Fate of Planetesimals1. Ejection from Sol. Sys.2. Collision with planets3. Capture as satellites, or into resonant orbits

(e.g., the Trojan asteroids)4. Fragmentation5. Preservation to today (however, not necessarily

in “pristine” condition)– 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 1758Halley’s comet!

Comet Ikeya Zhang

Nature of CometsWHAT “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 QuestionLong-period comets have orbits

a) the same as the orbits of short-period comets b) that are circular c) always in the ecliptic d) randomly oriented with respect to the ecliptic e) of low eccentricity

Comet Close-up• Giotto, a European probe, obtained 1st

close-up images of Halley’s comet during its 1986 passage

– Peanut shaped15 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