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Comets
Comets are relatively small icy bodies, often only a few kilometers in
extent, that formed in the outer solar system where temperatures are cold
enough to sustain (predominately water) ices. They represent the leftover
bits and pieces from the outer solar system formation process that took
place some 4.6 billion years ago.
Over long time periods, some comets are
perturbed from their distant orbits and sent close enough to the sun that
their ices begin to vaporize. This out-gassing of gas and dust from a
comet's nucleus produces an atmosphere (i.e., coma) often extending many
hundreds of thousands of kilometers. Because of the reflection of sunlight
from its dust particles and the fluorescence of its excited gases, this
atmosphere glows with a "fuzzy" appearance when viewed from the ground. As
this coma material continues to expand away from the solid cometary
nucleus, the gas component is eventually "blown" away from the sun by a
high speed stream of charged particles from the sun (solar wind). The
comet's dust component is also blown away from the sun - this time by the
pressure of sunlight on the tiny dust particles. Thus a comet can have
both a gas tail and a dust tail.
Comets originate in the outer regions of our planetary system with one
group forming in the region near the current orbits of Uranus and Neptune
and another group, called the Kuiper belt objects, forming somewhat more
distant to the sun - beyond the orbit of Neptune. As a result of their
interactions with the outer major planets, the comets in the first group
can be thrown out to the distant Oort cloud some 50,000 to 150,000 times
further from the sun than the Earth. Close passing single stars and the
gravitational interaction with our Milky Way disk of stars can then nudge
these comets back into the inner solar system where they can arrive with
any inclination with respect to the Earth's orbital plane. Sometimes these
objects can be seen as impressive, long-period comets like comet Hale-Bopp
that was easily observable to the naked-eye in 1997.
So-called long-period
comets orbit the sun with periods ranging from 200 to several million
years. Comets that form in the so-called Kuiper belt (or Edgeworth-Kuiper
belt after the two researchers who hypothesized these comets in the mid
twentieth century) are also acted upon gravitationally by the massive outer
planets and they often evolve into the short-periodic comets, whose orbital
inclinations are usually relatively close to the Earth's orbital
plane. With their orbital periods of about 5-7 years, these short-period
comets orbit the sun frequently, lose much of their volatile ices, and are
often far less visually impressive than their long-period cometary cousins
that arrive fresh from the Oort cloud.
Read this short article by Don Yeomans
to learn why comets are particularly interesting and
why we should study these primitive bodies.
Then, learn about some of the great comets
of the past in this article by Don Yeomans.
Orbits: Diagrams & Elements
The orbit of any comet (or asteroid) can be viewed
using our java-based orbit applet.
Start with our small-body browser to find the asteroid of interest,
then select the Orbit Diagram link.
For example, here is the
orbit diagram for comet 1P/Halley.
Orbital elements and related parameters are also
available for any comet (or asteroid) using our small-body browser.
In addition, custom tables of orbital elements and/or physical parameters
are available using our
small body database search engine.
We also provide fixed-format ASCII tables of elements.
Warning: If you intend to use cometary orbital elements in a two-body propagation to
compute future/past position (ephemerides), your results will be inaccurate
and in some cases, completely incorrect.
The motion of comets is affected by their so-called non-gravitational forces
(the rocket-like force from outgassing of material from the comet while close to the sun).
Thus, it is especially important to use HORIZONS to compute comet ephemerides.
Physical Parameters
Physical parameters for comets are not well known primarily because
these bodies are too small for ground-based observing when the comet
if far enough from the sun that it's coma does not shroud it's surface.
The only parameters determined for nearly all comets are their magnitude
parameters (M1,K1 and/or M2,K2).
However, a few comets have other parameters determined including
rotation period,
and geometric albedo.
Known physical parameters for any given small body are
are available from our small-body browser.
Ephemerides
Comet ephemerides are available using JPL's HORIZONS system.
Discovery Circumstances
Discovery circumstances for many comets are also available
using our small-body browser.
Discovery data include the date of discovery, who discovery the comet,
and where it was discovered.
Mission Targets
Spacecraft missions to small-bodies provide
valuable scientific data ultimately improving our understanding
of these primitive solar system bodies.
A list of asteroids and comets
targeted by spacecraft missions (past, present, and future)
is presented on this page.
Radar Astrometry
Radar astrometry for selected comets
is available in tabular format. A table showing data for only
comets is presented on this page.
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