Space Station Orbit
and Mission Control
(Launchspace Staff)
As we all know, the International Space Station (ISS) is a habitable artificial
satellite in low Earth orbit. It follows the Skylab and Mir stations, and is, in fact,
the ninth space station to be inhabited by astronauts from multiple countries. If
you have watched the development of the ISS, you know that it is a modular
structure whose first component was launched in 1998, with completion in July
2011 when Atlantis delivered the Multi-Purpose Logistics Module. Overall, the
station consists of pressurized modules, external trusses, solar arrays and other
components. All of the large modules were delivered by the Space Shuttles, with
some smaller elements carried on Proton and Soyuz vehicles.
As you can imagine, ISS mission operations are very complex. One of the critically
important operations is maintaining a nearly circular orbit with a minimum mean
altitude of 330 km, and a maximum of 410 km. Its orbit has an inclination of 51.6 degrees to ensure that
Soyuz and Progress spacecraft may be successfully launched from the Baikonur Cosmodrome. The
selection of this inclination permitted participation by the Russians. Space shuttles launched from
Florida had a capability of reaching inclinations between 28.5 and 57 degrees.
The ISS is travelling at about 7.7 km/sec, circling the Earth almost 16 times per day. Even though the
station is in the vacuum of space, this vacuum
is not perfect. Due to its high speed and
relatively low orbital altitude, ISS does
experience a slight continuous atmospheric
drag. Thus, the station’s orbit must be
periodically raised, or it would eventually deorbit. For Space Shuttle missions to the station
the ISS’s altitude was allowed to fall in
preparation for these missions. Orbital boost
burns would generally be delayed until after
the shuttle's departure. This allowed the
shuttles to carry maximum payload masses.
In the post-Shuttle era, orbit-raising can be done at more convenient times. Orbital boosting can be
performed either by the station's two main engines on the Zvezda service module or by the Russian or
European spacecraft docked to Zvezda's aft port. Each orbit-raising maneuver takes approximately two
orbits to complete.
The Russian Orbital Segment of the ISS contains the station's engines and control bridge, which handles
Guidance, Navigation and Control for the entire station. The Zvezda module contains the ESA built DMSR Data Management System. Two fault-tolerant computers are used to compute the station's position
and orbital trajectory. Earth horizon sensors, solar horizon sensors and star trackers are used for
attitude and position sensing. Zvezda also uses gyroscopes and thrusters to turn itself around.
As you can imagine, operations of the ISS require a great deal of oversight and careful decision making
processes in order to maintain orbit and attitude of the largest space structure ever assembled.