%small changes by TEH on 7/9/02, note MB suggests freezer bags work better
\section{Introduction}
Comets represent some of the earliest material left over from the
formation of the solar system, and are therefore of great interest to
planetary astronomers.  They can also be beautiful objects to observe in
the night sky, unlike their darker and less spectacular cousins,
asteroids. They therefore often capture the attention of the public.  The
objective of this lab is to teach you more about these fascinating
objects.

\begin{itemize}
\item{{\it Goals:}  to discuss the composition, components, and types of comets;
to build a comet and test its strength and reaction to light}
\item{{\it Materials:}  freezer bag, bucket, spoon, towel, mallet, light source, a rock, water, sand, ammonia, corn syrup, dry ice}
\end{itemize}
 
\section{Composition and Components of a Comet}

Comets are composed of ices (water ice and other kinds of ices),
gases (water, carbon dioxide, carbon monoxide, hydrogen, hydroxyl,
oxygen, carbon, silicon, and so on), and dust particles.  The dust
particles are smaller than the particles in cigarette smoke.  In
general, the model for a comet's composition is that of a ``dirty
snowball.''

Comets have several components that vary greatly in composition,
size, and brightness.  These components are the following:
\begin{itemize}
\item{{\it nucleus}: made of ice and rock, roughly 5-10 km across}
\item{{\it coma}: the ``head'' of a comet, a large cloud of gas and dust,
roughly 100,000 km in diameter}
\item{{\it gas tail}: straight and wispy; gas in the coma becomes ionized
by sunlight, and gets carried away by the solar wind to form a
straight blueish tail.  The shape of the gas tail is influenced by
the magnetic field in the solar wind.  Gas tails are pointed in the
direction directly opposite the sun, and can extend 10$^{8}$ km.}
\item{{\it dust tail}: dust is pushed outward by the pressure of sunlight
and forms a long, curving tail that has a much more uniform
appearance than the gas tail.  The dust tail is pointed in the
direction directly opposite the comet's direction of motion, and can
also extend 10$^{8}$ km from the nucleus.}
\end{itemize}

These various components of a comet are shown in the diagram, below.
\begin{figure}[h]
\centering
\leavevmode
\epsfysize=0.3\textheight
\epsfbox{COMET/partsofcomet.eps}
\caption{The main components of a comet.}
\end{figure}


\section{Types of Comets}

Comets originate from two primary locations in the solar system.  One
class of comets, called the {\bf long-period comets}, have long orbits
around the sun with periods of $>$ 200 years.  Their orbits are random
in shape and inclination, with comets entering the inner solar system
from all different directions.  These comets are thought to originate
in the {\bf Oort cloud}, a spherical cloud of icy bodies that extends
from $\sim$ 20,000 -- 150,000 AU from the Sun.  Some of these objects
might experience only one close approach to the Sun and then leave the
solar system (and the Sun's gravitational influence) completely.

\begin{figure}[h]
\centering
\leavevmode
\epsfysize=0.3\textheight
\epsfbox{COMET/Oort_cloud.eps}
\caption{The Oort cloud.}
\end{figure}

In contrast, the
{\bf short-period
comets} have periods less than 200 years, and their orbits are all
roughly in the plane of the solar system.  Comet Halley has a 76-year
period, and therefore is considered a short-period comet.  Comets with
orbital periods $<$ 100 years do not get much beyond Pluto's orbit at
their farthest distance from the Sun.
Short-period comets cannot survive many orbits around the Sun before
their ices are all melted away.  It is thought that these comets
originate in the {\bf Kuiper Belt,} a belt of small icy bodies beyond
the large gas giant planets and in the plane of the solar system.
Kuiper Belt objects have only been definitely confirmed to exist in
the last several years.

\begin{figure}[h]
\centering
\leavevmode
\epsfysize=0.3\textheight
\epsfbox{COMET/kuiper.eps}
\caption{The Kuiper Belt.}
\end{figure}

\section{Exercise}

\subsection{Building a Comet}
In this portion of the lab, you will actually build a comet out of
household materials.  These include water, ammonia, corn syrup, and
dry ice (CO$_{2}$ ice).  Be sure to distribute the work evenly among all
members of your group.  Follow these directions: ({\it 12 points})\\
\begin{enumerate}
\item Cut open a freezer bag and use it to line your bucket.
\item Place 1 cup of water in the bucket.
\item Add 2 spoonfuls of sand, stirring well.  ({\bf NOTE:} Do not
stir so hard that you rip the freezer bag lining!!)
\item Add a dash of ammonia.
\item Add a dash of organic material (dark corn syrup).  Stir until
well-mixed.
\item  Your TA will place a block or chunk of dry ice inside a towel and crush 
the block with the mallet and give you some crushed dry ice.
\item  Add 1 cup of crushed dry ice to the bucket, while stirring
vigorously.  ({\bf NOTE:} Do not
stir so hard that you rip the freezer bag!!)
\item  Continue stirring until mixture is almost frozen.
\item  Lift the comet out of the bucket using the plastic liner and shape
it for a few seconds as if you were building a snowball.
\item  Add 1/2 cup water and wait until mixture is frozen.
\item  Unwrap the comet once it is frozen enough to hold its shape.
\end{enumerate}

