Orbital mechanics

An orbiting object moves sideways while gravity bends its path. With the right speed and direction, it keeps missing the surface. The ISS is not outside gravity; it is moving in continuous free fall. That distinction explains why its crew appear to float.^[1]

The ideal two-body problem models motion under the mutual gravitational attraction of two point masses or spherically symmetric bodies. Bound trajectories are ellipses, with circles as a special case. Orbital energy and angular momentum constrain the trajectory; position alone is insufficient to specify an orbit without the velocity vector.^[1]

A weather satellite watching one region and a mapping satellite covering the globe need different views. Geostationary orbit keeps a spacecraft above roughly the same equatorial longitude. Near-polar orbits let Satellites observe different parts of Earth as the planet rotates below them.^[2]

Orbit selection trades coverage, revisit time, resolution, radiation exposure, launch energy, and ground access. A geostationary orbit is approximately circular, equatorial, and prograde with a sidereal-day period. Sun-synchronous orbits use nodal precession to maintain an approximately constant local solar time of passage; not every polar orbit is Sun-synchronous.^[2]

A spacecraft usually changes its orbit by changing velocity. A brief engine firing affects the path that follows, so the resulting motion may seem counterintuitive. Catching another satellite requires matching its position and velocity, not simply pointing at it and accelerating.^[3]

Impulsive maneuvers idealize thrust as an instantaneous velocity change. A tangential burn changes orbital energy and typically the opposite apsis. Rendezvous also requires phasing and relative-velocity control. Low-thrust trajectories need a continuous-thrust treatment rather than an unqualified application of impulsive transfer formulas.^[3]

Useful first calculations often treat a planet as a simple gravitational source. Real missions must also account for drag, uneven gravity, other bodies, and sunlight pushing on surfaces. Voyager’s flybys and Webb’s orbit show how varied mission geometry can become.^[3][2]

Perturbations include non-spherical gravity, third-body attraction, atmospheric drag, and solar radiation pressure. Their relative importance depends on altitude, duration, and spacecraft properties. The two-body solution is a reference model, not a universal operational prediction. Navigation updates and maneuver planning reduce the consequences of model and measurement uncertainty.^[3][2]