Download Theme 10.1 -- Leftovers: Comets

Theme 10.1 --
Leftovers: Comets
Introduction and History
The word “comet” comes from Latin and Greek roots meaning “wearing long
hair.” In other words, a comet looks like a star with long tendrils of
hair dangling from it, as is clear in this picture. Not surprisingly,
comets have only recently become understood physically. For thousands
of years they were seen in a variety of ways. Aristotle, for one,
believed that they were caused by gases rising up from the Earth into
the atmosphere and igniting in some way. The ancient Chinese believed
that they were celestial omens in the heavens that marked important
events such as the death of kings. That view was also expressed in
Europe until around the time of the Copernican Revolution. Here's a
quote from Shakespeare that expresses that thought.
Looking back in the historical record we've been able to trace the fact
that Halley's Comet has been observed since about 2000 years ago at
least. It actually shows up in a couple of interesting historical
renderings. The Bayeux Tapestry in the top left here was woven shortly
after the Battle of Hastings in 1066 A.D. What you see in the picture
is men marveling at a star in the sky: the Latin ISTI MIRANT STELLA
means these men are marveling at the star. The star can be seen in the
upper right, and it is almost certainly a depiction of Halley's Comet,
which passed shortly before the Battle of Hastings and was taken as an
omen, for good or for ill. On the bottom right we see Giotto's
Adoration of the Magi, a nativity scene from about 1305. At the top of
the picture we see his representation of the Star of Bethlehem, which
was very likely motivated by his observation of Halley's Comet, which
passed at about that time.
The truly scientific study of comets began with Halley. He was the
first to look at historical records of many old comets and he
recognized that three of them were, in fact, the same comet. It
appeared, originally, in the same part of the sky and seemed to follow
similar paths across the sky. So he made the bold prediction that this
would return again in 1758, an event that he did not live to see, and
understood this in terms of Newton's new laws that governed the nature
of the solar system. The link at the bottom of the page will take you
to a little animation that shows the orbit of Halley's Comet around the
solar system and a couple of other comets as well.
The Physical Nature of Comets
Comets are really only conspicuous when they're near the sun, and
here's a representation of how they appear at that time. From the
central nucleus, which is very small as we'll learn, frozen material
evaporates and expands to form an enormous coma, or head, which can be
as big as the Earth itself, tens of thousands of kilometres in
diameter. From the head, a long tail streams out in the direction that
points away from the sun. The tail, as you see, has two parts, which
we'll discuss in a moment. The coma and the tail reflect a lot of
sunlight but they also fluoresce like a neon lamp: they absorb
ultraviolet light from the sun and re-emit it in the visible so the
comet becomes very bright and conspicuous.
Far away from the sun, however, the comet is completely inconspicuous
and can be thought of as a “dirty snowball,” to use the phrase first
introduced by astronomer Fred Whipple in the 1950s. The nucleus would
be about 10 kilometres in diameter and consist of pebbles and small
stones in a matrix of frozen ices: water, ammonia, methane, frozen
carbon dioxide, and so on. If you imagine snow covering the Maritime
Provinces, as in the picture here, and then rolling it up into a loose
ball, picking up bits of gravel and dirt with it, forming an object
about 10 kilometres across, that would be very much like a comet -except that, on Earth, that would, of course, be nothing but pure water
ice. A comet contains other species as well.
Near the sun, comets form tails as we saw, and there are two points to
consider about the tails of comets. Let's compare them to animal
tails. As you know, when animals move, their tails are behind them.
Here we see tadpoles moving to the upper left with their tails wiggling
behind them. A comet, on the other hand, can move in any direction
with a tail pointing away from the sun. On the top right we see three
photographs of Comet Hyakutake taken on three successive days. The
comet is moving to the upper left, but the tail points away from the
sun, which is to the lower left. The tail gives no indication of the
direction of motion of the comet. Secondly, we noted earlier that
comets can have two tails, as seen on the bottom right. Two-tailed
animals are very rare. It's a genetic fluke, as for the cat on the
lower left, but two tails in comets are quite common.
We understand why comets have two tails: one of them consists of ions,
which are charged particles, single atoms or simple molecules, that are
very low in mass and they readily push straight out, directly away from
the sun at high speed. They have a characteristic bluish colour. The
other tail is made of dust and pebbles that are small, solid lumps, and
because they're more massive they move more slowly. This results in a
long, curved tail that lags behind the comet as it moves along its
orbit.
