Fate of Planetesimals • 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) • 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!
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 theyapproach, comets are thought to be part of S.S., in contrast to interstellar wanderers
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
Long-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 Share Question
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