For several years, Virgin Galactic has been working on a low-cost method of sending small satellites to space. The system consists of the company's LauncherOne rocket traveling into the upper atmosphere via a plane, where it's then launched into space mid-flight, reducing the amount of fuel needed compared to launching the same rocket from the ground.
Virgin Galactic's satellite launching system was initially set to use WhiteKnightTwo, a twin fuselage jet aircraft capable of flying at high altitudes. However the company has now announced that WhiteKnightTwo is getting replaced on these satellite missions by a fairly standard Boeing 747-400.
The Boeing 747-400, which was previously part of Virgin's commercial service since 2001, has been chosen as it's more suitable than WhiteKnightTwo for the LauncherOne's upgraded payload and flight frequency. The 747 is also a very common aircraft, which will inevitably reduce Virgin Galactic's operating and maintenance costs on these missions.
Actual satellite launches using this 747-400, nicknamed 'Cosmic Girl', are still several years away. The LauncherOne rocket and its factory are still being built, though when that's complete, Virgin Galactic will begin launching 648 internet satellites for OneWeb as soon as early 2017.
WhiteKnightTwo will continue to be used, though instead of launching satellites, it will be exclusively used for SpaceShipTwo commercial flights to the edge of space. Although SpaceShipTwo was destroyed during a test flight in 2014, resulting in the death of its pilot and injuries to the co-pilot, Virgin is still moving ahead with their commercial space operations.