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 Asteroid belt. Same as Jupiter, but separated by 60º - Trojans 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? 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 Ostro et al. 1995 Asteroid light curves. Radar mapping.

Asteroid Encounters Three fly-bys of asteroids: Two orbiters: 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 NEAR Shoemaker took this image of asteroid 433 Eros from a range of 1,150 meters (3,773 feet). The image is 54 meters (177 feet) across. The large rock at lower left is 7.4 meters (24 feet) across. NEAR Shoemaker's image of asteroid 433 Eros taken from a range of 700 meters (2,300 feet). The image is 33 meters (108 feet) across. The large, oblong rock casting a big shadow measures 4.3 meters (14 feet) across. NEAR Shoemaker's image of asteroid 433 Eros taken from a range of 250 meters (820 feet). The image is 12 meters (39 feet) across. The cluster of rocks at the upper right measures 1.4 meters (5 feet) across. This is the last image of asteroid 433 Eros received from NEAR Shoemaker. Taken from a range of 120 meters (394 feet), it measures 6 meters (20 feet) across. What we can see of the rock at the top of image measures 4 meters (12 feet) across. The streaky lines at the bottom indicate loss of signal as the spacecraft touched down on the asteroid during transmission of this image.

Mathilde

Eros

Itokawa

Composition Asteroids are classified into a number of types according to their spectra (and hence their chemical composition) and albedo: 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 S-type, 17% relatively bright (albedo .10-.22) metallic nickel-iron mixed with iron- and magnesium-silicates M-type, most of the rest bright (albedo .10-.18) pure nickel-iron 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? Albedo, size, distance, mass, density, composition. Albedo, size, distance, mass, density. Albedo, size, distance, mass. Albedo, size, distance. 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) 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

Processed: stony-iron Meteorites Primitive 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? Emission of visible light from a particle that has not yet entered Earth's atmosphere. The flash that occurs when a speeding rock from space hits the ground. A star that has suddenly shot across the sky. The glow from a pea-size particle and the surrounding air as the particle burns up in our atmosphere. 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 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.