Download Comets - from the Greek kome, meaning “hair”. Only visible when far

Some pictures
of famous
comets:
Comets - from the Greek
kome, meaning “hair”.
Only visible when far from
the Sun due to reflected
light. As they near the
Sun, comets emit light
of their own.
Comets have highly
elliptical orbits. They
develop tails as icy matter
in the comet becomes
heated and sublimes away.
A comet’s tail always points
away from the Sun.
If a comet survives its
close approach to the Sun
(some are completely
broken apart or crash into
the Sun), it continues
to extreme distances
from the Sun.
The orbits extend far beyond
Pluto, perhaps 50,000 A.U.’s.
Most take hundreds of
thousands to millions of years
to orbit the Sun. A “short”
period comet is a comet with
an orbit less than 200 years.
Comets orbit at all
inclinations and
orientations, both
prograde and retrograde.
For each comet we see,
there are many more so far
from the Sun that they are
invisible from Earth.
There may be a huge
cloud of comets,
beyond the orbit of
Pluto, called the Oort
Cloud. Most comets
spend their entire lives
in the cloud.
Occasionally the
gravity of a passing
star “kicks” a comet
into an orbit that
brings it close
to the Sun.
Halley’s Comet Edmund Halley
realized that this comet
visits every 76 years
and predicted its
reappearance in 1758.
He did not live to see his
prediction proved correct,
but the comet was named
in his honor. Sightings of
Halley’s comet have been
traced back to 240 B.C.
The tail of Halley’s comet can
reach almost one A.U. in
length, stretching tens of
degrees across the sky.
The 1986 visit was not
good for viewing from
Earth, but spacecraft
did visit it at this time.
The main solid body
of a comet is called
the nucleus. It is
typically only a few
kilometers in length.
The Sun’s heat causes the
nucleus to form a diffuse
coma of dust and evaporated
gas. The coma can measure
as much as 100,000 km
in diameter (almost as large
as Jupiter).
An invisible hydrogen
envelope surrounds
the coma and
stretches millions
of km into space.
The tail stretches
almost an A.U. The tail
and the coma are the
only parts visible from
Earth. Most of a
comet’s light comes
from the coma.
Comets are
of two types,
distinguished
by their tails:
Type I and Type II.
Type I (ion, or
plasma) tails:
very straight,
made of glowing,
linear streams.
Type II (dust)
tails: broad,
diffuse, gently
curved, only
reflects light.
Many comets have both
types mixed. Comet
Kahoutek (1975) was a
highly publicized flop
because its large
dust tail scattered the
light from its ion tail.
The tail of a comet always is
directed away from the Sun
as it is produced by the solar
winds. Ions in the type I tail
are more influenced by the
solar winds, so they are
always directed in a
straight line from the Sun.
The dust particles
of the type II tail are
heavier, so they have
more of a tendency to
follow the comet’s orbit,
making them slightly
curved.
In 1986, a number of spacecraft
visited Halley’s comet. Vega
2(Russian) went through the
tail, and Giotto(European)
moved within 600 km of the
nucleus(this damaged Giotto’s
camera). They each
imaged Halley’s nucleus.
Halley’s nucleus is irregular,
potatoe-shaped and is almost
jet black. Jets of matter are
expelled from small areas on
the sunlit side.
These jets are what causes
the nucleus to rotate once
every 53 hours.
Comets have masses
15
ranging from 10 to
19
10 g (much like small
asteroids), but a
comet’s mass
decreases over time.
Comets that move within 1
A.U. of the Sun typically lose
107 grams of material every
second. That is a loss of 10
tons of cometary material for
every second the comet
spends near the Sun.
Halley’s comet
will be gone in
about 40,000
years.
Sun-grazing comets
may break apart
when close to the
Sun or may even
plunge into the Sun.
Cometary nuclei are believed
to be composed of dust
particles trapped within a
mixture of methane,
ammonia, and ordinary water
ice. “Dirty snowballs” is the
term coined by comet expert
Fred Whipple.
Fred Whipple also
said, “Comets are
like cats, they both
have tails, and they
do whatever they
want!”
Eugene Shoemaker's passion was
Astrogeology. He dreamed of going to the
Moon. Credited with inventing the branch of
Astrogeology within the U.S. Geological
Survey, his contributions to the field and the
study of impact craters, lunar science,
asteroids, and comets are legendary.
Though his own career as an
astronaut/geologist was sidelined by a
health problem, he helped train the Apollo
astronauts in geology and the investigation
of the lunar surface.
Seen here at Meteor Crater, Arizona in the
mid 1960s, Shoemaker was killed in a tragic
car accident in July 1997. He is survived by
his wife and professional colleague, Carolyn,
and children. In a fitting tribute conceived by
a former student, Eugene Shoemaker's
ashes were placed on-board the Lunar
Prospector spacecraft which has now
successfully reached a polar mapping orbit
around the Moon. After completing its
scientific mission, the spacecraft will
ultimately impact the lunar surface.
The Lunar
Prospector crashed
to the Moon’s
surface in late
1999.
Gene Shoemaker
traveled to the Moon
as he wished. He
ultimately became what
he had spent his life
studying, a crater.