MCC PHS 142 M01 Astronomy Homework Ch.14-15      
Adj Prof Astronomy: Sam Wormley <>

Background Material

  Textbook - Read Chapters 14-15
  Textbook -
  Textbook -
      (take the Multiple Choice Quiz for for each chapter)

  Web - 
  Web - 
  Web - 
  Web - 
  Web - 

      Comet Hyakutake by Jim Bonser

Jim Bonser, a local amateur astronomer, made this images of comet
Hyakutake on April 9, 1996 using a 50mm lens at f2.8 and kodak Royal
Gold 1000 print film. Too bad the color is not reproduced here. A comet
is basically a ball of ice and dust. The typical comet is less than 10
kilometers across. Most of their time is spent frozen solid in the
outer reaches of our solar system. The graphic below shows all of the
components of a comet. At the stage being discussed at this point, the
comet is nothing more than the nucleus. Except for a few suspected dead
comets, and a couple of suspicious asteroids that occasionally show gas
emissions like a comet, the nucleus is never really seen from Earth. By
the time a comet becomes bright enough to be seen from our planet, it
is usually exhibiting a coma. 

The Comet Nucleus 

Do look at:

After the spacecraft Giotto photographed the nucleus of Halley's comet
back in 1986, we now know that a comet's nucleus probably has a surface
that is best described as a black crust. Although the length of the
nucleus of Halley's comet is about 12km, it is believed that comet
nuclei can range from 1 km to perhaps 50km across. Comet Hale-Bopp of
1997 had a nucleus that was perhaps 40km across.

The black crust of the nucleus helps the comet absorb heat, which in
turn causes some of the ices under the crust to turn to a gas. With
pressure now building beneath the crust, the serene, but frozen
landscape begins to bulge in places. Eventually the weakest areas of
the crust shatter from the pressure beneath, and the gas shoots outward
like a geyser and is referred to by astronomers as a jet. Any dust that
had been mixed in with the gas is thrown out as well. As more and more
jets appear, a tenuous gas and dust shell forms around the nucleus and
this is called the coma.

The Coma 

Comets can typically display a coma several thousand kilometers in
diameter, with the size being dependent on the comet's distance from
the sun and the size of the nucleus. The latter is important because
since jets generally spring up on the side of the nucleus facing the
sun (that side gets warmest), and since large nuclei have a greater
surface area facing the Sun, then there is the potential for larger
numbers of jets and greater amounts of gas and dust feeding the coma.
One of the largest comets in history was the Great Comet of 1811. It
was one of the few comets in history to be discovered with a relatively
small telescope at an unusually great distance from the sun, in this
case over half-way to the planet Jupiter's orbit. The nucleus has been
estimated as between 30 and 40 kilometers in diameter. At one point
during September to October 1811 the coma reached a diameter roughly
equivalent to the diameter of the sun and was a very notable naked-eye
object seen by people around the world. Even though the coma can become
quite large, its size can actually decrease about the time it crosses
the orbit of Mars. At this distance the particles streaming out from
the Sun provide enough force so as to act as a wind and will literally
blow the gas and dust particles away from the nucleus and coma. This
disruption is the process responsible for a comet's tail, the most
spectacular feature of a comet. 

The Tail 

When you have a large comet that moves well inside the orbit of Earth,
you have the potential for a long tail. The current record holder for
longest tail length is the Great Comet of 1843. Its tail extended more
than 250 million kilometers. What this means is that if the comet s
nucleus were placed in the center of the sun the tail would have
stretched passed the orbits of Mercury, Venus, Earth, and Mars! 

Where Do Comets Come From?

Our solar system began as a vast cloud of gas and dust. Several billion
years ago, this cloud slowly rotated around our very young sun and
particles within the cloud collided with one another. During this time
some objects were obliterated by these collisions, while others grew in
size and were to later become the planets. 

Throughout this early period, comets probably filled the solar system.
Their collisions with the early planets played a major part in the
growth and evolution of each planet. The ices that make up comets
appear to have been the very building blocks that formed the early
atmospheres of the planets, and scientists now very strongly believe
that it was the collisions of comets that brought water to our world
and enabled life to begin. 

Over the years, comets actually became rarer within our solar system.
They no longer fill the skies as they did 4 billion years ago, and
today a prominent naked-eye comet can be expected only about once a
decade. Astronomers with powerful telescopes can see many more comets,
but even in this case it is still rare for as many as 15 or 20 comets
to be detectable in the sky at any one time. 

