Comet Halley and the Orionid Meteor Shower.
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Comet Halley and the Orionid Meteor Shower.
By Sig Kutter
Mountain Meadows Circle #917, Ashland, OR 97520.
970-547-9610, skutter@colorado.net.
Oct. 1, 2014.
Introduction:
The Orionids are an annual meteor shower that becomes active in October and early
November. In 2014 the meteor shower will reach its peak during the nights of Oct. 20 and
21; and, due to a new Moon at that time, is predicted to put on a show worth watching,
provided of course that the night sky is clear. The source of the Orionids is Comet Halley,
named after the British astronomer Edmond Halley (1656-1742).
Comet Halley during its last apparition, in 1986.
Credit: The Guardian.
http://www.bing.com/images/search?q=comet+Halley&FORM=HDRSC2#view=detail&id=262B2020A917
602C4E14225DC128B4F7DD9FFA38&selectedIndex=26
But let us back up and introduce the subject. A "meteor shower" is the appearance in the
night sky of a large number of "shooting stars." Shooting stars, called "meteors" by
astronomers,1 are small rocky particles that enter the Earth's atmosphere at high speeds.
Friction with the atmosphere heats them to incandescence, usually vaporizes them, and
temporarily, while they streak across the sky, makes them shine like stars.
At their peaks, meteor showers may consist of up to some dozens to hundreds of shooting
stars per hour; or in rare instances even thousands, in which case one speaks of "meteor
storms." The showers can last from a day or two to several weeks. The source of the meteor
particles is in most cases a comet; though asteroids are known to be sources, too.
Comet Halley and the Orionid Meteor Shower Oct. 1, 2014 Page 1
Sig Kutter
Gemini meteor shower of 2012,
over the Dashanbao Wetlands, China.
Credits: Jeff Dai.
http://sci-universe.tumblr.com/post/69876749083/
geminid-meteor-shower-of-2012-over-dashanbao
Every year, we are treated to about a dozen impressive meteor showers, though the total
number is several times larger. Most showers are named after the constellation in which
they appear in the night sky; but in a few instances the naming references a bright star.
The point in the night sky where the meteors appear is known as the shower's "radiant." For
instance, the radiant of the Gemini meteor shower shown in the above image is located in the
constellation Gemini, which lies beyond the image's upper right-hand margin; and the meteors
themselves are known as the Geminids. The radiant of the meteor showers spawned by
Comet Halley, which is the focus of this article, lies in the constellation Orion; and, as already
mentioned, the meteors are known as the Orionids.
The Structure of Comets:
The solid cores of comets, known as "comet nuclei" or "cometary nuclei," are irregularly shaped
rocky-icy bodies up to several tens of miles in size and left-over debris from the formation of the
Solar System. They have densities of roughly 60% that of water or somewhat less, indicating
that besides rock and water ice, their compositions include low-density frozen gases, such as
those of carbon dioxide (i.e., dry ice), carbon monoxide, and methane. Due to their makeup,
comet nuclei are often referred to as "dirty snowballs" or "icy dirtballs."
Most comet nuclei orbit the Sun at distances well beyond the Sun's planets. But some follow
elongated elliptical paths that carry them through the inner Solar System and close to the Sun,
and then back again to where they came from.
When a comet nucleus gets closer to the Sun than approximately Jupiter, solar radiation
becomes sufficiently strong to sublimate2 the ices in its outer layers, causing the venting of
Comet Halley and the Orionid Meteor Shower Oct. 1, 2014 Page 2
Sig Kutter
gases and fine dust, and allowing particles of rock to break loose. The gases and dust form
a thin and highly extended atmosphere around the nucleus, known as the comet's "coma."
Forces exerted on the coma by the Sun's radiation and solar wind 3 stretch the coma's outer
layers into two long "tails." One of the tails, known as the "gas tail," is straight and points
directly away from the Sun. The other tail, known as the "dust tail," is curved, but also
points in the general direction opposite to that of the Sun.
In the case of some comets, the Sun's illumination of the coma and tails creates the
spectacular displays in the night sky that we typically associate with these celestial visitors,
as for example comets Halley and Hale-Bopp during their apparitions in1986 and 1997 (see
illustrations on p. 1 and below).
Left: Comet nucleus, illustrating the irregular shape of these bodies and
the venting of gases and dust.
Credit: GCSE, Physics. http://www.passmyexams.co.uk/GCSE/physics/comets.html
Right: Comet Halley's gas and dust tails at different positions along its orbit. Both tails
always point away from the Sun; but the gas tail is straight, while the dust tail is curved.
Credits: Why is the Universe in Dynamic Balance?
http://regenerating-universe-theory.org/11)_Why_is_universe_in_Dynamic_Balance.htm
Comet Hale Bopp, the Great Comet
of 1997, with its bright coma and
magnificent gas and dust tails.
