Asteroid Origins, Types, and Orbits: Key Insights from Phil Plait

Name: Asteroids: Crash Course Astronomy #20
Uploaded: 2015-06-04T21:00:00+00:00
Duration: 11 min 33 s
Channel: CrashCourse
Description: Summary and key takeaways on Asteroids: Crash Course Astronomy #20: Summary & Key Takeaways, covering Discovery and Definition Ceres was the first asteroid

CrashCourse

Jun 04, 2015

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11 min video

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Ceres was the first asteroid discovered in 1801, initially thought to be a planet. The name “asteroid” means “star‑like,” describing the dot‑like points seen through early telescopes. Although no hard‑and‑fast definition exists, asteroids are generally rocky or metallic bodies that orbit the Sun up to Jupiter’s distance.

Composition and Structure

About three‑quarters of asteroids are carbonaceous, one‑sixth are silicaceous, and the remainder are metallic, primarily iron and nickel. Many asteroids are “rubble piles,” loose collections of rocks held together by gravity rather than solid monoliths. The Main Belt is mostly empty space; on average, sizable asteroids are millions of kilometers apart, and the belt’s total mass is far smaller than the Moon’s.

Orbital Mechanics

The Main Belt contains gaps known as Kirkwood Gaps, where Jupiter’s gravitational tugs at simple orbital period multiples clear out asteroids. Families of asteroids share similar orbits, indicating they are fragments of a single disrupted parent body. Trojan asteroids occupy stable Lagrange points 60° ahead of or behind a planet’s orbit, while co‑orbital objects like Cruithne share Earth’s orbital path without being true moons.

Near‑Earth Objects

Mars‑crossing, Apollo, and Aten asteroids are classified by how closely their orbits approach Earth. Proximity alone does not guarantee collision; most have tilted or non‑intersecting trajectories that keep them safely away.

Notable Objects

Ceres, roughly 900 km across, likely contains a rocky core and a water‑ice mantle. Vesta, the third‑largest asteroid, is an oblate spheroid and the second most massive. The Japanese Hayabusa mission visited Itokawa, confirming its rubble‑pile nature. Earth’s Trojan 2010 TK7 measures about 300 m and orbits 800 million km away, while Cruithne can approach within 12 million km of Earth.

Mechanisms and Explanations

During solar system formation, material clumped into mid‑sized bodies that differentiated, allowing heavy metals to sink to cores. Subsequent high‑speed collisions shattered many of these bodies, creating the current asteroid population. Repeated, slower collisions over billions of years cracked and disrupted asteroids without fully dispersing them, producing the “bag of gravel” rubble‑pile structures. Lagrange points act as gravitational balance zones, comparable to an “egg in a cup,” where objects can remain stable for long periods.

Hard Facts and Numbers

Ceres: ~900 km diameter, rocky core, water‑ice mantle.
Vesta: third‑largest, second‑most massive, oblate shape.
Main Belt: hundreds of thousands of known asteroids, likely billions larger than 100 m.
Kirkwood Gaps: regions where orbital periods are simple fractions of Jupiter’s 12‑year period.
2010 TK7: Earth’s Trojan, ~300 m across, 800 million km distant.
Cruithne: co‑orbital object, can come within 12 million km of Earth.

“It’s weird to think of some asteroids as being not much more than free‑floating bags of gravel, but the Universe is under no obligation to adhere to our expectations.”
“If you took all the asteroids in the main belt and lumped ‘em together they’d be far smaller than our own Moon!”
“On average, decent‑sized asteroids are millions of kilometers apart; so far that if you stood on an asteroid, odds are good you wouldn’t even be able to see another one with your naked eye.”

Takeaways

Asteroids are remnants of early solar system bodies, formed from material that never coalesced into planets, and they range from carbon‑rich to metallic compositions.
The term “asteroid” means “star‑like,” reflecting their point‑source appearance in early telescopes, and no strict definition exists beyond being rocky or metallic bodies orbiting the Sun inside Jupiter’s orbit.
Most main‑belt asteroids are “rubble piles,” loose aggregates held together by gravity, and the belt’s total mass is far less than the Moon’s, with individual objects spaced millions of kilometers apart.
Kirkwood Gaps arise from Jupiter’s gravitational resonances, while asteroid families and Trojan groups trace back to common parent bodies or stable Lagrange points, illustrating complex orbital dynamics.
Near‑Earth asteroids such as Mars‑crossers, Apollos, and Atens approach Earth’s orbit but rarely collide because their paths are often tilted or non‑intersecting, and co‑orbital objects like Cruithne illustrate alternative orbital relationships.