Today, most comets are located outside our solar system in part of the
original cloud of dust and gas that has remained virtually untouched
for billions of years. These regions are referred to as the Oort Cloud
and the Kuiper Belt. The Oort Cloud was first theorized by the Dutch
astronomer Jan Oort in 1950. His investigation of the orbits of comets
with very long orbital periods brought him to conclude that a large
"cloud" of comets existed far outside the solar system, possibly within
the range of 5-8 trillion kilometers (or more) from the sun. The total
number of comets within this belt was estimated as a trillion. It is
thought that objects within this cloud are occasionally ejected either
by collision with one another, or by the gravitational forces of
stars.  Many of the ejected objects probably never cross the paths of
the planets, and still more do not come close enough to be seen with
even the largest telescopes. However, a few do manage to travel into
the inner solar system and are subsequently seen from Earth. This cloud
remains a theory only, as it has never been directly detected. 

The Kuiper Belt is a region first theorized by the Dutch-American
astronomer Gerard Kuiper in 1951. Seeing that Oort's cloud of comets
did not adequately account for the population of comets with short
orbital periods (making complete orbits around the sun in less than 200
years), Kuiper conjectured that a belt of comets probably existed
outside the orbit of Neptune within the range of 30 to 50 astronomical
units (2.8 to 4.6 billion miles) from the Sun. Collisions and
perturbations by the planets of our solar system are believed to be the
reasons for the ejection of bodies from this belt. Around 1988,
astronomers David Jewitt (University of Hawaii) and Jane Luu
(University of California at Berkeley) began searching for members of
the Kuiper belt using modern electronic cameras attached to a large
telescope on Mauna Kea, Hawaii. The equipment was capable of detecting
extremely faint objects. After nearly 5 years of systematic searching
they found a distinct image on 1992 August 30, which was subsequently
designated 1992 QB1. The object was moving very slowly, and
calculations eventually revealed the object took 291 years to orbit the
sun at an average distance of 43 AU. Since, the discovery of that
object over three dozen additional objects had been found as of the end
of 1996.

Meteors - The Leonids: A Celestial Lottery With a Potential Huge Payoff

Traveling in parallel, the meteors appear to us to radiate from a spot
in the constellation Leo the Lion. Note the Sickle just above center.
It resembles a backward question mark and represents the Lion's mane. 
1998 Shigemi Numazawa, Japan Planetarium Laboratory. November will mark
the return of cool weather, colorful foliage, ripe apples... and the
debris trails of the Leonid meteor shower. The chance of seeing a
spectacular 1,000-meteor-per-hour outburst is slim in most parts of the
world, but the possibility is there. It's sort of a celestial lottery.
A meteor is a momentary streak of light in the sky caused by a piece of
cosmic flotsam burning up as it plows into Earth's atmosphere at high
speed. The doomed object itself, a meteoroid, is typically a piece of
rock or clod of dust between the size of a sand grain and a pebble. On
any clear, moonless night, a skygazer far from city lights will see at
least a few sporadic meteors. But at certain times of the year we
encounter swarms of meteoroids that have boiled off of comets and now
litter their orbits. When we do, tens or hundreds of meteors can be
observed over just a few hours. This is a meteor shower.

Every year around November 17th Earth's orbit around the Sun carries us
though the Leonid meteor stream, which originates from Comet
55P/Tempel-Tuttle. In most years we encounter relatively sparse parts
of the stream, and the Leonid shower (so named because the meteors
appear to come from the direction of the constellation Leo) is not
particularly impressive. But a thin, dense ribbon of cometary debris
accompanies Tempel-Tuttle itself, which swings in a highly elongated
orbit around the Sun every 33 years. When our planet happens to pass
through the debris trail shortly before or after the comet has gone by,
we plunge right through this rich concentration of meteoroids, and the
normal Leonid drizzle can be replaced by a torrential meteor storm in
which thousands of shooting stars might flash overhead every minute.

The last great Leonid meteor storm took place in 1966, one
Tempel-Tuttle orbit ago. The comet passed through the inner solar
system most recently earlier this year, so expectations are high that a
Leonid storm will break out this year or next. For the last 1,000 years
the return of Comet Tempel-Tuttle every 33 years or so has often
signaled a better-than-usual November meteor shower, and the Leonids
have indeed been picking up strength in the last few years. At the very
least, therefore, skywatchers anywhere in the world with clear weather
and a dark sky free of light pollution can reasonably expect to see a
nice shower on the mornings of November 17th and 18th, with rates of
perhaps a meteor per minute. The Moon is at Third Quarter on November
18th and will rise around midnight on the November 17-18 in the
constellation Leo. 

