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Comets
Death by Photoshop
From archives.org
Comets’ Appearance
• Dirty snowballs
•Do not streak across the sky!
• Move slowly from night to
night (fast enough to have to
track a long camera exposure
such as Comet West, 1976,
shown here)
• Named after discoverer(s)
Comet Orbits
• Orbits highly eccentric—
some parabolic or
hyperbolic
• Period years to millions of
years
• By Kepler’s 2nd law, spend
most of their time far from
the Sun
• Also range in inclination—
the tilt of the orbit with
respect to the ecliptic
Fuzzy Discoveries
• Slow background motion
• Charles Messier made his list of
objects (M-numbers…) that are not
comets
Components
• solid ice/rock
nucleus
• as approaches
sun, heats up and
gaseous coma
expands
• solar wind blows
it into tail …
A Tale of Two Tails…

A dust tail arced due to
motion of comet (like
water from a hose), and
shines by reflection.

Ion or gas tail points
away from Sun, and
glows by line emission.
We went through the tail
of Halley’s comet in 1910.
Hale-Bopp 1995
Tail stays pointed away from the Sun
Ice Sublimates
A 1-km
nucleus loses
~3 meters of
depth per
passage by
the sun
Sun-grazing comets
Most famous: Halley’s Comet
Seen since 684 AD, earlier by Chinese
Bayeux Tapestry
Tells of William
of Normand
(France) , who
defeated the
Saxon king
Harold II of
England in 1066
(Battle of
Hastings)
Dates to at least 1476
The Adoration of the Magi, Giotto, 1305
Hevelius,
c. 1661
Edmund Halley
Predicted the return of the
comet – did return after his
death.
Post-Halley painting
Still feared!
1857 woodcut
Still feared?
Last passage: recovered in ‘82
Spacecraft encounters
Lick Obs. January 1986
January 1986 Disconnection Event
(When solar wind reverses polarity)
Soviet Vega-2 Image and a Model
Why not round??
Comet Hyakutake (1996)
Many comets discovered
every year, as I have told
my students in the past…
Shoemaker-Levy
Eugene and Carolyn Shoemaker, David Levy discovered
a comet orbiting Jupiter that would hit it in 1994
String of Pearls…
Impact!
Deep Impact! – NASA probe



Launched an impact probe
Impacted Comet Temple 1 on
July 2005
800 lbs of mostly copper, hit at
10 km/s (Cu to control
emission spectrum)
Asteroids
•Also called “minor planets”
•Discovered accidentally or with
surveys
• Few thousand have
determined orbits
Gaspra a representative
•Asteroids a few to a few
hundred kilometers in
size (Gaspra 12x20x11)
• Note the craters
Ida and its ‘moon’ Dactyl
Dactyl
Dactyl 1.5 km diameter; image by Galileo probe
Asteroid belt
• Belt asteroids
•Trojans-a
solution to the
“restricted 3body problem” in
celestial
mechanics….
… Bode’s Law
Take the series 0.0, 0.3,
0.6, 1.2 … and add 0.4 to
each to reproduce the
spacing of the planets
Missing: 2.8
Just a fluke?
NEO’s
Earth-crossing
“Near Earth
Objects” are of
great concern
Their sizes…
NEO Sizes
Near Earth Asteroid Rendezvous
(NEAR-Shoemaker)
Mathilde
Eros in stereo…
Actual dates later…
Eros Orbit and Final Descent
Descent/impact was on February 12, 2001