View Full Version : Turning Tail?

Peter Wilson
2006-Oct-11, 12:04 AM
Comet tails "always" point away from sun.

However, according to Poynting-Robertson effect (http://en.wikipedia.org/wiki/Poynting-Robertson_effect), if dust is moving at high velocity with respect to radiation field, it should fall into sun, not away from sun. This should occur for sun-grazing comets as they near the sun at high velocity.

Has tail-reversal of a sun-grazing comet ever been observed?

2006-Oct-11, 12:23 AM
From the article.

There is a critical size at which small objects are so affected by radiation pressure that the latter actually cancels the Sun's gravitation altogether. For rocky particles, this size is about 0.5 µm in diameter [1]. If the particles are already in motion at their creation, radiation pressure does not need to cancel gravity completely to move the particles out of the solar system, so the critical size becomes a bit larger. The Poynting-Robertson effect still affects these small particles, but they will be blown out of the solar system by the Sun's light before the Poynting-Robertson force produces any significant change in their motion.

Would this affect the tail?

2006-Oct-11, 12:45 AM
Actually, comets have two tails.

The gas tail, a bunch of single molecules, is the one that points straight away from the sun.

Then the dust tail, those particles being heavier don´t get so much acceleration from the solar wind, so it begins to form in the direction of the gas tail but starts to lag behind, curving "backwards" (the tail, not the particles´ trajectories which follow forward and out) according to the orbit.

Besides the previous two, when it is close enough to the sun, it has jets of vapour and hot water mainly on the sunny side. Do you mean these?

2006-Oct-11, 07:20 AM
Here is Comet Hale-Bopp as taken on 7 March 1997. The blue tail is the ion tail, while the white one is the dust tail. It does look a little like the blue one has a slight arc to it though.

2006-Oct-11, 11:36 AM
The small dust like particles of a comet are ripped away from the comet by the solar wind and, yes always look like they point away from the sun. There is a dragging effect which gives the curve observed. Some of this mater will end up being drawn into the solar mass. We will not see this as the particles become less dense and disperse to the point where we can no longer see them. Some of this material remains in the solar orbit for many years.
When the Earth passes through one of these we see a meteor shower.