by MarkBour » Tue Jul 21, 2020 12:07 am
I enjoyed looking at Dr. Nemiroff's "Lecture #11 on Comets and Meteors (
http://asterisk.apod.com/viewtopic.php?f=24&t=18013)
That lecture did not give much detail about the cause for the shape of a comet's tail. Tentatively, here's my view of it:
A bright comet with two bright tails is a chance to watch a gravitational density-separation process in action. Considering an outbound comet:
- The heat causes sublimation and out-gassing, which also kicks lots of dust and small particles off of the main nucleus.
- At the outset, all of these are travelling in the same orbit, and could stay roughly together, as a nucleus with an atmosphere, or exosphere, anyway. But the solar wind and solar radiation both press on this collection. Apparently, the gas molecules are apt to get ionized, which I guess increases the forces they experience from the solar wind. No doubt, the ions can return to the ground state, but they still experience force.
- So, the gas molecules, being the lightest, are getting a much greater force per unit mass than the rest. And they are thus being accelerated to a higher orbit. Indeed, coming off from the path of the comet, they look almost like a straight line outward.
- Next, the dust and small particles are getting much less force per unit mass than the gas molecules. (Their surface for impact of the wind and radiation is much smaller than it would be if they had been dissolved into a gas of those same molecules, and they have heavier elements, so they are accelerated far less than the gas.) Nevertheless, the small particles are getting a lot more pressure per unit mass than the nucleus is. So, they spread, each at their own rate, but all generally being pushed into higher orbits than the nucleus. The spread of this tail is not only from the variances in the dust, but also they are spread throughout the differing times of release.
- The nucleus itself is also being pushed by these forces, but since it is a large dense body, the push it receives is very small. By comparison to the other parts, it is just travelling in an orbit, and stays below all of the rest.
An inbound comet would be undergoing very similar effects. I find it more complex to think about what the tail would look like right after perihelion. Not at all sure how the skewed ""higher" orbits would look as all of this rounds perihelion in comparison to the material coming out on the outward journey.
Chris mentioned doing some very detailed simulations of this. I wonder if his models painted a different picture.
I enjoyed looking at Dr. Nemiroff's "Lecture #11 on Comets and Meteors ([url]http://asterisk.apod.com/viewtopic.php?f=24&t=18013[/url])
That lecture did not give much detail about the cause for the shape of a comet's tail. Tentatively, here's my view of it:
A bright comet with two bright tails is a chance to watch a gravitational density-separation process in action. Considering an outbound comet:
[list][*]The heat causes sublimation and out-gassing, which also kicks lots of dust and small particles off of the main nucleus.
[*]At the outset, all of these are travelling in the same orbit, and could stay roughly together, as a nucleus with an atmosphere, or exosphere, anyway. But the solar wind and solar radiation both press on this collection. Apparently, the gas molecules are apt to get ionized, which I guess increases the forces they experience from the solar wind. No doubt, the ions can return to the ground state, but they still experience force.
[*]So, the gas molecules, being the lightest, are getting a much greater force per unit mass than the rest. And they are thus being accelerated to a higher orbit. Indeed, coming off from the path of the comet, they look almost like a straight line outward.
[*]Next, the dust and small particles are getting much less force per unit mass than the gas molecules. (Their surface for impact of the wind and radiation is much smaller than it would be if they had been dissolved into a gas of those same molecules, and they have heavier elements, so they are accelerated far less than the gas.) Nevertheless, the small particles are getting a lot more pressure per unit mass than the nucleus is. So, they spread, each at their own rate, but all generally being pushed into higher orbits than the nucleus. The spread of this tail is not only from the variances in the dust, but also they are spread throughout the differing times of release.
[*]The nucleus itself is also being pushed by these forces, but since it is a large dense body, the push it receives is very small. By comparison to the other parts, it is just travelling in an orbit, and stays below all of the rest.
[/list]
An inbound comet would be undergoing very similar effects. I find it more complex to think about what the tail would look like right after perihelion. Not at all sure how the skewed ""higher" orbits would look as all of this rounds perihelion in comparison to the material coming out on the outward journey.
Chris mentioned doing some very detailed simulations of this. I wonder if his models painted a different picture.