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 – from lack of parallax (as Earth rotates), Brahe deduced that comets are farther than the Moon 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 successfully predicted its return in 1758 Halley’s comet!

Comet Ikeya Zhang

Comet Lovejoy

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, so 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 the Sol. Sys., in contrast to interstellar wanderers

The Oort Cloud

Example of a long-period comet out of the ecliptic Comet Hyakutake Example of a long-period comet out of the ecliptic

Comet Components Nucleus: the “snowball” core of a comet, 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, a pressure induced by photon momentum

Anatomy of a Comet

Comet Holmes (2007)

Tail Development

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

Model for comet tumbling and gas jets Comet Hale-Bopp Hale-Bopp from 1997 passage Model for comet tumbling and gas jets

Chunks of Shoemaker-Levy 9

Comet Impacts at Jupiter

Comet Ison (2013) Comet Ison enters from lower right, is disrupted during close passage of Sun, and emerges at upper right (time lapse picture).

Sun Grazing Comets Sungrazing comets are fairly common. These were discovered by the SOHO satellite that constantly stares at the Sun.

Stardust Mission

Share Question Long-period comets have orbits a) that are mostly circular b) that are always in the ecliptic c) that are randomly oriented with respect to the ecliptic d) of low eccentricity

Oumuamua interstellar interloper

‘Oumuamua interstellar interloper Approached within 0.22 AU of Sun in Sep 2017 About 230 x 35 meters Has a tumbling rotation Lacked a cometary tail, so is likely metal-rich