About comets
At the heart of a comet is its small (0.5 - 50 Km across - in comparison to the Earth's 12 756 Km) nucleus, composed of dust embedded in frozen volatile materials (water, methane, ammonia, carbon dioxide, carbon monoxide and other gases) that orbits the Sun typically in a highly elliptical orbit (squashed, rather than circular - with the closest and furthest distances from the Sun being very different).
As it approaches the Sun some these volatile evaporate off the nucleus of the comet to form its coma - a very large (100 000s Km across) diffuse, dusty "atmosphere" that is steadily streams off into space (the comet's gravity being insufficient to hold on to it). Very near the Sun the coma is often blown away by solar radiation to form the dust tail - with the solar wind dragging ionized gas away in a slightly different direction to form the plasma tail. Without this (visibly lost) volatile material it wouldn't be comet - but an asteroid. Some asteriods are old comets that have lost all their volatile materials through too many trips passed the Sun.
When it's more distant from the Sun (as it is for the vast proportion of the time) the comet dims dramatically as the evaporation to form the coma slows and it is less illuminated. At sufficient distances from the Sun the evaporation ceases and a passing observer would see just its sooty dark nucleus.
Comet orbital periods vary from just a few years to over 100 000 years. Most comets are discovered as they approach the Sun and become visible (to telescopes). Small comets are quite common, with about a dozen discovered every year, but most of these aren't visible to the naked eye. Some of us can look forward to the return of medium/short period dramatic comets like Halley (period: 76 years - next due in 2061) and about once a decade a spectacular longer period comet like Hyakutake (period 29 500 years) or Hale-Bopp (period 2 380 years) turns up unannounced from the outer solar system.
More about comets:
zoomschool comets
"nine planets" comets
Astronomy Notes on comets...
comets
comet orbits - Oort Cloud and Kuiper Belt
comet beginnings and ends
review questions
ScienceWeb:
teachers tips
comet questions
Windows to the Universe comet table
Why do published comet orbits change
Halley's comet
The return of Halley's comet
"nine planets" Halley' comet
comet Hale-Bopp
Teacher's newsletter preview
Changing estimates of orbit data
3D view of orbit
current comets:
Cometography.com current comets
About the simulation
This simulation adds a comet (an imaginary one) to the Solar system. It shows the comet's orbit - it doesn't (yet) show what you would see of an real comet (as long as it was bright enough); its coma and tail as it passes near the Sun.
The simulation displays two simultaneous views of the Solar system; a selection of mostly 3D views and and a choice of (top, front or side) plane views that allow you to change the velocity objects in the simulation.
The plane views show the velocity (blue arrows) of and force (red arrows) on the Sun, planets and the comet.
To change the orbit of any object,
pause the simulation
then use the mouse on any the
the plane views
(top, front or side) to alter its velocity.Experiment changing the comet's trajectory - and watch what happens. Click
rewind to return
the simulation to its original state.The energy and orbital period of the comet is shown in a scrolling graph. While the comet moves closer to the Sun its gravitational potential energy is converted into kinetic energy. As it moves away from the Sun the kinetic energy is converted back to gravitational potential energy. All the while the comet's total KE+PE remains constant (except when there are interactions with other objects in the Solar system).
As long as the comet's total energy is less than zero, it will remain in a closed orbit around the Sun (providing it doesn't crash into it, or get diverted by another object in the Solar System).
If you adjust the comet's velocity so that its total energy is greater than zero - then it will completely escape the Sun's gravitational pull.
Notes:
The comet's gravitational potential energy is always registered as negative - below the horizontal "zero" line on the graph. This is because, by convention, we say that the gravitational potential energy between two objects is zero when they are an infinite distance apart. Since this is also their maximum potential energy, the potential energy for objects a finite distance apart is less than zero.
This may seem confusing, but it is actually the simplest way of defining gravitational potential energy. Otherwise we'd have to specify some arbitrary separation for zero PE, and we'd have to deal with both positive and negative PE.
The "top" view looks "down" on the ecliptic plane - and shows velocity and force in that plane.
The ecliptic plane is the plane of the Earth's orbit around the Sun. It is so named because (lunar or solar) eclipses can only happen as the moon crosses this plane.
Worksheet questions:
1/ How does the comet's orbit differ from planets?
2/
a) How does the speed of the comet change through its orbit?
b) Where does the comet spend the most time?
c) Where does the comet spend the least time?
3/ How long does its orbit take? How does this compare to actual comets?
4/ In the Nth year of the simulation, the comet's orbit changes noticably. What has caused this?
5/
a) How does the mass of a typical comet compare with the masses of different planets?
b) How does this affect the interactions between the comet and planets?
c) Which planets is the comet likely to be most affected by - and why?
6/ Why is the comet's orbit unstable? What sort of orbits are more stable?
7/ What things might happen to the comet in the future?
8/ In web pages about comet Hale-Bopp some say its orbital period is 4000 years, others 2380 years. Why might this be?
data from http://www.seds.org/nineplanets/nineplanets/datamax.html