Elon Musk's SpaceX is on the verge of flying humans to space, with a critical test flight underway.

A SpaceX Falcon 9 rocket with the company's Crew Dragon spacecraft onboard is seen as it is rolled out of the horizontal integration facility at Launch Complex 39A as preparations continue for the Demo-1 mission at the Kennedy Space Center in Florida, February 28, 2019. /VCG Photo

A SpaceX Falcon 9 rocket with the company's Crew Dragon spacecraft onboard is seen as it is rolled out of the horizontal integration facility at Launch Complex 39A as preparations continue for the Demo-1 mission, at the Kennedy Space Center in Florida, February 28, 2019. /VCG Photo