Kalpana One, by Al Globus, Ankur Bajoria, and Nitin Arora, is intended to be the first, and smallest, of a family of space colonies.
The Kalpana One structure is a cylinder with a radius of 250m and a length of 550m and is designed to house 5000 people. The size is determined by the limited rotation rate humans are assumed to tolerate, 2rpm. The rotation rate drives the radius to achieve 1g pseudo-gravity, and the radius drives the length due to angular moment of inertia requirements. For later, larger colonies in the Kalpana family, the rotation rate may be reduced, increasing the radius and the allowable length.
Kalpana One solves some of the problems found in earlier designs: excessive shielding mass, large appendages, lack of natural sunlight, rotational instability, lack of wobble control, and some catastrophic failure modes. Much is left to be done before a practical space colony can be fully designed and built.