hogg@utcsri.UUCP (John Hogg) (12/20/85)
Comets are highly visible because a proportion of their mass is spread across an appreciable region of space at each pass. This implies that they eventually wear out. Can any of the experts reading this group tell me typical or range values for the following? a) initial comet mass and size b) loss of mass and size at each orbit c) number of orbits with "significant" tail d) final mass and size, assuming a "dead" core exists (does it?) Or to be more specific: when is Comet Halley likely to wear out, presuming that we don't accidentally take it out with a probe first? -- John Hogg Computer Systems Research Institute, UofT ...utzoo!utcsri!hogg Standard disclaimer: the above may or may not contain sarcasm, satire or irony. It does not contain smiley-faces. If you're illiterate, don't flame me.
jeff@utastro.UUCP (Jeff Brown the Scumbag) (12/25/85)
As our local comet people are away for the holidays and the news retention time on our system is so short they may not see this, I will run the gauntlet of trying to answer these. Nearly all my information is from a paper in the latest (October '85) Publications of the Astronomical Society of the Pacific by Armand Delsemme and is based on a review talk he gave at last summer's A. S. P. meeting. It is direct enough so that I think most reasonably literate people (and most people on the net qualify, I think :-) could read most of it; it's not as bad as a lot of technical literature. > a) initial comet mass and size We wish we knew. All we have now is *present* cometary measures, of couse, though periodic comets are supposed to be old and parabolic ones new. Delsemme says most periodic comets are 0.6 < (radius in kilometers) < 4. Parabolic comets are larger by about a factor of two. Radar measurements of comet IRAS/Araki/Alcock (which came within .03 AU of Earth in 1983) indicated a diameter of about 7 km. Masses are even more uncertain; the lower limits are based on estimates of the masses of meteor streams and are about 10 to the 16 or 17 power grams. The upper limits are so large as to be useless. I suspect that from the passage of the ICE spacecraft past Comet G-Z a couple of months ago that a good estimate of that comet's mass will appear very soon (it may be out already and I didn't see it). The Halley probes should give similar information. > b) loss of mass and size at each orbit For Halley, the estimate (averaged over one whole orbit) is 113 kilograms per second; that is, 2.7 times 10(14) {10 to the 14} grams per orbit. > c) number of orbits with "significant" tail Observations show that Halley has been going for 2200 years. Delsemme says "...Halley has a present mass of 5 times 10(16) grams and its half-life is 13 centuries" and later says the uncertainty is more than a factor of 2. > d) final mass and size, assuming a "dead" core exists (does it?) Here you're asking about Apollo-Amor objects, another class of objects we wish we knew more about. Unfortunately, I wasn't able to find a value for the size of these, other than a table entry of 0.5 km in a source I'm not confident about. I suspect a real planetary science person would know this and I encourage any of them to sneer at my ignorance and wave the number in my face. > Or to be more specific: when is Comet Halley likely to wear out, presuming > that we don't accidentally take it out with a probe first? From everything above and other stuff I've heard/read, on the order of a thousand years, give or take a factor of two, I guess. Jeff Brown the Scumbag {allegra,ihnp4}!{noao,ut-sally}!utastro!jeff jeff@astro.UTEXAS.EDU Astronomy Department, U. of Texas, Austin