Transcript Goal: To understand what comets are and to explore the

Goal: To understand what comets are
and to explore the Oort cloud.
Objectives:
1) To learn about what comets are
2) To understand where comets come from
3) To explore what comets are made of
4) To examine the different tails of comets
5) To understand the evolution of comets
6) To Explore the edge of the solar system
What is a comet?
• Try to think deep here…
• Draw a picture too.
What is a comet?
• Size
• Structure
• Orbit
Where do comets come from?
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Short period comets (periods < 300 years):
Option 1: When 2 icy TNOs collide.
A debris cloud will be formed.
This “debris” cloud will have several rubble
piles which are a few km in size.
• These rubble piles will travel in random
directions. Some will fall in towards the
sun.
Short period comets – cont.
• Usually they will start with an orbit which takes
them to the gas giant region. Here the objects
will be called Centaurs.
• Eventually these Centaurs will have a close
encounter with a gas giant (don’t worry Jupiter, I
won’t name any names).
• At this point they either collide with the planet
(like the Shoemaker-Levy 9 comet), or a moon
of the planet, or they are tossed into the inner
solar system.
Short period comets – option 2
• Have a close encounter with Neptune.
• This can toss it into an orbit which goes
from Neptune to the orbit of the terrestrial
planets (like Earth).
• Or, it could become a Centaur for awhile.
Long period comets
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Orbits are millions of years.
None come from interstellar space.
These come from the Oort cloud.
Collisions are a possibility, but more than likely
these orbits started this way (we will see how
later).
• Although collisions are possible, they are not
likely. There is just too much space out there.
• The only other way to go from circular orbits to
extremely elliptical ones is to have perturbations
from passing stars (stars that occasionally get
within about 0.5 light years from the sun).
Structure of comets (APOD – Bradfield + LINEAR)
Hale Bopp + Andromeda (APOD)
Nucleus – comet Borrelly (APOD)
Deep Impact – Temple 1
After
impact
Wild 2 (Stardust)
Halley’s nucleus (APOD + Giotto)
• 6 m deep of material thrown off each pass
• Cause of the Orionids meteor shower.
• 15 km in size, Density 25% of water. Very dark
nucleus.
apart
What is the fate of comets?
• Eventually, comets eject all their icy
material near their surfaces.
• When this happens, they look more like
metal poor asteroids.
• So, they stop being “comets” like we know
them, and are just a floating rock pile with
some ice in the center quietly floating
through the solar system in its orbit.
Out to the edge of the solar system
• So far we have examined out to about 50
AU (5 billion miles).
• What lies beyond that?
• Sedna hints there may be more structure
out there.
• Long period comets tell us there is a group
of objects which go about 20% of the way
to the next star system.
• This is: the Oort cloud.
Oort Cloud
• Not a lot is known about the Oort cloud.
• We don’t know exactly how many objects
it has.
• We don’t know exactly what these objects
are like.
• We don’t know what structure there may
be (groups of Oort cloud objects)
• We don’t know exactly where it starts and
ends.
Why we don’t know
• Objects get dim as they get far away.
• What we look at is reflected sunlight.
• As you get further from the sun, the light
from the sun spreads out, so the amount
you get drops as the radius squared.
But
• Then, that light gets reflected, and sent
back to us.
• The further from the sun it is, the further
from us it is, and the amount of reflected
light we get here on Earth drops as the
radius squared.
• So, the brightness of an object drops AS
THE RADIUS TO THE FORTH POWER!
Translation:
• If Jupiter were as reflective as Pluto, then
Jupiter would appear the same brightness
as Pluto at a distance of 280 AU (7X
further from the sun as Pluto).
• If Jupiter were more than 28,000 AU away,
even our BEST telescopes would not be
able to spot it.
• Plus, once you get more than 1000 AU
away, the object would move across the
sky so slowly that we would not longer be
able to tell the difference between it and a
normal star.
So
• We cannot see Oort cloud objects when
they are in the “Oort cloud”.
• We can only see them if they get closer to
us.
What we do know:
• Long period comets (comets with periods of
millions of years) come from the Oort cloud.
• Oort cloud objects probably formed in the
gas giant region and were probably tossed
there by Jupiter.
• They are usually a bit bigger than short
period comets, and higher densities.
• A bright one comes into the inner solar
system every 5-10 years.
• Tend to be a bit brighter than short period
comets.
Oort Cloud objects
• Could they have been like this for 4.5
billion years?
• Answer is yes!
• For the long period comets observed to
date, most seem to go out to 50,000 to
100,000 AU.
• 1 light-year is about 60,000 AU, so this is
1-2 light-years.
• The orbital period is about 10 million
years.
Collisions with Jupiter
• Jupiter is 80,000 miles in diameter.
• Jupiter’s orbit has a diameter of 500
million miles (circumference of about 3
billion miles).
• If the Oort cloud object crosses the path
twice every 10 million years, then…
• The estimated collisional time is about 200
billion years.
# of
• Probably there are billions of them.
• If we see 1 bright one every 5-10 years,
and their orbital periods are 10 million
years, that is 1-2 million larger ones (> 10
km) that go into the inner solar system.
• How many don’t go to the inner solar
system?
• How many are a bit smaller?
• There are probably billions of them out
there.
conclusion
• Comets are mostly loosely held together
piles of ice and rock (“dirty snowballs”).
• Comets light up when they get inside the
orbit of Mars because they eject several
meters of ice off their surfaces through
jets.
• Short period comets probably come from
the Kuiper Belt.
• Long period comes probably are the Oort
Cloud.