The dynamical lifetime of comet P/Halley

Using an extension of Opik's techniques the stability of the orbit of Halley's comet under close encounters with the planets is investigated, and it is found that the orbital changes due to such encounters expected over a long time-base are of the same order as the effects of more distant planetary perturbations and nongravitational effects. For the present orbit the half-life of the comet against ejection from the solar system as a result of a close encounter is 68,000 apparitions; the half-life against planetary impacts or catastrophic disruption in a close approach is about a million apparitions: dynamical loss is therefore unlikely. For a past lifetime of 2000-3000 perihelion passages there is a probability of about 50 percent of a single large deflection into an orbit of (q = 0.5-0.7 AU), (a = 15-25 AU), or (i = 150-170 deg). During this period the perihelion distance has most probably remained between 0.5 and 0.7 AU, so that physical models based upon sensibly constant rates of material loss from the surface through its lifetime appear to be justified.