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Sunita Williams and Barry “Butch” Wilmore must have heaved a sigh of relief when they opened the hatch and on June 6 2024. It was the first time the Starliner spacecraft they arrived on had ever carried people.

Built by aerospace manufacturing giant Boeing, Starliner had become the second commercial spacecraft to carry astronauts to the ISS after , built by Elon Musk’s company SpaceX, which has already completed eight crew transportation flights to the orbiting outpost.

Nasa’s decision more than a decade ago to to private companies like SpaceX and Boeing was seen as a bold step. The private space sector is now booming, with fully private flights to Earth orbit already happening, and plans for in the pipeline.

Wilmore and Williams had been expecting to travel back to Earth on Starliner just eight days after reaching the ISS. Little did they know that, more than two months later, , waiting for Nasa and Boeing to complete a detailed analysis of whether it was safe to send them back on Starliner.

, the US space agency announced that it did not have enough confidence to send the two astronauts back on the Boeing spacecraft. So, what does this mean for the two astronauts, and for the future of commercial spaceflight?

First of all, Williams and Wilmore are not in danger. They are safe inside the ISS, with plenty of food, clean water and air. They are now expected to stay on the ISS for eight months and return next year on a Crew Dragon spacecraft.

This , called Crew-9, will launch in September and return in February 2025. It may now astronauts up to the ISS, to make room for the two unexpected crew members on the way back.

The fact that such an option is available is a triumph for commercial spaceflight. SpaceX has become a reliable choice for Nasa to ferry its astronauts to the ISS. In addition to the eight crew rotation flights to date, SpaceX has launched more than 20 uncrewed supply missions to the ISS since 2012.

Following the loss of 14 astronauts in two Space Shuttle disasters – Challenger in 1986 and Columbia in 2003 – Nasa can ill afford more fatal accidents against the backdrop of a perilous financial landscape. This is why Nasa administrator Bill Nelson decided that Starliner would not carry the astronauts back to Earth.

Starliner’s first crewed flight to the ISS launched on , having previously made two uncrewed test flights to it. This latest mission was – discovered both before launch and during flight – and malfunctioning thrusters, the engines used for manoeuvring Starliner in space. Helium is used to push propellant into Starliner’s thrusters, so these malfunctions could have jeopardised manoeuvres to approach and dock with the ISS.

Critical functions

The thrusters’ correct function is also critical to ensuring Starliner’s crew module, the part of the spacecraft in which astronauts travel, enters Earth’s atmosphere at a safe angle. Re-enter too steeply and the spacecraft could disintegrate at a speed of 17,500 miles per hour. Enter at too shallow an angle and it could skim off the atmosphere into space again.

For most of the mission, the crew module is which supplies the power and houses the thrusters. Issues with the thrusters in Starliner’s service module had already emerged during its , which nevertheless successfully docked to ISS.

This followed a which failed due to a combination of an onboard clock error and confused communication. In both cases, the crew module made it down to Earth in one piece.

With astronauts aboard, however, Nasa officials were taking no chances. The uncrewed return of Starliner, scheduled for early September, will allow engineers on Earth to monitor the behaviour of the thrusters once more. They will also look out for potential issues related to the helium leaks. These are quite common in rockets, since helium atoms are very small and do not stick well to other atoms – meaning they can wriggle through tiny pores rather easily.

Unfortunately, the service module separates from the crew module during the return to Earth, burning up in the atmosphere. So, it will not be possible to inspect the thrusters on the ground.

Competitive field

Having been awarded US$4.2 billion (£3.2 billion) by Nasa’s Commercial Crew Program in 2014, Boeing has taken a decade to accomplish its first – ill-fated – crewed mission to ISS, using a United Launch Alliance Atlas V rocket. SpaceX – on a US$2.6 billion (£2 billion) contract from the same Nasa program – managed this feat faster, more cheaply (for Nasa at least), and more comprehensively.

SpaceX has also managed to to the ground, landing them using engines in an upright position on a barge in the ocean.

The Falcon 9 consists of two powered sections: the first-stage booster carries the second stage of the rocket to a predetermined altitude and speed. Returning the first stage allows it to be re-used, bringing down launch costs. Before SpaceX incorporated the technology for re-use, rocket stages were generally discarded.

The company is also , which consists of a huge space vehicle intended to carry humans or cargo on missions to the Moon or Mars, as well as the most powerful rocket ever built.

With other private companies such as Sierra Space and Blue Origin (Amazon founder Jeff Bezos’s space company) also making big steps in the human spaceflight business, Boeing is facing strong competition. The company has also had to deal with heavy criticism following of Boeing 737 Max aircraft and a during a Boeing 737 Max 9 flight.

However, SpaceX received unwelcome news on August 28 when a returning first-stage Falcon booster . The Federal Aviation Administration has now grounded all of its Falcon 9 rockets – one of which is meant to carry Crew-9 to the ISS in September.

Above all, Nasa’s decision to prioritise the safety of Williams and Wilmore, and to work with a commercial partner in finding a solution, is another example of giving the private space industry the confidence and trust required for commercial crewed spaceflight to keep expanding.

It sends a message to all that trying something new does not necessarily mean being reckless. In this sense, it is a triumph of wisdom and caution.

Jacco van Loon does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.