Orbital Mechanics

Orbital mechanics influence more than the path a spacecraft follows. Its orbit or trajectory also helps determine the environment in which its computers and other systems must operate.

Radiation exposure, sunlight and eclipse cycles, communication opportunities, and distance from Earth can all change with a spacecraft's location. Computing systems must therefore be designed around the conditions expected throughout the mission.

Low Earth Orbit

Low Earth Orbit (LEO) generally extends from the lowest practical orbital altitudes to about 2,000 kilometers (1,200 miles) above Earth. Spacecraft in this region travel around the planet relatively quickly, with vehicles at lower altitudes completing an orbit in roughly 90 minutes.

Depending on the orbit, a spacecraft may repeatedly pass between sunlight and Earth's shadow, creating regular changes in power generation and thermal conditions. Communication with individual ground stations may also be available only during portions of each orbit, requiring onboard data storage and careful transmission scheduling.

Geostationary Orbit

A spacecraft in geostationary orbit (GEO) travels above Earth's equator at an altitude of approximately 35,786 kilometers (22,236 miles). Its orbital period matches Earth's rotation, allowing it to remain above nearly the same location on the surface.

This geometry can provide continuous communication coverage across a broad region, but spacecraft at this altitude encounter a different radiation environment from those in low Earth orbit. Long mission lifetimes also place greater emphasis on component durability and dependable computing systems.

Polar and Sun-Synchronous Orbits

Spacecraft in polar or near-polar orbits can pass over large portions of Earth's surface as the planet rotates beneath them. A sun-synchronous orbit is designed so that a spacecraft crosses particular regions at approximately the same local solar time, providing consistent lighting for certain observations.

For computing systems, these missions may require the management of large volumes of observational data while adapting to changing communication, power, and thermal conditions during each orbit.

Beyond Earth Orbit

Spacecraft traveling to the Moon, other planets, or more distant destinations operate in environments very different from those of Earth-orbiting satellites. Communication delays increase with distance, and sunlight availability changes as a spacecraft moves closer to or farther from the Sun.

These missions may require greater onboard independence because time-sensitive decisions cannot always wait for instructions from Earth. Their computers must also be designed for the radiation, power, thermal, and communication conditions along their specific trajectories.

Orbit and Space Computing

No single computing architecture is ideal for every orbit. A spacecraft with frequent access to ground stations can handle data differently from one that communicates only occasionally. A vehicle that regularly passes through eclipse faces different power challenges from one following another type of trajectory.

Orbit is therefore part of the computing problem. By understanding where a spacecraft will travel and the conditions it will encounter, engineers can determine how much autonomy, storage, processing capability, fault tolerance, and resource management its onboard computer will need.