ASTR100 (Spring 2008) Introduction to Astronomy Asteroids, Comets, and Dwarf Planets

1. ASTR100 (Spring 2008) Introduction to AstronomyAsteroids, Comets, and Dwarf Planets Prof. D.C. Richardson Sections 0101-0106

2. Why is there an asteroid belt?

3. Asteroid Facts • Asteroids are rocky leftovers of planet formation. • Largest is Ceres, diameter ~1,000 km. • 150,000 in catalogs, and probably over a million with diameter >1 km. • Small asteroids are more common than large asteroids. • All the asteroids in the solar system wouldn’t add up to even a small terrestrial planet.

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5. Asteroids are cratered and not round.

6. Eros

7. Asteroids with Moons • Some large asteroids have their own moon(s). • Asteroid Ida has a tiny moon named Dactyl. Ida (approx. 30 km long) Dactyl!

8. Binary 90 Antiope

9. Triple 87 Sylvia

10. Bonus Topic: Origin of Binary Asteroids • If a fragile asteroid passes too close to a terrestrial planet, tides may pull it apart! • Impacts and spin-up can also make binaries…

11. Asteroid Orbits • Most asteroids orbit in a belt between Mars and Jupiter. • Trojan asteroids follow Jupiter’s path. • Orbits of near-Earth asteroids approach or cross Earth’s orbit.

12. Thought Question Why are there very few asteroids beyond Jupiter’s orbit? • There was no rocky material beyond Jupiter’s orbit. • The heaviest rocks sank toward the center of the solar system. • Ice could form in the outer solar system. • A passing star probably stripped away all of those asteroids, even if they were there at one time.

13. Thought Question Why are there very few asteroids beyond Jupiter’s orbit? • There was no rocky material beyond Jupiter’s orbit. • The heaviest rocks sank toward the center of the solar system. • Ice formed in the outer solar system. • A passing star probably stripped away all of those asteroids, even if they were there at one time.

14. Thought Question Which explanation for the asteroid belt seems the most plausible? • The belt is where all the asteroids happened to form. • The belt is the remnant of a large terrestrial planet that used to be between Mars and Jupiter. • The belt is where all the asteroids happened to survive.

15. Thought Question Which explanation for the asteroid belt seems the most plausible? • The belt is where all the asteroids happened to form. • The belt is the remnant of a large terrestrial planet that used to be between Mars and Jupiter. • The belt is where all the asteroids happened to survive. But WHY didn’t they form a little planet?

16. Orbital Resonances • Asteroids in orbital resonance with Jupiter experience periodic nudges. • Eventually those nudges moves asteroids out of resonant orbits, leaving gaps in the belt.

17. Origin of the Asteroid Belt • Rocky planetesimals between Mars and Jupiter did not accrete into a planet. • Orbital resonances stirred up asteroids, scattering most away and preventing the rest from forming a planet.

18. How are meteorites related to asteroids? • Most meteorites are pieces of asteroids.

19. Meteorite: A rock from space that reaches Earth’s surface. Meteor: The bright tail of hot debris from the rock as it plunges through Earth’s atmosphere.

20. Peekskill, NY: October 9, 1992

21. Meteorite Types • Primitive: Unchanged in composition since they first formed 4.6 billion years ago. • Processed: Younger, have experienced processes like volcanism or differentiation.

22. Primitive meteorites: simple, all ingredients mixed together

23. Processed meteorites: shattered fragments of larger objects Iron from a core. Volcanic rock from a crust or mantle.

24. What do we learn from meteorites? • Primitive meteorites reveal the age of the solar system. • Processed meteorites show that differentiation and volcanism happened on asteroids.

25. Meteorites from the Moon and Mars • A few meteorites arrive from the Moon and Mars. • Composition differs from asteroid fragments, matches planet samples. • A cheap (but slow) way to acquire Moon and Mars rocks!

26. How do comets get their tails?

27. Comet Facts • Formed beyond the frostline, comets are icy counterparts to asteroids. • The nucleus of a comet is like a “dirty snowball.” • Most comets do not have tails. • Most remain forever frozen in the outer solar system. • Only comets that enter the inner solar system grow tails.

28. Nucleus of a Comet • A “dirty snowball.” • Source of material for comet’s tail.

29. Anatomy of a Comet • Coma is atmosphere that comes from heated nucleus. • Plasma tail is gas escaping from coma, pushed by solar wind. • Dust tail is pushed by photons.

31. Deep Impact • Mission to study nucleus of Comet Tempel 1. • Projectile hit surface on July 4, 2005. • Many telescopes studied aftermath of impact.

32. Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet’s orbit.

33. Meteor showers appear to emanate from the same area of sky because of Earth’s motion through space.

34. Where do comets come from?

35. Only a tiny number of comets enter the inner solar system —most stay far from the Sun. Kuiper belt: Orderly orbits from 30–100 AU in disk of solar system. Oort cloud: Random orbits extending to about 50,000 AU.

36. How did they get there? • Kuiper belt comets: disk leftovers that share the orderly motions of planets. • Oort cloud comets: disk leftovers kicked far from the Sun by the jovian planets to form giant spherical cloud of comets.

37. How big can a comet be?

38. Is Pluto a planet? • Much smaller than the 8 major planets. • Not a gas giant like the outer planets. • Has an icy composition like a comet. • Has a very elliptical, inclined orbit. • Pluto has more in common with comets than with the 8 major planets.

39. Pluto’s Orbit • Pluto’s orbit is tilted and significantly elliptical. • Neptune orbits 3 times during the time Pluto orbits twice—resonance prevents a collision.

40. Discovering Large Iceballs • In summer 2005, astronomers discovered Eris, an iceball even larger than Pluto. • Eris even has a moon: Dysnomia.

41. Other Icy Bodies • There are many icy objects like Pluto on elliptical, inclined orbits beyond Neptune. • The largest ones are comparable in size to Earth’s Moon.

42. Kuiper Belt Objects • These large, icy bodies have orbits similar to the smaller objects in the Kuiper Belt that become short-period comets. • So are they very large comets or very small planets?

43. Is Pluto a Planet? • In 2006, the International Astronomical Union decided to call Pluto and objects like it “dwarf planets.”

44. What are Pluto and other large objects of the Kuiper Belt like?

45. What is Pluto like? • Its largest moon Charon is nearly as large as Pluto itself (probably made by a major impact). • Pluto is very cold (40 K). • Pluto has a thin nitrogen atmosphere that refreezes onto the surface as Pluto’s orbit takes it farther from the Sun.

46. HST’s View of Pluto and its Moons

47. Other Kuiper Belt Objects • Most have been discovered very recently so little is known about them. • NASA’s New Horizons mission will study Pluto and a few other Kuiper Belt objects in a planned flyby.

48. ASTR100 (Spring 2008) Introduction to AstronomyThe Impact Hazard Prof. D.C. Richardson Sections 0101-0106

49. Have we ever witnessed a major impact?

50. Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen. Tidal forces tore it apart during a previous encounter with Jupiter.