May 06, 2008
Comet Holmes 17P Startles Astronomers
The recent million-fold brightening of Holmes 17P has put another dent
in conventional comet theory. Should the "dirty snowball"
hypothesis be discarded in favor of electrical activity?
Astronomers
recently announced the unexpected brightening of a small
comet as it passed through the constellation Perseus. Called
Holmes 17P, the comet began a significant energetic outburst
on October 24, 2007. At that time, observers reported that it had begun to
increase in luminosity "for no apparent reason." Overall, it
gradually brightened from 17th
magnitude to about 2.5, bringing it into naked-eye
visibility.
Edwin Holmes discovered
the comet that bears his name on November 6, 1892, as he was
observing the Andromeda galaxy. The comet was apparently
experiencing a similar increase in brilliance during that
time, since Holmes reported it to be of approximately 5th
or 6th magnitude. A
historical narrative providing several highlights in the
story notes that the comet is periodic, orbiting the sun
every 6.9 years and staying between Mars and Jupiter.
In a
previous Thunderbolts Picture of the Day article about
the fissioning of Comet West in 1976, we noted that comets
tend to split or to undergo anomalous displays when they are
approaching their farthest distance from the sun. Because
conventional theories of the solar system based exclusively
on gravity expect disruptions only at close approaches to
the sun, the activity of comet West was a surprise.
Comet Linear defined another
mystery in July of 2000 when it actually blew apart. The
strangest thing about its fragmentation was that it occurred
at a distance of over 100 million kilometers from the sun
and not when it passed by during perihelion. As a matter of
observation, the majority of cometary disruptions take place
when they are far from the sun. Another counter-intuitive
reflection is that so-called "sun-grazer"
comets do not break apart despite approaching to within
50,000 kilometers of the sun's surface in some cases.
The large comet Hale-Bopp, also
did not obey the standard theory of cometary activity. Four
years after its encounter, in a region past Jupiter's orbit,
the comet displayed an ion tail, several jets of bright
material spewing into space and a glowing coma. The "dirty
snowball" theory cannot account for such activity at
distances where solar energy emissions are so weak that ice
will not melt.
If solar heating were
responsible for cometary discharges at such distances, then
all the frozen moons of Jupiter would be as dry as deserts
and would look more like our own moon than the icy bodies
that they are. If the sun's warmth is not the impetus for
Hale-Bopp's display, then what is it that provides the
energy for supersonic blasts of dust and ice when it is so
far away?
Comets spend most of their time
far from the sun where the charge density is low. Because
comets move slowly, their electric charges reach equilibrium
with the weak, radial solar field. As they get closer to the
sun, however, their nuclei speed into regions of increasing
charge density and varying electrical flux. Their polarity
and charge characteristics respond to the increasing solar
forces, so a coma (charge sheath) forms around them.
Discharge jets flare up and move across the surface very
much like the plumes of Jupiter's moon, Io. If the charge
imbalance becomes too great, the nuclei may explode like an
over-charged capacitor, breaking into fragments or vanishing
forever.
By Stephen Smith
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