Transcript Day-39

Astronomy 1010
Planetary Astronomy
Fall_2015
Day-39
Course Announcements
•
SW-chapter 11, 12 due: Mon. Dec. 7
•
1st Thursday Art Walk – 5-8pm on Study Day
•
Exam-4 & Final Exam: 1030-1230: 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 2014.
 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 highaltitude 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.
PROCESS OF SCIENCE
 Scientific decision-making must follow the
evidence, not affection or popular sentiment.
MATH TOOLS 12.1
 We can relate the eccentricity of an orbit to
the object’s closest approach (perihelion)
and farthest approach (aphelion).
CONNECTIONS 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.
MATH TOOLS 12.2
 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 1014 kg
and a speed of 20 km/s have?