This asteroid was visited by the NEAR Shoemaker probe, which orbited it, taking extensive photographs of its

Transcription

1 Chapter 9 Part 1 Asteroids and Comets

2 Why is there an asteroid belt? This asteroid was visited by the NEAR Shoemaker probe, which orbited it, taking extensive photographs of its surface, and, on February 12, 2001, at the end of its mission, landed on the asteroid's surface using its maneuvering jets. Objects in an orbit like Eros can exist for only a few hundred million years before the orbit is perturbed by gravitational interactions. So how can they last?

3 Asteroid Facts Asteroids are rocky leftovers of planet formation. The largest is Ceres, diameter ~1,000 km. There are 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.

4 Asteroids are cratered and not round.

5 Asteroids with Moons Some large asteroids have their own moon. Asteroid Ida has a tiny moon named Dactyl. These are caught in a gravitational dance that isn t too strong (pull in the moon and boom!)

6 Asteroid Orbits Most asteroids orbit in a belt between Mars and Jupiter. Trojan asteroids follow Jupiter s orbit. Orbits of near-earth asteroids cross Earth s orbit.

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

8 Origin of Asteroid Belt Rocky yplanetesimals between Mars and Jupiter did not accrete into a planet. Jupiter s gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet.

9 Meteor Terminology Definitions: Meteorite: A rock from space that falls through Earth s atmosphere. Meteor: The bright trail left by a meteorite. Types: Primitive: Unchanged in composition since they first formed 4.6 billion years ago Processed: Younger, have experienced processes like volcanism or differentiation

10 Pi Primitive iti Meteorites

11 Processed Meteorites

12 Meteorites from Moon and Mars Martian meteorite EETA79001 A few meteorites arrive from the Moon and Mars Composition differs from the asteroid fragments A cheap (but slow) way to acquire moon rocks and Mars rocks

13 How do comets get their tails?

14 Comet Facts Formed beyond the frost line, comets are icy counterparts to asteroids. The nucleus of a comet is like a dirty snowball. Most comets do not have tails. Most comets remain perpetually frozen in the outer solar system. Only comets that enter the inner solar system grow tails.

15 Nucleus of Comet A dirty snowball Source of material Source of material for comet s tail

16 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.

17 Growth of Tail

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

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

20 Meteors in a shower appear to emanate from the same area of sky Meteors in a shower appear to emanate from the same area of sky because of Earth s motion through space.

21 Where do comets come from?

22 Only a tiny number of comets enter the inner solar system; most stay far from the Sun. Oort cloud: On random orbits extending to about 50, AU Kuiper belt: On orderly orbits from AU in disk of solar system

23 How did they get there? Kuiper belt comets formed in the Kuiper belt: flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the planets. Oort cloud comets were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets: spherical distribution, ib ti orbits in any direction.