(CNN) -- When Mark Kelly blasts off for space next year, he will join his identical twin, Scott Kelly, who is already on a space station mission.
It will be the first time that twins have been in space at the same time.
Scott Kelly commands the International Space Station, and Mark Kelly will command the last scheduled space shuttle flight.
Scott Kelly blasted off for the space station last week, aboard a Russian Soyuz spacecraft. He'll be there for six months. Mark Kelly is scheduled to deliver supplies and the Alpha Magnetic Spectrometer to the station in February 2011.
The spectrometer will be used to study the universe's origin, by searching for antimatter, dark matter, strange matter and measuring cosmic rays, NASA says
The brothers are scheduled to work together in orbit for eight days before the shuttle returns to Earth.
The twins are 46. Mark Kelly is the elder of the two -- by six minutes -- but his brother beat him into space.
Scott Kelly has flown on two previous shuttle missions. He piloted a Hubble Space Telescope servicing mission in 1999 and commanded a mission in 2007. Mark Kelly has been on three previous shuttle missions -- as the pilot in 2001 and 2006, and as the commander in 2008.
They were born in West Orange, New Jersey, to two police officers.
The Kellys are both captains in the U.S. Navy, but they started off on divergent career paths. Scott Kelly thought he'd become a doctor, and Mark Kelly headed to the U.S. Merchant Marine Academy.
"I went to the University of Maryland for a year," Scott Kelly said in a NASA interview. Then he "decided my other interest was maybe flying airplanes in the Navy and just kind of changed my mind and changed schools and changed majors and decided to focus 100 percent on that."
His brother recalls it differently: "Actually remember going to visit him at the University of Maryland, probably towards the end of his first year and showing him all these pictures of Navy airplanes and tellin' him I was going and, go ahead and do this."
"That's not how I remember it," said Scott Kelly, who ended up going to the State University of New York Maritime College.
Mark Kelly said: " ... and then he said something like. ... "
Scott Kelly: "Yeah, right."
Mark Kelly said with a chuckle: "That sounds a lot better than being a doctor."
"That's not how I remember it," said Scott Kelly.
There's no friction, though, Scott Kelly said.
"We've always been pretty close and not very competitive with each other," he said.
OK, maybe there is a whisper of competition.
Asked who threaded the needle the best for docking as mission commanders, Scott Kelly said, "I don't know."
Mark Kelly said, "I watched his approach, which looked really, really good. I think it was almost as good as mine."