nearparabolic

Nearparabolic is a term used in celestial mechanics to describe an orbit whose eccentricity is extremely close to 1, effectively a near-parabolic trajectory. This description is most commonly applied to long-period comets originating in distant reservoirs such as the Oort cloud. In a nearparabolic orbit, the eccentricity e can be written as e = 1 − ε with ε small and positive, which implies an exceptionally large semimajor axis a and an orbital period that can be millions of years. The perihelion distance q remains finite, since q = a(1 − e).

Dynamics and characteristics: Nearparabolic orbits are highly sensitive to small perturbations from planets, passing stars, and galactic tides. When such an object enters the inner solar system, gravitational interactions can noticeably alter its orbital elements, potentially converting a nearparabolic, bound trajectory into a more tightly bound elliptical orbit with a long period, or, less commonly, producing a hyperbolic trajectory (e ≥ 1) that ejects the body from the solar system.

Origin and significance: The nearparabolic population is largely associated with comets from the outer reaches of the solar system. Studying these orbits provides insight into the structure and mass of the Oort cloud, the history of solar-system formation, and the influence of external perturbations over vast timescales. Nearparabolic orbits illustrate how minute gravitational effects can govern the fate of bodies on trajectories spanning astronomical distances.

See also: Oort cloud, long-period comet, parabolic orbit, hyperbolic trajectory.