SMP3: Comets
Michael Lonergan
Xavier Fu
Tao, Jingwei
Comets have long been the intriguing wonderment of astronomers and its observers,
who saw not only its beauty and elegance, but also the many stories it told. Being the leftover
building blocks of our solar system, they are a gateway to our knowledge of the origin of the
solar system. Having the substances they are composed of, they may yield the key to our
origin. And from the collision effects they are capable of, they can also hold the key to our
extinction. As a comet is viewed in our night sky, astronomers and observers alike cannot help
but become captivated by the power of creation and destruction it holds.
Aside from the Sun, the 8 planets, and Pluto, our solar system also contains comets,
asteroids, meteoroids, meteors, and meteorites. With the amount of debris in the region, it is
important that they be distinguished individually. NASA differentiates the 5 stellar objects as
noted:
Asteroid
Comet
Meteoroid
Meteor
A relatively small, inactive, rocky body orbiting the Sun.
A relatively small, at times active, object whose ices can vaporize in sunlight
forming an atmosphere (coma) of dust and gas and, sometimes, a tail of dust
and/or gas.
A small particle from a comet or asteroid orbiting the Sun.
The light phenomena which results when a meteoroid enters the Earth's
atmosphere and vaporizes; a shooting star.
Meteorite
A meteoroid that survives its passage through the Earth's atmosphere and lands
upon the Earth's surface.
Table obtained from NASA.1
There are two traditional classifications of comets: short period and long period. Short
period comets have periods lower than 200 years and often only about 20 years. They have
orbits comparable to Jupiter’s orbit, and are theorized to be from the Kuiper Belt. Long period
comets have periods of more than 200 years
and have orbit size comparable to Neptune’s
orbit. They are theorized to originate from the
Oort cloud. Nevertheless, our understandings
of the period of the comets are limited, for our
accuracy on the predictions of comets’
whereabouts has only generally been available
for about 200 years. Short period comets’
orbits are inclined to at most 400 to the ecliptic
plane, while long period comets tend to enter
the inner solar system from all directions. As
such, short term comets orbit around the Sun
in the same direction as the Earth’s orbit, while Figure 1: The general characteristics of a comet.
many long term comets move in the opposite
1
NASA, “Frequently Asked Questions”, Near Earth Object Program, Last Updated Nov 25,2007, Accessed Nov 25.
<http://neo.jpl.nasa.gov/faq/#diff>
direction.
Although many comets have passed close enough to be observed, we still do not know
much about them. Most of the information we have presently are mostly from Halley’s Comet,
the first comet to be probed. Due to this, the knowledge astronomers have about comets may
not be accurate with all comets. Nevertheless, the information we have are still a vital source
for future uses.
Properties of Comets
A comet consists of tails, a hydrogen cloud, a coma, and a nucleus (Figure 1). The
temperature of a comet nearing Earth is around 300K. They are composed of around 80%
water, 10% carbon monoxide, 3.5% carbon dioxide, a few percent polymerized formaldehyde
((H2CO)n), and amounts of other substances.
The most magnificent features of a comet are of course its tails, which can reach
lengths of about 1 AU or more. When observed, we can see two tails of the comets, the dust
tail and the plasma tail. The dust tail reflects sunlight, and thus appears yellow. The plasma tail
consists of carbon monoxide ions, which fluoresce in sunlight, causing the tail to be blue.
The dust tail is usually seen as an arc, about 1 to 10 million km long, and is composed
of dust grains that are about the size of smoke particles. The dust particles of the tail is ejected
from the nucleus, with its direction away from the nucleus determined by its ejection velocity,
the Sun’s gravity, the outward push of solar radiation pressure, and the masses of the individual
dust particles.
The plasma tail is usually seen as straight and about 10-100 million km long. The
direction of the tail is away from the Sun. The tail consists of a concentration of a dense and
cold mixture of electrons and molecular ions. It is theorized that the plasma tail originated when
the comet hits the solar wind, causing some of the materials in the coma to be trapped within
the solar wind’s magnetic field. This causes the field to be burdened with more mass, thus
decelerating and wrapping around the nucleus forming the plasma tail. When a comet’s plasma
tail disconnects from its region near the nucleus, it is called a disconnection event. This occurs
when the solar-wind magnetic field strength changes at the heliopheric current sheet, or sector
boundary. During the crossing, adjacent field lines within the tail cross and reconnect,
disconnecting the plasma tail.
