Comets are small, fragile, irregularly shaped bodies composed of
a mixture of non-volatile grains and frozen gases. They have highly
elliptical orbits that bring them very close to the Sun and swing
them deeply into space, often beyond the orbit of Pluto.
Comet structures are diverse and very dynamic, but they all develop
a surrounding cloud of diffuse material, called a coma, that usually
grows in size and brightness as the comet approaches the Sun. Usually
a small, bright nucleus (less than 10 km in diameter) is visible
in the middle of the coma. The coma and the nucleus together constitute
the head of the comet.
As comets approach the Sun they develop enormous tails of luminous
material that extend for millions of kilometers from the head, away
from the Sun. When far from the Sun, the nucleus is very cold and
its material is frozen solid within the nucleus. In this state comets
are sometimes referred to as a "dirty iceberg" or "dirty snowball,"
since over half of their material is ice. When a comet approaches
within a few AU of the Sun, the surface of the nucleus begins to
warm, and volatiles evaporate. The evaporated molecules boil off
and carry small solid particles with them, forming the comet's coma
of gas and dust.
When the nucleus is frozen, it can be seen only by reflected sunlight.
However, when a coma develops, dust reflects still more sunlight,
and gas in the coma absorbs ultraviolet radiation and begins to
fluoresce. At about 5 AU from the Sun, fluorescence usually becomes
more intense than reflected light.
As the comet absorbs ultraviolet light, chemical processes release
hydrogen, which escapes the comet's gravity, and forms a hydrogen
envelope. This envelope cannot be seen from Earth because its light
is absorbed by our atmosphere, but it has been detected by spacecraft.
The Sun's radiation pressure and solar wind accelerate materials
away from the comet's head at differing velocities according to
the size and mass of the materials. Thus, relatively massive dust
tails are accelerated slowly and tend to be curved. The ion tail
is much less massive, and is accelerated so greatly that it appears
as a nearly straight line extending away from the comet opposite
the Sun. The following view of Comet West shows two distinct tails.
The thin blue plasma tail is made up of gases and the broad white
tail is made up of microscopic dust particles.
Each time a comet visits the Sun, it loses some of its volatiles.
Eventually, it becomes just another rocky mass in the solar system.
For this reason, comets are said to be short-lived, on a cosmological
time scale. Many scientists believe that some asteroids are extinct
comet nuclei, comets that have lost all of their volatiles.
This color photograph of the comet Kohoutek was taken by members
of the lunar and planetary laboratory photographic team from the
University of Arizona. They photographed the comet from the Catalina
observatory with a 35mm camera on January 11, 1974. (Courtesy
These Hubble Space Telescope images of comet Hyakutake were taken
on March 25, 1996 when the comet passed at a distance of 9.3 million
miles from Earth. These images focus on a very small region near
the heart of the comet, the icy, solid nucleus and provide an exceptionally
clear view of the near-nucleus region of the comet.
The left image is 2070 miles across (3340 km) and shows that most
of the dust is being produced on the sunward-facing hemisphere of
the comet. Also at upper left are three small pieces which have
broken off the comet and are forming their own tails. Icy regions
on the nucleus are activated as they rotate into sunlight, ejecting
large amounts of dust in the jets that are faintly visible in this
image. Sunlight striking this dust eventually turns it around and
"blows" it into the tailward hemisphere.
The bottom-right image is an expanded view of the near-nucleus
region and is only 470 miles (760 km) across. The nucleus is near
the center of the frame, but the brightest area is probably the
tip of the strongest dust jet rather than the nucleus itself. Presumably,
the nucleus surface lies just below this bright jet. The top-right
image shows pieces of the nucleus that apparently broke off. The
image shows at least three separate objects that are probably made
up of coarse-grained dust. Large fragments of the nucleus would
not be accelerated into the tail, which appears to be the case in
this image. (Credit: H. A. Weaver--Applied Research Corp., HST
Comet Hyakutake Observing Team, and NASA)
First X-Rays From Comet Hyakutake Discovered
This image shows the discovery of a strong X-ray radiation signal
coming from comet Hyakutake. The image was made on March 27, 1996
using Germany's orbiting ROSAT satellite. The comet was near its
closest approach to the Earth at a distance of less than 10 million
miles when X-ray emmisions were first detected by ROSAT. The strength
and rapid changes in intensity of the comet's X-ray emission both
surprised and puzzled astronomers. "We had no clear expectation
that comets shine in X-rays," said Dr. Michael J. Mumma of NASA's
Goddard Space Flight Center, Greenbelt, MD. X-rays have never been
found from a comet before, and scientists had optimistically predicted
an intensity that turned out to be about 100 times weaker than the
radiation actually detected by ROSAT. Strong changes in the brightness
of the X-rays were another surprise. There were pronounced increases
and decreases in the X-ray brightness from one ROSAT observation
to another, typically over a time difference of a few hours.
Still another puzzle is the nature of the physical process that
generates the X-rays, but the ROSAT image may contain clues to this
process. In the image, the X-rays from the comet seem to come from
a crescent-shaped region on the sunward side of Comet Hyakutake.
One preliminary theory is that X-ray emission from the Sun was absorbed
by a cloud of gaseous water molecules surrounding the nucleus of
the comet, and then were re-emitted by the molecules in a process
physicists call "fluorescence." According to this idea, the cloud
is so thick that its sunward side absorbs nearly all the incoming
solar X-rays, so that none reach the remainder of the cloud. This
could explain why the cometary X-ray emission has the form of a
crescent, rather than that of a sphere around the nucleus. A second
possible explanation is that the X-rays are produced from the violent
collision between the comet material and the supersonic "wind" of
plasma and particles streaming away from the Sun.
Comet 1993a Mueller
This is a CCD image of comet 1993a Mueller, taken on October 6,
1993 with a 288mm f/5.2 Schmidt-Cassegrain telescope. The comet
has a coma diameter of 3' and a fan-shaped tail, up to 7' long.
(Courtesy Erich Meyer and Herbert Raab, Austria)
Comet West (1975)
This photograph was taken by amateur astronomer John Loborde on
March 9, 1976. This picture shows two distinct tails. The thin blue
plasma tail is made up of gases and the broad white tail is made
up of microscopic dust particles.
Comet West (1975)
This image of comet West was taken by John Laborde at the Tierra
Del Sol Observatory site in San Diego County. The exposure was 30
minutes with a 135 mm Nikon lens.
These NASA Hubble Space Telescope pictures of comet Hale-Bopp show
a remarkable "pinwheel" pattern and a blob of free-flying debris
near the nucleus. The bright clump of light along the spiral (above
the nucleus, which is near the center of the frame) may be a piece
of the comet's icy crust that was ejected into space by a combination
of ice evaporation and the comet's rotation, and which then disintegrated
into a bright cloud of particles.
Although the "blob" is about 3.5 times fainter than
the brightest portion at the nucleus, the lump appears brighter
because it covers a larger area. The debris follows a spiral pattern
outward because the solid nucleus is rotating like a lawn sprinkler,
completing a single rotation about once per week.
This image of comet Hale-Bopp was taken by John Laborde with his
home designed and built, 8.8" f/3.7 Wright Schmidt Camera. The picture
was taken at the Tierra Del Sol Observatory site in San Diego County
with a 25 minute exposure on Kodak PPF400 film.
This image of comet Ikeya-Seki was taken by John Laborde in Poway,
California just before dawn. The exposure was 15 minutes with a
55 mm Nikon lens.