We've talked about terrestrial planets. 
We've talked about jovian planets. 
we'll probably not have time to talk 
about the many wonderful moons in the 
solar system but I hope you have the 
tools to go out and read about them and 
understand what the criteria are that one 
would apply to try to understand what 
they're like. 
But there are two types of object that I 
still want to mention. 
The first is asteroids. 
So, asteroids are what we call 
planetesimals that, remember, never made 
it to planets. 
most, but not all also never melted and 
differientated and so they're still 
planetesimals, they never made it to 
protoplanet and 
these are of great interest to 
astronomers, because even the moon or a 
museum was chemically differentiated, 
so the chemistry on its surface reflects 
chemistry billions of years ago, but not 
the chemistry of the original nebula 
whereas, some asteroids might preserve 
the actual intact chemistry of the solar 
nebula because their surface has not 
undergone differentiation. 
You will find exactly the traces of 
whatever it was that was around. 
So, there are absolutely perfect museums, 
if you wish. 
some of them some of the rocks flying 
around in space maybe the debris of late 
collision, so do not reflect necessarily 
all of the early structure. 
There are many ways to generate rocks. 
Some never accreted, some might be the 
result of collisions later, and some did 
melt, and those are called dwarf planets. 
a dwarf planer, 
for the record, is something that is in 
solar orbit, was big enough to melt and 
be in hydro, ellipsoid in hydrostatic 
equilibrium, but is not big enough to 
have cleared its orbit of everything 
else. 
This is the definition that caused poor 
Pluto to be demoted from actual planet to 
a dwarf planet. 
the asteroid Ceres as we'll see, is a 
dwarf planet. 
what do asteroids look like? 
Well, this is an image of asteroid PA8, a 
near-Earth asteroid. 
we see as it rotates, we see various 
sides of the asteroid. 
There's a chunk of rock, these are radar 
images. 
This is a near-Earth asteroid, but not a 
very close call. 
This one at its closest was seventeen 
times the distance to the moon from 
Earth. 
But it was good practice at taking 
pictures of an object as it approaches 
the earth. 
these are Vesta, of which we saw 
beautiful images from the Dawn spacecraft 
previously, and Ceres, and we see that 
unlike Vesta, which is a planetesimal, 
Ceres is actually differentiated, 
it's a protoplanet, it's spherical. 
This is the largest of the asteroid, and 
it's the only object for now in the 
asteroid belt that qualifies for dwarf 
planet status. 
And we expect to learn much more about 
it, when Dawn arrives at Ceres soon. 
So, various sizes of objects, 
where do we find them? 
Well, most of them are found in the belt 
between two and three one-half 
astronomical units away from the sun. 
But as this image to the right shows, 
this is actually an image as of December 
1st, 2012, I believe. 
every green dot here is an individual 
asteroid. 
Every yellow dot is an individual 
asteroid. 
The red dots are also asteroids. 
These are asteroids that are near-Earth 
orbit, that come close to the orbit of 
Earth and, as you can see, of course, 
their sizes do not reflect the actual 
sizes of the asteroids on this scale, 
they'd be much too small for us to see. 
But as you can see, there are many, many 
hundreds of thousands of objects that we 
have studied and more and more are being 
discovered. 
I'll give you a link to the rate of 
discovery. 
There's a beautiful animation that makes 
this clear. 
And so, most of them move and then there 
are these gaps Kirk, Kirkwood gaps in the 
asteroid belt in which are very little 
orbits. 
Although, there is the possibility of a 
stable resonance with Jupiter and there 
are some asteroids that orbit in orbits 
that are resonant with Jupiter, 
these that we see in the first part of 
this animation orbit in a two to three 
resonance with Jupiter so that they go 
around three times each time Jupiter goes 
around once. 
We've here, frozen everything so that 
Jupiter appears stationary, so the 
picture is rotating. 
Together with Jupiter, we see the in, in, 
inner planets running around, and Jupiter 
is stationary. 
And we see these populations and note 
that in addition to streaming from sort 
of one accumulation point to the other, 
periodically one of them because of 
interactions with the rest of the swarm, 
gets shot down into the inner solar 
system. 
This collection are the Trojans. 
These orbit in a one to one residence 
with Jupiter. 
You know they're not going anywhere, 
they're just orbiting these points 60 
degrees west and east of the planet along 
its orbit. 