Frequently Asked Questions

Why are Kirkwood Gaps empty regions in the asteroid belt?

Kirkwood Gaps are zones where Jupiter’s gravity repeatedly nudges asteroids whose orbital periods are simple fractions of Jupiter’s 12‑year cycle, destabilizing their paths and clearing those regions of objects. The resonant interactions cause asteroids to be ejected or shifted into different orbits.

What does it mean that many asteroids are "rubble piles"?

A “rubble pile” asteroid consists of a loose collection of rocks and dust held together mainly by its own gravity rather than a solid, monolithic structure. Repeated low‑speed collisions over billions of years break apart larger bodies, leaving a loosely bound “bag of gravel” that can still orbit as a single object.

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Helpful resources related to this video

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

Beginner Amateur Astronomy Telescope Recommended

Allows the user to observe celestial bodies like asteroids and planets, helping them visualize the objects discussed in the lecture.

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The Planets By Andrew Cohen

A book that provides context on the formation of the solar system, which is essential for understanding how asteroids were created.

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Star Chart Planisphere For Stargazing

Helps the user locate constellations and track the movement of objects in the night sky, providing context for asteroid discovery.

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Astronomy Binoculars For Stargazing

Provides a wider field of view than a telescope, making it easier for beginners to scan the sky for moving objects.

