Many comets are made up of frozen water and frozen
gases. They have been referred to as "dirty snowballs" and may
contain a variety of other materials (metals and rock). I guess if they
were made up entirely of hard, cohesive, solid stuff then they wouldn't
have those nice tails and we would be more tempted to classify them as
"asteroids".
In general, they lose material to space when they come close to
the sun. The sun's heat melts and vaporizes part of the comet's
material, and it is blown away from the comet by the solar wind -- a
constant outwards flux of charged and neutral particles -- mostly
protons and electrons, but perhaps a tiny amount of heavier nuclei. The
solar wind should not condense on a comet, and so the comet should only
lose material as it passes by the sun.
Yes, comets do in fact become smaller and smaller and eventually
vanish below the level of observability. One might ask that if the
solar system really is many billions of years old, why are there any
comets left at all? The main idea is that there are lots and lots in
relatively stable orbits out in the Kuyper belt, which starts just
beyond the orbit of Neptune, and the Oort cloud, which is even farther
away. If an icy object in the Kuyper belt interacts with another
gravitationally, and gets kicked close to Neptune's orbit, it can be
kicked into an elongated, elliptcal orbit taking it close to the sun.
The same is true for icy objects farther out. One hypothesis is that
there is a massive, dark planet out in the Oort cloud, kicking comets
inwards, called "Nemesis". I'm not sure what the current status of this
hypothesis is.
Comets should not increase their rate of rotation (or at least not
that I'm aware of), since there is a lack of external torques on any
part of the comet -- evaporating gases should carry away the angular
momentum they had before they evaporated and so the remainder should
not spin up.
Nonetheless, tidal forces can tear apart a weakly-held-together
comet if it passes near a planet or the sun. For example, Comet
Shoemaker-Levy broke apart into 21 discernable fragments before
impacting Jupiter -- it was torn apart by the differnece in
gravitaitonal forces from one side of the comet to another as it passed
Jupiter on an earlier orbit, and the pieces traveled together,
spreading out a bit, before impacting more directly in 1994.
Tom
(published on 10/22/2007)
