COMETS are presumed to enter the inner Solar System from the Oort cloud, a repository of comets more than 104 AU from the Sun. Provided the perturbing effects of planetary encounters are taken into account, the original orbital energy of a comet can be calculated, and is negative or positive according to whether the comet's orbit is bound or unbound. The Oort effect1 is the tendency for the original energies of long-period (>200-yr) comets to fall within a narrow range: about 25% of such comets have original energies in the upper 0.2% of the total energy range. In addition, 10% of long-period comets are unbound, and it has been found2 that positive original energy correlates with distance of closest approach to the Sun. We report here a further correlation: unbound comets are more likely to move in retrograde orbits. We suggest that this anomaly comes about because of the omission from the orbital energy determination of non-gravitational effects arising from enhanced volatility.