"If all these countries are safely manned, flights will be possible in four years. If we encounter challenges, the manned flights will be postponed for another two years," he writes further.
Musk says that it is only possible to travel from Earth to Mars every other year, when the planets are aligned.
"This increases the difficulty of the task, but also serves to protect Mars from many catastrophic events on Earth."
Musk's goal is for everyone to be able to visit Mars.
"That means you or your family or friends – everyone who dreams of great adventures."
Musk mentioned already two weeks ago that the first Starship spacecraft would be sent to Mars in two years when Earth and Mars are aligned with each other next time.
Space X has an explicit strategy of streamlining its development through "trial and error", where you quickly get to testing and learn from any mistakes.
