Virgin Galactic is sending its supersonic plane to the edge of space for the second time later today.
The firm went to space in December last year and its billionaire founder said he intends to be on-board one of the flights 'within six months'.
Virgin returning to altitudes exceeding 50 miles puts the company one step closer to conquering commercial spaceflight and beating commercial rivals SpaceX and Blue Origin in the race to ferry paying customers to space.
This journey will enable scientists working for Virgin to take detailed readings, including of the vehicle's centre of gravity, which will help inform future flights.
The precise launch time remains unknown but Virgin has said the 'window' for the test flight is now open.
Virgin Galactic reached the edge of space for the first time in 2018 (pictured). The milestone accomplishment for Richard Branson's space tourism endeavours is now set to be repeated
Virgin Galactic plans to conduct two or three more tests of this kind, and move its operations to New Mexico before sending Branson flying more than 250,000 feet above Earth.
The aerospace firm's SpaceShipTwo craft reached a boundary more than 50 miles above Earth last year and today will hope to replicate that.
Branson said the achievement marks the first time a crewed craft designed to carry private passengers has made it to space.
The company's definition of the edge of space, however, sits lower than the widely accepted space boundary said to be at 62 miles high.
Virgin instead uses the altitude of 50 miles (about 80 kilometers), which is used by NASA and the US Air Force for awarding astronaut wings.
In any case, the achievement puts Virgin well ahead of its competitors - Elon Musk's SpaceX and Jeff Bezos' Blue Origin - who have yet to carry crew to a point so high using their passenger craft.
This run will see the flight run for longer and the pane will be heavier - approaching its fully-laden commercial weight.
Virgin Galactic said: 'With the usual caveats that apply to all test flights, and with the added uncertainties of weather at this time of year in Mojave, the window for our fifth supersonic powered test flight opens tomorrow, Wednesday 20 February.'
The blog post also shed light on the scientific goals of the flight.
Virgin returning to space puts the company one step closer to commercial spaceflight and ferrying paying customers. This journey will enable scientists working for Virgin to take detailed readings which will inform future flights
It read: 'So, for this flight, we will be expanding the envelope to gather new and vital data essential to future tests and operations, including vehicle centre of gravity.'
'As we prepare for this next flight, we do so with meticulous care and great attention to detail.
'As always, there are no guarantees that everything will go to plan or work perfectly and the team's overriding priority is always to bring crew and spaceship home safely.
'Whether we complete all our objectives during the next flight or not, we remain committed to completing the final stages of flight test as quickly, but more importantly as safely, as possible.'
Virgin Galactic's SpaceshipTwo can be seen as it takes off for a suborbital test flight of the VSS Unity on December 13, 2018, in Mojave, California
During the test flight, VSS Unity will be co-piloted by Mike Masucci and Dave Mackay.
When the first milestone of spaceflight was achieved, the firm revealed its further goals in using the craft closer to its limits.
'We will now push on with the remaining portion of our flight test program, which will see the rocket motor burn for longer and VSS Unity fly still faster and higher towards giving thousands of private astronauts an experience which provides a new, planetary perspective to our relationship with the Earth and the cosmos,' Branson said after the flight.
Virgin has invested heavily in commercial space exploration and has had a chequered past.
'Ever since I watched the moon landings as a child I have looked up to the skies with wonder,' Branson said after the first successful flight to space. 'Today, as I stood among a truly remarkable group of people with our eyes on the stars, we saw our biggest dream and our toughest challenge to date fulfilled'
The company first promised to fly tourists into space by the start of 2009, but multiple delays and a fatal test flight crash in 2014 have pushed its
