Download Comets: Ancient Beliefs and Modern Discoveries and more Exams Astronomy in PDF only on Docsity! 1 Astro 120 Fall 2005: Lecture 23 page 1 Reading: Chapter 13, Section 13.4; Chapter 15 Exam: Final Grades by Thursday Last Time • Astrology & Pseudoscience - hallmarks of science (section 3.4) explanations based on natural causes creation and testing of predictive models Today • Comets Astro 120 Fall 2005: Lecture 23 page 2 Detail from Bayeux Tapestry Through the ages, comets have been superstitiously regarded as heralds of strange events. Comets – ancient ideas Astro 120 Fall 2005: Lecture 23 page 3 Comets – ancient ideas Earliest recorded comet was in 1059 BC in China. “a broom star appeared with the handle pointing east.” Aristotle (384-322 BC) believed that comets were “atmospheric exhalations” in the sublunary realm. Chinese comet atlas (c. 300 BC) What experiment could you construct to test this idea? Astro 120 Fall 2005: Lecture 23 page 4 Tycho Brahe: “Comets are Distant” 2 Astro 120 Fall 2005: Lecture 23 page 5Comet Halley • Edmond Halley (1656-1742) was the first to realize that comets orbit the Sun, as do the planets. •Comets of 1531, 1607, and 1682 are similar •So… another comet in 1682+76 years 1758/9 ? •Halley computes the orbits for 24 comets. Astro 120 Fall 2005: Lecture 23 page 6 • First recorded in China in 240 BC • Closest to Earth 837 A.D. (0.03 AU) • Last return to inner solar system 1986 • Next return to inner solar system 2062 • Orbital period is 76 years Comet Halley Astro 120 Fall 2005: Lecture 23 page 7 Comets – some questions What are they made of? Why do they change appearance? Where do they come from? Why do astronomers care about them? Astro 120 Fall 2005: Lecture 23 page 8 Composition • Ices mostly H2O, also CO2, CO, NH3, CH4, H2CO Rock & Dust - silicates, carbon compounds Nucleus • Typical size: a few km; smaller probably common; rarely 10 km - 100+ km; Coma • surrounds nucleus when near the Sun – HUGE! • sublimated gas & dust Comets are ice-rich planetesimals (bits left over from the formation of the solar system) – ice mixed with rock and dust, a “dirty snowball” Comets – modern ideas 5 Astro 120 Fall 2005: Lecture 23 page 17Comet Hale-Bopp Astro 120 Fall 2005: Lecture 23 page 18Most Comets Look Like This… Comet Encke (3.3 yr. period) Astro 120 Fall 2005: Lecture 23 page 19ESA’s Giotto spacecraft photographs Comet Halley on March 13, 1986 at a distance of 600 km from the nucleus. Potato-shaped Halley has a nucleus that is roughly 15 km across. Astro 120 Fall 2005: Lecture 23 page 20Comet Borrelly from Deep Space 1 9/22/01 Size: 8 x 4 km 6 Astro 120 Fall 2005: Lecture 23 page 21 Comet Wild 2 from Stardust Nucleus is roughly 5 km across. Astro 120 Fall 2005: Lecture 23 page 22 Comet Tempel 1 50oC-15oC Nucleus is roughly 5 km x 7 km in size. Astro 120 Fall 2005: Lecture 23 page 23 • If a comet loses about 0.1% (0.001) of its mass every time it passes the sun how long can it last? • Consider roughly what percentage of comet Halley has been lost 760 years ago, 7600 years ago and 76000 years. Where do they come from? What conclusion can you draw from this calculation? Astro 120 Fall 2005: Lecture 23 page 24 Short- Period Comets: Tempel 1 Halley Long-Period Comets: Hyakutake 7 Astro 120 Fall 2005: Lecture 23 page 25 Origin of the Long Period Comets – Oort Cloud Formed from icy objects ejected from the early solar system Scale – up to 100 000 AU (~ 1.5 ly !) Astro 120 Fall 2005: Lecture 23 page 26 Found primarily in a thick band between 30 and 50 AU. Origin of Short Period Comets – Kuiper Belt Scattered population Orbit between 35 and 100’s AU High inclinations, eccentricities