\subsection{Comets and Light}

Observe the comet as it is sitting on a desk.  Make note of some of
its physical characteristics, for example:
\begin{itemize}
\item{shape}
\item{color}
\item{smell}
\end{itemize}

Now bring the comet over to the light source (overhead projector) and
place it on top.  Observe what happens to the comet.

\subsection{Comet Strength}

Comets, like all objects in the solar system, are held together by
their internal strength.  If they pass too close to a large body, such
as Jupiter, their internal strength is not large enough to compete
with the powerful gravity of the massive body.  In such encounters, a comet
can be broken apart into smaller pieces.  In 1994, we saw evidence of
this when Comet Shoemaker-Levy/9 impacted into Jupiter.  In 1992,
that comet passed very close to Jupiter and was fragmented into
pieces.  Two years later, more than 21 cometary fragments crashed
into Jupiter's atmosphere, creating spectacular (but temporary) ``scars'' on
Jupiter's cloud deck.

\begin{figure}[h]
\centering
\leavevmode
\epsfysize=0.3\textheight
\epsfbox{COMET/sh-levy.eps}
\caption{The Impact of "Fragment K" of Comet Shoemaker-Levy/9 with Jupiter.}
\end{figure}

{\it Question:} Do you think comets have more
or less internal strength than asteroids, which are composed primarily
of rock?  [Hint: If you are playing outside with your friends in a
snow storm, would you rather be hit with a snowball or a rock?]

{\bf Exercise:}  After everyone in your group has carefully examined
your comet, it is time to say goodbye.  Take a sample rock and your
comet, go outside, and drop them both on the sidewalk.  What happened
to each object? ({\it 5 points})

\vspace*{2cm}

\newpage

\section{Questions}

\begin{enumerate}

\item  Draw a comet and label all of its components.  Be sure to
indicate the direction the Sun is in, and the comet's direction of motion.  ({\it 10 points})
\bigspace

\item  What are some differences between long-period and short-period
comets?  Does it make sense that they are two distinct classes of
objects?  Why or why not?  ({\it 10 points})
\bigspace

\item  List some properties of the comet you built.  In particular,
describe its shape, color, smell and weight relative to other common
objects (e.g. tennis ball, regular snow ball, etc.). ({\it 10 points})
\bigspace

\item Describe what happened when you put your comet near the light
source.  Were there localized regions of activity, or did things
happen uniformly to the entire comet?  ({\it 5 points})
\bigspace

\item  If a comet is far away from the Sun and then it draws nearer as
it orbits the Sun, what would you expect to happen?  ({\it 10 points})
\bigspace

\item  Which object do you think has more internal strength, an asteroid or a
comet, and why?  ({\it 8 points})
\bigspace

\end{enumerate}

\section{Summary}

({\it 35 points}) Summarize the important ideas covered in
this lab.  Questions you may want to consider are: 

\begin{itemize}
\item{Why are comets important to planetary astronomers?}
\item{What can they tell us about the solar system?}
\item{What are some components of comets and how are they affected by the Sun?}  \item{How are comets different from asteroids?}
\end{itemize}

\noindent
Use complete sentences, and proofread your summary before handing in the lab.

\section{Extra Credit}

Look up one (or more) of the following current spacecraft missions on the
web and
briefly describe the mission, its scientific objectives, and the significance
of these objectives: ({\it 3 points each})

\begin{itemize}
\item Ulysses (\htmladdnormallink{http://ulysses.jpl.nasa.gov/}{http://ulysses.jpl.nasa.gov})
\item Genesis (\htmladdnormallink{http://genesismission.jpl.nasa.gov/}{http://genesismission.jpl.nasa.gov})
\item Mars Odyssey (\htmladdnormallink{http://mars.jpl.nasa.gov/odyssey}{http://mars.jpl.nasa.gov/odyssey})
\item CONTOUR (\htmladdnormallink{http://www.contour2002.org/}{http://www.contour2002.org})
\end{itemize}

%\end{document}



