Though frequently beautiful, comets traditionally have stricken terror as often as they
have generated wonder as they arc across the sky during their passages around the
Sun. Astrologers interpreted the sudden appearances of the glowing visitors as ill
omens presaging famine, flood or the death of kings. Even as recently as the 1910
appearance of Halley's Comet, entrepreneurs did a brisk business selling gas masks to
people who feared Earth's passage through the comet's tail.
In the 4th century B.C., the Greek philosopher Aristotle concluded that comets were
some kind of emission from Earth that rose into the sky. The heavens, he maintained,
were perfect and orderly; a phenomenon as unexpected and erratic as a comet surely
could not be part of the celestial vault. In 1577, Danish astronomer Tycho Brahe carefully
examined the positions of a comet and the Moon against the stars during the
evening and predawn morning. Due to parallax, a close object will appear to change
its position against the stars more than a distant object will -- the same effect that you
see if you hold up a finger and look at it while closing one eye and then the other. The
Moon appeared to move more against the stars from evening to morning than the
comet did, leading Tycho to conclude that the comet was at least six times farther
A hundred years later, the English physicist Isaac Newton established that a comet
appearing in 1680 followed a nearly parabolic orbit. The English astronomer Edmond
Halley used Newton's method to study the orbits of two dozen documented cometary
visits. The orbits of three comets seen in 1531, 1607 and 1682 were so similar that he
concluded they in fact were appearances of a single comet wheeling around the Sun in
a closed ellipse every 75 years or so. He successfully predicted the next visit in 1758-9, and the comet thereafter bore his name.
Since then, astronomers have concluded that some comets return relatively frequently,
in intervals ranging from 3 to 200 years; these are called "short-period" comets.
Others have enormous orbits that bring them back only once in hundreds of millennia.
In the mid-1800s, scientists also began to turn their attention to the question of comets'
composition. Astronomers noted that several major meteor showers took place when
Earth passed through the known orbits of comets, leading them to conclude that the
objects are clumps of dust or sand. By the early 20th century, astronomers studied
comets using the technique of spectroscopy, breaking down the color spectrum of light
given off by an object to reveal the chemical makeup of the object. They concluded
that comets also emitted gases as well as molecular ions.
In 1950, the American astronomer Fred L. Whipple (1906-2004) authored a major
paper proposing what became known as the "dirty snowball" model of the cometary
nucleus. This model, which has since been widely adopted, pictures the nucleus as a
mixture of dark organic material, rocky grains and water ice. ("Organic" means that
the compound is based on carbon and hydrogen, but is not necessarily biological in
origin.) Most nuclei of comets range in size from about 1 to 10 kilometers (1/2 to 6
miles) in diameter.
If comets contain icy material, they must originate somewhere much colder than the
relatively warm inner solar system. In 1950, the Dutch astronomer Jan Hendrick Oort
(1900-1992) used indirect reasoning from observations to predict the existence of a
vast cloud of comets orbiting many billions of miles from the Sun - perhaps 50,000
astronomical units (AU) away (one AU is the distance from Earth to the Sun), or nearly
halfway to the next nearest star. This region has since become known as the Oort
A year later, the Dutch-born American astronomer Gerard Kuiper (1905-1973) pointed
out that the Oort Cloud is too distant to act as the nursery for short-period comets. He
suggested the existence of a belt of dormant comets lying just outside the orbits of the
planets at perhaps 30 to 100 AU from the Sun; this has become known as the Kuiper
Belt. (Other astronomers such as Frederick Leonard and Kenneth Edgeworth also
speculated about the existence of such a belt in the 1930s and 1940s, and so the
region is sometimes referred to as the Edgeworth-Kuiper Belt, the Leonard-Edgeworth-Kuiper
Belt, and so on.) Close encounters with other dormant comets sometimes
change their orbits so that they venture in toward the Sun and fall under the influence
of the gravities of the giant outer planets -- first Neptune, then Uranus, then Saturn and
The Oort Cloud, by contrast, would be the home of long-period comets. They are periodically
nudged from their orbits by any one of several influences - perhaps the gravitational
pull of a passing star or giant molecular cloud, or tidal forces of the Milky Way
In addition to the length of time between their visits, another feature distinguishes
short- and long-period comets. The orbits of short-period comets are all fairly close to
the ecliptic plane, the plane in which Earth and most other planets orbit the Sun.
Long-period comets, by contrast, dive inwards toward the Sun from virtually any part of
the sky. This suggests that the Kuiper Belt is a relatively flat belt, whereas the Oort
Cloud is a three-dimensional sphere surrounding the solar system.
Read more about why comets are important targets of study.
This text is extracted from the NASA Deep Impact Launch Press Kit. More information on the Deep Impact mission is available
on the project home page.