done. “I think it’s really, really critical that we get the capability to monitor as best we can debris in orbit.”

While Burbank and Pettit are in space, an important anniversary will take place. February 20 will mark the 50th anniversary of John Glenn’s historic flight, when he became the first American to orbit the earth. “Amazing,” said Pettit, “a short flight in a capsule he couldn’t even unstrap from. You look at what we’re doing now and it’s just amazing in terms of the orbital operations with station and the number of people flying.”

A ceremony marking the anniversary is planned for the Kennedy Space Center. It’s hoped Glenn and Scott Carpenter, the last two surviving Mercury astronauts, will attend.