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Q: Comet orbit questions. Why does comet go around the sun instead of into the sun? ( Answered,   6 Comments )
Subject: Comet orbit questions. Why does comet go around the sun instead of into the sun?
Category: Science > Astronomy
Asked by: bravestdog-ga
List Price: $2.00
Posted: 27 May 2004 19:33 PDT
Expires: 26 Jun 2004 19:33 PDT
Question ID: 352996
Why does a comet go around the sun in a nice smooth trajectory instead
of going into the center of the sun?

When a comet goes out into space after orbiting the sun, how far
(planet wise) does it go and what happens when it reaches it's
farthest distance?

Does it come to a standstill like throwing a ball in the air and start
falling back towards the sun or does it continue to travel at 20,000
mph...? and at the end of it's farthest point from the sun it then
starts moving towards the sun again?

Is it's trajectory speed constant or does it change? Alot, a little...?

I've heard the long journey comets trajectories are cigar shaped and
small journey comets are rounder...?

I'm just trying to get a rough idea of how it travels, why it doesn't
crash into the center of the sun, what it's trajectory is like. Does
the speed change or only the travel direction...?

Do comets have orbits that are in different planes compared to the
planets and is this because they come from the ort cloud that
surrounds our solar system...?

Where did the water come from that formed the dirty snowball? Is it
pure water? Did it come from rain?

Please add anything relative to this topic.

If anyone else wants to add to this topic, please do so.


Clarification of Question by bravestdog-ga on 29 May 2004 10:41 PDT
Thanks for all your responses.

Could anyone answer "Why doesn't the comet go directly into the center
of the sun instead of around the outside of it?"

If the suns gravity is attracting the comet then why doesn't the comet
go into the center of the mass of the sun?

Why does the comet go around the sun instead of into the center of the sun?

And if anyone knows of any astronomy forums where I could post these
questions, that would be helpful.

Subject: Re: Comet orbit questions. Why does comet go around the sun instead of into the sun?
Answered By: siliconsamurai-ga on 31 May 2004 14:57 PDT
Your initial question basically required the entire mechanics 101
course in physics but your more limited request for an explanation
about why an object with a specific velocity (speed and direction, not
just speed) avoids colliding with the center of the controlling
gravitational force (or, in Relativistic terms, takes a world line not
intersecting with the body) is answerable for your price.

I am presuming you have no knowledge of physics or you wouldn?t ask
this so I will make this as easy to understand as I can with an
example rather than giving the full answer which is, by necessity,

The first thing to understand is that to a physicist, a comet is
simply another object with a certain mass, there?s nothing special to
differentiate it from a satellite, or any random piece of space junk.

The basic situation is easily understood using this simple experiment. 

Take an object on a string or rope and whirl it around your head ? the
object is being impelled away from your hand while being constrained
to an ?orbit? by the string.

This is an example of the exact opposite of your question where the
force impels the object in orbit to move toward the larger object but
is otherwise very similar ? there is a force on the string caused by
the object being swung which, by Newton?s first law, is trying its
hardest to go in a straight line.

Now substitute an elastic bungee cord or a rubber band for the string
? if you swing it faster the object will try to move further away,
stretching the band ? move slower and it will tend to come a lot
closer to your head. Stop spinning entirely and it will just dangle
from the restraining cord ? the equivalent of hitting the sun.

That demonstration holds the core answer to your question ? the comet
doesn?t strike the center of the sun because it is moving quickly
enough and in the correct direction to avoid this fate.

Any comets which were going just as fast as the one in question but
which were aimed at the sun rather than away from its center would
have hit the sun and are no longer available as examples.

I?ll take a quick shot at another of your original questions ? no, the
water didn?t come from rain on this planet, but many asteroids and
comets are thought to have come from a planet which used to orbit
between Mars and Jupiter and which broke up in the past. Anytime you
have hydrogen and oxygen in close proximity then you are very likely
to see them combine into their only stable compound, H2O, or water
(H2O2, or hydrogen peroxide isn?t stable.)

