CEO of SpaceX space technology company Elon Musk has said it will take 20 years and 1,000 spacecraft to build a city on Mars, which is his long-term vision.

Musk wrote on Twitter that SpaceX will need to build about 1,000 spaceships for Mars, which will need to transport cargo, infrastructure and crew to Mars over the course of 20 years since planetary alignment only really allows for a realistically achievable Mars flight once every two years, Tech Crunch reported.

The launch costs about $ 2 million per spacecraft, and Musk says the ultimate goal is to build a "self-sustaining city of Mars."