Before 2I all of the comets

[Nick James]

Before 2I all of the comets that we know with very slightly hyperbolic orbits acquired the extra velocity through planetary perturbations when they were in the inner Solar System. We would expect comets falling in from the Oort cloud to be in parabolic orbits, i.e. have a velocity of zero at infinity. The largest eccentricity known prior to 2I was 1.057 for C/1980 E1 (Bowell). This large excess velocity (around 3.8 km/s) was acquired  from a Jupiter encounter in 1980 December. Comet C/1956 R1 (Arend-Roland) had an exit eccentricity of 1.0002. The pre-perihelion eccentricity was indistinguishable from parabolic. The ultimate end for our comets is to either fall into the Sun or be ejected from our Solar System and become an interstellar comet for some one else.