Slashdot Mirror
Comet Catalina To Pass By Earth For the Final Time
StartsWithABang writes: Originating from the Kuiper belt and the Oort cloud, comets are generally thought of as periodic objects, with their initial trajectories having been perturbed by either Neptune, another distant object or a passing star or rogue planet. But most comets aren't periodic; they're transient instead, where a trip into the inner Solar System gives them additional gravitational perturbations, causing them to either fly into the Sun or gain enough kinetic energy to escape entirely. This latter fate is the case for Comet Catalina, which reaches perihelion on November 15th and then heads out of the Solar System after putting on one final show for observers on Earth.
It's a chaotic system. The odds of the gravitational perturbation increasing the speed of the comet (and by extension, pushing it away and increasing its period) aren't much different than the odds of the perturbation decreasing it. And the effect is small, that a given orbit might bring a comet that has near escape velocity over the edge (as apparently is happening here) but Halley's Comet has nowhere near that.
Why doesn't this happen to Halley's Comet? It gets perturbed by the gravity from the gas giants, yet has managed to retain a period of 74-79 years since 240 BC. That's a lot of trips through the solar system. Because it's a short period comet, it spends more time around the gas giants than a long period comet. That should subject it to more gravitational perturbations, but it's still remained very periodic. Why is that?
Luck, mostly. An object will either be expelled or gravitationally (tidally) torn apart by the sun after a few trips around the solar system, with an exponential distribution describing how many trips it makes. As with all exponential distributions, the curve flattens out to the right, and if any particular object has just the right orbital parameters to make i.e. 10 or 20 or 50 passes, then it's pretty much fallen (by luck) into the sweet spot of orbital resonances to keep making more passes. The chances are astronomically low that any particular object will have this luck, but there is a huge number of objects out there, so some do beat the odds.
Halley's comet is simply one of the few that beat the odds.
It is dangerous to be right when the government is wrong.