A
Visitor from the Oort Cloud
by
Kenny A. Chaffin
All Rights Reserved © 2013 Kenny A. Chaffin
(Note this post was written before the feeble appearance and flyby of Comet ISON, nevertheless it contains some relevant information about comets, the Oort Cloud etc.)
This November you may see the most
spectacular celestial event of your life. Comet ISON (officially known as
C/2012 S1) is due to make its closest approach to the Sun on November 28, 2013
and if indications are correct it will be as bright as the full moon and may even
be visible in daylight with a magnitude of -11.6.
Comet ISON is named after the
Russian observatory - the International Scientific Optical Network (ISON) near
Kislovodsk where it was discovered by Vitali Nevski and Artyom Novichonok on
September 21, 2012. At that time it was still 625 million miles from Earth.
This event may very well be one
they are talking about 1000 years from now long after you and I are gone. But as
with all comets though there is much guesswork involved. If it should breakup
before it gets close to the Sun there may be no sky-show at all. The
spectacularation is also dependent on the comet’s composition which of course varies
from comet to comet. Comet hunter David Levy says, “Comets are like cats, they
have tails, and they do precisely what they want.” We’ll simply have to wait
and see what happens but given the indicators (such as the photographs by the
NASA probe Deep Impact on January 17 and 18) it will be truly awesome. I’m betting
it will outshine my experience of Halley’s Comet which I saw put on a show in
1986.
Comet ISON on April
10, 2013 taken by the Hubble Space Telescope.
Image courtesy of NASA.
Comets are things of imagination,
fascination and lore. With their long luminous tails sweeping across the sky,
their unusual movement against the fixed stars of the night sky and their unpredictable
appearances they were clearly of a fantastical and dream-like nature and have
long been thought of as bad omens. Still the biblical star of Bethlehem may
have been a comet and the 1066 comet over the Battle of Hastings was no less
than the well-known Halley’s Comet itself, perhaps the most famous of all
comets and one that was key in establishing the nature and science of comets. Halley’s
is a short period comet and that was helpful in identifying it as a recurrent
visitor to our inner solar system. By tracking its path and calculating its
orbit Halley in 1705 was able to predict that it would reappear every 76 years
and also confirm many historical observations. It has been reliably recorded since 240 BCE and
will next appear in mid-2061. Odds are not in my favor for being around
barring significant medical breakthroughs in age extension, but hopefully you
will be there to see it.
Long period comets like Comet ISON are somewhat
more difficult to predict as small variations in their paths and changes in
their mass as they approach the sun can result in significant variations in
their predicted return. These long period comets may not return for hundreds or
thousands of years. The difference in short and long period comets is in their
origin and the path they follow around the sun. They each come from different
regions of our solar system. The short period comets originate and travel from
an area known as the Kuiper Belt which is just past Neptune within the outer
reaches of our planetary space. The long period comets like ISON originate in
the Oort Cloud which is vastly further away.
Artists rendering of the Kuiper Belt and Oort Cloud.
Courtesy NASA.
No one has seen the Oort cloud; not
Hubble, not Sagan, not Voyager, not even Halley though the comet named for him
was a large impetus for its conception. There have been ancient and historical
accounts of comets appearing in the skies, the Chinese observed Comet Halley in August and September
of 12 BC. Because comets are so unusual and different with their movement and huge
glowing and streaming tails they were terrifying to the ancients. People would hide
or perform sacrifices or other bizarre behaviors when a comet appeared in the
sky; though that may not be much different than today given the strange cult
behavior during recent comet appearances. Eventually though more scientific
oriented minds began to ask what are they? Where do they come from? These
thoughts led Ernst Opik in 1932 to postulate the existence of a vast cloud of
material surrounding our solar system and for Jan Hendrik Oort to come to the
same conclusion in 1950. The behavior of comets – the ‘boiling away’ of their
mass as they approach the sun and its warmth creates their spectacular tails
and this thought to preclude them being formed near the sun. The hypothesis was
then that they were from some far domain, some cometary cloud that had existed since
the formation of our solar system and only some gravitational perturbation
would cause a comet to begin a journey to the gravity well of our sun. Many of
those journeys would end up with collisions of the comet with planets or a
final plunge into the sun itself. Others on slightly different trajectories
would enter the solar system swing around the gravity well of the sun and be
returned one again to the far reaches of the solar system -- only to return
again. Oort used the trajectories of the comets along with his knowledge of the
solar system and of the cosmology knowledge of the day to describe and
determine the distance, extent and makeup of the speculative cloud that came to
bear his name.