Credit: Fred Espenak.
http://www.mreclipse.com/Comets/
HaleBopp.html
Comet Halley and the Orionid Meteor Shower Oct. 1, 2014 Page 3
Sig KutterAdditionally, the rocky particles shed by the nucleus, most of which are smaller than grains
of sand but larger than the dust motes of comets' dust tails, gradually spread out along the
comet's orbit and keep travelling around the Sun. These orbiting particles are called the
comet's "particle trail." When, in the course of its annual orbit around the Sun, the Earth
passes through such a trail, some of the particles are swept up by the terrestrial atmosphere,
get heated to incandescence, and become the shooting stars of a meteor shower.
Comet Halley -- Source of the Orionid Meteor Showers:
The source of the Orionids are rocky particles from Comet Halley's particle trail. The sizes
of the Comet's and the particles' orbits range from about 35 AU 4 from the Sun, which lies
beyond Neptune's orbit, to slightly less than 0.6 AU, which is closer to the Sun than Venus.
The orbits are inclined by nearly 18o relative the Earth's orbit, as illustrated in the diagram
below. Furthermore, the orbital motions of Comet Halley and its particles are retrograde,
meaning that they travel around the Sun in the direction opposite to that of the Earth.
Comet Halley takes 75 to 76 yrs. to complete one orbit. It last appeared in the inner Solar
System in 1986. In 2024 it will reach its greatest distance from the Sun and then "fall" once
again toward the inner Solar System, which it will reach in 2061. I chose the word "fall"
deliberately in order to indicate that the force that keeps Comet Halley and other comets
orbiting is the same as the one that causes an apple to fall from a tree here on Earth, keeps
us bound to the Earth, and keeps the planets orbiting the Sun -- namely, the force of gravity.
Comet Halley's orbit around the Sun and its positions for selected years.
Notice the Comet's retrograde motion compared to that of the planets.
Its last closest approach to the Sun occurred on Feb. 9, 1986.
Credit: Steve R. Majewski, Univ. of Virginia.
http://www.astro.virginia.edu/class/majewski/astr1230/LECTURES/LECTURE8/lecture8E-s13.html
In October and early November of 2014, Earth will pass through Comet Halley's particle
trail at a speed of roughly 150,000 mph. This extreme speed is due to the Comet's
retrograde orbit -- i.e., the Comet's particles and Earth will collide nearly head-on. 5
Consequently, frictional heating of the rock particles, as they enter the Earth's upper
Comet Halley and the Orionid Meteor Shower Oct. 1, 2014 Page 4
Sig Kutteratmosphere, will also be extreme; and they are expected to get unusually bright. Some of
the larger particles may even explode and create so-called "fire balls" or "bolides." Thus,
the prediction that the Orionids are likely to put on a show worth watching seems well
justified.
To conclude: The best time to view the Orionids will be during the early morning hours of
Oct. 20 and 21. At those times, go out to a dark place away from city lights, and find the
constellations Orion and Gemini in the night sky. The Orionids' radiant lies between these
two constellations; and that's where you will find the greatest concentration of meteors.
The radiant of the 2014 Orionid Meteor Shower
and the directions in which the meteors will
appear, shown as a red disk and lines.
Credit: Meteor Blog. http://www.meteorblog.com/
________________________
1
A meteor is a "shooting star" -- i.e., it is rocky particle, whose size may vary from less than that
of a grain of sand to that of a pebble and, in rare cases, even bigger -- passing through the
terrestrial atmosphere, getting heated to incandescence, and temporarily becoming bright like
a star or brighter. While such a rocky particle orbits the Sun, it is called a meteoroid. While it
passes through the atmosphere, it's called a meteor. And if it survives the passage through the
atmosphere and impacts on the ground, it is called a meteorite.
2
Sublimate refers to the change of ice into gas, without passing through the liquid state.
3
The solar wind is a stream of ionized particles ejected by the Sun at about 106 mph that travel
through the Solar System. Solar wind particles interact with the Earth's magnetic field and are
responsible for the aurora borealis and aurora australis (i.e., the Earth's northern and southern
lights).
4
AU stands for "astronomical unit," with 1 AU referring to the average Sun-Earth distance, which is
about 93 mill. miles.
5
Currently, the orbits of Earth and Comet Halley do not intercept. Thus, when Earth passes through
Comet Halley's particle stream in late October, 2014, creating shooting stars, Earth and the Comet
will not collide. This may change in the future due to perturbations of the Comet's orbit by Jupiter
and the other planets; but no one knows for sure.
Comet Halley and the Orionid Meteor Shower Oct. 1, 2014 Page 5
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