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NASA’s WISE Mission and Hands On Universe

NASA’s WISE Mission and Hands On Universe. Space Rocks! A Teacher’s Resource Primer. The asteroid belt. Between orbits of Mars and Jupiter Resonance: “Should” be a planet there? Jupiter’s gravitational perturbations probably prevented coalescence into a planet. Click on image!. What it is.

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NASA’s WISE Mission and Hands On Universe

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  1. NASA’s WISE Missionand Hands On Universe Space Rocks! A Teacher’s Resource Primer

  2. The asteroid belt • Between orbits of Mars and Jupiter • Resonance: “Should” be a planet there? • Jupiter’s gravitational perturbations probably prevented coalescence into a planet Click on image!

  3. What it is If it’s in orbit about the Sun it’s Minor Planet, size doesn’t matter If it’s rocky it’s an asteroid If it’s icy it’s a comet If it’s a comet debris field in space it’s a meteoroid If it’s in the atmosphere it’s a meteor or “shooting star” producing a meteor shower If it hits the planet it’s a meteorite

  4. Trojan asteroids The asteroid belt

  5. How did asteroid belt get there in the first place? • Current mass 0.0005x mass of Earth • original belt may have been 100 - 1000x more massive • Jupiter’s gravity strongly perturbed the orbits of almost all asteroids • Most nudged into highly eccentric orbits, either leaving Solar System or headed inwards toward Sun • A fraction headed inwards may have hit early Earth!

  6. Asteroids are quite far apart (not like Sci Fi) • About 100,000 asteroids larger than 1 km • Not much mass: if gathered in a sphere, they would make a body less than 1000 km in diameter • Mean distance between asteroids is several million km! • If on an asteroid and looking up, you could see at most one other asteroid • Bennett estimates there is ONE major collision in the asteroid belt every 100,000 years

  7. How do we study asteroids? • Detection: streaks on time-exposed images • Spacecraft: directly measure size, shape, etc (only a hand full of asteroids so far) Click on Image: How to use Astrometrica!

  8. Sorting by Classification M S C

  9. S asteroids (silicaceous) 951 Gaspra 433 Eros Ida (and Dactyl) (true color) • 19 x 12 x 11 km 33 x 13 x13 km 58 x 23 km (1km) • Galileo flyby, 199 NEAR orbit/landing Galileo flyby, 1993 • Grooves, curved near-Earth asteroid, member of Koronis depressions, ridges space weathering family, first ID of (Phobos-like) effects documented asteroid ‘moons’

  10. C Asteroids (carbonaceous) • 253 Mathilde; 66 x 48 x 46 km, visited by NEAR Shoemaker • Surface as dark as charcoal; typical outer belt asteroid

  11. Asteroid Composition M - metalP - dirty ice S - silicate D - very dark ice C - carbonaceous

  12. Diversity of Asteroids

  13. Plot of the position of all known asteroids in the inner solar system. Green: Main belt asteroids. Red: Earth cross- ing asteroids Blue (small): Trojan asteroids

  14. Asteroid Groups Earth crossing Atens Apollos Mars crossing Amors Asteroid Belt Trojans - near Jupiter Centaurs Kuiper Belt Centaurs Temporary orbits at 20-50 AU, from Saturn to Neptune Kuipers Stable region Around 35,000 objects of size 100-300 km around Neptune and Pluto (30-50 AU) Belt extends to 400 AU A flattened belt!

  15. Near Earth asteroids come in three varieties:

  16. Light curves of asteroids • Asteroids spin (typically in 8 hours) changing brightness as longer or shorter side reflects more or less sunlight • By measuring light curves (brightness vs. time) from different viewing angles, can get 3 dee shape of asteroid

  17. Asteroid Light Curves SHAPE EFFECT MATERIAL EFFECT

  18. Asteroid Photometry Sample Light curves

  19. Spectra of Some Asteroids

  20. Binary Asteroids

  21. Binary asteroids show additional “bumps and wiggles” in lightcurves due to eclipses/occultation

  22. Bodies in space mutually orbit each other, like ice skaters, producing “wobbles” and other effects

  23. 3D shapes from Lightcurves

  24. An Asteroid Skips in the Atmosphere Poland: May 31, 2009

  25. Whoops! Forgot to duck!

  26. Tunguska Impact Event, Siberia June 30, 1908

  27. Tunguska Event • 10 – 50 m diameter • Probability of impact: 1 in 5 years

  28. They fell like rain . . . • China 1490 10 000 deaths • 100 m diameter • Every 1 000 years

  29. A Bad Day for Dinosaurs • 15 km diameter • 1 in every 65 million years

  30. Protecting the Earth

  31. June 2009 Earth-asteroid encounters:

  32. Measuring asteroid sizes • Angular size of asteroids is very small, so not generally possible to measure sizes directly • Amount of light reflected depends on size, distance and reflectivity (albedo) • Compare infrared (reradiated) and optical (reflected) luminosities to determine albedo.

  33. Rotation speeds • What is the maximum rotation rate an asteroid may have, before it flies apart?

  34. Rubble Piles

  35. 25143 Itokawa How are these features different than a typical moon or asteroid?

  36. Asteroid jiggles like a jar of mixed nutsResearchers say Itokawa seismically active, despite being so small YORP Effect: slow and steady sunlight can over the eons substantially alter an asteroid's rotation rate and axis orientation in semi-predictable ways Asteroid Shimmy “cashew effect”

  37. Asteroid Spins • Most large (> 150 m) asteroids spin slower than the rubble breakup limit. Pravec & Harris 2000 • 3.0 g/cc

  38. Measuring albedos • First, we need to know how much solar flux is intercepted by the asteroid at its distance from the Sun, r. • The total energy reflected depends on both the cross section and albedo of the body. In the visible wavelength region we have: • where R is the asteroid’s radius and AV is the visual albedo • Any flux not reflected is absorbed and will go to heating the asteroid which will then emit thermal radiation; so we can write:

  39. Measuring albedos • Now assume that the Earth is between Sun and asteroid (like the full Moon) and that we see the radiation reflected over 2π steradians. This means that what we actually observe is: • Once we have measured fth and fref we can determine AV: • where d is the distance to the asteroid. At opposition: • D = rasteroid - rSun • For the case of the thermal radiation from the asteroid we use similar logic, in this case assuming that the asteroid rotates rapidly enough that it is uniformly heated and the thermal radiation we observe is similarly normalized, this time by a factor of 4π:

  40. Large craters and low density of Mathilde imply high porosity.  ~59 km  NEAR Low Densities • Many asteroids appear underdense, particularly C-class asteroids.

  41. Densities • Requires mass measurement, from orbiting spacecraft or small moons • E.g. Eugenia (below) • Orbit of small moon can be measured. • From this orbit we can get a pretty good mass estimate: 6x1018 kg • Inferring a size from its brightness, we find a density of only about 1200 kg/m3. • Probably a loosely bound rubble pile.

  42. Asteroid densities • Low-density Mathilde: • Huge crater evidence of large collision • Probably broke up the asteroid, but it is still loosely held together

  43. Latest Evidence • Galileo flyby of Amalthea revealed bulk density of just 1 g/cc for this 270 km moon. Leading Trailing

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