The Oort cloud itself is a
spherical cloud of material surrounding our solar system centered on the Sun. Its outer edge lies approximately one light
year or 50,000 Astronomical Units (1 A.U. = 93 million miles) from the sun. An
additional reason the Oort Cloud is of note is that its outer edge marks the furthest
extent of gravitational influence from the Sun and planets. In attempting to
comprehend this vast distance we might think about the current furthest man-made
objects from the Earth -- the Voyager spacecraft -- currently 123 AU from the
sun well beyond the Kuiper belt and are just entering (as of December 2012) ‘interstellar’
space yet still far from the Oort Cloud. Voyager’s trip so far has taken 35 years
and will require another 10,000 years to reach the Oort Cloud – far longer than
humanities recorded history on Earth.
Since no one has ever seen the Oort
cloud our information about it is based on secondary observations such as those
long-period comets like ISON, simulations and logical inferences about the
formation of the Solar system and we speculate that other solar systems have
Oort clouds as well, but given the density and the difficulty we have with even
detecting planets around other stars detecting an extra-solar Oort cloud is pretty
much out of the question given our current capabilities. We believe the Oort Cloud
consists of two parts, a spherical outer shell some 20,000- 50,000 AU and an
inner doughnut-shaped cloud of 2000- 20,000 AU from the Sun. This inner portion
is called the Hills Cloud for J.G. Hills who proposed its existence in 1981.
Based on the comets we have
observed it is believed that the cloud is comprised of trillions of icy objects
larger than 1 km. The ices are thought to be water, methane, ethane, carbon
monoxide and hydrogen cyanide though some may be rocky asteroid-type objects. All
together it is thought the mass of the Oort cloud is about 5 times the Earth’s
mass – virtually nothing given the volume of space it covers. It is no wonder
we can’t see it! Still a few objects have been detected and are thought to be
part of the Oort Cloud, the most well know and biggest is Sedna discovered in
2003 and thought to be some 1500 km in diameter.
One reason
comets are of interest – other than making disaster movies and creating
end-of-life-as-we-know-it scenarios and speculation is the possibility that
comets were the bearers of water to our planet and possible even the bearers of
life itself or the components thereof. This idea of course has been questioned
and continues to be questioned. Since we have established that comets do
contain a significant amount of water as well as other ices this seems a
reasonable possibility. But it is also just as possible is that the Earth and
comets formed from the same protoplanetary substances and thus both contain
water. This second scenario however does not explain is why we don’t see water
on the other solar system planets and moons – though there may be other
explanations for that. This cometary delivery of water has also been questioned
because of the make-up of the water ice in the comets which we’ve been able to
determine using spectral analysis. Earth’s oceans contain a significant
percentage of ‘heavy’ water - deuterium. Our analysis of comet Halley and comet
Hyakutake, both originating in the Oort cloud give ratios of deuterium to
normal hydrogen that is twice that of Earth’s oceans. Conversely analysis of Comet
Hartley2 and Comet LINEAR (originating in the Kuiper belt) show it has the proper
ratio. It is thought the origin of these comets from different parts of our
solar system accounts for their difference in composition. So there appear to
be at least two measurable differences in the types of comets we see, their
composition and their orbital period – either short or long period comets. This
seems to make sense in that comets from vastly different parts of the solar
system (Kuiper belt or Oort cloud) would have different compositions based on
the way the initial protoplanetary disk originated and formed the objects in our
solar system.
Interestingly then based on
composition it would appear Halley’s Comet actually originated in the Oort cloud
but has through gravitational perturbations become a short period comet now
traveling from the Kuiper belt around the sun while Comet ISON remains a long
period comet and will only be visiting us for a short time later this year and then
beginning its long climb out of the solar well to the far reaches of the Oort
cloud.
As far as ISON, it is still 475
million miles away as of January 18, 2013 but already has a tail more than
40,000 miles long. It is going to be spectacular! Plan your comet watching
party now while there is still time. I’m very much looking forward to it and as
it may be the last really spectacular celestial light show I’ll see and I’d
love for it to outshine my memories of those 60’s and 70’s concert light-shows!
I've got a feeling it just might. Take care and enjoy the show!
References/Links:
Oort Cloud:
Oort Cloud Image:
Kuiper Belt and Oort Cloud:
Comets:
Where did Earth get its water:
Comet ISON:
Comet ISON Image:
About the Author
Kenny A. Chaffin writes
poetry, fiction and nonfiction and has published poems and fiction in Vision Magazine, The Bay Review, Caney
River Reader, WritersHood, Star*Line, MiPo, Melange and Ad Astra and
has published nonfiction in The
Writer, The Electron, Writers Journal and Today’s Family. He grew up in
southern Oklahoma and now lives in Denver, CO where he works hard to make
enough of a living to support two cats, numerous wild birds and a bevy of
squirrels. His poetry collections No
Longer Dressed in Black, The
Poet of Utah Park, The Joy of Science, A Fleeting Existence, a collection of science essays How do we Know, and a memoir of growing up on an Oklahoma farm - Growing
Up Stories are all available at Amazon.com: http://www.amazon.com/-/e/B007S3SMY8. He
may be contacted through his website at http://www.kacweb.com.
He may be contacted through his website at http://www.kacweb.com.
