A cross generation ship is a variant of a generation ship that takes into account obstacles that slower than light generation ships might have.

The major foreseeable obstacles of a generation ship are that:

• It would taken an enormous amount of resources to build.
• There are no guarantees that the ship would last the amount of time it was designed for, up to centuries.
• There may be no time for centuries of testing of the generation ship.
• There are no backups in the closely linked biosphere required for an efficient generation ship.
• The inhabitants would experience a sense of isolation
• The lack of genetic diversity in the biosphere (though this can also be countered in ordinary generation ships by the inclusion of a sperm bank).

The cross generation ship is a concept that attempts to solve these problems by eliminating the idea of colonizing with a single ship. The idea is that after one ship has been launched on its voyage, the home world would continue to develop better technologies including better equipment, advances in biology, and spacecraft propulsion. The home world would then launch another ship with the same destination but with faster engines that would in time catch up to the first ship sent. The newer ship might carry more load than the older one and provide the older ship with better technologies, supplies, and replacement parts. The homeworld would continue to send these ships until a colony has been established or another destination for colonization was found.

Since any one of these ships would not be designed to be self-sufficient forever, they are expected to cost less and take less time to design, built, and test than a true generation ship, although a fully self-sufficient generation ship could take part in a cross generation ship fleet. Also, with the constant arrival of new ships with new material, technologies, and colonists, the traveling colony-convoy might develop a sense of comradery and culture that would make them better prepared for cooperation and survival. The arrival of new colonists, plants, and animals would also ensure genetic diversity. In emergencies, colonists would be able to get help from other cross generation ships in the fleet, or in extreme cases move to other ships.

The major disadvantage of this concept is that a fleet of these ships is a long term commitement of resources that might be draining to the home world.

See also: Generation ship