Supersynchronous orbit
A supersynchronous orbit is a geocentric orbit characterized by an orbital period exceeding Earth's sidereal rotation period of 23 hours 56 minutes, resulting in a semi-major axis larger than that of a geosynchronous orbit and, for circular cases, altitudes above 35,786 kilometers.[1] Such orbits differ from geosynchronous ones, where the period matches Earth's rotation, by drifting westward relative to the planet's surface due to the mismatch in angular velocities.[2] Supersynchronous orbits find primary application in transfer trajectories for geostationary satellites, known as supersynchronous transfer orbits (SSTO), where launch vehicles impart excess energy to achieve an apogee well beyond geosynchronous altitude—often two to three times higher—allowing the payload to circularize at geostationary orbit using minimal onboard propellant.[3] This approach optimizes launcher performance by reducing the satellite's required delta-v for inclination correction and perigee raise, as demonstrated in missions like NASA's ORION, which utilized an SSTO with apogee over three times geosynchronous altitude.[2] Additionally, supersynchronous orbits serve as disposal locations for end-of-life geostationary satellites, forming part of the graveyard orbit regime to mitigate space debris accumulation in the congested geosynchronous belt; international guidelines recommend elevating defunct spacecraft by at least 235-300 kilometers above geosynchronous altitude to prevent collisions and ensure long-term stability against perturbations like lunar-solar gravity.[4] While stable over operational lifetimes, supersynchronous disposal orbits face challenges from third-body perturbations and potential fragment cascades, prompting studies on minimum safe disposal distances to isolate debris from active geosynchronous regions.[5] These orbits thus balance operational efficiency with debris mitigation, supporting sustainable access to high-value equatorial slots amid growing satellite constellations.[4]Fundamentals
Definition and Parameters
A supersynchronous orbit is an Earth-centered orbit with a semi-major axis exceeding that of a geosynchronous orbit, which measures 42,164 km.[6] This configuration yields an orbital period greater than one sidereal day, equivalent to 23 hours, 56 minutes, and 4 seconds (86,164 seconds).[7] The defining parameter is thus the semi-major axis a, where a > 42,164 km, distinguishing it from subsynchronous orbits (a < 42,164 km) and synchronous orbits (a = 42,164 km).[8] Key orbital parameters for supersynchronous orbits follow standard Keplerian elements, with the semi-major axis as the primary differentiator: eccentricity e (often near zero for circular disposal orbits but elevated in transfer configurations), inclination i (typically matching the originating orbit), longitude of the ascending node Ω, argument of perigee ω, and true anomaly ν or mean anomaly M. Orbital period T scales with a via Kepler's third law: T = 2π √(a3/μ), where μ ≈ 3.986 × 1014 m³/s² is Earth's gravitational parameter.[7] These orbits are commonly elliptical during transfer phases, with apogee altitudes surpassing geostationary levels (e.g., >35,786 km) to facilitate efficient propulsion for disposal or lunar trajectories.[1]Comparison to Synchronous and Subsynchronous Orbits
Supersynchronous orbits are characterized by an orbital period exceeding Earth's sidereal rotation period of 23 hours, 56 minutes, and 4 seconds, resulting in a semi-major axis larger than that of synchronous orbits for equivalent eccentricities.[7] In contrast, synchronous orbits, such as geosynchronous orbits, have periods precisely matching this rotation rate, enabling satellites to maintain fixed positions relative to Earth's surface when equatorial and circular (geostationary). Subsynchronous orbits feature shorter periods and smaller semi-major axes, causing satellites to complete more revolutions per day than Earth's rotations, leading to eastward drift in their ground tracks relative to fixed ground observers.[1] The differences in orbital dynamics arise from Kepler's third law, where the cube of the semi-major axis is proportional to the square of the orbital period: a^3 \propto T^2. For circular orbits around Earth, this yields altitudes above approximately 35,786 km for supersynchronous paths, compared to exactly 35,786 km for geostationary synchronous orbits and lower values for subsynchronous ones, such as medium Earth orbits around 20,000 km. Supersynchronous configurations thus exhibit slower angular velocities relative to Earth's inertial frame, producing westward ground track progression, whereas subsynchronous orbits advance eastward.| Orbit Type | Period Relative to Sidereal Day | Typical Circular Altitude (km) | Ground Track Drift Direction |
|---|---|---|---|
| Subsynchronous | Shorter | < 35,786 | Eastward |
| Synchronous | Equal | 35,786 | None (stationary if GEO) |
| Supersynchronous | Longer | > 35,786 | Westward |