Richard Branson’s Virgin Galactic is gearing up to finally send its tourism rocket ship to the edge of space.

If successful, it would be a major step toward the long-delayed dream of commercial space tourism.

The next test flight could come as early as Thursday (US time), with two pilots taking Virgin Space Ship Unity high above California’s Mojave Desert.

In a statement, the company said the next stage of testing aimed to “reach a space altitude for the first time”.

“Although this could happen as soon as Thursday morning, the nature of flight test means that it may take us a little longer to get to that milestone,” it said.

Space begins at an altitude of 100 kilometres; Unity’s last test flight was at 52 kilometres.

Reaching that threshold would demonstrate significant progress toward the start of commercial flights that were promised more than a decade ago.

The window for our 4th powered test flight opens on Thursday. We plan to burn the rocket motor for longer than we have in flight before, but not to its full duration. Some background on this next phase of our test flight program can be found here. https://t.co/4MY6YEFWe3 pic.twitter.com/4xJI64reEe

— Virgin Galactic (@virgingalactic) December 12, 2018

Virgin Galactic’s development of its spaceship took far longer than expected. It also endured a major setback when the first experimental craft broke apart during a 2014 test flight, killing the co-pilot.

More than 600 people have committed up to $US250,000 ($A346,000) for rides in the six-passenger rocket, which is about the size of an executive jet.

Mr Branson isn’t alone in the space tourism business: Jeff Bezos’ Blue Origin is planning to take space tourists on suborbital trips, using the more traditional method of a capsule atop a rocket that blasts off from a launch pad.

SpaceX’s Elon Musk also recently announced plans to take a wealthy Japanese entrepreneur and his friends on a trip around the moon.

-AP