Orbital Phase

Author: Tianjiang Shuo

Website: https://cislunarspace.cn

Definition

The orbital phase is the flight stage during which a spacecraft enters free-flight or orbital operation after its main engine has ceased firing. For ballistic missiles, the free-flight segment following the powered phase can be regarded as a special orbital phase. During this stage, the spacecraft primarily undergoes conic motion under the influence of Earth's central gravitational force.

Core Elements

Motion Characteristics

The essence of orbital-phase flight is conic motion under the influence of Earth's central gravity. Depending on the energy acquired at the end of the powered phase, the trajectory can be:

  • Elliptical orbit: negative energy; the spacecraft is bound by Earth's gravity and orbits periodically (artificial satellite)
  • Parabolic orbit: zero energy; exactly at escape velocity
  • Hyperbolic orbit: positive energy; the spacecraft will escape Earth's gravitational field

Perturbation Factors

In actual flight, the spacecraft is also subject to various perturbation factors that cause the trajectory to deviate from the ideal conic section:

  • Earth's non-spherical gravity: Earth is not a perfect sphere; higher-order gravity terms such as J2J_2 cause orbital plane precession and apse line rotation
  • Atmospheric drag: low-orbit spacecraft experience subtle drag from the upper atmosphere, causing gradual orbit decay
  • Lunar and solar gravity: gravitational perturbations from the Moon and Sun affect high-orbit and deep-space spacecraft
  • Solar radiation pressure: solar radiation pressure has a significant effect on spacecraft with a high area-to-mass ratio (e.g., solar sails)

Orbital Maneuver

To eliminate perturbation effects or meet specific mission requirements, the spacecraft must change its state of motion under the action of its control system through orbital maneuvers. Orbital maneuvering is one of the core research topics in spacecraft orbital mechanics.

Free-Flight Segment of Ballistic Missiles

For ballistic missiles, the free-flight segment (also called the passive phase) begins after the powered phase ends. If the warhead is not equipped with a propulsion system or control system, it relies entirely on the energy acquired at the end of the powered phase for inertial flight, with a trajectory that approximates an ellipse intersecting the Earth. Modern warheads can perform maneuvering flight during the free-flight segment to improve penetration probability.

Application Value

The orbital phase is the primary operational stage for artificial satellites, space stations, and other spacecraft, as well as a critical stage for ballistic missile flight. The equations of motion, orbital elements, orbit prediction, and orbit design during the orbital phase are core topics in spacecraft orbital mechanics.

References

  • Zheng W, An X Y, Zhou X, He R Z. Aerospace Flight Mechanics[M]. National University of Defense Technology, 2026.
  • Ren X, Xiao F. Satellite Orbital Mechanics[M]. National University of Defense Technology Press.