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?

• 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• 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.