Until recent decades, many comets were discovered by amateurs who would
survey the skies with small telescopes. The man at the top centre here
is David Levy, who is actually a product of Queen's University although
he studied English here, not astrophysics. He has a very handsome
record of finding comets. On the top right, though, is the world
record holder, William Bradfield of Australia, now deceased, who
discovered 18 comets, a record that will never be broken by any
individual because comets nowadays are found routinely by sensitive
electronic detectors in all-sky studies. The role of the dedicated
amateur has been much diminished.
As I mentioned, David Levy is a product of Queen's University in the
English Department but he went on to a very fine career in
observational astronomy, studying comets, in particular with the couple
you see here, Gene and Carolyn Shoemaker. Gene was the first to
persuade scientists that lunar craters are from impacts, not volcanoes,
and we'll see them in the context of a very important impact event
shortly.
The Fate of Comets
Comets cannot last forever. Each time they pass by the sun, some of
the frozen material in the nucleus evaporates and moves off into the
coma and the tail. So, eventually, they're bound to be disrupted. In
fact, Comet Halley has an expected lifetime of only about 30,000 years.
That sounds a long time but is very small in astronomical terms.
Eventually comets can be disrupted in this way and vanish.
Comets can suffer a fate other than being disrupted by evaporation.
They're very lightweight objects, and as they move around the solar
system their orbits can be strongly perturbed by the gravity of the
planets, in particular Jupiter. The new orbits may lead to their
ejection from the solar system, or to them colliding with planets or
even the sun itself. The link here shows a comet moving in towards the
sun and having an apparent collision with it. (The sun itself is
hidden by an occulting disk so that its brightness doesn't dominate the
picture, and this is a sequence of images taken from a satellite in
orbit.)
Finally, comets can be disrupted by the tides of a major planet, in
particular Jupiter. We discussed this earlier in the context of tides,
and here we see the example that we described then. This is the comet
Shoemaker-Levy, which was found in 1994. It had already been broken
into pieces in a previous near pass of the planet Jupiter and,
interestingly, the pieces were now moving like cars on a train, one
after another, in an orbit that took it back to collisions with Jupiter
itself, one piece after another.
As you can see from these images, it's difficult to discern any detail
of the pieces of Comet Shoemaker-Levy as it moves, and astronomers were
not able to tell whether these were chunks of kilometre size or larger,
and whether they were fairly solid or, indeed, fragile objects -- and
therefore it was very difficult to predict exactly what would happen as
each of these, in turn, fell into the gaseous atmosphere of Jupiter.
One possibility was that they would plunge in without much effect at
all, just as one might throw a brick from a plane, see it plummet into
the clouds of the Earth with no effect. On the other hand, it was
possible that the energy released on impact would be very considerable.
So astronomers waited with interest to see what would happen.
Here we see on the upper left the record of the impacts on the planet
Jupiter. This is an infrared image, and the object to the upper left
is, in fact, the hot moon Io glowing in the infrared. But at the bottom
of the planet you can see the spots which mark the points at which the
various chunks of Comet Shoemaker-Levy plunged into the atmosphere of
Jupiter. The enormous energy released at that time led to this
infrared glow. The pieces do not fall in onr after another in the same
spot because of the rotation of the planet, so they are in different
locations, as you see. Any one of these pieces landing on the Earth
would have released enough energy to effectively eradicate all human
civilization and have a very profound effect on the biosphere. The
picture on the right shows an ordinary visible light photograph of
Jupiter where the dark spots are where the pieces plunged into the
gases. The chemistry was affected: different compounds were formed,
and the colours were changed as a result, but those affects have long
since vanished so Jupiter does not show any trace of the impacts today.
The Replenishment of Comets
As we have seen, comets can evaporate and be disrupted, or they can
collide with planets or escape the solar system entirely. The
interesting question, then, is why we still see any comets at all.
After 4.6 billion years why are there any still left in the solar
system? The answer is that there is a steady replenishment of comets.
This requires a mechanism for injecting new comets, and a reservoir, a
source of primitive comets. There are, in fact, two such sources,
known as the Kuiper Belt and the Oort cloud, both named after Dutch
astronomers, as it happens.
Here is an artist's rendition of the Kuiper Belt, which consists of
many billions of cometary nuclei in orbits out beyond the orbit of
Neptune. You'll see that it's a flattened distribution. Also shown is
the orbit of Pluto, which is inclined and quite eccentric, as you
notice -- very different from the orbit of the eight inner planets.