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Full Transcript YouTube

When you look at a diagram of the solar system,
you’ll see a big gap between Mars and Jupiter.
A few centuries ago, that gap bugged astronomers;
they really wanted there to be a planet in there.
On the first day of the 19th century—January
1, 1801—they got their wish. Kinda. Italian
astronomer Giuseppi Piazzi found a point of
light moving at just the right speed to be
the desired planet, but it was just a dot,
and too faint to physically be a terribly
big object. He suspected it might be a comet,
but follow-up observations showed it wasn’t
fuzzy. The object was given the name Ceres…
but was it really a planet?
Well...
Hopes were high that Ceres was the wished-for
planet between Mars and Jupiter. But then
something rather amazing happened: A little
over a year later, in 1802, another one was
found. Then, in 1804, astronomers spotted
a third one, and a fourth in 1807.
It was becoming clear that a new class of
solar system object had been discovered. Given
that they were all just dots in the telescopes
of the time, points of light like stars, they
were given the name “asteroids”, which
literally means star-like.
By the end of the 19th century more than 450
had been found in total. The rate of discovery
has accelerated over the years, and now, today,
we know of hundreds of thousands. There are
probably billions—yes, billions—of them
larger than 100 meters across in the solar
system, and over a million larger than 1 km
in size.
So what are we dealing with here? What are
these asteroids?
There’s not really a hard-and-fast definition
of what’s an asteroid and what isn’t.
But generally speaking, it’s a class of
smaller bodies that are rocky or metallic
that orbit the Sun out to Jupiter. Objects
past Jupiter have special designations that
we’ll get to in the next episode.
Over the centuries we’ve learned a lot about
them by scrutinizing them with telescopes.
Asteroids come in a few basic flavors. Most,
of them, about 3/4, are carbonaceous, which
means they have lots of carbon in them. About
1/6th are silicaceous—heavy on the silicon-based
materials, y’know, rock. The rest are lumped
into one catch-all category, but are dominated
by metallic objects, literally loaded with
iron, nickel, and other metals.
So many of them orbit the Sun between Mars
and Jupiter that this region is now called
the Main Belt. The Main Belt has structure;
for example, there are very few asteroids
about 425 million kilometers from the Sun.
An asteroid at that distance would have an
orbital period of about 4 years; a simple
fraction of Jupiter’s 12 year period. Any
asteroid there would feel a repeated tug from
Jupiter’s mighty gravity, pulling it out
of that orbit. The resulting gap is called
the Kirkwood Gap, and there are several such
asteroid deserts, all with simple multiples
of Jupiter’s period. In this way, the main
belt is like Saturn’s rings, whose gaps are
carved out by the gravity of the orbiting moons.
Another way to group asteroids is by orbit;
some have similar orbits and may have formed
from a bigger, parent asteroid that got disrupted
by an impact. These groups are called families,
and there are a few dozen known. For example,
the Eunomia family has over 400 members, and
are silicaceous, rocky asteroids and probably all formed
from a parent body that was about 300 km across.
When you watch movies, they always show spaceships
dodging and swooping through asteroid belts,
trying to evade the bad guys. But in reality
our asteroid belt is mostly empty space! On
average, decent-sized asteroids are millions
of kilometers apart; so far that if you stood
on an asteroid, odds are good you wouldn’t even
be able to see another one with your naked eye.
And despite their huge numbers, they don’t
add up to much. If you took all the asteroids
in the main belt and lumped ‘em together
they’d be far smaller than our own Moon!
Ceres is the biggest, at about 900 km across.
It’s round, nearly spherical due to its
own gravity crushing it into a ball.
A funny thing about Ceres: As we write and
record this episode, it’s being visited
for the first time, by a spacecraft named
Dawn. That means everything I tell you about
this asteroid is probably about to be obsolete.
But we do know a few things. Ceres probably
has a rocky core surrounded by a water ice
mantle. The amount of water in it is staggering;
probably more than all the fresh water on
Earth! It may even be liquid under the surface,
like the oceans of Enceladus and Europa.
Early images by Dawn as it approached the
asteroid show its surface is heavily cratered,
and some craters are very bright; they may
be exposing ice under the surface, or just
fresher, brighter material. There are tantalizing
observations of localized water vapor on the
surface, which may be from sublimation; ice
turning directly into a gas due to the Sun’s
heat, or it might indicate cryovolcanoes.
Dawn also visited Vesta, which is the third
largest but second most massive asteroid known.
Vesta is round…ish, what’s called an oblate
spheroid, flattened a bit like a ball someone’s
sitting on. The southern hemisphere got hammered
by impacts long ago, leaving a huge basin there.
Several other main-belt asteroids have been
visited by spacecraft, mostly via flybys.
Lutetia, Gaspra, Steins, Mathilde. Ida is
another, and was discovered to have a small
moon orbiting it. In fact, a lot of asteroids
have moons or are actually binary, with two
similarly-sized bodies in orbit around each
other. Kleopatra, a weird dog bone-shape rock,
has two moons!
You might think asteroids are just giant versions
of rocks you might find in your garden; tough,
solid, singular bodies. But it turns out that’s
not the case. A few years ago scientists realized
that asteroids have spent billions of years
whacking into one another -- sometimes in
high-speed collisions, sometimes more slowly.
Slower hits can disrupt the asteroid, crack it,
but not necessarily be strong enough to
actually disrupt it so that it breaks apart.
Over time, enough hits like that can leave
behind what’s called a rubble pile: Individual
rocks held together by their own gravity,
like a bag of gravel, or a car window that’s
been cracked and still holds its overall shape.
This became more clear when the Japanese Hayabusa
spacecraft visited the asteroid Itokawa, and
