- The sudden apparition, changing
shapes, and unpredictable movements of comets have puzzled humanity for
centuries. To ancient cultures they were harbingers of disaster and portents
of great events.
- Comet Halley has returned to
fascinate and frighten the world for more than 2000 years.
comets, with orbital periods greater than 200 years, have been tossed into the
planetary realm from a remote, spherical shell, named the Oort cloud, located
about a quarter of the way to the nearest star.
- A million million, or 1012, invisible comets have been hibernating in
the deep freeze of the Oort comet cloud since the formation of the solar system
4.6 billion years ago.
- Many Jupiter family comets, with
orbital periods of less than 20 years, probably came from the Kuiper belt, which lies in
the outer disk of the planetary system beyond the orbit of Neptune and may
contain more than a billion unseen comets.
light up and become visible for just a few weeks or months, when their orbits
bring them near the Sun. The solar heat then vaporizes some of the comet water
ices, permitting the comets to grow large enough to be seen. The water ice
sublimates, or turns directly from solid ice to water vapor.
- The solid comet nucleus is just a
gigantic ball of water ice, other ices, dust and rock. Some of the comet
nuclei are about the size of Paris or Manhattan and roughly one billionth the
mass of the Earth. Others comets are much smaller.
- No two comets ever look
identical, and every comet changes shape and form as it whips around the Sun,
but they all develop a glowing spherical cloud of gas and dust, known as the
coma, when moving close enough to the Sun.
- The comet coma can be larger than
the Earth and as big as the Sun, and around the coma there is an even larger
envelope of atomic hydrogen, known as the hydrogen cloud, that shines in
- Some comets develop tails that
flow away from the Sun, briefly attaining lengths as large as the distance
between the Earth and the Sun, but other comets have no tail at all.
- Comets can have two kinds of
tails, the long, straight ion tails, that re-emit sunlight with a faint blue
fluorescence, and a shorter, curved dust tail that shines by reflecting yellow
- The Giotto, Deep Space 1, Stardust, and Deep Impact spacecraft have respectively peered into the icy heart
of four comets, Halley, Borrelly, Wild 2 and Tempel 1, showing that their nuclei
are blacker than coal and reflect just a few percent of the incident sunlight.
- Gas and dust jet out from the
sunlit side of the nucleus of comet Halley, from fissures in its dark crust,
but nearly 90 percent of the surface of its nucleus was inactive at the time of
the Giotto encounter.
- Comet Borrelly is covered with a
dark, unreflective carbon-rich material, and contains surface features that are
most likely supported by solid water ice.
- Comet Wild 2 has a relatively young
surface, exposed just a few times to the Sunís intense heat, and it has a dark,
pockmarked surface with pits, craters and jets of gas and dust.
- The Stardust spacecraft gathered dust from the coma of comet Wild 2 in
January 2004, returning the dust in a capsule that was parachuted to Earth two
years later. The returned comet dust contains a mix of minerals formed at both
cold and high temperatures, two types of nitrogen-rich organic molecules, and
the amino acid glycine.
- The Deep Impact spacecraft collided with comet Tempel 1, on 4 July
2005; spectroscopic examination of the ejected cloud of dust revealed fine
porous material, water vapor, water ice, carbon dioxide, hydrocarbons, and
silicates or sand.
- When a bright comet nears the
Sun, it turns on its celestial fountains, spurting out about a million tons of
water each day.
- The recoil effect of jets of
matter ejected from a cometís spinning, icy nucleus can push a comet along in
its orbit or oppose its motion, causing the comet to arrive closest to the Sun
earlier or later than expected.
- Most of the comets seen during
recorded history will vanish from sight in less than a million years, either
vaporizing into nothing or leaving a black, invisible rock behind.
- About 40 thousand tons of small,
cosmic dust particles fall onto the Earth in a typical year, wafting gently
through the atmosphere to the ground.
- Visible comets are in their death
throes, but they may carry the residues of creation in their ice and dust.
- Meteor showers, commonly known as
shooting stars, are produced when sand-sized or pebble-sized pieces of an icy
comet burn up in the atmosphere, never reaching the ground.
- Comets strew particles along their
orbital path as they loop around the Sun, and when the Earth passes through one
of these meteoric streams a meteor shower occurs, recurring at the same time
Copyright 2010, Professor Kenneth R. Lang, Tufts University