NASA said on Saturday that two NASA astronauts who traveled to the International Space Station in June aboard Boeing’s defective Starliner capsule will need to return to Earth on a SpaceX vehicle early next year, considering the issues related to the Starliner propulsion system to be too risky to bring the first crew home.

The agency's decision to rely on Boeing's main competitor in the space sector to return the astronauts is one of NASA's most significant decisions in years. Boeing had hoped the test mission would save the Starliner program after years of development issues and more than $1.6 billion in budget overruns since 2016.

Boeing is also struggling with quality issues in commercial airplane production, its most important products.

NASA veteran astronauts Butch Wilmore and Sunita Williams, both former military test pilots, became the first crew to ride Starliner on June 5 when they were launched to the International Space Station for what was expected to be an eight-day test mission.

However, the spacecraft's "Starliner" propulsion system suffered a series of failures that began within the first 24 hours of its voyage to the International Space Station, causing months of successive delays. Five of the 28 propulsion engines failed, and several helium leaks occurred, which is used to pressurize the propulsion engines.

In a rare reshuffling of astronaut operations at NASA, the astronauts are now expected to return in February 2025 aboard a SpaceX Crew Dragon spacecraft scheduled for launch next month as part of a routine astronaut crew rotation mission. Two of the four astronaut seats in Crew Dragon will be kept vacant for Wilmore and Williams.

Starliner will separate from the International Space Station without a crew and attempt to return to Earth as it was to do with astronauts on board.

Boeing has struggled for years to develop Starliner, a candy-shaped capsule designed to compete with Crew Dragon as a second American option for ferrying astronaut crews to and from low Earth orbit.

Starliner failed a 2019 uncrewed test launch to the International Space Station, but largely succeeded in the 2022 attempt, which also faced some propulsion issues. A crewed mission was needed before NASA would issue a certification for the capsule for routine flights, but now the path to Starliner’s certification has been turned upside down.

Since Starliner's docking with the International Space Station in June, Boeing has rushed to investigate the causes of its engine failures and helium leaks. The company arranged ground tests and simulations to gather data that it used in an attempt to convince NASA officials that Starliner is safe to bring the crew home.

However, the results of this testing raised more difficult engineering questions and ultimately failed to allay NASA officials' concerns about Starliner's ability to make a crewed return flight—the most difficult and complex part of the test mission.

The decision by NASA, and the now uncertain path for Starliner to achieve certification, will add to the crises facing Boeing's new CEO, Kelly Ortberg, who took the helm this month with the goal of rebuilding the company's reputation after a passenger 737 Max door panel exploded significantly in mid-air in January.