A mathematical explanation based on simplistic Newtonian physics is found at: (Why do
objects stay in orbit?) but I feel my simple experiment probably is
what you were looking for ? a way to understand the science on an
intuitive level.

Einstein explained things differently but the results are the same and
explaining relativistic mechanics is far beyond the scope of this
answer ? I am fully prepared to do so but the cost of such an answer
would be considerably higher. An episode of Fox TV?s The Simpsons
actually gave an excellent visual explanation.

Google search term:
Newton law

Why do object orbit

The Oracle ThinkQuest answer is at the top of the search: (Why do
objects stay in orbit?)

I hope this was of help and I hope you continue to explore this
fascinating field of physics.
Subject: Re: Comet orbit questions. Why does comet go around the sun instead of into the sun?
From: aht-ga on 27 May 2004 21:40 PDT

You have asked for a lot of information and research here, so you may
want to consider upping your list price to match the amount of effort
that a Researcher would have to put into providing you with a useful

In the meantime, you might like this interactive demonstration of a comet's orbit:
Subject: Re: Comet orbit questions. Why does comet go around the sun instead of into the
From: rnt20-ga on 28 May 2004 02:26 PDT
Comets follow orbits around the sun, just like the planets and
asteroids in our Solar System (and lately a few interplanetary space
probes too!). Orbits are very simple -- in fact every time you throw a
ball you are essentially putting it into orbit around the centre of
the Earth, it's just that the orbit happens to intersect the surface
of the Earth (or the person you're throwing the ball at) so it doesn't
get very far on its orbit. Orbits are always eliptical (assuming the
body they are orbiting is spherical). In everyday physics we
approximate the path of a ball to be a parabola -- in this case we are
ignoring the curvature of the Earth which would turn the parabolic
path into an ellipse.

Imagine you are floating in space a long way above the Earth, and
throw a ball in a direction perpendicular to the direction of the
Earth (i.e. not towards or away from the Earth, but sideways). The
ball wouldn't go in a straight line, because the gravity of the Earth
would bend the path of the ball, like this:

             ----__\ (ball)

    /  \
    \__/ (Earth)

The ball would miss the Earth because you had thrown it sideways. The
path of the ball would carry on curving due to the gravity of the
Earth until it was beside the Earth in this diagram:

    __                          \
   /  \                          |
   \__/ (Earth)                 \|/ (ball)

The gravity of the Earth keeps bending the path of the ball, but
because the ball is moving sideways the ball keeps missing the Earth.
This is called an orbit, and it's what the International Space Station
is doing all the time.

In the same way, comets keep orbiting the sun. The shape of the orbit
depends on how fast an object was thrown to start with and where it
was thrown from. All orbits are elliptical. The orbits of the planets
around the sun are *almost* circular, except for Pluto.

If you tried to fire a ball really fast at the sun from the outer
Solar System, it would be really very difficult to hit the sun
(because it would be so far away, and look so small). If you missed
the sun (which you probably would if you fired the ball fast) the ball
would whizz past the sun with its path bent by the Sun's gravity so
that the ball went round the back of the sun and headed straight back
towards you. This would be the type of orbit that comets are on.

Hope that's helpful!
Subject: Re: Comet orbit questions. Why does comet go around the sun instead of into the
From: transiter-ga on 28 May 2004 03:25 PDT
To take each part of your question one stage at a time:
 The reason a comet follows a smooth trajectory as you describe it is
due the the gravitational attraction between the comet and the Sun. 
The orbit is not necessarily smooth and in fact purturbations (small
deviations from the small orbit) occur in the comets trajectory due to
the graviational influence of the planets.  The general pathe of the
comet is governed by the attraction between it and the Sun as I said
though and the magnitude of the gravitational force between the comet
and the Sun is given by Newton's Graviational law:

F=(G*M(sun)*M(comet))/(R*R)  where G is Newtons gravitational constant
and R is   the distance between the comet and the Sun.

Some comets do not orbit around the Sun though and do crash into it
and indeed into planets - take for example comet Shoemaker - Levy 
which spectacularly crashed into Jupiter in 1994.