So the Kuiper Belt, beyond the orbit of Neptune, is flattened like the
solar system itself and contains many of these tiny pristine comets
that have never yet approached the sun. It also, of course, contains
some larger Kuiper Belt objects, or KBOs as they're known. Some of
them are shown here. These have been discovered telescopically and
notice that at least one of them is at least as big as the planet
Pluto. Pluto is, in fact, a Kuiper Belt Object, not a planet in the
conventional sense of the word, which is why astronomers redefined it
some years ago.
Far beyond the Kuiper Belt, however, there is an enormous reservoir
known as the Oort cloud: a cloud of cometary nuclei that surrounds the
solar system and stretches out a considerable distance towards the
nearest stars. This cloud is also spherical: there's no flattening to
it at all. The comets orbit at random around the solar system at the
centre.
Given these reservoirs of comets, how do they provide replenishment of
the comets in the inner part of the solar system? The answer is that
individual objects may occasionally be given a ‘nudge.’ A nearby
passing star could have a gravitational effect on a comet in the Oort
cloud. Kuiper Belt comets may be affected by the gravity of one of the
larger KBOs. In any event, a tiny adjustment to the orbit of one of
them may lead to it falling in towards the centre of the solar system.
Of course, a comet falling in towards the centre of the solar system
has no guarantee of being captured. As we see on the left, one might
expect it to fall in, picking up speed as it goes because of the sun's
gravitation, then to come whipping around and go back as far out as it
originated from, thanks to the conservation of energy. The difference
is that some of these will pass near the planet Jupiter as they fall
in, and the gravitational effect of Jupiter will put them into new,
smaller orbits. Of course, the passage also has a tiny effect on the
motion of Jupiter itself but, again, this is microscopic by comparison.
So the comet takes up a new orbit that is close and now in the inner
solar system, replenishing some of the others that have been lost over
time.
Here then is a birth-to-death story of a comet. After formation in the
original solar system nebula, 4.6 billion years ago, a cometary nucleus
may spend essentially all of that time in the Oort cloud or the Kuiper
Belt, only to feel some small gravitational perturbation relatively
recently that will direct it inward. The gravity of an inner planet,
most likely Jupiter, can change its orbit again so its captured into an
orbit a much shorter period in the inner solar system; and then
repeated passages around the sun fairly quickly lead to its destruction
through evaporation or perhaps through gravitational interactions of a
further sort.
This panel shows what happens to a comet in its repeated orbits around
the sun. Far from the sun it is a small nucleus. As it moves in, it
starts to heat up, form a coma and a tail; material evaporates away and
so on. Eventually the comet disrupts entirely.
Missions to Comets
Much of this sounds speculative in the sense that it is very difficult
to image an object that is merely 10 km in diameter at enormous
astronomical distances. We'd like to know more about comets in detail.
First of all, we can examine the composition of comets by looking at
the spectrum of light that they emit when they're heated by the sun.
This will show what sorts of gases are emitted, thanks to the
fluorescence mechanisms as they absorb ultraviolet light. Here's the
spectrum of Comet Hyakutake, which shows it contains the common
elements of oxygen, hydrogen, carbon, nitrogen, and so on. No
surprises there.
To learn more, however, astronomers in the space age had set the goals
of actually visiting comets. We'll now consider three examples in
which this has been accomplished: first of all, fly-by missions to
Comet Halley in 1986; secondly, the so-called Deep Impact Mission to
Comet Temple in 2005; and, finally, the Rosetta Probe and Philae Lander
in 2014-15.
The return
On the top
the bottom
sky during
of Halley's comet in 1986 was, of course, long anticipated.
left, we see a picture of Halley in beautiful close-up. On
right we see Halley's comet as it appeared to the eye in the
that passage.
Halley's comet presents one very big problem for space probes: that is,
it moves in a retrograde orbit, in the sense opposite to the Earth's
motion around the sun. This makes it very difficult to send a space
probe to move alongside and perhaps even land on Halley's comet,
because we first have to cancel the motion that the probe has by virtue
of its motion with the Earth. We have to cancel all of that motion,
get the probe moving rapidly the other direction, and then catch up to
Halley's comet. This was energetically impossible in the 1980s, at the
time of Halley's last visit.
Given that, the Halley missions became simple fly-bys, the best known
of which was the Giotto Probe, which came closest to the comet itself.
In this instance, the probe was sent out from Earth in the forward
direction and simply flies past Halley's Comet as it moves the other
way. This limits the quality of observation we can make, and an
analogy is to think of yourself driving down the highway, then trying
to glance into the face of the driver of the car opposite as it whizzes
past and perhaps trying to gauge the colour of his eyes. We get only a
very brief glimpse as the two pass each other very quickly.