And again, once in a while because of 
interactions with the swarm, one of 
these, or several, will shoot down into 
the inner solar system or be shot way 
out. 
and this is where near-Earth asteroids 
can come from. 
Now, 
when they get deflected and they come 
moving near-Earth, they become meteors. 
When they hit our atmosphere, bits of 
rock in our atmosphere heat up due to 
friction. 
Most of them burn up. 
We see what we call shooting stars which 
are meteors hitting the atmosphere. 
some of them are large enough that even 
after burning, some survives to hit the 
ground, 
that makes them a meteorite. 
So, a meteorite is a meteor that made it 
to the ground. 
farther out, there are more objects. 
there are objects between the orbit of 
Jupiter and Neptune, those are called 
Centaurs. And beyond Neptune, we have 
trans-Neptunian objects. 
They include the Kuiper belt. 
Here is a, a display of known Kuiper belt 
objects of which over a thousand are 
known. 
they stretch out from beyond Neptune's 
orbit at 30 astronomical units all the 
way out to 50 and are strongly influenced 
by Neptune's gravity. 
That's what qualifies them. 
As one would expect from objects way out 
in the distant outer regions of the solar 
nebula, they are very rich in ices so a 
lot of them is made of ice. 
these provide the source of short-term 
comets. 
Once in a while, gravitational 
interaction will shoot one of these into 
the inter solar nebulae and because 
they're so icy, rather than rocky, the 
big difference between a Kuiper belt 
object and an asteroid really is that, an 
asteroid is made of rock and a Kuiper 
belt object is rich in ices. 
And the icy these, these forms of source 
comets, as we will see, the prevalence of 
short period comets leads one to estimate 
that there are over a 100,000 Kuiper belt 
objects with radii above a 100 
kilometers. 
So there are a lot of things out there, 
and then the long period comets as I 
said, predict this Oort cloud, which 
extends out to 50,000 astronomical units 
and is spherically symmetric so 
the solar system goes on for a while. 
And investigating what happens to these 
things, when they do get dipped into the 
solar system, is sufficiently interesting 
that it's worth doing. 
So, when something, collisions or the 
perturbation due to Neptune slows one of 
these trans-Neptunian objects down into 
an eccentric object or by taking them 
into the inner solar system, then they 
encounter, you have this ice ball. 
dusty snowball is what they're called 
with a radius of a few kilometers to a 
100 kilometers, say. 
And this now encounters the more intense 
solar radiation and the more intense 
solar wind in the interior of the solar 
system. 
And this is what creates a comet. 
This is what causes these objects to be 
both visible and sometimes, as the image 
nicely shows, very brilliant. 
they, the the interaction with the solar 
nebula, with the solar radiation, and the 
solar wind creates a coma, the head of a 
comet, 
which is the the brightest part we see. 
And then as we can see nicely in the 
picture, 
two tails, a whitish curved dust tail, 
and a straight blue-glowing ion tail. 
And let's discuss what it takes to make a 
comet. 
So, the nucleus of a comet, when it's 
visible, is hidden inside the coma. 
It's very hard to see. 
Only in the past few years have space 
missions been able to go and orbit the 
nuclei of comets far from the Sun. 
This is Comet Temple. 
And 
some of the things they found were 
somewhat surprising. 
This dirty snowball seems to have 
cratering in it, so it must be a more 
rigid thing than was imagined. 
In fact, somewhat brittle. 
So, there's this picture that what we 
have is sort of an interlaced structure, 
with ices and dust and ices and dust, ice 
serving as the glue, and what happens 
when this object approaches the sun is 
that the ices sublimate. 
They're heated. 
They turn to gas. 
At high pressure, they are ejected in 
jets carrying away dust. 
this is not a dust jet. 
This is the image of the Deep Impact. 
In fact what the Deep Impact spacecraft 
did is it shot a projectile into Comet 
Temple in order to generate a crater and 
study what the ejecta were to try to 
understand the inner structure of the 
comet. 
Studies of the out, of the results of 
this are still ongoing. 
Now, but this comet is closer to the sun. 
We see the sun light hitting it from the 
right and what we see is that on the side 
that the sun warms, we find jets emitted 
of these sublimated volatiles carrying 
away dust in jets. 
And this creates a tenuous dusty 
atmosphere around the comet that because 
the comet's gravity is very weak, this is 
an object a few tens of kilometers in 
radius 
this dust sort of drifts around in a huge 
cloud that can be the size of the Sun. 
And because it's so full of dust, 
reflects sunlight. 
