The role of Elon Musk's SpaceX in an ambitious new U.S. missile defense system is in question following the dramatic feud last week between the billionaire entrepreneur and President Donald Trump, according to three people familiar with the project.

The SpaceX launch of four private astronauts has been delayed again. The Ax-4 mission to the International Space Station (ISS) had been scheduled to launch atop a SpaceX Falcon 9 rocket from Florida's Space Coast on Wednesday morning (June 11), but a propellant leak in the booster nixed that plan.