Expert: Tom Whiting - 9/24/2009

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