jay@npois.UUCP (Anton Winteroak) (01/07/86)
Someone asked if we can plot backward to find when the close encounter that put Halley's Comet in approxomately it's modern orbit was. The answer is that it is unlikely that we will ever know. Even with all the very accurate data about the last several orbits, the time of Halley's return this time was uncertain by more than a month. The further back we look at the records of Halley's Comet, the worse the time and position data is. Also, if you note when the closest approaches were, you'll see that the period has varied considerably. Most comets keep irregular orbits, since the jetting of material towards the sun, and with the direction of rotation, causes a small but signifigant change in momentum. Clearly a single event capture would require the comet to get very close to a planet. This planet would most likely be Earth or Venus since only those two planets are close to where Halley's comet crosses the ecliptic. The Earth travels nearly two million miles a day, and Venus moves even more quickly. The capture event would probably have brought the Comet within a few hundred thousand miles of the Earth or Venus, perhaps closer still. We can't know the position of the comet that well before recorded astronomical history. If the Earth captured the comet, I'm sure it looked spectacular.
gv@hou2e.UUCP (A.VANNUCCI) (01/10/86)
Mark Brader asks > As we all know, Halley's Comet is known to have been observed in every > appearance since 240 BC. And according to Isaac Asimov's essay "The > Long Ellipse" (in the collection "'X' Stands for Unknown), we also have > records of the 467 BC appearance, though not the two intervening ones. > > My question is whether anyone has tried to work out how many orbits the > comet had made before that. > > After all, a comet coming in from the Oort cloud (or wherever) doesn't > just go into a 76-year orbit spontaneously; there must have been an > encounter with one of the planets from Venus to Neptune, most likely > Jupiter, of course. The orbits of the planets are well known and we > must have, from all the historical records and from calculating backwards, > some fairly good idea of what the orbit of Halley's was like then (despite > all the perturbations since -- or do they cancel out anyway?). > > So has anyone calculated back to find the decisive encounter that is > responsible for the 76-year orbit? Anton Winteroak answers: > The answer is that it is unlikely that we will ever know. Even > with all the very accurate data about the last several orbits, the time > of Halley's return this time was uncertain by more than a month. > The further back we look at the records of Halley's Comet, > the worse the time and position data is. Also, if you note when the > closest approaches were, you'll see that the period has varied considerably. > Most comets keep irregular orbits, since the jetting of material > towards the sun, and with the direction of rotation, causes a small but > signifigant change in momentum. > Clearly a single event capture would require the comet to get > very close to a planet. This planet would most likely be Earth or Venus > since only those two planets are close to where Halley's comet crosses > the ecliptic. > The Earth travels nearly two million miles a day, and Venus moves > even more quickly. The capture event would probably have brought the > Comet within a few hundred thousand miles of the Earth or Venus, perhaps > closer still. We can't know the position of the comet that well before > recorded astronomical history. > If the Earth captured the comet, I'm sure it looked spectacular. > > This is a paragraph from an article titled: "P/Halley: The Quintessential Comet" The article appeared in the 13 December issue of SCIENCE magazine. "P/Halley moves in an orbit that evolves quickly on astronomical time scales. Nevertheless, its dynamical age is quite old compared to the 3000-year orbital lifetimes of most periodical comets. Studies of its long-term orbital behaviour and brightness indicate that its nongravitational reaction forces and perihelion brightness have shown no perceptible change for the last 2000 years. One must take the orbit back over 210 revolutions (~16,000 years) in order to find an indication that the comet had a close gravitational encounter with Jupiter, the most probable planet to have captured the comet into its present orbit. But even this would have been a very unlikely event, and it is reasonable to assume that P/Halley has been around for a long time". I would like to note that the Earth or Venus are unlikely to have captured Halley. The relative speed between Earth and a comet coming from the Oort cloud would be around 50 Km/sec. At that speed a **grazing** encounter would barely be sufficient to deflect the comet by a non-negligible amount. An encounter that brings the comet within a few hundred thousand miles of Earth (the distance between Earth and Moon) is pitifully inadequate to capture the comet. The situation is even worse for Venus, which is smaller than Earth; also, it is closer to the sun so that the relative speed is even higher. Jupiter, on the other hand, is about one thousand times more massive than Earth and five times farther from the sun. This gives it a great edge at deflecting comets ! Giovanni Vannucci AT&T Bell Laboratories HOH R-207 Holmdel, NJ 07733 hou2e!gv