Space travel has long been the preserve of governments and sci-fi fans, but in recent years a crop of new commercial ventures, often backed by billionaire entrepreneurs, has sought to get into the race.

The list of so-called thrillonaires has only grown, along with their ambitions: Jeff Bezos, the Amazon founder who set up Blue Origin to lower the cost of space technology; Elon Musk, who founded SpaceX with the aim of going to Mars one day; and Richard Branson, who started the space tourism company Virgin Galactic.

But two recent accidents involving commercial rockets have underscored the high risks and soaring costs involved in any spaceflight.

On Friday, a Virgin Galactic space plane exploded during a test flight over the Mojave Desert, killing one pilot and injuring another. Days earlier, an Orbital Sciences rocket carrying a supply vessel to the International Space Station blew up seconds after it was launched.