hyperbolic comet

A hyperbolic comet is one with an orbit that forms a hyperbola rather than the usual ellipse. Its orbital eccentricity is greater than 1. The term applies to interstellar objects that are comets, but it has been used for comets with orbits very near a parabolic orbit (with an orbital eccentricity very near 1), the limit between elliptical and hyperbolic. The terms parabolic comet and near-hyperbolic comet are also used for these. A few thousand such comets are known, and though some may have come from beyond the solar system, that is not clear: an orbit from the furthest reaches of the Oort Cloud is naturally extremely close to parabolic, and it is possible that their speed has been induced to increase, e.g., an interaction with another celestial object. It is also unclear whether they will travel beyond the solar system: such an interaction may well slow them on their way out and they are passing through the interplanetary medium and interstellar medium. The recently-spotted interstellar objects had eccentricities not as close to 1: the first interstellar interloper had an eccentricity of 1.2 (and the subsequent two even higher), whereas the comets not declared to be interstellar have had eccentricities less than 1.1, with only a handful above 1.01.