Transcript Fate of Planetesimals 1. 2. 3. Ejection from Sol. Sys. Collision with planets Capture as satellites, or into resonant orbits (e.g., the Trojan asteroids) Fragmentation Preservation to today (however,

Fate of Planetesimals
1.
2.
3.
Ejection from Sol. Sys.
Collision with planets
Capture as satellites, or into resonant orbits (e.g., the
Trojan asteroids)
Fragmentation
Preservation to today (however, not necessarily in
“pristine” condition)
4.
5.
–
–
–
comets
asteroids
meteors
Comets
• “comet” from “kome” (Gr.), meaning
“hair” being descriptive of comet tails
• Comets have been seen since ancient
times, often considered bad omens
• 1577 – Brahe deduced that comets
are farther than the Moon from lack of
parallax (as Earth rotates)
• 1704 – Edmond Halley uses
Newton’s gravity to discover that
comets move on long elliptical orbits:
Comets seen in 146, 1531, 1607, and 1682
are the same [also records for 66 and 451],
with Porb= 75 yrs – Halley predicted its return
in 1758
Halley’s comet!
Comet Ikeya Zhang
Nature of Comets
WHAT “dirty snowballs”
WHERE
• The Oort cloud – named after discoverer Jan
Oort (Dutch), a swarm of “dormant” comets at
~50,000 AU
• Long periods (using P2=a3, P ~ 105-107 yrs)
• Since they travel at less than vesc from the solar
system as they approach, comets are thought to
be part of S.S., in contrast to interstellar
wanderers
The Oort Cloud
Comet Hyakutake
Comet Components
• Nucleus: the “snowball”, of a few km in diam.
• Coma: halo of gases enveloping the nucleus,
about 106 km in diam.
• Tails: can extend to over 1AU; directed away
from Sun
– Plasma tail – driven back by ionized solar
wind
– Dust tail – repelled by sunlight, like mini-solar
sails
Anatomy of a Comet
Comet Holmes (2007)
Tail Development
Share Question
Long-period comets have orbits
a)
b)
c)
d)
e)
the same as the orbits of short-period comets
that are circular
always in the ecliptic
randomly oriented with respect to the ecliptic
of low eccentricity
Comet Close-up
• Giotto, a European probe, obtained 1st close-up
images of Halley’s comet during its 1986
passage
– Peanut shaped
15 x 7 x 10 km
– Jets of gas and dust
Halley’s Comet
Comet Hale-Bopp
Chunks of Shoemaker-Levy 9
Comet Impacts at Jupiter
Stardust
Mission