Elon Musk’sSpaceX is due to undertake a repeat attempt at launching two Americans into orbit from Florida, in a mission that would mark the first spaceflight of Nasa astronauts from US soil in nine years.
The mission’s first launch attempt on Wednesday was called off with fewer than 17 minutes remaining on the countdown clock, due to stormy weather around the Kennedy Space Center at Cape Canaveral.
The second attempt is due to take place on Saturday, although the forecast is also precarious. Mission managers plan to make an earlier decision on weather hazards to avoid wearing out the crew with another full day of launch preparations.
The Nasa chief, Jim Bridenstine, told a news conference on Friday that “back-to-back wet dress rehearsals” disrupt the astronauts’ sleep cycles.
Barring weather or other unforeseen problems, the 24-storey-tall Falcon 9 rocket is due to lift off at 9.22pm GMT, propelling astronauts DougHurley and Bob Behnken on a 19-hour ride to the International Space Station.
They will be carried inside the newly designed Crew Dragon capsule, making its first flight into orbit with humans on board.
The launch pad is the same one used by Nasa’s final space shuttle flight, piloted by Hurley in 2011. Since then, Nasa astronauts have had to hitch rides aboard Russia’s Soyuz spacecraft.
Bridenstine said resuming launches of US astronauts on American-made rockets from US soil is the space agency’s top priority.
For Musk, the launch represents another milestone for the reusable rockets his company pioneered to make spaceflight less costly. If successful, it would also mark the first time commercially developed space vehicles, owned and operated by a private entity rather than Nasa, have carried Americans into orbit.
The last time Nasa launched astronauts into space on a new vehicle was 40 years ago at the beginning of the shuttle program.
The US president, Donald Trump, and vice-president, Mike Pence, visited the Kennedy Space Center for the first launch attempt. Trump said he plans to return for Saturday’s attempt.
If the launch is scrapped again, the next window would be Sunday afternoon, with weather forecasts appearing more favourable.
Musk, the South African-born entrepreneur who made his fortune in Silicon Valley, is also the CEO of electric carmaker and battery manufacturer Tesla. He founded California-based SpaceX, formally known as Space Exploration Technologies, in 2002.
Hurley, 53, and Behnken, 49, are Nasa employees under contract to fly with SpaceX. They are expected to remain at the space station for several weeks, assisting a crew aboard the orbital laboratory.
Boeing is producing a launch system in competition with SpaceX. It is expected to fly its CST-100 Starliner with astronauts on board for the first time next year. Nasa has awarded nearly $8bn (£6.4bn) in total to SpaceX and Boeing for development of their rockets.