Comets

1
Comets
Death by Photoshop
From archives.org
2
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)

3
Comet Orbits

• Orbits highly eccentricsome parabolic or
  hyperbolic
• Period years to millions of years
• By Keplers 2nd law, spend most of their time
  far from the Sun
• Also range in inclinationthe tilt of the orbit
  with respect to the ecliptic

4
Fuzzy Discoveries

• Slow background motion
• Charles Messier made his list of objects
  (M-numbers) that are not comets

5
Components

• solid ice/rock nucleus
• as approaches sun, heats up and gaseous coma
  expands
• solar wind blows it into tail

6
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
  Halleys comet in 1910.

Hale-Bopp 1995
7
Tail stays pointed away from the Sun
8
Ice Sublimates
A 1-km nucleus loses 3 meters of depth per
passage by the sun
9
Sun-grazing comets
10
Most famous Halleys Comet
11
Seen since 684 AD, earlier by Chinese
12
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
13
The Adoration of the Magi, Giotto, 1305
14
Hevelius, c. 1661
15
Edmund Halley
Predicted the return of the comet did return
after his death.
Post-Halley painting
16
Still feared!
1857 woodcut
17
Still feared?
18
Last passage recovered in 82
19
Spacecraft encounters
20
Lick Obs. January 1986
21
January 1986 Disconnection Event
(When solar wind reverses polarity)
22
Soviet Vega-2 Image and a Model
Why not round??
23
Comet Hyakutake (1996)
Many comets discovered every year, as I have told
my students in the past
24
Shoemaker-Levy
Eugene and Carolyn Shoemaker, David Levy
discovered a comet orbiting Jupiter that would
hit it in 1994
25
String of Pearls
26
Impact!
27
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)

28
Asteroids

• Also called minor planets
• Discovered accidentally or with surveys
• Few thousand have determined orbits

29
Gaspra a representative

• Asteroids a few to a few hundred kilometers in
  size (Gaspra 12x20x11)
• Note the craters

30
Ida and its moon Dactyl
Dactyl

• Dactyl 1.5 km diameter image by Galileo probe

31
Asteroid belt

• Belt asteroids
• Trojans-a solution to the restricted 3-body
  problem in celestial mechanics.

32
Bodes 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?
33
NEOs
Earth-crossing Near Earth Objects are of great
concern Their sizes
34
NEO Sizes
35
Near Earth Asteroid Rendezvous (NEAR-Shoemaker)
36
Mathilde
37
Eros in stereo
38
Actual dates later
39
Eros Orbit and Final Descent
Descent/impact was on February 12, 2001