Actually, both tails are pushed away from the sun. Neither traces out the path of the comet (though the aggregate of the remaining dust does trace this out, and is the source of meteor showers). The ion tail goes straight back; the dust trail is curved away from being straight back from the sun by the motion. The text below is taken from Comet, by Carl Sagan and Ann Druyan; page 160
Comet tails point away from the Sun whether the comet os approaching the Sun or receding from it, a conclusion apparently first drawn by Chinese astronomers during the apparition of Halley's Comet in the year 837. After perihelion passage the comet flies tail first out of the solar system. The tails of comets are much more like the effluvia from an industrial smokestack blown back on a windy day than the long hair of a bicyclist flowing behind her as she coasts down a hill on a windless day: it is not the motion of the comet through some resisting gas that determines the orientation of the tails, but rather the comet's motion combined with something like a wind blowing out from the Sun.
There are two kinds of cometary tails: long, straight, faintly blue tails, pointing almost perfectly straight back from the Sun; and usually shorter, curved, faintly yellow tails. Before their nature was understood, they were called Type I and Type II tails, respectively. [...] The Type II tails are yellow because they reflect sunlight back to us; but the Type I tails give off a blue light of their own.
Continuing on page 165 the book talks about the Type II, or dust, tail:
Tiny particles–clusters of still smaller motes of silicates and organics–are jetted off the cometary nucleus, and redirected back away from the Sun. Newtonian gravity will then produce the graceful curved tail. But what is this mysterious influence driving the particles back? The first person to guess the right answer was Johannes Kepler, who held that the tail of a comet is pushed away by the pressure of sunlight. The net result, he argued, was that the comet would ultimately dissipate into interplanetary gas.
Radiation pressure is not a factor in everyday life. Even very small people are not thrown to the ground by sunlight as on a cloudless day the step out of doors. The force on radiation pressure is the equivalent of the weight of a layer one atom thick at the surface of the Earth. Radiation pressure amounts to very nearly nothing. But if you are made of almost nothing in the first place, radiation pressure can push you around. [...] [I]n free space, a sufficiently small particle will feel the force of the radiation pressure driving it outward from the Sun more strongly than the Sun's gravity pulling it inward toward the Sun.
On page 169 is an explanation of the solar wind shaping the ion tail:
This wind from the Sun carries with it a magnetic field that sweeps up ions in its path. The moment a molecule is ionized by sunlight, it is captured by the magnetic fields carried with the solar wind. However, those molecules that are not ionized, that remain electrically neutral, are unaffected by magnetic fields. Thus the ion tails of comets are weather vanes in the solar wind, always blowing straight back; but occasionally an irregular magnetized cloud form the Sun catches up with the tail and stirs up the ions [accounting for the knots and other structure seen in the ion tail].
Hope this wasn't too long-winded, Jim ---- Jim Cobb james@cobb.name On Jan 10, 2005, at 7:05 PM, Chuck Hards wrote:
Jim Gibson asked:
Can anyone explain why there appears to be 2 tails in the animation that Rich posted?
Jim Cobb's explanation is basically correct. The Ion tail, sometimes called the plasma tail, is composed of atomic particles and is driven by the solar wind; it always points away from the sun (more or less). The dust tail, composed of solid particles of dust and rubble, (and the source of earthly meteor showers when we intersect a comet's orbit) basically streams along behind the comet in it's orbital path. Imagining the geometry in your mind's eye, you can see that the two tails can thus diverge by gross angles as the comet travels in it's path. Add to that the changing position of our viewpoint on the moving earth, relative to the comet, and the relative positions of the two tails change even more. Ion tails are typically bluish because they are generating their own photons at a specific wavelength, dust tails whitish to yellowish because they are just reflecting the broad-spectrum sunlight.