by Anton Wylie
Published Sunday 18th November
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 sky-watchers as it went from a dim 17th
magnitude then suddenly to 3 magnitude"
wrote Theo from the Pacific Northwest.*
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.
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 (below).
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
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
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" - centered on the
Sun - which extends beyond Pluto, and which is modified by the
magnetic and electrical contribution of the planets.
In this account, cometary coma 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 dispatched 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?