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.