Comet Composition, Orbits, and Their Role in Life’s Origins

Name: Comets: Crash Course Astronomy #21
Uploaded: 2015-06-18T21:00:02+00:00
Duration: 11 min 54 s
Channel: CrashCourse
Description: Summary and key takeaways on Comets: Crash Course Astronomy #21: Summary & Key Takeaways, covering Historical Perception Ancient cultures interpreted comets as

CrashCourse

Jun 18, 2015

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11 min video

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2 min read

YouTube video ID: yB9HHyPpKds

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Ancient cultures interpreted comets as omens that foretold human events. Depending on the society, a comet could be seen as a good or bad sign; the 1066 appearance of a comet was linked to the Norman invasion.

Physical Composition and Structure

Comets are “dirty snowballs” made of rock, dust, and frozen gases such as water, carbon dioxide, carbon monoxide, methane, and ammonia. The solid core is called the nucleus, while the surrounding gaseous cloud is the coma—Latin for “hair.” As a comet nears the Sun, ice undergoes sublimation, turning directly into gas.

Tail Formation and Dynamics

When solar ultraviolet light ionizes gas molecules, an ion tail forms and is pushed straight away from the Sun by the solar wind. Sunlight pressure drives dust particles into a separate dust tail that lags behind the comet’s orbital path. Both tails are extremely tenuous, containing only a few hundred atoms per cubic centimeter.

Orbital Classification

Comets with orbital periods shorter than 200 years are short‑period comets; they travel in the same plane and direction as the planets. Those with periods longer than 200 years are long‑term comets, whose orbits are highly tilted and can appear anywhere in the sky. “Sundivers” or “Sungrazers” skim the Sun’s surface and are often fragments of larger, broken‑up comets.

Close‑up Exploration

The 1980s Halley missions (Vega 1, Giotto) revealed a dark, irregular nucleus about 15 × 8 km that reflects only 4 % of sunlight. The Rosetta mission’s 2014 encounter with 67P/Churyumov‑Gerasimenko showed a double‑lobed “rubber ducky” shape, gas vents, and a surprisingly hard surface crust. Surface features proved inhomogeneous, with venting confined to specific cracks rather than the entire nucleus.

Impact on Earth and Life

Comets may have delivered large quantities of water to early Earth billions of years ago. NASA’s Stardust probe collected samples from Comet Wild 2 that contained complex organic molecules, including amino acids. These findings suggest that comet impacts could have supplied essential ingredients for the origin of life.

Mechanisms Explained

Sublimation converts ice directly into gas as solar heating intensifies. Ion tail formation occurs when solar ultraviolet light ionizes gas molecules and the solar wind’s magnetic field carries the charged particles away from the Sun. Dust tail formation results from solar radiation pressure pushing dust particles outward, causing them to trail behind the comet’s orbit. Near the Sun, porous ice inside a comet can harden into a crust as surface ice warms and restructures.

Takeaways

Comets are composed of rock, dust, and frozen gases, forming a nucleus surrounded by a coma that sublimates as the comet approaches the Sun.
Two distinct tails arise: an ion tail driven by solar wind and a dust tail pushed by sunlight pressure, both extremely low in density.
Short‑period comets orbit within 200 years in the planetary plane, while long‑term comets have tilted, often unpredictable paths, and sungrazers skim the Sun’s surface.
Space missions such as Giotto, Rosetta, and Stardust have revealed dark, irregular nuclei, venting activity, and complex organic compounds in comet samples.
Cometary delivery of water and organic molecules may have supplied key ingredients for life on early Earth.

Frequently Asked Questions

How do ion tails and dust tails form differently on a comet?

Ion tails form when solar ultraviolet light ionizes gas molecules, and the solar wind pushes the charged particles directly away from the Sun. Dust tails arise from sunlight pressure that nudges dust particles outward, causing them to lag behind the comet’s orbital path.

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If you want to practice or explore the concepts discussed in the video, these commonly used tools may help.

Beginner Amateur Astronomy Telescope For Beginners Recommended

Allows users to observe comets and celestial bodies directly, helping them visualize the orbital dynamics discussed in the lecture.

