by neufer » Mon Dec 14, 2015 2:58 pm
JohnD wrote:
<<And the dust tail is composed of particles ejected by the core as it is is heated by the Sun. Those will leave the core at a random velocity, almost unaffected by the core's minute gravity, into free fall in the same orbit as the core. So why does a tail form, instead of a cloud around the core that gets bigger and denser until it moves away from the Sun?
The particles are mostly tiny so may be affected by light pressure, in which case they would move from the orbit away from the Sun, forming a tail at an angle to the orbit.
Is the dust tail along the core's orbit, or at an angle to it?>>
The dust tail is pushed away from the Sun by radiation pressure
just at a
much slower rate than the ion tail (pushed by the ~400 km/s solar wind).
https://en.wikipedia.org/wiki/Comet_tail#Tail_formation wrote:
<<As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.
The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, always points along the streamlines of the solar wind as it is strongly affected by the magnetic field of the plasma of the solar wind. The ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.>>
JohnD wrote:
<<A rider Q is about the meteors that are associated with many comets. They are said to be spread out along the comet's orbit, which makes me think that the answer to my last is that the dust cloud is deposited along the orbit, not spread out away from it by light pressure. Is there an orbital mechanics explanation? It can't be tidal can it, in the way that Shoemaker's broke up and "spread out" along its orbit?>>
The primary effect of radiation pressure is to spread the cometary material into a more circular orbit with a similar period as the comet's elliptical orbit. This is because the force is mostly radial and therefore is not changing the angular momentum. One should end up with more or less a circular ring feature in the plane of the comet's orbit.
[quote="JohnD"]
<<And the dust tail is composed of particles ejected by the core as it is is heated by the Sun. Those will leave the core at a random velocity, almost unaffected by the core's minute gravity, into free fall in the same orbit as the core. So why does a tail form, instead of a cloud around the core that gets bigger and denser until it moves away from the Sun?
The particles are mostly tiny so may be affected by light pressure, in which case they would move from the orbit away from the Sun, forming a tail at an angle to the orbit.
Is the dust tail along the core's orbit, or at an angle to it?>>[/quote]
The dust tail is pushed away from the Sun by radiation pressure
just at a [b][u]much slower[/u][/b] rate than the ion tail (pushed by the ~400 km/s solar wind).
[quote=" https://en.wikipedia.org/wiki/Comet_tail#Tail_formation"]
[float=right][img3="[b][color=#0000FF]A comet's orbit showing the different directions of the gas and dust tails as the comet passes the Sun[/color][/b]"]https://upload.wikimedia.org/wikipedia/commons/c/cb/Cometorbit.png[/img3][/float]<<As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.
The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail, made of gases, always points along the streamlines of the solar wind as it is strongly affected by the magnetic field of the plasma of the solar wind. The ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.>>[/quote][quote="JohnD"]
<<A rider Q is about the meteors that are associated with many comets. They are said to be spread out along the comet's orbit, which makes me think that the answer to my last is that the dust cloud is deposited along the orbit, not spread out away from it by light pressure. Is there an orbital mechanics explanation? It can't be tidal can it, in the way that Shoemaker's broke up and "spread out" along its orbit?>>[/quote]
The primary effect of radiation pressure is to spread the cometary material into a more circular orbit with a similar period as the comet's elliptical orbit. This is because the force is mostly radial and therefore is not changing the angular momentum. One should end up with more or less a circular ring feature in the plane of the comet's orbit.