Starship Asterisk*

Introducing Comet ISON

Explanation: Could this dim spot brighten into one of the brightest comets ever? It's possible. Alternatively, the comet could break up when it gets closer to the Sun, or brighten much more modestly. Sky enthusiasts the world over are all [url=http://3.bp.blogspot.com/-Ok7KAaeDjoI/T9a_eWgdZNI/AAAAAAAAVUE/YBJJI1_DW0Q/s1600/Cat-whispering-to-dog.jpg" >abuzz, though, from the more <a href="http://www.ast.cam.ac.uk/~jds/12s1.ukl]optimistic speculations[/url] -- that the newly discovered C/2012 S1 (ISON) could develop a spectacular tail or briefly approach the brightness of the full Moon toward the end of 2013. Comet ISON currently is very faint but is just visible at magnitude 18 in the above image. The comet, discovered just over a week ago from Russia by Vitali Nevski (Belarus) and Artyom Novichonok (Russia), is currently falling toward the Sun from between the orbits of Jupiter and Saturn. In early 2013 October it will pass very near Mars and possibly be visible to rovers and orbiting spacecraft. Comet ISON appears on course to achieve sungrazer status as it passes within a solar diameter of Sun's surface in late 2013 November. Whatever survives will then pass nearest the Earth in late 2013 December. Astronomers around the world will be tracking this large dirty snowball closely to better understand its nature and how it might evolve during the next 15 months.

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Are there any historical accounts of two comets being simultaneously visible?

csw wrote:Are there any historical accounts of two comets being simultaneously visible?

There have been many times when two or more naked eye comets were simultaneously visible (I remember a few). Do you mean "great" comets, the exceptionally bright ones that capture the imagination of the public at large, not just astronomers?

Dirty snowball? haven't close encounters pf the last few years discounted that idea? you people are so funny.

tannaberton wrote:Dirty snowball? haven't close encounters pf the last few years discounted that idea? you people are so funny.

Not at all, although some comets are now perhaps better described as snowy dirtballs. But fundamentally, the makeup of a comet has been understood for a long time: a mix of rocks and dust, carbonaceous silicates, and frozen volatiles. Close encounters have clarified how these things are distributed.

Chris Peterson wrote:

tannaberton wrote:
Dirty snowball? haven't close encounters pf the last few years discounted that idea?

Not at all, although some comets are now perhaps better described as snowy dirtballs.

After Tunguska I'd think twice before calling any comet a snowy dirtball.

"Could this dim spot brighten into one of the brightest comets ever?"

Why? What features of this comet predict how it will develop?
JOhn

JohnD wrote:"Could this dim spot brighten into one of the brightest comets ever?"

Why? What features of this comet predict how it will develop?

Its brightness suggests it is relatively large, its high eccentricity orbit suggests that it has never passed the Sun before, and its upcoming perihelion will be very close to the Sun. This all means that there may be a lot of volatiles which will be released. On top of this, the comet's path takes it quite close to the Earth. These are all factors that we'd expect to find in a very bright comet.

Thanks, Chris!
There's none of that in the APOD blurb!
Make sit sound all the more exciting.
John

I wonder what perturbed it out of its orbit in the Ort Cloud....


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I don't understand the implication of the several green/red/yellow "spots" in the enlarged blue window. Has the comet already fragmented into several pieces?

Oh, just great. You gotta hand it to those Mayans.
They were only off by a year...

Boomer12k wrote:I wonder what perturbed it out of its orbit in the Ort Cloud....

Bodies in the Oort cloud are just a gnat's fart below solar escape velocity; a near pass by another Oort object could do the trick, or a rogue extrasolar planet, or maybe even a resonance involving another star and Jupiter.

Chris Peterson wrote:

Boomer12k wrote:
I wonder what perturbed it out of its orbit in the Ort Cloud....

Bodies in the Oort cloud are just a gnat's fart below solar escape velocity; a near pass by another Oort object could do the trick, or a rogue extrasolar planet, or maybe even a resonance involving another star and Jupiter.

But when this body was in quasi-circular Oort orbit it had about a thousand times the angular momentum that it currently has. I wonder what sort of perturbation could so efficiently damp its angular momentum by a factor of ~1000.

neufer wrote:I wonder what sort of perturbation could so efficiently damp its angular momentum by a factor of ~1000.

Moveing the decimal point to the left 3-places.

neufer wrote:But when this body was in quasi-circular Oort orbit it had about a thousand times the angular momentum that it currently has. I wonder what sort of perturbation could so efficiently damp its angular momentum by a factor of ~1000.

The multiplicative change in angular momentum is not a good way of looking at this, since the angular momentum is very small to begin with. It's better too look at it as a delta-V problem. It only takes a change of about 1 m/s to move an object from the outer Oort cloud into a parabolic orbit with a perihelion close to the Sun. Even tidal effects from the Milky Way are on that order when combined with planetary or stellar resonances.

biddie67 wrote:I don't understand the implication of the several green/red/yellow "spots" in the enlarged blue window. Has the comet already fragmented into several pieces?

