Posted 01 December 2012 - 06:24 PM
Posted 01 December 2012 - 06:55 PM
Posted 01 December 2012 - 08:53 PM
Bill in Flag
Posted 02 December 2012 - 12:54 AM
Posted 02 December 2012 - 06:46 AM
Does anyone know how they can tell that Comet ISON is indead a comet? Wouldn't an asteroid/dwarf planetoid appear the same in photos at that great a distance?
The terminology isn't very precise nor very important for such distant objects.
The defining characteristic of a dwarf planet (if you subscribe to that newfangled category) is its size. It's easy to tell that ISON isn't a dwarf planet because we know how far it is (by its orbit) and how bright it is (by direct observation). Anything Pluto-sized or bigger would be thousands of time brighter at that distance.
The distinction between asteroids and comets is fuzzy indeed. Asteroids are generally considered to be made mostly of rock and metal, whereas comets are largely or mostly water (frozen, of course).
Generally speaking, stuff in the inner solar system tends to be made of rock and metal because ice boils off when you're that close to the Sun.
Small bodies in the outer solar system tend to be made mostly of ice, for the simple reason that hydrogen and oxygen are the first and third most abundant elements. (The universe is 74% hydrogen, 24% helium, 1% oxygen, and 1% everything else.) So H2O is the most common substance in the solar system that solidifies under normal cosmic conditions.
Things are usually called comets only when they come close to the Sun and start to boil, producing atmospheres and tails. If something knocked Pluto in toward the Sun, it would produce a tail (a whopping huge one!) and most likely be called a comet. Or it might just knock our whole system of nomenclature flat on its face and force us to rethink our categories.
So the trillion-some-odd objects orbiting about 1 light-year from the Sun are perhaps better called proto-comets or potential comets than real comets. Until something pushes one in toward the Sun, where it becomes a comet, like ISON.
Posted 02 December 2012 - 10:10 PM
I think comets like this one give away their identity by being much more reflective than the alternatives at the same distances. ISON looks to be a REALLY good comet because it's been found while still very far out.
No, ISON was designated a comet because it displayed a coma in some of the earliest images.
Bill in Flag
Posted 02 December 2012 - 10:39 PM
I suspect it is based on orbital paths. Comets orbit in a highly elliptcal path (and in fact ISON may be a one-time commet that will be flung completely out of the solar system after one pass) while asteroid orbits are generally much less eliptical like a planet's. Once a new object is detected they can do a rough orbital calculation after only a few days' observations.
While most comets do have highly eccentric orbits, there are comets having low eccentricity (0.2 or less) orbits. So, while most comets do have highly elliptical or parabolic orbits, it is the display of a coma and/or tail that earns a newly discovered minor planet a cometary designation.
Bill in Flag