Comets also consist of a third tail, which was discovered in Hale-Bopp. This tail is
dominated by sodium. It is long and narrow and ran from the sun through the comet. This tail
originated from radiation pressure of sunlight accelerating the sodium atoms in the comet,
causing the tail to be yellow in color.
Another amazing feature of a comet is the hydrogen cloud that surrounds it. These
clouds span millions of kilometres, and can be larger than the Sun. The hydrogen in the cloud
emit high amounts of ultraviolet radiation in the neighbourhood of the Earth, allowing
astronomers to estimate that comets can produce more than 1029 hydrogen atoms per second,
with an outflow speed of 8 km per second (10 times faster than the materials ejected from the
nucleus). Due to this high speed, it is theorized that the hydrogen cloud originated from the
dissociation of hydroxyl radicals (OH, which can be derived from water) by sunlight.
The coma is a dense spherical envelope of gas and dust surrounding the nucleus of the
comet. The coma gases flows outward from the nucleus at speeds of 0.5 to 1 km per second,
dragging dust into space, and span 100,000 to 1,000,000 km from the nucleus. The coma
cannot be observed until it is 3 AU away from the Sun, where the water ice of the comet
sublimates. This outflow of water and dust from the comet causes a comet to lose about 0.1 to
1% of its mass per passage through the inner solar system. However, this loss of comet
materials only spreads out to about one dust particle per cubic centimetre of the coma.
Since comets are surrounded by the vast amount of dust and water particles variants, it
is hard to view and study the heart of a comet, its nucleus. To date, we have only got
evidence and view of Halley’s nucleus. However, Halley’s nucleus is not ideal for every comet,
so other comets’ nuclei are approximated by calculations and assumptions. In simplicity, the
comet’s nucleus is a very black, potato shaped lump of snow and dust, about 1 to 10km in size.
As a comet nears the inner solar system, sunlight heats the nucleus, allowing the dust and ice
to sublimate. A dusty crust then forms, which insulates the nucleus and regulates the
sublimation. The materials in a comet’s nucleus are not constant, thus causing the sublimation
to occur faster in some areas, and thus causing the irregularity of the nucleus’ surface features.
The masses of comets were usually estimated by assuming a density of 1g/cm3.
However, the discovery of Halley Comet’s nucleus led to the implication that its density was
0.25g/cm3, which led to the fact that the nucleus must be porous and fragile. The recent
viewings of the fragmentation of comets (ex. Shoemaker-Levy, the comet that collided with
Jupiter) supports this implication, but does not justify it, for the reasons for the fragmentation is
still not understood. Nevertheless, if comets are porous and fragile and can split up, it implies
that the comet is made up of collections of smaller, loosely bounded building blocks.
Kuiper Belt and Oort Cloud
The Kuiper belt lies roughly at 35AU to 50AU from sun, just outside of Neptune’s orbit.
It is composed of small objects that are remnants of the protoplanetary disk; those that was not
dense enough to fall and compose a planet. Due to the belt’s closure to Neptune, Kuiper Belt
Objects (KBOs) in orbits between 35AU and 40AU are greatly influenced by Neptune’s
gravitational forces. This gravitational influence can cause the object to move toward the inner
solar system. If the object can move into Neptune’s orbit, it becomes a so-called Centaur
planet. Due to the great influence of the nearby large planets’ gravitational force, the orbit of a
Centaur planet can become a total chaos. If the change of the Centaur planet orbit causes it to
feel Jupiter’s gravitational force, there is a possibility that it will move further toward the Sun
and become a short period comet. However, there is also the possibility for the planet to be
scattered away into the Scattered Disk which has a high eccentricity.
A scattered disk object is an object with perihelion no smaller than 35AU. The formation
and construction of a scattered disk is still not clear but it is highly possible that the objects in it
were originally KBOs that got their orbit reshaped due to the gravitational force of Neptune.
