A space station is thought of as an orbital vehicle capable of supporting one or more humans for an extended period of time. These generally lack major propulsion, reentry and landing systems. Such vehicles have docking ports to allow rendezvous and docking of other spacecraft for the purpose of crew transfer and resupply activities.
Most space stations have been launched in connection with technology and science projects. A few military missions have also employed stations. Currently, only one fully operational and permanently inhabited space station is in low Earth orbit and that is the 22-year-old International Space Station (ISS).
This station provides an orbiting facility for studying the effects of spaceflight on the human body and for conducting a number of long-length scientific studies. China, India, Russia and the U.S. are planning other stations for the coming decades.
All prior space stations have been flown in orbits that cover most of the populated Earth. In other words, their orbits are inclined relative to the equatorial plane. The ISS is in an orbit that is 51.6 degrees away from the equator.
This orbital inclination was selected a accommodate Russian launch-site constraints. Without Russian participation the ISS orbit could have been inclined at 28.4 degrees, the latitude of U.S. launch site at Cape Canaveral, FL. Unfortunately, the use of inclined orbits imposes certain limitations on launch timing and orbital operations.
The use of an equatorial launch site would relieve such constraints. For example, all spacecraft that are sent to the ISS have very narrow launch windows because lift off can only occur as the launch site passes through the plane of the ISS orbit.
If a planned launch window is missed, there is a one-day delay. A launch from an equatorial launch site to an equatorial space station can occur at any time because the launch site is always in the plane of the station. This geometrical convenience is also true for reusable transport vehicles, i.e., reentry will always bring the vehicle back to the equator.
A number of on-orbit operations can be simplified when operating in equatorial orbits. Maneuvering propellant is minimal because no plane changes are required for rendezvous and servicing of satellites.
Rescue operations can be conducted at any time. On the adverse side, surveillance of the Earth's surface is limited to near-equator regions and launch operations would require an equatorial spaceport. Nevertheless, there are important advantages for over-the equator flights.