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Astronomy Book For Beginners Space Science

Provides foundational knowledge on solar system formation and comet composition, reinforcing the lecture's educational content.

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High Quality Star Chart Planisphere

Helps users locate constellations and track the movement of objects in the sky, which is essential for understanding orbital paths.

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Solar System Model Kit For Adults

Provides a physical representation of planetary orbits and comet trajectories, making complex orbital dynamics easier to grasp.

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Full Transcript YouTube

Hey, Phil Plait here and this is Crash Course
Astronomy.
Since humans have been human we’ve looked
to the skies for portents of the future.
The Sun, the Moon, the planets, the stars;
they’ve all been used for prognostication.
And so have comets.
A fuzzy blob, moving slowly across the stars?
How could soothsayers resist?
But now we know a lot more about comets.
They’re beautiful, fascinating, and can bring
both life and death upon our little world.
Comets have been seen the sky since antiquity.
Comet Halley, for example, is shown in the
Bayeux Tapestry, which depicts the Norman
invasion of the British Isles in the year
1066.
It was seen by ancient Chinese and Greeks, too.
In general, and like everything else in the
sky, comets were considered omens or
harbingers of human events.
Sometimes they were good omens —
William the Conqueror liked his chances in
1066 after seeing one —
and sometimes bad — that same comet didn’t
do so well for King Harold II.
A comet bright enough to see with the naked
eye shows up in the heavens
every few years or so, and some can get spectacularly
bright.
In 2007, I saw Comet McNaught very near the
Sun in broad daylight!
When you think of a comet, you probably picture a
fuzzy blob and a long tail stretching away from it.
Fair enough. But there’s a bit more to them
than that.
Comets are in many ways similar to asteroids.
They’re roughly hewn chunks of stuff left
over from the formation of the solar system.
Unlike asteroids, which are mostly rock with
a dash of ice and maybe metal,
comets are a more balanced mixture of ice
and rock.
And by “ice” I mean frozen water -- but
also frozen carbon dioxide, carbon monoxide,
methane, ammonia -- things we normally think
of as gases on Earth.
And by “rock” I do mean rocks, but also
gravel and dust.
In fact, astronomers sometimes call comets
“dirty snowballs,” which isn’t a half-bad term.
It’s that ice that makes comets, well, comets.
When they’re way out in deep space they’re so cold
that they’re basically inert lumps of ice and rock.
But many are on elliptical orbits that take
them from those sub-freezing
depths into our neck of the woods, where the
Sun can warm them.
As they heat up the ice turns directly into
a gas — a process called “sublimation.”
The gas then flows away from the comet, creating
a cloud around it.
This makes the comet look fuzzy, and actually
in the past they’ve been called “hairy stars.”
I like that term too, and in a sense we still
use it.
The solid part of the comet is called the
nucleus, and the gaseous cloud around it is
called the coma — Latin for “hair.” In fact,
that’s why we called them “comets.”
As the ice sublimates, the bits of rock and
gravel and dust embedded in it can be freed
and leave the nucleus as well.
This material is what forms the comet’s
tail, but how that happens depends on which
material you’re talking about.
The gas and the dust from comets form two
different tails.
Gas molecules emitted by the comet get ionized
by the Sun’s ultraviolet light.
That means they lose electrons, becoming charged,
and charged particles are highly susceptible
to magnetic fields.
The solar wind is a stream of charged particles blown
out by the Sun, and carries a magnetic field with it.
As the wind hits the ionized gas from the
comet, it picks up those particles
and carries them downstream, away from the
Sun.
The solar wind is usually moving far, far
faster than
the comet, so this “ion tail” winds up
pointing directly away from the Sun.
The dust, on the other hand, is influenced
more by sunlight.
Light from the Sun exerts a small but inexorable
pressure, and this pushes on the dust particles.
The dust streams away, but because the pressure
isn’t as intense as the solar wind is on the gas tail,
the dust tail blows away more lazily, and
tends to lag behind the comet in its orbit.
That means the two tails usually point in
two different directions!
