Halley's comet

Question

What causes comets to die out?

Answer

Do you mean over their multi-million year total life span or do you mean as they exit the inner Solar System and head back out into interplanetary space? 1. For the first part, all comets are composed of frozen ices, frozen gas and dust. (Basically a dirty snowball). Eventually after hundreds or thousands of close passes around the sun, a comet loses all it's volatile material (frozen solid ices and gases) and becomes just a used-up comet. {Check the internet for asteroid 3200 Phaethon, thought to be an extinct comet nucleus that causes the Geminid meteor shower every December 13th, which has lost all it's volatile material}. The Halley Comet nucleus will end up that way, someday.

Notice I did not use the phrase 'burn up' but rather, used up...comets do not burn...they sublimate; the frozen gases and ice go directly from the solid to the vapor state...called sublimation, like our common frozen carbon dioxide, also called dry ice. In fact, comets are visible...all by reflected sunlight, they do not generate their own heat. The sun sublimates the frozen gases, releasing dust, and the solar wind pushes the dust and gas away from the nucleus to form a long tail....actually 2 tails, the dust tail and the gas (or ion) tail. (That's why the tail ALWAYS points away from the sun, and is a NON-indicator of direction of flight. For if it were an indicator, the comet would be either heading into or directly away from the sun. Comets do neither...they orbit the sun in very cigar-shaped, elongated elliptical orbits.

 Many times these two tails are seen separately, sometimes they merge into one tail. And what we see as a tail is just the dust and gas reflecting sunlight...there is no burning or fire involved here.  The nucleus may get just hot enough to glow a little when the comet rounds perihelion very close to the sun, but even the nucleus and coma of the comet is still mostly just....reflecting sunlight.

2. Well, they really don't die out after perihelion...as they leave the heat of the sun, they just quit sublimating, and eventually all that is left (as they head back out into the cold of space) is the cold nucleus, having probably lost several feet of frozen ices and gases and dust. And it will remain in that "hibernation" state until the next time it draws near the sun again, and the process is repeated. For more on Halley, and a spacecraft that visited Halley on it's last return, Google the European Giotto Mission in 1986 for actual pictures of the dark, black nucleus.

Clear skies, Tom Whiting Erie, PA

Question

When did Halley's comet look the best and explain how it looked?

Answer

Hi Terell, Halley's comet looked the best in the 1910 return because we, the Earth, actually zoomed through the tail, a very close encounter. Pictures of the spectacular 1910 return are probably found on the internet. Correspondingly, the 1986 return was the poorest on record because the Earth was exactly opposite the sun from the comet at perihelion...(we were on the wrong side of the sun at that time), so all we observers got to see was an average bright fuzzball with not much of a tail from the period of October 1985 thru April 1986. (Of course, all us astronomers knew of that well in advance of the comet anyway, so we were prepared for a poor showing).

It was a rather small apparent tail because most of the time the tail pointed directly away from the comet, the sun, and the Earth as it rounded the sun. So the tail appeared very 'foreshortened' from our viewpoint for nearly the entire apparition. By the time we were viewing the comet broadside, it had dimmed below naked eye on it's way outbound in May and June of 1986. And of course the tail had almost disappeared too, by then. Hope this helps, Clear Skies, Tom Whiting Erie, PA

Question

What makes Haley's comet so different from other comets

Answer

Hi Terell, There are two types of comets, periodic - ones that return at regular intervals, and non-periodic comets that make one pass and never (or perhaps take thousands of years) come back. Halley's Comet is the brightest of the first type, the periodic comets. Also, Halley's Comet nearly intercepts our Earth's orbit at 2 points, both it's inbound leg in Orion and it's outbound leg in Aquarius. Thus it's remains (comet dust from tails) produces two annual meteor showers every year; the Eta Aquarids of early May and the Orionids of late October. Other famous periodic comets typically only produce one meteor shower a year, (if any) like the Perseids of August 11th and the Leonids of November 17th. It's also one of the few comets, perhaps the only one, where the actual discoverer never actually saw his comet. All other comets are named either after the person who first discovers them (and properly reports them), or the name of the satellite or equipment that first spots them, thus we have the IRAS (Infrared Astronomy Satellite) comet of 1983 and a lot of "LINEAR" comets now. Clear Skies, Tom Whiting Erie, PA

FOLLOW UP: Oh, and it's Halley's comet (rhymes with alley), not haley's comet (hay lee)...some people mistakenly call it the latter, because there was a famous USA Rock Group of the 1950's era called "Bill Haley and the Comets".... for what it's worth. Clear Skies, Tom

Question

what makes Halley's comet so famous?

Answer

Hi Terell, It was the first comet determined to be in an orbit around the sun, and therefore would return at regular intervals for us to see. This was done by Edmund Halley back in the early 18th century by reviewing old records of past comets. One bright comet kept repeating about every 75-80 years or so, with an average period of 76 years, so he was able to predict it's next return to the inner Solar System on it's next pass in 1758, which it did. Thus it was named after him, even though he actually never saw his comet. He only asked that people remember that it was an Englishman who predicted it's return. Halley's comet was last seen in 1910 and 1986 so it's next return is around 2061 or 2062. Clear Skies, Tom Whiting Erie, PA

Question

Where was Halley's comet observed?

Answer

Hi Terell, When? Tom

FOLLOW UP: I first observed Halley in February 1986 from Erie, PA...then we took vacation to southern Florida and saw it again from Sanibel Island just south of Fort Myers, in March of 1986.

Since then, I believe large telescopes have photographed it passing Saturn's orbit outbound, down around 24th magnitude, but I haven't seen a recent picture of it. Generally, all parts of the world get to see a bright comet, (except a polar region where it is continuous daylight) as they only move a degree or several degrees per 24 hours, so they are around quite a while...several weeks or months. (Unlike meteors that appear and are gone in a flash). Comets DO NOT streak across the sky, they move very leisurely across the sky, slowly changing their position a little, from night to night...similar to our moon, but not even that fast. (The moon moves about 12 degrees eastward every 24 hours in our sky.) Comets typically move only one degree from night to night. Clear Skies, Tom