And this is Comet Holmes, which in 2008 
underwent presumably some kind of 
collapse of an ice cave or something 
raising a big cloud of dust and 
generating a coma about the size of the 
Sun that was visible to the naked eye as 
a blurry object. 
but this is why you can not see the 
nucleus of a comet from Earth. Once a 
comet is visible, it's carrying around 
this huge tenuous atmosphere that is the 
coma. 
And then, besides that, there are two 
tails and we saw the two of them. 
the image of a comet zooming across the 
sky with its tail strung out behind it is 
very natural and intuitive but, of 
course, that intuition is based upon 
running around on Earth in the presence 
of an atmosphere. 
a comet does not zoom. 
A comet orbits. 
the orbital periods of comets range from 
a few decades to millions of years and so 
their motion across the sky is very slow. 
They rise and set like stars, of course, 
and then they move across the celestial 
sphere at a rather dignified slow pace, 
a, a comet and a shooting star are very 
different objects. 
shooting stars, as I said, are 
terrestrial. 
We'll talk about that in a second. 
but they do sprout tails. 
These tails are the ejecta gas and dust, 
ejected from the planet being pushed away 
from the sun by both the pressure of 
solar radiation, light carries energy and 
also momentum, and light applies a 
pressure, and so the solar radiation as 
well as the solar wind push some of the 
ejecta out, stringing it out in a tail 
that can be a few astronomical units long 
so astronomical unit, 150 million 
kilometers, the distance from Earth to 
the sun, these are solar system scale 
objects, these tails that always point 
away from the sun so that as a comet is 
approaching the sun, the tail stretches 
behind it. 
As it is moving away from the sun, the 
tail precedes it. 
this is not a problem. 
the two tails have to do with the two 
mechanisms. 
dust, which is indeed, propelled by the 
pressure of the sun 
of both radiation and of the solar wind 
is propelled away from the sun but as the 
comet moves along its trajectory, the 
dust it ejected weeks ago, remains away, 
pointed away from the sun at the position 
where the comet was so it produces this 
magnificent arching structure. 
So here, the sun is sort of has just set. 
So, it's below. 
the dust is being ejected in an upward 
direction but the comet itself is moving 
from right to left so that the dust it 
ejected a few days ago is now far to your 
right. 
on the other hand, the ion tail the gases 
that are pushed away and, and the dust 
tail is, of course, visible because dust 
reflects sunlight, so you see this 
beautiful white tail. 
the ion tail, not evident in this picture 
of comet Mcnaught, but certainly evident 
in this picture does not curve. 
It is always pointed directly straight 
away from the sun. 
the gases are ionized by solar wind, and 
then once the up charged particles, the 
interactions are governed by magnetic 
interactions essentially, 
the comet acquires a magnetosphere. 
The magnetosphere interacts in a 
complicated way with the solar wind. 
And the result is that the ion tail 
always points directly away from the sun 
and is redirected, if you will, as the 
comet moves so it does not curve. 
And ionized gas recombination, the same 
kind of ghostly blue glow that we saw in 
our electron tube is how you characterize 
the ion tail. 
So, these are these beautiful objects, 
comets, but a comet can't last forever, 
it's a this ice ball. 
As the ice evaporates, remember, ice was 
perhaps the glue that held things 
together. 
And a comet can't last forever, how do 
comets end? 
Well, one of four possible ends, I guess. 
One is, 
these things are at highly eccentric 
elliptic orbits. 
They can come close to the wrong object, 
Jupiter, Neptune Uranus. 
And collide gravitationally in such a way 
that they are completely ejected from the 
solar system. 
This certainly can happen especially to 
the Oort cloud comets, that are rather 
weakly bound so a small perturbation can 
send them completely out of the solar 
system. 
And then, they won't come back. 
Another possible end is extinction. 
Eventually, a comet might run out of 
volatiles, have them all sublimated away 
or maybe some of them might be sealed 
deep inside the comet but inaccessible 
and so you have basically a chunk of rock 
now or dust, orbiting the sun. 
It'll never make a comet any more. 
some suggestions are that as the ice 
leaves, the thing compacts and maybe some 
of what we call asteroids are essentially 
extinct comets 
having been deprived of their volatiles 
after passage through the sun. 
another possibility is disintegration. 
as you lose the icy glue that held 
together the dust, either the recoil from 
these jets submitted on the sun side or 
the tidal forces of the sun or some 
planet near which you come, can cause a, 
a comet to disintegrate as in these 
Hubble images brilliantly, Comet 73P. 
