|MadSci Network: Astronomy|
Hi Cathy! Your question is a good one, and like all good questions I can think of two answers. Firstly, the theoretical answer: Halley's comet follows an extremely elliptical path around the sun. The eccentricity is 0.967 for Halley, compared to 0.0167 for the Earth. The higher the number, the more "squashed" the orbit. At its closest, Halley's comet passes within 0.6AU of the Sun (1AU is the Earth-Sun distance) while at its furthest, the comet is some 35AU away - about as far away as Pluto. A photograph of Halley was taken in 1994, eight years after the last closest approach to the Earth, at a range of nearly 19AU. This web page details the event http://www.sc.eso.org/~ohainaut/nice/halley94.html. At this range the comet is cold and it's not surrounded by the gas and dust that are emitted during the closer part of its orbit due to the warming effect of the sun. It's this gas and dust which makes comets so bright, so the picture is only of the comet's nucleus, and therefore it's correspondingly dark. Indeed, the visual magnitude was listed as 26.5 +/-0.2 at this range, making it a very dim object, and one that required the "shutter" on the photograph to be open for nearly four hours. A Google search on "visual magnitude scale" will provide plenty of information on how these magnitude numbers work, if you're unsure. A good general guide is here: http://cfa-www.harvard.edu/icq/MagScale.html. At the comet's furthest point from the sun, the nucleus will have a magnitude of approximately 29.3. I calculated this using a flash-based piece of software that I wrote, which is here: http://www.thirteenthcentury.com/flash/brightness.swf. Set the albedo to be 0.04, the diameter to be about 11000m, leave the solar output as 1, and put 35.5AU for the object distance and viewer distance.) This is just about the dimmest that an eighteen-hour exposure on the Hubble Space Telescope can detect. Therefore, it's theoretically possible to view Halley's Comet throughout its entire 74 to 79 year-orbit, though the practical value of seeing it at such great distances, when to all intents and purposes it's dormant, and its orbital parameters are well-known, is another matter. And now for the more practical answer! If, by viewing the comet, you mean with the naked eye, then clearly it's only going to be visible at night, when it's actively emitting dust and gas, and when it's close to the Earth. Due to the physics of orbits, comet Halley is travelling at its fastest when it's closest to the sun, so it doesn't remain in our neighbourhood for very long. The orbit of Halley is also highly inclined with respect to that of the Earth and other planets, so where you are on the Earth will determine whether the comet is visible or not. And finally, the comet can pass near the Earth both on its trip towards the Sun and on its return from its minimum distance, so there can be two opportunities to see it in the night sky for any single orbit. Naturally all these factors are inter-related, but calculable; so while they vary from orbit to orbit, a good rule of thumb is that a comet like Halley can be (but not may not be!) an easy visual target in the night sky for several weeks for any one encounter, moving from night-to-night by up to a few degrees across the background stars. The boundaries for these viewing opportunities are the naked-eye visibility limit (about magnitude 6.0 for dark skies) and when it's lost in the dawn or dusk glare of the sun. I hope this answers your question, and that you enjoy the 2061 return of Comet Halley in fifty-six years' time! Andy Goddard
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