"Newfound Comet SWAN could soon fizzle out of view"

During the next couple of weeks we'll have a chance of seeing a new comet as it sweeps past the sun. 

The comet's name is SWAN, an acronym for the Solar Wind Anisotropies camera on NASA's Solar and Heliospheric Observatory (SOHO). Officially designated as C/2020 F8, the comet was discovered by Australian amateur astronomer, Michael Mattiazzo, while exploring SWAN imagery on March 25. 

Comet SWAN initially caught Mattiazzo's attention because it apparently was undergoing a sudden outburst of hydrogen gas — something that SOHO's SWAN instrument is particularly well adapted to picking up. Water ice from the comet's nucleus evaporates as the comet approaches the sun. Solar ultraviolet radiation splits up water molecules, and the liberated hydrogen atoms glow in ultraviolet light.

The comet was 135 million miles (217 million kilometers) from the sun when Mattiazzo first saw it, but it will ultimately come to within 40.2 million miles (64.6 million km) of our star when it arrives at its perihelion, its closest point to the sun, on May 27. 

In scanning the Internet I've seen comments about Comet SWAN describing it as (potentially) putting on a "splendid" show in the coming days, and how it could be the "best comet in years" or the "brightest in decades." 

Well ... don't count on that.

Sorry if I sound like the wet blanket at the party, folks, but it seems to me that many who are ballyhooing Comet SWAN as a possible celestial showpiece are forgetting some of the old basics of forecasting what a lot of comets tend to do: Tease us, and more often than not end up underperforming and disappointing.

In recent days, some very impressive images of Comet SWAN have been making the rounds on different astronomy sites, all showing a comet with a large glowing head (called the "coma"), trailed by a long and beautiful gossamer tail. 

I'm sure that one look at those photographs, combined with the promise that viewers could see such a spectacle with their own eyes, will have many making a special effort to go out and see it for themselves.

But what you see in those photographs, is not what you're going to get.

Is it a dusty comet, or a gassy one? 

Comets are composed of frozen gases — methane, ammonia, water vapor and many others — that are heated as they approach the sun and made to glow by the sun's light, much in the same manner as phosphorescent paint glows under an ultraviolet lamp. Mixed within the gas are particles of mostly fine-grained and dusty material. 

Now the best comets — the ones that put on a good show — contain a lot of dust. Dust, you see, is a very good reflector of sunlight. As the gasses warm and expand, the solar wind (a stream of subatomic particles rushing out from the sun) blows the expanding material out into space to form the comet's beautiful tail.

If the comet is a dusty one, the coma and resulting tail will be bright and easy to see. On occasion you may even perceive a slight yellow or pinkish tint. The dust may actually lag behind the comet head and impart the shape of a gentle arc. Comet Bennett (1970), Comet West (1976), Comet Hale-Bopp (1997) and Comet McNaught (2007) are all examples of bright and dusty comets. 

But if the comet is primarily composed of gas, it will generally appear much dimmer; more "ghostly" than anything else. Such comets usually appear as nothing more than a fuzzball to the eye, and the resulting tail will tend to be a faint and narrow appendage, stretching straight out from behind the coma. 

Moreover, to the naked eye and even through binoculars or a small telescope, the gas tail (usually of a bluish tint) tends to be so faint that it is hardly evident at all. Ironically, long-exposure photographs will bring out the gaseous tail quite nicely. Such tails can be rather long and stretch for many degrees across the sky, although what the camera sees is deceptive, for visually with your eyes, only a fraction of that length tends to be evident, again because of the faintness of the gas projected against the background of the sky.

Now, guess what type of comet SWAN happens to be? If you said "gas" you're (unfortunately) correct.