Nasa is to make a second attempt to launch its “Mars spacecraft”.

The Orion crew capsule is being touted by the US space agency as the essential first step in its ambition to land humans on Mars by the mid-2030s.

If it goes ahead as planned, Friday’s test launch will be uncrewed. The capsule will circle the Earth twice before splashing down in the Pacific Ocean off the coast of Baja California. It will be recovered by Nasa and the US Navy.

The first attempt to launch the mission on Thursday was plagued by technical and weather problems. Mission control eventually postponed the launch by 24 hours.

Friday’s launch window opens at 7.05am (12:05 GMT) and lasts for 2 hours and 39 minutes.

One of the problems that delayed the initial launch was the appearance of a boat in the restricted area of the Atlantic, off the coast of Florida near Cape Canaveral. Strong gusts also triggered several automatic aborts as the wind speed rose above 20 knots (about the speed of the sprinter Usain Bolt).

The mission was finally aborted because of a technical issue in the rocket engine. A number of valves that regulate the flow of liquid hydrogen fuel into three core boosters of the rocket’s first-stage engine failed to close. Normal operation calls for them to remain open during the fuelling and then shut tight a few minutes before liftoff to seal the fuel tank. This did not happen.

Engineers from Lockheed Martin, which built the Delta IV Heavy launch vehicle, have been working on the problem since.

“We’re very confident we’re going to be able to exonerate the equipment,” said Dan Collins, chief operating officer of United Launch Alliance.

Preparations for refuelling began on Friday morning around 08:00 GMT. The fuel is a mix of liquid oxygen and hydrogen which is ignited to create the thrust needed to lift the spacecraft into orbit.

Even if the technicians fix the problems, however, the weather could still prove problematic. Initial forecasts predicted a 60% chance of favourable conditions. This was downgraded during the night to 40%.

Air Force meteorologists are concerned about a weather system moving up from the south, which could bring rain and higher winds that would scupper the lift-off.

If the launch goes ahead, Orion will reach a maximum altitude of 3,600 miles (5,800km), about 15 times higher than the International Space Station. From this height, it will plunge into Earth’s atmosphere at 20,000mph. This will generate temperatures around 2,200C (3990F), which is representative of a re-entry from lunar orbit, and test whether the newly designed heat shield can protect the spacecraft during re-entry.

If all goes well, the next test flight is scheduled for 2017, when another unmanned Orion craft will be sent around the far side of the moon and back again.

Humans are not expected to fly on Orion until 2021. This is slated to be a mission to lunar orbit, possibly to rendezvous with a small asteroid that Nasa hopes to capture and tow into the orbit using a robotic spacecraft.

The Orion programme, and its associated rocket, the Space Launch System, is costing Nasa $3bn a year for 10 years. Any eventual Mars mission will require large additional funds to build space habitats and landing craft.