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\begin{Large}
\section{\bf Building a Comet}
\end{Large}

\subsection{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 are also beautiful objects to observe in
the night sky, unlike their darker and less spectacular cousins,
asteroids, and therefore 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:}  ziplock bag, bucket, spoon, towel, mallet, light source,
 water, sand, ammonia, potting soil, dry ice, gloves}
\end{itemize}

\subsection{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.''

\noindent 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 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 opposite the comet's direction of motion, and can
also extend 10$^{8}$ km from the nucleus.}
\end{itemize}

\subsection{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.\\
\\
\noindent 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]
\centerline{{\includegraphics[width=12cm]{kuiper3.jpg}}}
\caption{Different origins for long and short period comets.  Image courtesy of NASA.} 
\end{figure}

\subsection{Exercises}

\subsubsection{Building a Comet}
In this portion of the lab, you will actually build a comet out of
household materials.  These include water, ammonia, potting soil, and
dry ice (CO$_{2}$ ice).  Be sure to distribute the work evenly among all
members of your group.  Follow these directions:\\
\begin{enumerate}
\item Put the ziplock bag in your bucket.
\item Place 1 cup of water in the bucket.
\item Add 2 spoonfuls of sand, mix well.  ({\bf NOTE:} Do not
stir so hard that you rip the bag!!)
\item Add a dash of ammonia.
\item Add a dash of organic material (potting soil).  Stir until
well-mixed.
\item  The TA will place a block or chunk of dry ice inside a towel and crush 
the block with the mallet.
\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 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 water slowly, and as needed, to get the mixture frozen into a ball.
\item  Unwrap the comet once it is frozen enough to hold its shape.
\end{enumerate}

\subsubsection{Comets and Light}

Observe the comet as it is sitting on a desk.  Make note of some of
its physical characteristics, for example: ({\bf 5 points})
\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 and record what happens to the comet.  ({\bf 5 points})

\subsubsection{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 that case, 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.\\
\\
\noindent 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? ({\bf 5 points})
\vspace{1.5in}

\subsection{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.  ({\bf 10 points})
\vspace{2.5in}

\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?  ({\bf 10 points})
\vspace{1.5in}

\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.). ({\bf 15 points})
\vspace{1.5in}

\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?  ({\bf 10 points})
\vspace{1.5in}

\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?  ({\bf 10 points})
\vspace{1.5in}

\item  Do you think comets (``dirty snowballs'') 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 10 points})
\vspace{1.5in}

\end{enumerate}
\vspace{8in}
~~~\\

\newpage

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\subsection{Take-Home Summary}

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, and
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
{\bf Type this summary.}  Use complete sentences, and proofread your summary before handing in the lab. {\bf (30 points)}

\subsection{Extra Credit}

Use the internet to look up one (or more) of the following current or planned spacecraft missions to comets and briefly describe the mission, its scientific objectives, and the significance of these objectives.  {\bf DO NOT copy the information from the web sites; put it into your own words!  (3 points each)}

\begin{itemize}
\item Stardust ({\tt http://stardust.jpl.nasa.gov/})
\item Deep Impact ({\tt http://deepimpact.umd.edu/home/index.html})
\item Rosetta ({\tt http://sci.esa.int/science-e/www/area/index.cfm?fareaid=13})
\item CONTOUR ({\tt http://discovery.nasa.gov/contour.html})
\end{itemize}

\vspace{8in}
~~~\\

