Comets:
Primitive Hairy Objects
Water ice,
other frozen volatiles, and dust. Formed in cold outer solar system
History
Portended
catastrophe
Classical
Greeks thought heavens unchanging \ comets
atmospheric phenomena
Tycho
Brahe: comets seen in same celestial location from different places on Earth
Þ no
noticeable parallax \ must be
much farther away than in atmosphere
Halley
First realized
orbits closed ellipses - could reappear periodically.
Concluded
Great Comets of 1531, 1607, 1682, all same comet with 76 year period
Correctly
predicted next appearance in 1758, after his death
Comets
called "dirty snowballs"
Reveal
early history of the solar system.
Activated
near sun; several distinct parts:
- nucleus:
solid, mostly with embedded dust
and other solids
- coma:
dense cloud of water, carbon dioxide and other neutral gases sublimed from
the nucleus;
- hydrogen cloud:
millions of km in diameter but tenuous envelope of neutral hydrogen;
- dust tail: up
to 10 million km long - smoke-sized dust particles driven off the nucleus
by escaping gases; most prominent to unaided eye; curved
- ion tail: as
much as several hundred million km long. Composed of plasma. Displays rays and streamers caused by
interactions with the solar wind. Straight.
Origins
Oort
cloud - to about 50,000 au, roughly spherical - long period comets
Kuiper
belt - Starts at Neptune's orbit - short and intermediate period
comets
Comets invisible until near the Sun.
Most
comets are long period comets: have highly eccentric orbits, which take
them far beyond the orbit of Pluto - seen once and then disappear for millennia
or longer. Orbits nearly parabolic.
Close
encounter with massive planet like Jupiter can:
Slow
it down Þ short or intermediate
period (20 - 200 yrs) comets). Stay within the orbit of Pluto for a significant
fraction of their orbits.
Speed
it up Þ escape velocity from solar
system
Impact
the planet (or the Sun)
Fragment:
Shoemaker-Levy 9, 1994. Had passed 35,000 km from Jupiter in 1992 
breaking
into 20 fragments, altering orbit to
Jupiter orbit.
Comets'
orbits unstable near sun due to perturbations from planets, non-gravitational
forces - jets
Images
excerpted from Chaisson McMillan
500
or so passes near the Sun Þ most
volatiles, ices and gas, is lost
leaving a rocky object looking like an asteroid.
About
half of the near-Earth asteroids may be "dead" comets.
Meteors
and Meteor Showers
Meteor:
streak of light high in atmosphere - trail of glowing gas ionized by passage
of asteroidal or meteoric debris,
usually meteoric.
Sporadic
meteors: material from random orbits
Meteor
Showers: directly linked to periodic comets - meteoric streams intersect Earth
orbit. Reoccur each year
Radiant
(apparent origin of meteors) in constellation determines name: Perseus ,Þ Perseids,
August 11, from comet Swift-Tuttle