Transcript Document
Asteroids Not just for kids anymore. Goals • What are asteroids? • How do we know? • Why do we care? Asteroids • All planets and moons have been modified chemically and geologically. • Where do you look for a piece of the original “stuff” of the solar system? • Asteroids and comets. • Small objects – Little internal heat, little to no geological activity. – Little gravity, little to no atmosphere. Orbits 1. Asteroid belt. 2. Same as Jupiter, but separated by 60º Trojans 3. Elliptical orbits that pass Earth • Earth-crossing asteroids: – Near-earth asteroids (NEAs) – Near-earth objects (NEOs) Asteroid sizes • How big are they? – – – – Largest (Ceres) is 940 km in diameter Three larger than 500 km About a dozen larger than 250 km Number increases rapidly with decreasing size • Total volume of all asteroids ~ much smaller than moon. How do we know? • Compare IR light to visible light. – Visible light: what light a body reflects. – IR light: what a body emits because of its temperature = what light it absorbs. • Albedo = function of vis/(vis + IR) • Size = function of (vis/albedo)*distance – Assume all asteroids have same albedo and at same distance: Size ~ vis – Allow different distances: Size ~ vis*distance – Allow different albedo: Size ~ (vis/albedo) Concept Test • Suppose you discover two asteroids that are equally bright in the visible but IR observations tell you Asteroid#1 is more reflective than Asteroid#2. What can you conclude? a. b. c. d. e. Asteroid#1 is larger than Asteroid#2. Asteroid#1 has a lower albedo than Asteroid#2. Asteroid#1 is farther away than Asteroid#2. Asteroid#1 is warmer than Asteroid#2. None of the above. What do asteroids look like? • Shape generally depends on size. • Gravity tries to make things spherical (hydrostatic equilibrium). – Largest (Ceres) is 940 km in diameter - spherical – Three larger than 500 km (Vesta) – mostly spheroidal – Smaller than 250 km irregular • Gravity Total mass of all asteroids ~ 5% of the moon Ceres - HST Vesta – Thomas et al. HST Shapes • Asteroid light curves. • Radar mapping. Ostro et al. 1995 Asteroid Encounters • Three fly-bys of asteroids: – Gaspra by Galileo in 1991 – Ida by Galileo in 1993 – Mathilde by NEAR in 1999 • Two orbiters: – Eros by NEAR in 2000 – Itokawa by Hayabusa in 2005 Eros Eros Scale Eros Landing Mathilde Eros Itokawa Composition • Asteroids are classified into a number of types according to their spectra (and hence their chemical composition) and albedo: 1. C-type, includes more than 75% of known asteroids: – extremely dark (albedo 0.03) – approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles 2. S-type, 17% – relatively bright (albedo .10-.22) – metallic nickel-iron mixed with iron- and magnesium-silicates 3. M-type, most of the rest – bright (albedo .10-.18) – pure nickel-iron 4. There are also a dozen or so other rare types Masses • Kepler’s Third Law – “moon” – spacecraft Density • • • • • • • • Calculate Density Rock ( ~ 3g/cm3) vs. metal (~7g/cm3). Solid vs. rubble pile. Ida = 2.6 g/cm3 Eros = 2.4 g/cm3 Itokawa = 1.9 g/cm3 Mathilde = 1.5 g/cm3 Eugenia = 1.12 g/cm3 Concept Test • I discover an asteroid all by itself. Without sending a spacecraft there, what can I determine about the asteroid? a. Albedo, size, distance, mass, density, composition. b. Albedo, size, distance, mass, density. c. Albedo, size, distance, mass. d. Albedo, size, distance. e. Albedo, size. Meteorites • Want real sample of this material. • Hayabusa sample return – one asteroid. • Meteorites potentially many asteroids. • Really piece of asteroids? – Compare spectra. – Compare trajectories (observed falls). Copyright - Wally Pacholka Meteorites • • • • Meteoroid – the particle in space. Meteor – the fiery streak through the sky. Meteorite – the rock on the ground. Types (2 main) 1. 2. • Primitive – mix of rock and metals Processed – rocky or metallic (from differentiated asteroid or “parent body”). Compare spectra to find parent body: – – – Asteroid (e.g. Vesta) Moon Mars Meteorites Primitive Processed: stony-iron Processed: iron Peekskill Meteorite Copyright – Pierre Thomas (1992) Copyright – Anne Arundel (1992) Peekskill Orbit • Parent body aphelion = 2.1 AU • Martin Beech et al. (Univ of Western Ontario) 1995 Concept Test • When you see the bright flash of a meteor, what are you actually seeing? a. Emission of visible light from a particle that has not yet entered Earth's atmosphere. b. The flash that occurs when a speeding rock from space hits the ground. c. A star that has suddenly shot across the sky. d. The glow from a pea-size particle and the surrounding air as the particle burns up in our atmosphere. e. None of the above. Concept Test • I find a meteorite that is composed entirely of rock (no metal). Assuming it’s from an asteroid, what type of parent body is it probably from? a. b. c. d. e. A very small asteroid. A part of the core of a very large asteroid. A part of the outer layers of a very large asteroid. From the heart of a differentiated asteroid. It is not possible to tell. Homework #19 • Due Wednesday 19-Nov: • Read Bennett 12.1 - 12.3. • Do 6, 8, 27, 28, and 32.