LONG BEACH, Calif. -- Southern California-based Virgin Orbit has reached a milestone in developing its airborne orbital launch system.

The company says this week it mated a LauncherOne rocket to a special Boeing 747 at Long Beach Airport and will soon begin a series of flights that will culminate with a drop test in which the booster will be released from beneath the jet's left wing.

The system is intended to carry small satellites into orbit.

Virgin Orbit is a sister company of Virgin Galactic, which is developing an air-launched rocket plane for carrying tourists on suborbital flights into space.

Virgin Orbit says it already has hundreds of millions of dollars worth of launches on contract for a wide range of customers including NASA and the U.S. Defence Department.