The comments came one day after Virgin Galactic founder Richard Branson made history as the first person to blast off in his own spaceship. He beat Jeff Bezos, the richest person on the planet, by a slim margin – just nine days.
"Those who attack space maybe don't realize that space represents hope for so many people," Musk tweeted.
Branson was supposed to fly later this year on the second of three more test flights planned by Virgin Galactic before flying ticket holders next year. But on Sunday, he catapulted ahead of his rivals, reaching an altitude of 53.5 miles over the New Mexico desert with five crewmates.
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On July 20, Bezos will follow Branson's lead and head to space on New Shepard's 16th flight into space alongside his brother, Mark, a $28 million auction winner and Wally Funk, one of the last surviving members of the Mercury 13 who was chosen as his "honored guest."
Although the three billionaires have similar goals, Musk's plan is a bit different.
SpaceX, which is already launching astronauts to the space station for NASA and building moon and Mars ships, plans to take tourists into orbit around the Earth for days, with seats costing well into the millions. The company’s first private flight is set for September.
And even though the three men are competitors, they have publicly supported each other in recent weeks.
Branson – who has insisted this isn't a space race – tweeted an image Sunday morning of Musk visiting him before his big launch.
Bezos also directed some kind words to Branson on social media.
"@richardbranson and crew, congratulations on the flight. Can’t wait to join the club!" Bezos wrote on Instagram alongside an image of Virgin's rocket.
The Associated Press contributed to this report.