After years of waiting, Richard Branson’s journey to space earlier this month on a Virgin Galactic vessel was supposed to be a triumphant homecoming. Instead, the jaunt attracted significant criticism – about its carbon footprint.

With Earth’s wealthiest man Jeff Bezos spent a few minutes outside the planet’s atmosphere Tuesday on Blue Origin, and Elon Musk’s SpaceX planning an all-civilian orbital mission in September, the nascent space tourism industry finds itself facing tough questions about its environmental impact.

Compared to Virgin Galactic’s SpaceShipTwo spaceplanes, Blue Origin’s are much cleaner, according to a recent paper by scientist Martin Ross of Aerospace, which Bezos’ company plugged on Twitter.