The mission itself was remarkably successful. On the left, we see a
photograph of the nucleus of Halley's Comet taken by the Giotto
Mission. (The colours are not realistic, by the way.) On the right, a
diagram shows what we are seeing. Notice the jets of material that are
coming off on the sunlit side; and notice also the size of the comet,
about 10 km in diameter, just as had been suggested by Fred Whipple:
the “dirty snowball” model.
Twenty years later, in 2005, a probe was sent out to Comet Tempel in
what is known as the Deep Impact mission. The objectives here were to
send the probe out to catch up to Comet Tempel and travel close to it,
sending back pictures of the comet, and then to drop a large, solid
object, an “impactor,” into the path of the comet so the comet would
run into it. The impactor is, of course, very much smaller than the
comet and will have no real effect on it, except that material will be
sputtered off the surface in the impact, and the nature of that dust
and light given off will give us some sense about the rigidity,
structure, composition and so on of the surface of the comet itself.
On the top left here we see the path followed by the Probe as it
travelled to Comet Tempel. The bottom left shows the comet itself, and
on the right an image of the probe that was sent out with the impactor
separated at the bottom. It had a mass of about a ton. Here's an
image that was sent back from the probe at the moment that the impactor
collided with the surface of the comet. You can see the flash of light
from the release of energy. The link at the bottom will take you to a
small movie realization that reminds you how this all worked.
What did we learn from that mission? Well, Comet Tempel is much as we
imagined: it may have more fluffy dust particles, and a bit less icy
material than thought, but the spectra tell us it’s much as we thought.
In other words, there were no surprises again, just a fine-tuning of
our understanding. What was really wanted, however, was to land on and
study a comet in considerable detail. This was realized with the
Rosetta Probe.
The Rosetta Probe came to fruition in the fall of 2014, but it should
be pointed out that the mission launch was a full decade ahead of that.
It takes an enormous amount of time to reach these astronomical
targets, and the planning and engineering is very complex indeed. The
project was intended to send a probe out to Comet ChuryumovGerasimenko; to put the probe into orbit around the comet travelling
along with it, surveying its surface in detail; and eventually to
select a location onto which a lander, called Philae, would be dropped
down for a soft landing. The lander would then be able to experiment
on the material of which the comet is made, drilling into the surface
and carrying out a variety of scientific tests.
The name Rosetta was chosen, by the way, because of the Rosetta Stone,
which as you may know was found in Ancient Egypt: a stone that
contained on it inscriptions in three languages. The knowledge of
Ancient Greek allowed archaeologists at the time to work out what the
equivalent words were in Ancient Egyptian, and this provided the first
interpretation of hieroglyphic writings. We, likewise, believe that
studying comets -- pristine objects from the birth of solar system -will give us a look back into the formation of the solar system itself,
a look back in history.
Here you can see the path that was taken by the Rosetta Probe as it
went on its way to the comet, and you notice it's a very complex path,
which actually was planned in a way that allowed the probe to pass
other interesting objects en route, but much of its time was spent
dormant: it was revived shortly before arrival at the comet.
Here are two images showing the comet itself on arrival in the upper
left. It is several kilometres in size and, as you can see, quite
irregularly shaped. On the right, we see pictures of the surface in
some detail, as seen by the orbiter around the comet.
This picture shows the size of the comet compared to the city of
Toronto. Although it is small compared to the planet, it's large on
human scales and, in fact, if an object of this size were to hit the
planet at astronomical speed the effects would be truly catastrophic -a point we'll come back to in a subsequent discussion.
As you may remember from the fall of 2014, the Philae lander was
successfully detached from the orbiter and, indeed, reached the
surface, but, unfortunately, it bounced on arrival and eventually wound
up settling in a shaded area next to a vertical wall. This limited the
available solar power and thus the time that was available for direct
experiments: the batteries ran down without recharging but from the
solar cells. So not all of the observations were successfully
completed, although there was a very rich return of scientific
information. In fact, overall this is one of the great success stories
of modern space technology and experimentation. Here's the mission
website if you want to learn the very latest news.
The orbiter was designed to continue its motion along with the comet as
it neared the sun, and as the comet nears the sun, of course, it
produces jets of gas. The comet is starting to be active; and Philae,
the lander, came back to life because of sunlight so more information
will flow from it as well.