When the first baby is born off-Earth, it will be a milestone as momentous as humanity's first steps out of Africa. Such a birth would mark the beginning of a multi–planet civilization for the human species.

For the first half-century of the Space Age, only governments launched satellites and people into Earth orbit. No longer. Hundreds of private space companies are building a new industry that already has US$300 billion in annual revenue.

I'm a professor of astronomy who has written a book and a number of articles about humans' future in space. Today, all activity in space is tethered to the Earth. But I predict that in around 30 years people will start living in space—and soon after, the first off-Earth baby will be born.

The players in space

Space started as a duopoly as the United States and the Soviet Union vied for supremacy in a geopolitical contest with loud military overtones.

China's rover Zhurong, named after the mythological fire god, successfully touched down on Mars on May 14—the first time that China has successfully landed a rover on the red planet.

On May 19, China's National Space Administration issued the first images the rover had taken on Mars.

After a summer of Mars launches in 2020, and with 2021 shaping up to be a successful one for landers and orbiters, it might seem like landing on Mars is routine.

Yet to understand why a first successful landing is such a huge achievement, we need to look back at the complicated history and legacy of landing on Earth's smaller neighbor.

Seven minutes of terror

"Mars is hard" has become a meme now, thrown around during Mars landings. It's also terrifyingly true. Three things make Mars landings difficult—the planet's gravity, Mars' atmosphere and our distance from the red planet.

FIRST PHOTOS FROM THE CHINESE MARS ROVER ZHURONG IS OUT! pic.twitter.com/6K8RQQqjPy

— Cosmic Penguin (@Cosmic_Penguin) May 19, 2021

Mars is less massive than Earth, but its atmosphere is also perilously thin.