The P, by the way, stands for periodic. 
is shown disintegrating and then there, 
the, the bottom image shows from the 
ground, shows that there are many more 
fragments scattered around, so there's a, 
a hierarchical fragmentation going on. 
And then the other possible end for a 
comet is a collision. 
famously in 1994, Comet Shoemaker-Levy 9 
this, did both. 
It first disintegrated into about 
nineteen pieces in the tidal, given the 
tidal force of Jupiter which it was 
approaching. 
And then, sequentially one after the 
other, all of these ploughed into Jupiter 
giving us a rare and at the time unique 
opportunity perhaps to see what ejecta 
they pushed out of the, pulled out of the 
atmosphere and to learn something about 
the 
constitution of Jupiter's atmosphere. 
Sadly, this collision occurred on the far 
side of Jupiter, so we couldn't see it. 
but every telescope on Earth was trained 
on Jupiter, because Jupiter rotates so 
rapidly, that within twenty minutes after 
the collision, the impacted areas faced 
the Earth. 
And in this Hubble telescope image, we 
see at least four of these dark spots in 
the lower right-hand corner of the 
planet, 
which are collision zones, where 
fragments of the comet collided with 
Jupiter. 
this was so successful that 
a decade or so later we dropped a probe 
of our own into Jupiter's atmosphere to 
try to understand the constitution and 
processes inside Jupiter's atmosphere 
with mixed results, I should say. 
so these are, one of these things is 
going to end a comet's career. 
A comet is not a stable object. 
It does not last billions upon billions 
of years like a planet. 
but it leaves a trace and these traces 
are interest, of interest to us. 
And if a comet does not collide with a 
planet like Jupiter, then, 
of course, there's always the option of 
collision with the sun. 
Here we see comet Lovejoy, ploughing on 
its way to the sun. 
We see the magnetic interactions between 
the solar wind and the tail, cause all 
this complicated dynamics in the tail the 
comet is on its way to a firey death. 
we see the comets diving into the sun 
with reasonable regularity. 
The exciting thing about comet Lovejoy, 
is that it actually survived the 
encounter, and came out the other end. 
And so, a year ago there was great joy 
among the sun grazing comet community, 
when Lovejoy actually survived the 
collision. 
You can look it up and find the briefless 
blogs. 
so, comets don't last forever, but they 
leave a trace. 
So, every time a comet comes near the 
sun, a bunch of it evaporates. 
It has this, it leaves behind this dust 
and gas that it ejected. 
all of this dust is scattered about the 
planet's orb, the comet's orbit. 
And as we discussed, what that does is 
the bits that are too close to the sun 
and inner orbits get stretched out ahead 
of the comet. 
These that are a little bit behind 
outside the orbit, become stretched out 
behind the comet, and essentially this 
dust forms a ring-like structure around 
the entire orbit of a comet. 
And after a few passes, the entire orbit 
of a comet is full of this orbiting space 
junk debris from prior passes near the 
sun. 
And what this means is that if a comets 
orbit happens to intersect or come close 
to the Earth's orbit, 
then, well, the timing is off. 
You might not collide with the comet. 
But once a year, when the Earth 
approaches that point in its orbit at 
which it is crossing the trajectory, the 
orbit of a comet then we will beats, we 
will experience an increased incidence of 
space junk, we call that a meteor shower 
and not only, and that's a predictable 
event because we know where the comet's 
orbit is, not only will the incident of 
space junk and shooting stars be 
increased, but because they are all 
orbiting in some particular orbit of a 
particular comet. 
The Earth is moving in a particular 
direction. 
There will be a 
predictable, repeatable direction from 
which all these shooting stars will 
appear to come because they all impinge 
the air, moving in roughly the same 
relevant direction. 
in this beautiful image from the 2001 
Leonid meteor shower in November 2001, 
which was particularly brilliant, 
what we see is indeed, there are shooting 
stars all over the sky. 
These meteors hit the atmosphere where 
they hit it but they all appeared to be 
coming from one point in the sky. 
This is called the quadrant, in this 
case, the quadrant for the Leonid is in 
the constellation Leo. 
They all appear to be coming from there, 
this is, 
if you look at this, 
you'll immediately see the perspective 
point of view. 
It's 
this is the direction from which all of 
these 
space debris is impinging upon the Earth 
and this is the source of our annually 
repeating periodic meteor showers.