NOTE - Possibly obsoleted by exovivaria - lift lines if they are better here.
Orbital terraria (a kind of biosatellite) will be managed ecosystems in orbit, tended through remote control and monitoring by teams of users on Earth. Management of the ecosystems will require both dedicated special-purpose equipment, and small, more general-purpose robots.
Terraria meet several criteria for "a world worth talking about" for Project Persephone. Ecosystems are not perfectly predictable. They are a source of surprises. Living things inspire affection when they are amusing or attractive, and disdain when they endanger those qualities of a valued ecosystem.
Terraria are also good candidates for reducing the costs of developing and maintaining an orbital habitat. To the extent that human spaceflight is expensive because of the need for safe delivery of human beings to and from orbit, terraria may offer significant economies per pound of launched biomass and life-support materials, because launch need not be as safe, and return from orbit is unnecessary.
With the relaxation of the requirement of man-rating the delivery system, other launch economies should be possible with orbital terraria. A terrarium might be "seeded" using propulsion modes with very high thrust-to-weight ratios (thus high accelerations) for reaching initial orbit, followed by propulsion using very low thrust-to-weight ratios (thus long travel times) to rendezvous with the terrarium. The basic materials for an ecosystem (water, atmospheric gases, minimal soils) and seedstocks (essential micro-organisms, plant seeds, insects, frozen ova for other small animals) feature mucher higher acceleration tolerances and much longer orbital "shelf-life" than living human beings. High acceleration tolerance favors projectile launch?, which may be the cheapest earth-to-orbit solution for unmanned launch. High "orbital shelf-life" means that the payload mass fraction can be higher, since the "wetted mass" of low-thrust, high-Isp propulsion such as ion engines is lower than that of conventional rockets.