In some comets, like 1997’s incredibly bright
and gorgeous Comet Hale-Bopp,
this is pretty obvious. The dust tails look white
or a teeny bit yellowish, due to reflected sunlight,
while the ion tail glows blue or green, depending
on the primary constituents of the gas.
Carbon monoxide tends to emit blue light,
while carbon molecules glow a ghostly green.
A comet’s tail can stretch for tens of millions
of kilometers.
But, despite their length, tails are incredibly
low density, as low as a few hundred
atoms per cubic centimeter. The air you breathe
is a million billion times denser!
In 1910, Earth passed through the tail of
Comet Halley.
This caused some public fear because cyanogen,
a deadly gas, had been detected in the tail!
But of course nothing happened; it turns out getting a
gazillionth of the toxic dose isn’t that a big of a problem.
Broadly speaking, comets are classified by
their orbits.
If they have orbital period less than 200
years they’re called short-period comets.
These tend to orbit the Sun in the same plane
as the planets,
and go around the Sun in the same direction
as well. From Earth,
we see them sticking near the ecliptic, the line across
the sky that marks the annual path of the Sun.
Comets that take longer than two centuries to
go around the Sun are called long-term comets,
and have orbits that are tilted every which-way.
That means they can appear anywhere in the sky.
But this raises an interesting point: Comets
go away.
Every time they get near the Sun and start
outgassing, they lose mass.
Over time they get smaller. Eventually, they
should… evaporate. Pfffft!
Some do this all at once because they dive
into the Sun, skimming our star’s surface.
We call those Sundivers or Sungrazers. Many
of those may actually be
pieces from a bigger comet that broke up in
space nearly a thousand years ago.
But besides those, we know of some comets
with orbits that can be short,
some with periods of just a few years. Even
a century is like
a single flap of a mosquito’s wing compared
to the lifetime of the solar system!
How can comets be billions of years old if their
orbits bring them close to the Sun all the time?
Astronomers wondered about this very thing.
Over the years they came up with an idea: Maybe,
out past Neptune, there’s a repository of comets.
Chunks of dirty ice, millions of them, billions,
orbiting the Sun where it’s perpetually cold.
They could have orbits that last for millennia
or more.
But then something tweaks them, makes them
fall toward the Sun.
In fact, there may be two such regions, since
we have both short period and long term comets.
Turns out: this idea is correct! We now know
enough about those distant regions of
the solar system that they deserves their own
episode, so we’ll dive into that topic later.
So. What do comets look like up close? Like,
really close?
Studying them from Earth is hard.
The coma obscures the nucleus, making it nearly
impossible to see it directly.
Ahhh, “from Earth”. If you instead send
your telescope to a comet, things change.
We first did that in the 1980s, the last time
Comet Halley came around.
Several nations sent spacecraft to fly past
the comet, and the Soviet mission Vega 1
was the first to successfully get pictures of the
nucleus. The low-resolution images revealed
a dark lump highlighted with two bright spots,
later determined to be jets of gas streaming away.
These images were used to better determine
the position of the nucleus,
and a few days later the European probe Giotto
zipped past the nucleus
at an incredibly close distance of just 600
kilometers.
Those pictures were more detailed, and showed
us a flying mountain,
an irregular chunk 15 x 8 kilometers in size.
And it was dark, reflecting only 4% of the
light that hit it.
That makes the nucleus as black as asphalt!
You might think that all that ice would be
shiny, but it’s not that simple.
Most of Halley’s nucleus is covered in thick
dust laced with darker molecules,
with only a few spots emitting gas. Most likely,
there are deposits of ice under the surface,
and only some of them receive enough heat
from the Sun to sublimate and blow out gas.
This has been seen with other comets as well;
the gas is emitted from specific spots on the comet,
venting out from cracks in the crusty surface.
The surfaces of comets must be inhomogeneous,
different in different places.
That fact was brought home magnificently in
2014 by another European mission, Rosetta.
It went into orbit around the comet 67P/Churyumov-
Gerasimenko, and found it to be a bizarre little object.
Measuring about 4 kilometers end-to-end, 67P
has two lobes connected by a narrow neck,