As for the distance a comet travels - you may find the site
useful as it describes the orbital parameters that you want.  To
calculate the farthest distance the comet gets from the Sun you need
to add 1 to the value of the orbital eccentricity and then multiply
this number by the semi major axis e.g. for comet Halley the
eccentricity is 0.967 so add 1 - 1.967 and multiply by the semi-major
axis (17.94 AU) and the result is 35.287 AU.  1 AU is the mean
distance between the Earth and the Sun so 35 AU is 35 times the
distance between the Earth and the Sun.

This page also gives you information on the inclinations of the orbits
i.e. the plane they orbit in - showing how varied they are.

Right onto the speed of the comet.  The speed of the comet is
constantly varying throughout its orbit.  It does not come to a
complete stop at the farthest point (or aphelion) in the orbit but it
is travelling at it slowest at this point and conversely it is
travelling at it maximum speed at perihelion or the closest distance
it gets to the Sun.  The speed and trajectory of the comet at any
point in the orbit is govenred by Kepler's laws, which I have adapted
for your question:

1.) The orbit of the comet about the Sun is elliptical with the Sun at
one focus of the ellipse.
2.) A line drawn between the comet and the Sun will sweep aout an
equal area over equal periods of time. (Thi therefore means that the
comet travels slower the further from the Sun it is)
3.)The square of the period of the comet is proportional to the cube
of the semi-major axis.  Or in its more correct form


This equation is hard to read from this text but go to
for a strightforward explanation.

In terms of their origins, indeed many comets do originate in the Oort
cloud but some are kuiper belt objects.  This is a band of frozen and
rocky bodies just beyond the ornit of Pluto.  Their composition is
believed to be as follows:
Nucleus - made of water ice, CO2 ice, rocky debris, and other frozen gasses
Coma - gaseous water, CO2 and other gasses

There are also the two tails a) the dust tail formed as the Sun's
radiation causes microscopic particles to be "blown" off the comet and
also ionised gasses form b) the ion tail.  Just to calrify the point
on the water - no it didnt come from rain it is from the gasses that
fill the regions between stars called the interstellar medium.

Hope this helps
Subject: Re: Comet orbit questions. Why does comet go around the sun instead of into the sun?
From: rnt20-ga on 29 May 2004 13:59 PDT
You asked:
Could anyone answer "Why doesn't the comet go directly into the center
of the sun instead of around the outside of it?"

If the comet was not moving to start with it would drop straight into
the sun. If the comet was moving to start with, it doesn't immediately
loose the speed it has (the momentum it has), so it will miss the sun.

Imagine standing holding a ball over your foot. If you let go of the
ball, it drops straight onto your foot. But if you release the ball
with some sideways motion (you throw it), it misses your foot. Even
though gravity is straight down, the ball moves sideways when you
throw it sideways. In the same way, if the comet is moving sideways to
start with, it carries on moving sideways instead of falling straight
onto the sun -- the result is that it "misses" the sun as is in fact
in orbit.

Try thinking about it and looking at the diagrams in the above
comments. You could also try some of the simulators on the web, e.g.

Orbits are the natural result of the pull of gravity between spherical
bodies, but it is not something people are familiar with in everyday

Hope that's helpful!
Subject: Re: Comet orbit questions. Why does comet go around the sun instead of into the sun?
From: neilzero-ga on 03 Jun 2004 10:04 PDT
With a few mimor exceptions the answer and the comments are correct.
Go to  scroll to the bottom, Click on forums. Enter
eliptial orbit in the search engine for the 'ask the astronomer'
board. Some of the information will be wrong, so sign up and get a
pass word so you can ask questions on the 'ask the astronomer' board.
I am ccpoodle on these boards.   Neil
Subject: Re: Comet orbit questions. Why does comet go around the sun instead of into the sun?
From: neilzero-ga on 03 Jun 2004 10:16 PDT
All orbits are slightly or very eliptical with minor bumps due to the
gravity of planets as well as the sun. The orbit would have only minor
bumps if the sun was cube shaped instead of very close to spherical. 
The planet Mercury has almost as eliptical and tilted orbit as Pluto. 
Mars is close to the ecliptic but about 15% eliptical.  Neil

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