What Are Orbits?

Orbits are the curved paths that objects follow under the influence of gravity when moving around a more massive body. In engineering and applied science, the term most often refers to the trajectories of artificial satellites, spacecraft, and natural moons around Earth or other celestial bodies. The geometry of an orbit is governed by six classical orbital elements: semi-major axis, eccentricity, inclination, right ascension of the ascending node, argument of perigee, and true anomaly, which together specify the shape, orientation, and instantaneous position of an orbiting body.

Orbital mechanics traces its formal foundation to Kepler's laws of planetary motion (published 1609 to 1619) and Newton's law of universal gravitation (1687), but the practical demands of the space age transformed the field into an engineering discipline with rigorous numerical methods for propagation, maneuvering, and collision avoidance. The European Space Agency's overview of orbit types provides a practical taxonomy of the orbit regimes used by operational spacecraft.

Low Earth Orbit

Low Earth orbit (LEO) encompasses altitudes from roughly 160 km to 2,000 km above Earth's surface. At these altitudes, orbital velocity is approximately 7.8 km per second and orbital periods run near 90 minutes. LEO is the most densely populated orbital shell: it hosts the International Space Station, Earth-observation constellations, and large broadband networks such as Starlink. Atmospheric drag is perceptible in LEO, gradually lowering perigee and limiting satellite lifetimes without periodic reboosting. The high revisit rates and relatively low launch energy requirements make LEO the preferred regime for imaging and low-latency communications.

Medium Earth Orbit and Geostationary Orbit

Medium Earth orbit (MEO) spans altitudes between the LEO boundary and geostationary orbit, roughly 2,000 km to 35,786 km. Navigation satellite constellations use MEO: the GPS satellites operate at about 20,200 km altitude with 12-hour orbital periods, while Europe's Galileo constellation is distributed at approximately 23,222 km. Geostationary Earth orbit (GEO) is a special case of geosynchronous orbit at exactly 35,786 km over the equator, where an orbital period equals one sidereal day. A satellite in GEO appears fixed over a point on the equator, making it ideal for telecommunications and weather monitoring coverage of a fixed geographic region. Just three GEO satellites can provide near-global coverage between roughly 70 degrees north and south latitude.

Highly Elliptical Orbits

Highly elliptical orbits (HEO) are defined by high eccentricity, meaning the orbit's perigee remains relatively low while the apogee extends to tens of thousands of kilometers. The Molniya orbit, developed for Soviet communications coverage of high-latitude regions beyond the reach of geostationary satellites, is the canonical example: a 12-hour period orbit inclined at 63.4 degrees with an apogee near 40,000 km. A satellite on a Molniya or Tundra orbit dwells near apogee for many hours per day, providing extended visibility over polar regions. The inclination of 63.4 degrees is chosen because it freezes the argument of perigee, preventing apsidal precession due to Earth's oblateness. The Space Foundation's overview of Earth orbit categories and the NASA SSRI orbit design and analysis knowledge base together discuss how engineers select and analyze orbit types across mission phases.

Applications

Orbits has applications in a wide range of fields, including:

• Global positioning and navigation via satellite constellations in MEO
• Broadband and direct-to-device telecommunications from GEO and LEO constellations
• Earth observation, weather monitoring, and climate science from sun-synchronous LEO orbits
• Scientific missions to outer planets and small bodies using hyperbolic departure trajectories
• Space situational awareness and debris tracking across all orbital shells