The identical twins are positioned to become NASA's first blood relatives to be in space together, if its shuttle schedule holds, according to reports.
Scott Kelly blasted off last week from Kazakhstan for the International Space Station, where he'll be the commander for six months. Mark Kelly is the shuttle Endeavour's next commander, scheduled to blast off in late February on NASA's final shuttle flight.
If the schedule holds, the 46-year-old Kelly twins will be at the space station together for about eight days in February, The Star-Ledger of Newark reported.
Dmitry Lovetsky, AP
U.S. astronaut Scott Kelly, left, is accompanied by twin brother Mark as he gets ready to board a rocket headed for the International Space Station. Mark is scheduled to join his twin at the space station next year.
"It's something we hoped would happen,'' Mark Kelly told The Associated Press. "It wasn't done by design. But we're fortunate. I think it will be fun for us.''
Mark is older by six minutes, according to a NASA interview, but Scott went into space first.
Scott Kelly flew a mission to service the Hubble Space Telescope in 1999 and led a 2007 mission. Mark Kelly has flown on three shuttle missions: as pilot in 2001 and 2006 and as commander in 2008.
Though they're identical twins and both are Navy captains and former fighter pilots, there is one prominent difference between the brothers: Mark has a mustache and Scott does not.
The brothers grew up in West Orange, N.J., where they shared a bedroom until age 12 and worked making pizza and volunteering with an ambulance crew, the Star-Ledger reported.
The space siblings said they want to mark their overlap by arm wrestling, but they think it might end up a tie because of their weightlessness, the AP reported.
The astronauts are thinking about their missions, not about pulling any pranks.
"We don't spend a lot of time thinking about the whole twin thing," Mark Kelly told the Star-Ledger.