Mission Categories

Spacecraft missions may be broadly categorised into three main types:

Working either from high geostationary orbits or in orbiting constellations configured to cover the area of service, communications satellites have long design lifetimes (up to 17 years) and usually low attitude-stabilisation requirements. Missions are simple: a spacecraft is launched to provide a particular service, maneuvered into the required orbit and activated to fulfil the intended purpose. Spacecraft have little autonomy beyond basic attitude control and are operated largely by telecommand. There is some low-level failure detection and recovery incorporated, but most decisions are made on the ground.

Earth Observation Missions

Spacecraft used to provide continuous coverage of a particular area, such as weather satellites, are placed in geostationary orbits with moderate stabilisation requirements. Other spacecraft, including military spy satellites, track across the surface of the earth are placed in low (usually sun-synchronous polar) earth orbits of 700-1000 Km scanning the earth using a variety of instruments from optical telescopes to Radar. The required lifetime varies (typically 2-10 years), dependent on the height of the observing orbit and the need for phenomena to be measured once or continuously. Stabilisation requirements can be high depending on the types of instrumentation on board. These spacecraft require a larger degree of autonomy as they are often out of communication with ground support. Stabilisation of the spacecraft is more complex because the lower orbits have shorter periods and atmospheric drag disturbance torques are increased.

Astronomy and Space Science Missions

Spacecraft are operated in an orbit or trajectory dependent on the particular mission. Missions observing the sky, such as Hubble or Hipparcos, are intended to have conventional circular orbits, progressively scanning the sky. Exploration missions have extremely complex orbits or trajectories designed to take advantage of the gravitational fields of local planets in order to explore deeply into the solar system, matching velocities with the objects they are sent to observe or sample. Missions are often long and involve a variety of operational phases. The proposed European Space Agency cometary lander mission, for example, goes through launch, checkout, cruise, intercept and observation phases during its nine year mission. Such spacecraft must have a degree of autonomy since communication delays to earth are measured in minutes or sometimes hours. Stabilisation systems typically have very high specification (high quality imagery and other sensitive observations require observing platforms stabilised to seconds of an arc).

These types of mission are among the most technologically challenging for spacecraft design. They require highly reliable spacecraft, performing a number of different activities and capable of safeguarding their own operation. Guidance and control systems must operate to the highest accuracies with the greatest level of autonomy.

Next: Mission Costs and reliability