Asteroids

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

13. Eros Scale

16. Eros Landing

17. Mathilde

18. Eros

19. Itokawa

22. 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

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

24. 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

25. 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.

26. 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

27. 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

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

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

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

31. 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.

32. 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.

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