Oort cloud occupies the outside edge of Kuiper belt (around 50AU) to almost 50,000AU
from the Sun. It can be divided into a spherical outer Oort cloud ranging from 20,000AU to
50,000AU and inner Oort cloud from 50AU to 20,000AU.
Objects in the Oort cloud are believed to originally form closer to the Sun than the
KBOs. They are believed to be the remnants of the protoplanetary disk inside Jupiter’s orbit
when the solar system formed. The gravitational intersection with huge gas planets like Jupiter
may result in an extremely ecliptic orbit. At the distance of over 20,000AU, the influence of the
nearby stars may further modify the shape of the orbit and lead to a spherical shape.
Halley’s Comet
The most famous periodic comet is known as Halley’s Comet, officially designated as
1P/Halley. Halley is classified as a short period comet and is the only short period comet that is
visible with the naked eye and certain to return in a human lifetime. Its orbital period is about
75.3a, it is seen every 75-76 years. The reason for its varied time for its approach to the sun is
caused from the gravitational pull from the major planets in the solar system. Halley has a
highly elliptical orbit which is focused on the Sun. Its aphelion distance is about 35.1AU
(roughly as far as Pluto) and its perihelion distance is about 0.586AU (between the orbits of
Mercury and Venus). Halley has an inclination of about 162.3°. Last time Halley appeared in the
inner solar system was in 1986 and its next approach is predicted for 2061. The 1986 passage
of Halley was least favourable for Earth observers in history, but we learnt the most of comets
in general because of space travel. Multiple spacecraft were sent to Halley in the mid-1980s.
The first ever pictures of a comets nucleus were taken by a Soviet probe.
Halley is thought to have been originally formed in the Oort cloud and captured by the
giant planets which sent it into the inner solar system. Its coma extends for millions of
kilometers into space but its actual nucleus is relatively small (barely 15km long, 8km wide, 8km
thick). It has an extremely low mass of about 2.2 x 1014kg and an average density of about
0.6g/cm3. Halley is about 4% albedo (ability of an object to reflect light). This means that only
about 4% of the light that hits it is reflected, which is similar to a piece of coal. As a result,
Halley is pitch black and is one of the darkest objects in our solar system.
As Halley approaches the inner solar system, the Sun warms it causing it to sublimate,
change directly from a solid to a gas, and jets of volatile material to burst from its dark surface.
Halley reaches a temperature of about 77°C when closest to the Sun. Several tons of gas and
dust are emitted each second in these jets. The gases ejected from the nucleus are consisted of
about 80% water vapour, 17% carbon monoxide, 3-4% carbon dioxide with traces of hydro
carbons.
Comet Hyakutake
Hyakutake is also known as “The Great Comet of 1996”, but it is officially designated as
C/1996B2. Hyakutake was discovered on January 30 1996. Hyakutake was so bright in the night
sky when it reached its perihelion, that it was seen worldwide! Hyakutake is a long period
comet with an orbital period of about 75,000 years. Its orbital period used to be about 15,000
years, but it has now increased significantly due to gravitational influences from the giant
planets. Hyakutake has an aphelion distance of about 4698.77AU and a perihelion distance of
about 0.230204AU (which is well inside the Mercury orbit). It was one of the closest approaches
by a comet in the previous 200 years. The comet was just 0.1AU away from the Earth on March
25 1996. Its tail length was reported to have stretched to about 80° across the sky near
perihelion.
Terrestrial observers found roughly equal amounts of ethane and methane in the comet,
which indicates the first time these gases were discovered in a comet. Another odd discovery
was x-ray emissions were detected from the comet, which was a first. These x-rays were
caused by ionized solar wind particles interacting with neutral atoms in the coma. The cause of
x-ray emission could also have been a part of cometary material evaporating from the nucleus.
Hyakutake’s nucleus is about 2km across, surrounded by a flurry of pebble-sized
particles ejected at a few meters per second. This is due to the fact that Hyakutake has a very
active surface. It also has a rotation period of about 6.23 hours. Despite its relatively small
nucleus, it has the largest known comet tail. Ulysses spacecraft unexpectedly crossed the tail
500 million km from the nucleus. This was detected because the spacecraft detected a large
drop of protons passing, and as well as a change in the direction and strength of the local
magnitude field.