saw what can only be described as a jumbled
mess. The asteroid had no craters on it, and
was littered with rubble and debris. It was
also very low density, just what you’d expect
for a loosely bound rock pile.
It’s weird to think of some asteroids as
being not much more than free-floating bags
of gravel, but the Universe is under no obligation
to adhere to our expectations. It’s full
of surprises, and we need to keep our minds
flexible.
So here’s a question: why is there even
a main asteroid belt at all?
The solar system formed from a disk of material,
and over time, that material started to clump
into bigger and bigger pieces. As planets
formed, they swept up and pulled in lots more
stuff, and grew large. Jupiter consumed a
lot of the material around it, but not all,
and left a lot of debris inside its orbit.
Some of this clumped together to form middling-sized
objects, probably smaller than the planets
we have now, but big enough to undergo differentiation:
Heavy stuff like metals sank to the middle,
and lighter stuff formed a mantle and crust.
Collisions broke almost all of them apart,
though, and that’s why we see asteroids
with different compositions: Some are from
the denser core, others from the lighter crust.
There was probably a lot more material between
Mars and Jupiter billions of years ago, but
it either got eaten by Jupiter, or the planet’s
immense gravity altered the asteroids’ orbits,
flinging them away. This may be why Mars is
so small, too; Jupiter robbed it of all of
its food as it formed.
While most asteroids live in the main belt,
not all of them do. Some have orbits that
cross that of Mars, taking them closer to the Sun.
We call those -- wait for it -- Mars-crossing
asteroids. Some have orbits that take them
even closer to the Sun, crossing Earth’s
orbit. We call those… Apollo asteroids.
Eh? Gotcha! They’re named after the asteroid
Apollo, the first of its kind to be found.
Some have orbits that are almost entirely
inside Earth’s orbit, called Aten asteroids.
Aten and Apollo asteroids can get pretty close
to Earth, so we call them Near-Earth Asteroids.
Now, while they get close to us, that doesn’t
mean they’ll hit us, because, for example,
their orbits may be tilted, so their orbits
and the orbit of the Earth don’t actually
ever physically cross.
But… some do have paths that literally intersect
Earth’s. That doesn’t mean they’ll hit
us every pass, either; after all, you can
walk across a street without getting hit by
a car. The problem comes when you try to occupy
the same volume of space as a car at the same time.
Astronomers, unsurprisingly, are very concerned
about asteroids that can hit us. That’s
why we have surveys, observatories scanning
the skies, looking for them. This is a pretty
important topic, and I’ll go into in more
depth in a future episode.
There’s another category of asteroid that
exists due to a quirk of gravity. When a planet
orbits a star, there are points along the
planet’s orbit and near it in space where
the gravitational forces are in balance. If
you place an object there, it tends to stay
there, like an egg in a cup. These are called
Lagrange points. One of them is along the
same orbit as the planet, but 60° ahead;
another is 60° behind.
The first such asteroid found was orbiting
60° ahead Jupiter, and was named Achilles,
after the Greek hero in the Trojan war. As
more were found, the naming convention stuck;
asteroids ahead of Jupiter were named after
Greek figures in the Trojan war, and those
behind Jupiter were named for Trojans, and
now we just call them all Trojan asteroids.
Trojan asteroids have been spotted for Jupiter,
Mars, Uranus, Neptune, and even Earth! Earth’s
was found in 2010 using observations by an
orbiting observatory called WISE, which scans
the skies in infrared light, where asteroids
glow due to their own heat. 2010 TK7, as it’s
called, is about 300 meters across and 800 million
kilometers away, orbiting the Sun ahead of the Earth.
There are also asteroids that have orbits
that are very similar to Earth’s, but are
slightly elliptical and tilted with respect
to ours. Because of this, they can stay relatively
near the Earth in space, but don’t really
orbit us; instead they sometimes get closer
and sometimes recede. It’s pretty weird,
but a natural outcome of orbital mechanics.
Some people say these asteroids are moons
of Earth, but it’s better to say they’re
co-orbital with us. Only a few are known,
the most famous being Cruithne, which can
get as close as 12 or so million kilometers
from us.
Oh, one more thing. Originally, asteroids
were named after female goddesses; Ceres,
Vesta, Juno, and so on. But as hundreds more
were found, and then thousands, we ran out
of names. Eventually astronomers who discovered
asteroids were allowed to name them -- through
a lengthy proposal and acceptance process
governed by the International Astronomical
Union. They also get a number assigned to
them as well.
A lot of astronomers have asteroids named
after them, including astronomers who study
asteroids, like my friend Amy Mainzer, who
works on the WISE mission—hers is 234750
Amymainzer—and Eleanor Helin, who discovered
quite a few asteroids and comets. Hers is
3267 Glo; for her nickname.
And this one? It’s a one-kilometer wide
rock in the main belt, and goes by the name
165347 Philplait.
Must be coincidence.
Today you learned that asteroids are chunks
of rock, metal, or both that were once part
of smallish planets but were destroyed after
collisions. Most orbit the Sun between Mars
and Jupiter, but some get near the Earth.
The biggest, Ceres is far smaller than the
Moon but still big enough to be round and
have undergone differentiation.
Crash Course Astronomy is produced in association
with PBS Digital Studios. Head over to their
channel for even more awesome videos. This
episode was written by me, Phil Plait -- I
hosted it too. You probably saw that. 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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Composition and Structure

Orbital Mechanics

Near‑Earth Objects

Notable Objects

Mechanisms and Explanations

Hard Facts and Numbers

Takeaways

Frequently Asked Questions

Why are Kirkwood Gaps empty regions in the asteroid belt?

What does it mean that many asteroids are "rubble piles"?

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