I'm not certain as to the color scheme but the inset resembles something other than the visible spectrum. The 3 closest Green/Yellow blobs in the inset are the 3 closest stars to the comet in the visual light image

biddie67 wrote:I don't understand the implication of the several green/red/yellow "spots" in the enlarged blue window. Has the comet already fragmented into several pieces?

BMAONE23 wrote:I'm not certain as to the color scheme but the inset resembles something other than the visible spectrum. The 3 closest Green/Yellow blobs in the inset are the 3 closest stars to the comet in the visual light image

Chris Peterson wrote:

neufer wrote:
But when this body was in quasi-circular Oort orbit it had about a thousand times the angular momentum that it currently has. I wonder what sort of perturbation could so efficiently damp its angular momentum by a factor of ~1000.

The multiplicative change in angular momentum is not a good way of looking at this, since the angular momentum is very small to begin with. It's better too look at it as a delta-V problem. It only takes a change of about 1 m/s to move an object from the outer Oort cloud into a parabolic orbit with a perihelion close to the Sun. Even tidal effects from the Milky Way are on that order when combined with planetary or stellar resonances.

Orbital velocity falls off inversely with the square root of distance not with the distance itself.

Hence orbital angular velocity increases with the square root of distance.

You are the one that have told us that astronomical objects such as global clusters don't collapse because of the conservation of angular momentum...now suddenly you are changing your tune?

http://en.wikipedia.org/wiki/Oort_cloud wrote:
<<The Oort cloud, or the Öpik–Oort cloud, is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun.>>

Earth's orbital velocity ~30 km/s

Oort cloud orbital velocity ~30/sqrt(50,000) = 134 m/s

neufer wrote:Orbital velocity falls off inversely with the square root of distance not with the distance itself.

Hence orbital angular velocity increases with the square root of distance.

You are the one that have told us that astronomical objects such as global clusters don't collapse because of the conservation of angular momentum...now suddenly you are changing your tune?

I'm not really sure of the point you're trying to make. Are you disputing that a delta-V as small as 1 m/s is sufficient to place a Oort cloud body in an escape orbit?

I don't recall arguing that globular clusters don't collapse because of the conservation of angular momentum. They don't collapse because their components are in orbit around the cluster's center of mass. What I have said is that globular clusters do fall apart because of the conservation of angular momentum, whereby small transfers of momentum between different bodies result in some exceeding their local escape velocity and therefore leaving the cluster- very much what happens in the Oort cloud as small perturbations occasionally do the same to icy bodies.

Earth's orbital velocity ~30 km/s

Oort cloud orbital velocity ~30/sqrt(50,000) = 134 m/s

Again, I'm not sure of the point you're making. It requires only a tiny delta-V for a body in the Oort cloud to have its orbit radically modified- typically into a highly eccentric ellipse or on open (parabolic or hyperbolic) one.

BMAONE23 wrote:I'm not certain as to the color scheme but the inset resembles something other than the visible spectrum. The 3 closest Green/Yellow blobs in the inset are the 3 closest stars to the comet in the visual light image

The human eye is not very good at distinguishing between different gray levels- we typically only see about a hundred distinctions between black and white. So it is common to use a pseudocolor scheme, which maps different intensity levels to different colors, allowing us to see much finer gradations in intensity. Unlike some false color schemes (e.g. the Hubble palette) there is no connection between the colors in this image and anything in the spectral quality of the objects. It is purely a mapping between intensity and color.

Thanks Chris,
I wasn't certain and didn't notice that particular tidbit of info in any of the associated texts of links

Chris Peterson wrote:

neufer wrote:
Earth's orbital velocity ~30 km/s

Oort cloud orbital velocity ~30/sqrt(50,000) = 134 m/s

Again, I'm not sure of the point you're making. It requires only a tiny delta-V for a body in the Oort cloud to have its orbit radically modified- typically into a highly eccentric ellipse or on open (parabolic or hyperbolic) one.

134 m/s is not a tiny delta-V

More importantly, for Comet C/2012 S1 (ISON) to achieve a tiny perihelion of 0.012 AU that delta-V must be precisely directed so that the orbital angular velocity is reduced to a mere 9 cm/s [= 30 m/s (x sqrt(2*0.012)/50,000)]. A randomly directed ~134 m/s delta-V perturbation has a 50% chance of knocking an Oort body out into interstellar space but it has only about a one in a million chance of deadening Oort body's orbital angular velocity so as to achieve a perihelion as small as 0.012 AU.

The whole physical process is something of a mystery to me, frankly.

neufer wrote:134 m/s is not a tiny delta-V :!:

But that isn't the delta-V required to put a distant Oort body into a parabolic orbit.