How to Watch 

Meteor watching is easy. You don't need (and shouldn't use) binoculars
or a telescope -- just your eyes. Plan to start your Leonid watch
around 1 or 2 a.m. local time on November 17th and 18th. By then the
shower's radiant (apparent point of origin) in Leo is getting fairly
well up in the east. The higher the radiant, the more meteors appear
all over the sky. The hour or two before dawn should be best of all, so
plan to get up very early, rather than to stay up very late. 

Meteor vigils under a clear sky can get surprisingly cold, so it's wise
to dress for the iciest depths of winter. Bring a reclining lawn chair
to a dark site with an open view of the sky. No trees or buildings
should intrude into your view except maybe at the very edges. Watch the
darkest part of the sky at least 50 up from the horizon; you don't
need to look in the direction of the shower's radiant. If your field of
vision remains free of trees and clouds, a meteor should catch your eye
from time to time. If its path, extended far enough backward, would
cross the Sickle of Leo, you've seen a Leonid. If not, you ve seen a
sporadic meteor. The darker your site and the longer you stay outside,
the more meteors you ll see. 

Whatever you do, set that alarm clock for the mornings of November 17th
and 18th! If you miss the Leonids this time around, don't count on
another chance 33 years hence. The years 1998 through 2003 are probably
the last for several more 33-year cycles when a Leonid storm is even
possible. A gravitational perturbation by Jupiter in 2029 will tug
Comet Tempel-Tuttle's orbit away from Earth, and not until 2098 or
perhaps 2131 will hopes for a Leonid storm again be justified.

The Bigger Picture of Solar System Debris

Not only does dust indicate the existence of asteroids and comets, it
can reveal planets.  In our solar system the gravity of the giant
planets is thought to create paterns in the Kuiper belt dust. An
outside viewer would see gaps and clumps in an infrared image [insert]
and deduce the presence of a giant planet. This simulated view is
similar to what astronomers have seen around other stars.

Homework Problems

Note the answers to the odd (Conceptual Questions, Problems and
Figure-Based Questions) are in the back of your textbook. It is
strongly suggested that you do some of those in every chapter so you
have immediate feedback as how well you are understanding the material.
There are online multiple choice quizzes for each chapter of your
textbook. Goto then click on

  Your book
  Student Edition
  Choose a chapter
  Multiple Choice Quiz
You are expected to do all of your own homework. Statistical patterns
showing copying or collaboration will result in no credit for the
homework assignment for all participants involved. The Code of Academic
Conduct for Iowa Valley Community College District is found in the
Student Handbook.

Physical Science classes require the use of mathematics. If you don't
know algebra, you sould NOT be taking this class. If you need to review,
look at Introduction to Algebra
WolframAlpha is way faster than a scientific calculator.

There is little excuse for turning homework in late. You have a whole
week between classes to read the chapters and do the homework. Homework
one week late - half credit. Two or more weeks late - no credit. Do the
homework during the week, not in class! You got homework questions,
email me 24/7.  Even if you don't have a homework 
question, email me anyway!

Problem 1: 
Using your Planisphere (starwheel), in what constellation is the Moon
at 9pm on your next birthday? In order to solve this problem, you
need the Right Ascension and Declination of the Moon. You could look up
the data in the Astronomical Almanac. Another way is to go to the
internet, such as:

Read the Right Ascension and Declination of the Moon and note where the
Moon is on your birthday.
Problem 2: 
Using your Planisphere (starwheel), determine what time Castor and
Pollux are overhead on your birthday.

Problem 3:
The density of Io is 3600 kg/m3 while that of Callisto is
1900 kg/m3. What do these densities tell us about the relative
amounts of rock and ice in Io and Callisto? 

Problem 4: 
Both the Moon and Triton raise tidal bulges on the planets they orbit.
Why does the tidal bulge on Neptune cause Triton to spiral inward
while the tidal bulge on the Earth causes the Moon to spiral outward?

Problem 5: 
Tell what happens to meteoroids (objects that become meteors) with
diameters of:
   (a) 10 m 
   (b)  1 cm 
   (c)  1 m  
   (d) 10 m  
beginning from the time they enter the Earth's atmosphere.

Problem 6: 
What is the relationship between meteors and asteroids?

Problem 7: 
What is the relationship between meteor showers and comets?

Problem 8: 
What is the orbital period of one of the Trojan asteroids?

Problem 9:  Assume that an Oort cloud comet has an
orbital period of 5 million years. What is its average distance from
the Sun? Hint: use Kepler's 3rd Law.