Celestia Forums

I thought that comet tails disappeared after the comet had traveled
approximately 5AU from the Sun. This is not the case. Halley STILL
emits its tail when it's 31 AU from the Sun. Is this being addressed?

Should be a relatively EASY fix I would think. This bothers me only
because I'm trying to examine a new texture for Halley, and the
corona, tail, whatever you call it gets in the way. I know I can simply
turn off the tail, but I shouldn't have to do this at this distance from
the Sun should I?

One other point... Comets also usually feature TWO tails. One for the
dust and gases they emit, and one for the ionization trail. Any
thoughts?

Thanks, Bob

The tail certainly vanished beyond 5 -8 AU solar distance after I had first implemented these features. Meanwhile Chris has introduced some additions/modifications I suppose... But I have not checked comets since.

Bye Fridger

Using Celestia built from CVS, I see a "tail" only on the sunward side of Halley, none trailing along behind. My suspicion would be that the recent code added to improve the shape of the coma isn't testing the distance from the Sun.

selden wrote:Using Celestia built from CVS, I see a "tail" only on the sunward side of Halley, none trailing along behind. My suspicion would be that the recent code added to improve the shape of the coma isn't testing the distance from the Sun.

Are you saying politely that the tail doesn't shut off?

I just tested Halley during his 1910 approach, both in 1.5.0pre4 and 1.5.0CVS. In both cases, the tail was gone around 8 AU distance from the sun. So I cannot make out any problems

Bye Fridger

I just visited Halley in its current position TODAY. This position is
approximately 39 AU away from the Sun and - as I mentioned - the
tail is still being shown.

Just FYI.

Thanks, Bob

BobHegwood wrote:I just visited Halley in its current position TODAY. This position is
approximately 39 AU away from the Sun and - as I mentioned - the
tail is still being shown.

Just FYI.

Thanks, Bob

Sorry Bob,

so did I and I DON'T see a tail. The comet is 31 AU away from Sol today, Nov. 19. I just see the coma. When I change 2007 -> 1910 I see a looooong tail.

Probably you use still an older version? I use 1.5.0 CVS, latest.

Cheers,
Fridger

Just checked it again just to be certain. When I simply select Halley,
and then go to it, I see a VERY bright corona.

As Selden mentioned though, this corona displays itself in FRONT of
the comet. There is NO tail following the comet. Also, the comet IS
located today at approximately 31 AU, and not 39 as I stated to you
previously.

Sorry, I was in a hurry, and just guessed at the AU.

This time, I checked it accurately and the corona DOES display itself
as described above. So, my question becomes should the corona
display itself in front of the comet at 31 AU?

Thanks, Bob

Ah good,

so we cleared the tail issue up. Well the corona was always like that and I am not aware of any established results concerning the vanishing of the corona. In any case the corona presently looks quite strange, more like a familiar plastic toy to play with on the lawn in spring

Bye Fridger

t00fri wrote:Ah good,

so we cleared the tail issue up. Well the corona was always like that and I am not aware of any established results concerning the vanishing of the corona. In any case the corona presently looks quite strange, more like a familiar plastic toy to play with on the lawn in spring

Bye Fridger

I agree...

Concerning the tails and despite their recent smoothing, I think the rendering is way too synthetic... I wonder if there is no way to add some noise or turbulence in the rendering to make them feel a bit more natural...

I also noticed the comet's head, even when the comet is VERY far away from Sol. I think this is unrealistic. As the tail, it should fade away with distance from Sol.

As Cham says above, I too thought that if the tail disappeared, then
so should the corona. Is this not correct? What evidence is there to
suggest that the corona will stay ahead of the comet if there is
nothing to disturb the surface of the "snowball?"

Are you telling me that comets emit coronas even when in deep
space and FAR away from any interference from solar emissions?

Thanks, Bob

Actually, that's "coma" not "corona". A comet's coma is its bright central glow. A corona is what the Sun has.

