Comets
Comets are icy bodies orbiting in the Solar System, which partially
vaporizes when it nears the Sun, developing a diffuse envelope of dust and
gas and, normally, one or more tails.
Ground-based observations of the behaviour of many comets, together with
results from the investigation in 1986 of Halley's Comet from space probes,
support the view first proposed by F. Whipple in about 1949 that the nuclei
of comets are essentially 'dirty snowballs' a few kilometres across. They
appear to be composed of frozen water, carbon dioxide, methane and ammonia,
in which dust and rocky material is embedded. As a comet approaches the Sun,
solar heating starts to vaporizes the ices, releasing gas that forms a
diffuse luminous sphere, called the coma, around the nucleus. The coma may
be up to a million kilometres across. The nucleus itself is too small to be
observed directly.
Dust and gas leave the comet nucleus from jets on the side facing the Sun,
then stream away under the Sun's influence. Electrically charged ionized
atoms are swept away directly by the magnetic field of the solar wind,
forming straight ion tails (alternatively called Type I, plasma or gas
tails). Variations in the solar wind cause the ion tail to take on
structure, or even break off in a disconnection event. Small neutral dust
particles are not carried along by the solar wind but get 'blown' gently
away from the Sun by radiation pressure. Dust tails (also called Type
II tails) are often broad and flat. The tail grows as a comet approaches the
Sun and are always directed away from the Sun: they can be as much as a
hundred million kilometres long. Large dust particles become strewn along
the comet's orbit and form meteor streams.
Description: In July 1994
the individual
fragments of Comet Shoemaker-Levy 9 impacted onto the planet Jupiter.
Images obtained from the 1m Jacobus Kapteyn Telescope clearly showed the
ejecta
plume of debris rising over the planet's limb from the impact site
of fragment L. At the same time the 2.5m Isaac Newton Telescope was used
to observe spectroscopic emission from the vaporised comet, revealing some
of its constituent elements. More information can be found on
Alan
Fitzsimmons' web page.
Credit: Alan Fitzsimmons.
Technical Description: Images taken using the JAG CCD camera on the Jacobus Kapteyn
Telescope.
Available formats: Photo 1 GIF (147 K) | Photo 2 GIF (167 K)
Description: Comet Hyakutake observed from La Palma on the night of 24 March 1996. At this time the
Comet was making an extremely close approach to the Earth passing within
14 million kilometers (about 9 million miles) from us. This is the closest
approach of a comet for 13 years and the brightest comet within the last
20 years. At this time the Comet extended some 30 degrees in the sky which
translates to a physical length of around 6 million kilometres (about 4
million miles) and was easily visible to the naked eye having an apparent
brightness equal to that of the brightest stars. Its apparent diameter
was equal to three full moons while its real diameter was around 250000
kilometres (150000 miles).
Credit: Don Pollacco.
Technical description: This image was taken using a CCD detector mounted
on a 35-mm telephoto lens.
Available formats: JPEG (72 K)
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Description: Comet Hale-Bopp was a spectacular object in the
evening skies during the spring of 1997. Image 1 was obtained
on the 25th of August, 1995 when the comet was 6.9 AU (1,030,000,000
kilometres) from the sun and 6.3 AU (940,000,000 kilometres) from the Earth.
A large number of stars are visible, as at this time the comet was in the
direction of the constellation of Sagittarius. On the 1st March, 1997
the William Herschel telescope took image 2 which shows a spiral jet and some dusty arcs ejected from the
nucleus. CoCAM camera,
the wide field imaging facility of ING, began to observe comet Hale-Bopp
on the first days of March, 1997. Image 3 and Image 4 are good samples of the observations carried
out. Finally, on the 16th April CoCAM discovered a
new
type of cometary tail, the sodium tail (the straight line from the
right bottom to the left top in image 5), which consists of neutral
atoms, never seen before.
Credits:
Photo 1: Alan Fitzsimmons.
Photo 2: John Telting.
Photo 3: Javier Méndez.
Photo 4: Javier Méndez.
Photo 5: European Comet Hale-Bopp Team.
Technical Information:
Photo 1: CCD camera on the Jacobus Kapteyn Telescope.
Photo 2: Prime Focus CCD camera+filter Z on teh William Herschel
Telescope.
Photo 3: CoCAM camera+filter centered at 618nm. Field of view: 10.5 degrees.
Photo 4: CoCAM camera+filter centered at 618nm. Field of view: 6 degrees.
Photo 5: CoCAM camera+sodium filter.
More images of comet Hale-Bopp:
The ING comet Hale-Bopp web pages
Alan Fitzsimmons' comet web
pages
