Complex sentence: While this will be Hawking's first time in space, it will not be the first time he will have experienced weightlessness: in 2007, he had flown into zero gravity aboard a specially-modified Boeing 727-200 aircraft.
Split into simpler sentences: While this will be Hawking's first time in space, it will not be the first time he will have experienced weightlessness. In 2007, he flew into zero gravity aboard a specially-modified Boeing 727-200 aircraft.