HOUSTON --NASA will mark another historic day early next year when two astronauts shake hands in space. That's because the two men are identical twin brothers and will become the first relatives ever to be in space together. For the past 14 years, NASA employees at times thought they were seeing double. "There are still people in the astronaut office who can't tell us apart," Scott Kelly said. Scott and Mark Kelly, who've separately commanded space shuttle missions, are identical twins, the only twins the Astronaut Corps has ever had. Growing up in New Jersey, the Kelly twins finished each other's sentences, and sometimes tried to finish each other. "I am not kidding you that we would fight, like fist fight, every single day until we were 15 years old, and our parents weren't home, so they would last eight hours," Mark said. But at high school graduation, they went their separate ways, Mark to the Merchant Marine Academy, and Scott to the New York maritime college. Both became naval officers and got master's degrees and by coincidence ended up in test pilot school together. "From the time we were 18 to probably when we showed up at test pilot school at 28, probably didn't see each other very much," Mark said. And fate brought both to NASA in 1996, when they joined the same astronaut class. Scott was the first in space, on Discovery in 1999. Mark didn't launch until 2001 but now has logged 38 days in space compared to his brother's 21 days. "Twenty-one days in space, I've spent 14 years training at some level," Scott said. "To be honest with you, it will be fun to be in space together," Mark said. "Yeah, that's going to be something," Scott added. That will happen next spring, when the Kelly twins become the first blood relatives ever to be in space together. Scott is headed to the space station in October, where he will be the station commander for six months. Mark will be the commander of what is now the last shuttle flight before the shuttle fleet is retired next year. They will meet and make history next February when the shuttle docks at the International Space Station. "From a historical perspective, I don't really think about it," Mark said. "I feel fortunate to be able to have the privilege of flying in space and representing our country as astronaut," Scott said. The twin brothers are so close they have never felt the need to shake each other's hand. But they'll shake up space history when the hatch to the space station opens to the shuttle for a final time next year. "We always thought that would be the coolest thing ever, and we always thought, no, no, that's not gonna happen, that's not gonna happen," said Mark's daughter, Claudia.