Comet Hale-Bopp
The most widely observed comet of the twentieth century is known as Hale-Bopp,
formally designated C/199501. It was visible to the naked eye for a record 18 months. The use
of the internet at this time played a huge role in Hale-Bopp’s public interest. At its perihelion,
Hale-Bopp was shown brighter than any other star in the night sky (except for Sirius). Its tail
stretched to about 30-40 degrees. Its aphelion distance is about 371AU and its perihelion
distance is about 0.91AU with an orbital period of about 2537a.
Hale-Bopp has an inclination to the ecliptic of about 89.4°. As a result, during its orbit
Hale-Bopp rarely passes any of the planets. However, in March 1996, Hale-Bopp came within
0.77AU of Jupiter which shorten its orbit considerably by gravity. A third type of tail was
discovered with Hale-Bopp, known as the sodium tail. This tail consisted of neutral atom and
extended 50 million km in length. Hale-Bopp’s rotation period is about 11 hours and 46
minutes.
Hale-Bopp set many records for comets. Hale-Bopp was the furthest from the Sun when
discovered. It has the largest nucleus of any other comet known. It was also visible to the
naked eye the longest.
Comet Holmes
The strangest comet in a long time is still visible to the naked eye as of now. This comet
is known as Holmes’ comet, or officially designated as 17P/Holmes. Holmes is a periodic comet
within our solar system. It has an aphelion distance of about 5.18AU and a perihelion distance
of about 2.05Au. Its orbital period is about 6.88 years, making it a short period comet. Holmes
has an inclination of about 19.11°. Its next perihelion is predicted for March in 2014. Its nucleus
is about 3.4 kilometers.
The reason why it is one of the strangest comets is the fact that it had the largest
outburst by a comet in history. In October of this year, Holmes went from magnitude of about
17 to magnitude 2.8. This is an increase in magnitude of about a million times. The true
diameter of the coma swelled over one million kilometers, larger than the sun. The reason for
the outburst is not completely understood. It is theorized to have been caused by a collision
with a meteorite or by a build up of gas inside the nucleus, which broke through to the surface.
Holmes has undergone this type of event before. In 1892, when it was first discovered, Edwin
Holmes observed the comet go through a similar outburst.
References
1) Beatty, J.K, Chaikin, A. & Petersen, C.C. The New Solar System. 4th Ed. Sky Publishing. 1999,
Cambridge.
2) Frei, P. & Mayor, M. New Worlds in the Cosmos: The Discovery of Exoplanets. Cambridge. 2003.
New York.
3) NASA, “Frequently Asked Questions”, Near Earth Object Program, Last Updated Nov 25,2007,
Accessed Nov 25.< http://neo.jpl.nasa.gov/faq/#diff>
4) Wikipedia, “Comets”, Wikipedia, Last Updated Nov 23, 2007, Accessed Nov 25.
< http://en.wikipedia.org/wiki/Comets>
5) Wikipedia, “Kuiper Belt”, Wikipedia, Last Updated Nov 21, 2007, Accessed Nov 25.
< http://en.wikipedia.org/wiki/Kuiper_belt.>
6) Wikipedia, “Oort Clouds”, Wikipedia, Last Updated Nov 21, 2007, Accessed Nov 25.
<http://en.wikipedia.org/wiki/Oort_cloud.>
7) Wikipedia, “Halley’s Comet”, Wikipedia, Last Updated Nov 25, 2007, Accessed Nov 25.
< http://en.wikipedia.org/wiki/Halley%27s_Comet>
8) Wikipedia, “Comet Hyakutake”, Wikipedia, Last Updated Nov 16, 2007, Accessed Nov 25.
< http://en.wikipedia.org/wiki/Hyakutake>
9) Wikipedia, “Comet Hale-Bopp”, Wikipedia, Last Updated Nov 22, 2007, Accessed Nov 25.
< http://en.wikipedia.org/wiki/Hale-bopp>
10) Wikipedia, “17P/Holmes”, Wikipedia, Last Updated Nov 23, 2007, Accessed Nov 25.
< http://en.wikipedia.org/wiki/Holmes_comet>