The stars are so far away that, at any speed we can reach, the journey would take far longer than a human life. Voyager 1, one of the fastest objects people have sent out of the Solar System, would need about 75 000 years to reach the nearest star. One answer is not to try to survive the trip, but to live it: a ship carrying a whole community, whose members are born, live and die on board, so that their distant descendants step out at the other end. This is a generation ship.
An old idea
The rocket pioneer Robert Goddard wrote about such voyages in 1918, in an essay called “The Ultimate Migration”. In 1928 the Russian Konstantin Tsiolkovsky described a multi-generation colony ship he called “Noah’s Ark”, and in 1929 the British scientist J. D. Bernal imagined people living in space for generations. Science fiction took up the theme, most famously in Robert Heinlein’s 1941 stories later collected as Orphans of the Sky, in which the crew of a vast ship have forgotten that they are on a ship at all.
How many people?
A generation ship must carry enough people to stay healthy and genetically varied for centuries. In 2002 the anthropologist John Moore estimated a minimum of about 160, or as few as 80 if couples delayed having children. In 2018 Frédéric Marin and Camille Beluffi ran simulations of a 6 300-year voyage and found that a crew of 98 could be enough. Other studies have suggested far larger numbers, up to tens of thousands, to guard against disease and accident.
The harder questions may be human ones: how to keep a society stable and purposeful for hundreds of years, and whether it is right to commit unborn generations to a journey they never chose.
Designs
In 2024 the Initiative for Interstellar Studies launched Project Hyperion, a competition to design a generation ship. The winner, announced in July 2025, was Chrysalis, by an Italian team: a ship 58 kilometres long carrying about 600 people on a voyage of at least 400 years, with gravity provided by rotation. Like every such design, it depends on technology that does not yet exist. Generation ships remain the slowest, and perhaps the most realistic, of all the ways imagined to reach the stars.