The mated jet and rocket ship lifted off from Mojave Air and Space Port shortly after 7 a.m. Thursday.
The jet will climb to an altitude near 43,000 feet and then release Virgin Space Ship Unity.
The pilots hope to fly the rocket ship to an altitude exceeding 50 miles (80 kilometers), which Virgin Galactic considers the boundary of space.
The company plans to eventually take paying passengers on short trips to space.
A rocket ship set to launch from California’s Mojave Desert could herald a giant step toward the long-awaited dream of space tourism for Virgin Galactic.
Two test pilots are scheduled to fly Virgin Space Ship Unity more than 50 miles (80 kilometers) into the sky Thursday morning from the company’s spaceport in Mojave. They should get several minutes of weightlessness and a view of Earth.
Virgin Galactic considers that altitude to be the threshold of space because it’s defined that way by the Air Force and other U.S. agencies, although others have long considered the boundary to be higher.
A successful flight will be significant progress toward the start of commercial flights that Virgin Galactic promised more than a decade ago.
Virgin Galactic’s first experimental spacecraft broke apart during a 2014 test flight, killing the co-pilot.