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Q & A: Comets breaking up

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Most recent answer: 10/22/2007
Q:
1) As a commet passes by the sun, does it loose its tail material to the space, or does it gather it up after passing the sun? If it looses material to the space, will it then vanish after several millions of years? 2) Does a commet spin faster and faster under the acceleration generated by its jets and ultimately fall apart into smaller commets orbiting its center of mass? Thank you.
- Mehran
Miami, USA
A:
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)

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