But what's a Comet, anyway?

©REUTERS/NASA, ESA, and H. Weaver/The Johns Hopkins University Applied Physics Laboratory/Handout
A Hubble image (R), taken November 4, 2007, shows the heart of Comet 17P/Holmes. The central portion of the image has been specially processed to highlight variations in the dust distribution near the nucleus. About twice as much dust lies along the east-west direction (the horizontal direction) as along the north-south direction (the vertical direction), giving the comet a "bow tie" appearance. The composite color image at left, taken on November 1, 2007, by an amateur astronomer shows the complex structure of the entire coma, consisting of concentric shells of dust and a faint tail emanating from the comet's right side. The normally sedate Comet Holmes made a bright splash in the sky about two weeks ago, unexpectedly becoming a million times brighter than normal overnight and causing a stir among astronomers.

The Web is abuzz about Comet Holmes.

Comet 17P/Holmes had been expected to make just another routine ultra low visibility departure from perihelion passage this year. But in a record for a comet, it unexpectedly brightened by a factor of around a million on October 24, making it more reminiscent of a stellar nova explosion.

"Amateur astronomers the world over have been stunned and amazed by the weirdest new object to appear in the sky in memory," wrote Sky and Telescope.

"The comet shocked skywatchers as it went from a dim 17th magnitude then suddenly to 3 magnitude" wrote Theo from the Pacific Northwest.(*)

Holmes brightened from below the threshold of binoculars to being clearly visible with the unaided eye. The outburst presented as a circular fuzzy patch in the constellation of Perseus, and has proceeded to grow in size. The comet was then reported to have spawned a tail.

More drama was to come.

Spaceweather currently reports that "last Friday, astronomers watched in amazement as the comet's tail broke off!".

And Comet Holmes may yet make further news, should this be a prelude to the full-on fissioning of the comet's core into several pieces. The comet is expected to remain visible to the naked eye for some weeks.

Why did Comet Holmes flare up?

Yet Comet Holmes has brightened up unexpectedly on a previous occasion: that of November 1892 led to its discovery by E. Holmes in England. It was subsequently determined to be a periodic comet, with a revolutionary period of about every seven years. You can see its orbit in relation to other planets courtesy of Larry's animation here.

Much less, though, has been made of the fact of where it was when the current flare-up occurred. The Armagh Observatory's website puts the comet at some 230m miles from the Sun - a distance from, and quite a long time after, its closest point to the Sun, which occurred on May 4, 2007.

In the accepted view, comets are thought to comprise rocky and icy solids in a mixture termed "rocky snowballs" by some and "icy rockballs" by others. They get brighter and sprout tails as the "icy" component is thought to sublimate from solid to gaseous state under the influence of solar heating, which typically begins affecting them at a point just inside the orbit of Jupiter. This gives rise initially to an expanding light-reflecting aura known as the coma, which is then "blown away" in the opposite direction from the sun by the solar "wind".

Multiple repeated thermally-induced mechanical stresses are thought to give rise to a cometary aging process, whereby a large single core disintegrates after multiple closer solar encounters into much smaller fragments. When such orbiting debris intersects the Earth's orbit, it can give rise to meteor showers.

Professionals, however, are well aware of exceptions.

Comet Wirtanen fragmented in 1957 when it was just inside the orbit of Saturn. Conversely, in December 1680, the eponymous comet studied by Halley and Newton passed intact within 100,000km of the Sun.

Things took a more serious turn, as reported in 1990 by New Scientist.

The object named Chiron, discovered in 1977 as an asteroid orbiting between Jupiter and Neptune, had begun to "look like a gigantic comet".

Forget for a moment the stuff comets are made of - here, the very categories of astronomy were under attack.

An ontological debate ensued, mostly in private, but recently resulting in the public ejection of Pluto from the category of "planet". In this debate, the retention of a solar system cosmogony - including attachment to a primeval outer comet reservoir with links to the present (the Oort Cloud) - has been a significant feature.

Why do comets split?

Yet as far back as 1989, Carl Sagan and Nancy Druyan had remarked on the fact that 80 per cent of comets which split do so when they are far from the Sun.

In their book Comet, they wrote:
The gravitational tides of the Sun or unequal heating cannot be sole causes of the splitting of comets. We still do not know why comets split.
Stranger still, Dr Brian Marsden, for many years astronomy's Mr Orbital Elements extraordinaire, had noted in the 1960s that two comets, 1882 II and 1965 VIII, looked as if they had split apart near aphelion (their farthest distance from the Sun), beyond the orbit of Neptune and above the ecliptic plane. Has nothing progressed?

Contemporary authorities such as David C. Jewitt of the Institute for Astronomy, University of Hawaii, concede that still "most splitting events occur without obvious provocation and their cause is unknown". (See pdf, 1.2MB.)

This has not stopped cometologists devising ever more complex structures for comets to account for the more evident facts: some comets are conceived of as comprising an icy core well sealed in a rocky shell; others as having a refractory mantle for their orbital phase. But can the counter-intuitive nature of such theoretical elaborations equal some of the more counter-intuitive facts which recent observations have adduced, such as the production of X-rays?

Some commentators think that the standard narrative of comets has been allowed to grow in an ad hoc manner for too long.

The Institute of Electrical and Electronics Engineers (IEEE) has for many years had a Nuclear and Plasma Sciences Society subgroup, and has published papers since 1986 on the application of plasmas to the phenomena of astronomy. A 7th series is underway.

Countering the astronomical consensus, plasma physicists account for comet features in terms of a varying electrical potential experienced by the comet as it changes location within the electrical plasma field. This is the "solar wind" - centred on the Sun - which extends beyond Pluto, and which is modified by the magnetic and electrical contribution of the planets.

In this account, cometary comae and tails are the result of electrical arcing, analogous to electrical discharge machining, rather than sublimation.

The dual flashes of light observed when the Deep Impact spacecraft mission despatched a copper impactor into Comet Temple 1 were consistent with the expectations of plasma physicists. Scientists observed two flashes: the second flash was due to the impact, but the first flash was due to electrical discharge between the comet and the impactor, and resulted in failure of the on-board data collection and transmission systems.

A compilation of predictions about the Deep Impact encounter from the point of view of a plasma universe can be found here.

So a possible fragmentation of comet Holmes is not the only thing to look forward to. Some are asking if God if not after all a crude materialist, but a subtle electrician?

Follow the link above Larry's page for a comet-locating starmap. UK readers should crane their necks round midnight to the zenith, and adjust the gaze slightly north-east. With binoculars the fuzzy coma is clearly visible. ®

(*) Bootnote

Unlike the rest of science, the "order" of stellar magnitude is not computed to base 10, but to Pogson's Ratio of 2.512. To know exactly why, you would need to build a time machine, travel back to the ancient Greek period, and ask them how they could conceive of a sixth rate star, when you or I might make do with third or fourth rate ones.