looking very much like a cosmic rubber ducky.
The surface is completely devoid of craters;
clearly the surface is very young.
Images show jets of gas emitted from very
specific places on the surface,
and there are wide circular pits here and
there which may be gas vents,
growing wider over time as the ice below is
depleted.
Surprisingly, the surface is fairly tough and hard,
when some scientists expected it to be fluffier.
The comet has a very low density, similar
to rubble-pile asteroids,
so it was expected that the surface would
be soft.
Rosetta sent down a lander named Philae to set down
on the surface, using harpoons to anchor itself,
but instead the lander bounced, unable to
penetrate the tougher-than-expected material.
One idea to explain this is that the ice is
porous and fluffier inside the comet,
but as it nears the Sun the ice at the surface warms
and changes its structure, forming that harder crust.
As for the double-lobed thing, well that’s a bit baffling.
We see some asteroids shaped that way as well.
It’s possible 67P used to be two separate comets
that had a low speed collision and stuck together.
Or maybe it used to be one big lump, but over
the eons the ice in the middle sublimated
more, leaving behind the two lobes.
Rosetta is the first time in human history
we’ve had a probe orbiting a comet,
studying it up close and long-term. We’re
still learning, still figuring this stuff out.
Incidentally, I mentioned earlier that a)
comets have lots of ice in them,
and 2) they also get really close to Earth
sometimes. In fact, they can hit us!
Now not to get all technical and scientificy,
but that is what we would call “bad,”
as we’ll discuss in an upcoming episode.
But billions of years ago lots of comets hit
the Earth not long after our planet formed.
Together with asteroids — many of which
are also rich in water ice —
they may have brought a significant amount
of water to Earth! Scientists are still wrestling
over the details of this,
and it may be a while before the actual numbers
are nailed down, but it’s an intriguing thought.
Even more interesting? In 2004, NASA’s Stardust
space probe physically passed through
the coma of comet Wild 2, collecting samples
that were returned to Earth.
Careful analysis of the material found the presence
of organic, carbon based molecules in them.
And not just any random molecules, but complex
ones, including amino acids!
These are the building blocks of all life on Earth;
amino acids are what our bodies use to create proteins.
It’s possible that the ingredients of life on Earth
didn’t start here, but instead were brought to
our planet from comet impacts. Or, at least,
there was a mix of the two.
If that’s the case, then in a sense, all
life on Earth is part alien. How about that?
But what gets me are the philosophical ramifications
of this. When we look into space,
when we examine our celestial neighbors, when
we send probes to comets and survey what we find,
we’re looking at our own origins. Comets are like
time machines, allowing us to investigate our past,
four billion years back, hinting at the secrets
of the origin of life itself.
And you thought astronomy was just lying out
in a field and looking up.
Well, it is, but if you let it, it’s also
a whole lot more.
Today you learned that comets are chunks of
ice and rock that orbit the Sun.
When they get near the Sun the ice turns into
gas, forming the long tail,
and also releases dust that forms a different
tail. We’ve visited comets up close and
found them to be lumpy,
with vents in the surface that release the
gas as ice sublimates.
Eons ago, comets (and asteroids) may have
brought a lot of water to Earth -- as well
as the ingredients for life.
Crash Course Astronomy is produced in association
with PBS Digital Studios.
They have a ton of good shows over on their channel
so you should head over there and take a look.
This episode was written by me, Phil Plait. The script
was edited by Blake de Pastino, and our consultant is
Dr. Michelle Thaller. It was directed by Nicholas
Jenkins, the script supervisor and editor is
Nicole Sweeney, the sound designer is Michael Aranda,
and the graphics team is Thought Café.

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Asteroids: Crash Course Astronomy #20

CrashCourse

Apr 24, 2026

Physical Composition and Structure

Tail Formation and Dynamics

Orbital Classification

Close‑up Exploration

Impact on Earth and Life

Mechanisms Explained

Takeaways

Frequently Asked Questions

How do ion tails and dust tails form differently on a comet?

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