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Asteroids

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.

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Asteroids

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  1. Asteroids Not just for kids anymore.

  2. Goals • What are asteroids? • How do we know? • Why do we care?

  3. 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.

  4. 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)

  5. 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.

  6. 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)

  7. 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.

  8. Vesta – Thomas et al. HST Ceres - HST 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

  9. Ostro et al. 1995 Shapes • Asteroid light curves. • Radar mapping.

  10. 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

  11. Eros

  12. Eros Scale

  13. Eros Landing

  14. Mathilde

  15. Eros

  16. Itokawa

  17. 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

  18. Masses • Kepler’s Third Law • “moon” • spacecraft

  19. 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

  20. 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.

  21. 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

  22. 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

  23. Primitive Processed: stony-iron Processed: iron Meteorites

  24. Copyright – Pierre Thomas (1992) Peekskill Meteorite Copyright – Anne Arundel (1992)

  25. Peekskill Orbit • Parent body aphelion = 2.1 AU • Martin Beech et al. (Univ of Western Ontario) 1995

  26. 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.

  27. 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.

  28. Homework #19 • Due Wednesday 19-Nov: • Read Bennett 12.1 - 12.3. • Do 6, 8, 27, 28, and 32.

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