Astronomy 1010 Planetary Astronomy Fall_2015 Day-39

Course Announcements SW-chapter 11, 12 due: Mon. Dec. 7 1 st Thursday Art Walk – 5-8pm on Study Day Exam-4 & Final Exam: : Mon. Dec. 7

 Asteroids are rocky planetesimals.  Most are in the asteroid belt between Mars and Jupiter.  Near-Earth asteroids have orbits that come close to the orbit of Earth.

 Amor, Apollo, and Aten asteroids have orbits near Earth.  Apollo and Aten asteroids have orbits that could cross Earth’s—could collide with Earth.  Called near-Earth objects.

 Asteroids are fragments of rock.  Generally not large enough to be spherical.  Erratic rotation periods.

 S- and M-type asteroids differentiated.  C-type asteroids did not.  S-type are similar to igneous rock; M-type to iron and nickel.

i_Clicker Question Solar System Debris: Asteroid Resonances

 Spacecraft have visited seven asteroids.  It is possible for them to have moons.

Comets  Comets are icy planetesimals found beyond the planets.  Far from the Sun, these objects remain small, icy bodies that are very hard to see from Earth.  Located either in the Kuiper Belt or the Oort Cloud surrounding the Solar System.

 Short-period comets:  Periods < few centuries.  Near ecliptic plane.  Prograde orbits, circular or somewhat elongated.  Kuiper Belt.

 Long-period comets:  Periods of almost 1000 to perhaps 1 million years.  Prograde or retrograde orbits, from the Oort Cloud.  Large tilts from the ecliptic, very elongated orbits.  Nucleus not “worn out.”

 Comet nucleus is an ice/rock mix.  “Dirty snowballs.”  Size of nucleus ranges from a few dozen meters to a several hundred kilometers.  This is the comet’s appearance when far from the Sun.

 When near the Sun: active comets.  Sun heats the icy nucleus, causing sublimation.  Sublimation forms: Coma. Ion tail. Dust tail.

 Comets have two tails.  Ion tail created by the solar wind interacting with ions of the nucleus.  Dust tail created from solar wind and sunlight.  Comet tails point away from the Sun.

 Seven comets have been visited so far by spacecraft, with one more in  A projectile hit Comet Tempel 1 in order to study its contents.

 Large collisions in space are not frequent, but they occur.  1994: Comet Shoemaker-Levy 9 crashed into Jupiter, leaving visible scars.

 Comet or asteroid impacts on Earth are infrequent, but devastating.  1908: Tunguska event was possibly the high- altitude explosion of an asteroid or comet.

i_Clicker Question Solar System Debris: Characteristic of Debris

 Meteorites are pieces of asteroids that have fallen to Earth.  In space, a meteorite is called a meteoroid.  While passing through the atmosphere, it is a meteor.

 Comet nucleus disintegration and asteroid collisions make debris.  Earth passing through a collection of debris results in meteor showers, such as the Perseids or Leonids.  Single pieces of debris result in sporadic meteors.

 Meteorites are pieces of asteroids.  Over 90% are stony, like Earth rocks.  Chondrites have chondrules; achondrites do not; some have carbon.  Can use meteorites to date the age of the Solar System (4.5 billion years).

 Iron meteorites have high concentrations of metal, with a melted and pitted appearance.  Stony-iron meteorites are a combination and are relatively rare.

 Sunlight reflects off ground-up asteroid and cometary debris in the inner Solar System.  This zodiacal dust in the plane of the Solar System makes the zodiacal light.

 Comets and asteroids have affected life on Earth in the past and may in the future.  In the early history of Earth, icy planetesimals most likely collided with Earth and deposited water.  Comets also possess complex organic material.  Life on Earth is threatened in the future by potential comet and asteroid collisions.

 Scientific decision-making must follow the evidence, not affection or popular sentiment. PROCESS OF SCIENCE

 We can relate the eccentricity of an orbit to the object’s closest approach (perihelion) and farthest approach (aphelion). MATH TOOLS 12.1

 There are areas in the asteroid belt that lack asteroids (Kirkwood gaps).  These correspond to orbital resonances with Jupiter.  Repeated tugs from Jupiter would prevent an asteroid from staying in that orbit. CONNECTIONS 12.1

 The energy that can be released by the impact of a comet nucleus is equal to its kinetic energy:  How much energy does a 10-km diameter comet nucleus with a mass of 5 x kg and a speed of 20 km/s have? MATH TOOLS 12.2