When comets are far enough out to be cold and frozen, there should be no coma, either, not just no tail(s). At large distances, the nucleus would be too cold to outgas. 5AU is the approximate distance when most comets' outgassing comas' fluorescence becomes greater than their reflected light.

I just checked all four of the v1.5.0 prereleases. They all display Halley with a glow around it. Since it's out at 31AU now, the same distance as Pluto from the Sun, almost all of its volatiles should be frozen solid, and certainly not fluorescing.

It'd be nice if Celestia's comets showed both outgassing (atmosphere) at large distances from the sun and fluorescence when they're closer, but I doubt that complexity could make it into Celestia for v1.5.0. It wasn't all that long ago when we were delighted to see any kind of comet tails!

selden wrote:Actually, that's "coma" not "corona".

Many thanks Selden. If I had checked the Galactic Dictionary before
posting, I would have caught that. That's alright though... You
at least understood my point here.

Am NOT complaining, just making an observation that you scientifically
educated types should have noticed long ago. <smirk>

Sorry, couldn't resist.

Thanks, Brain-Dead

Any chance to get the second tail in there?

selden wrote:Actually, that's "coma" not "corona". A comet's coma is its bright central glow. A corona is what the Sun has.

When comets are far enough out to be cold and frozen, there should be no coma, either, not just no tail(s). At large distances, the nucleus would be too cold to outgas. 5AU is the approximate distance when most comets' outgassing comas' fluorescence becomes greater than their reflected light.

I just checked all four of the v1.5.0 prereleases. They all display Halley with a glow around it. Since it's out at 31AU now, the same distance as Pluto from the Sun, almost all of its volatiles should be frozen solid, and certainly not fluorescing.

It'd be nice if Celestia's comets showed both outgassing (atmosphere) at large distances from the sun and fluorescence when they're closer, but I doubt that complexity could make it into Celestia for v1.5.0. It wasn't all that long ago when we were delighted to see any kind of comet tails!

Selden,

do you know a serious, published paper that makes general statements about coma fading based on either convincing astrophysics arguments or direct measurements?

If there exists convincing support, the distance-dependent fading is very easy to implement. But I would only want to implement this on the basis of real scientific investigations not folklore.

Bye Fridger

MKruer wrote:Any chance to get the second tail in there?

How about a general criterion when secondary tails are absent and present respectively, and how they are oriented in case of bi-tailed comets flying though a system with several suns...? In case of one tail, I discussed the orientation issue some time ago in a Maple worksheet in this forum. I have also implemented this into Celestia.


Coding is easy once such basic astrophysics issues have been settled first.

Bye Fridger

Fridger,

My quick search last night turned up lots of papers about specific comets, but many of their common characteristics were mentioned only in generic introductory articles which didn't bother to cite sources. I'll look further.

This publication seems to be one of the seminal documents about cometary structure:

wikipedia wrote:ESSAY ON COMETS, which gained the first of Dr. Fellowes's prizes, proposed to those who had attended the University of Edinburgh within the last twelve years. By David Milne. Publisher: Edinburgh, Printed for A. Black; 1828. ''(a searchable facsimile at the University of Georgia Libraries; [[DjVu]] & http://fax.libs.uga.edu/QB721xM635/1f/essay_on_comets.pdf in layered PDF format)"

From the reference material presented above by Selden:

Page 9

"So we also find, from HERSCHEL'S observations of the great Comet
of 1811, that in receding from the sun, the envelope, losing its high
degree of attenuation, from the gradual diminution of the solar heat,
at length subsided altogether upon the nucleus, which previously it
had surrounded at a considerable distance."

Thanks, Bob

Page 21:

From the powerful action of the sun's heat upon the nucleus,
at this part of the orbit, supplies of nebulous matter are continually
detached; and as this effect must continue for a considerable time
after the perihelion passage, it is evident that the tail will then be
the most extensive."

If the tail displays the most extensive displays from a brush with
the Sun, then doesn't it logically follow that the Coma will have been
dissipated previously to the tail(s)?

You know, I might actually READ this, Selden. Interesting stuff.