Asteroids: Crash Course Astronomy #20

Summary

This script explores the fascinating world of asteroids, from their discovery in the gap between Mars and Jupiter to the diverse types found in the solar system. It delves into the composition of these celestial bodies, the Kirkwood Gaps, asteroid families, and the largest asteroid, Ceres. It also discusses the concept of asteroid belts, their actual sparseness, and the various classifications of asteroids, including Near-Earth Asteroids and Trojan asteroids. The script concludes with the naming conventions of asteroids and their importance in astronomical studies.

Takeaways

• 📅 The discovery of Ceres in 1801 marked the beginning of the recognition of a new class of solar system objects, initially thought to be the 'missing planet' between Mars and Jupiter.
• 🌌 The term 'asteroids' was coined for these celestial bodies, which are like stars in appearance but are smaller and rocky or metallic in composition.
• 🔍 Over time, the number of known asteroids has grown exponentially, with hundreds of thousands identified and potentially billions larger than 100 meters across in the solar system.
• 🪨 Asteroids are categorized into types based on their composition: carbonaceous, silicaceous, and metallic, with the majority being carbonaceous.
• 💫 The region where most asteroids are found between Mars and Jupiter is known as the Main Belt, which has a structure influenced by gravitational interactions with Jupiter.
• 🚀 Spacecraft like Dawn have visited and are studying asteroids, providing new insights into their composition and history, such as the presence of water on Ceres.
• 🌑 Asteroids can form families with similar orbits, suggesting a common origin from a disrupted larger body, like the Eunomia family.
• 🌐 The Main Belt is not densely packed; asteroids are spaced millions of kilometers apart, contrary to popular depictions in media.
• 🌍 Some asteroids have orbits that bring them close to Earth, categorized as Near-Earth Asteroids, with some being Mars-crossing or even Earth-approaching.
• 🌑 Asteroids are remnants from the early solar system, possibly once part of larger bodies that were broken apart by collisions and gravitational disturbances.
• 🌐 Trojan asteroids, found in Lagrange points, share an orbit with planets like Jupiter, and even Earth, maintaining a relative position due to balanced gravitational forces.

Q & A

What significant event occurred on January 1, 1801, related to the solar system?

On January 1, 1801, Italian astronomer Giuseppi Piazzi discovered a point of light moving at the right speed to be the desired planet between Mars and Jupiter, which was later named Ceres.

What is the term used to describe the celestial bodies found between Mars and Jupiter, and what does it mean?

The term used to describe these celestial bodies is 'asteroids', which literally means 'star-like', as they were points of light like stars when viewed through telescopes.

How many asteroids are known to exist in the solar system today?

Today, we know of hundreds of thousands of asteroids, with estimates suggesting there could be billions of them larger than 100 meters across in the solar system, and over a million larger than 1 km in size.

What is the general definition of an asteroid?

An asteroid is generally defined as a class of smaller bodies that are rocky or metallic, orbiting the Sun out to Jupiter.

What are the three main compositions of asteroids?

The three main compositions of asteroids are carbonaceous, which contains lots of carbon; silicaceous, which is heavy on silicon-based materials or rock; and a catch-all category dominated by metallic objects rich in iron, nickel, and other metals.

What is the Kirkwood Gap and how is it formed?

The Kirkwood Gap is a region in the asteroid belt where very few asteroids are found, approximately 425 million kilometers from the Sun. It is formed due to Jupiter's gravity repeatedly pulling asteroids out of that orbit, as any asteroid there would have an orbital period that is a simple fraction of Jupiter's period.

What is the significance of the Ceres asteroid, and what spacecraft is currently visiting it?

Ceres is the largest asteroid, about 900 km across, and is nearly spherical due to its own gravity. It is currently being visited by a spacecraft named Dawn, which may provide new insights into its composition and structure.

What are asteroid families, and how do they form?

Asteroid families are groups of asteroids that have similar orbits and are thought to have formed from a larger parent asteroid that was disrupted by an impact.

Why is the asteroid belt mostly empty space, and how far apart are asteroids on average?

The asteroid belt is mostly empty space because, on average, decent-sized asteroids are millions of kilometers apart, so far that if you stood on an asteroid, you likely wouldn't be able to see another one with the naked eye.

What are Near-Earth Asteroids, and how are they categorized?

Near-Earth Asteroids are asteroids whose orbits bring them close to Earth. They include Apollo asteroids, which cross Earth's orbit, and Aten asteroids, which have orbits almost entirely inside Earth's orbit.

What are Trojan asteroids, and how are they named?

Trojan asteroids are those that occupy the Lagrange points along the same orbit as a planet, but 60° ahead or behind. They are named after Greek and Trojan figures from the Trojan War, with the first found being named Achilles.

What is the current naming convention for asteroids, and how does it relate to the International Astronomical Union?

Originally, asteroids were named after female goddesses, but as more were discovered, astronomers were allowed to name them through a lengthy proposal and acceptance process governed by the International Astronomical Union. They are also assigned a number.

What is the role of observatories and surveys in monitoring Near-Earth Asteroids, and why is it important?

Observatories and surveys scan the skies to monitor Near-Earth Asteroids to assess the potential risk they pose to Earth. This is important because some of these asteroids have orbits that intersect with Earth's, posing a potential impact threat.

Outlines

00:00

🌌 Discovery of the Asteroid Belt

The script begins by highlighting the historical quest for a planet between Mars and Jupiter, which was thought to exist due to the large gap observed in the solar system diagram. The discovery of Ceres by Italian astronomer Giuseppi Piazzi in 1801 initially sparked hope for a new planet, but further observations revealed it to be an asteroid. The subsequent discovery of more similar bodies led to the recognition of a new class of solar system objects, termed 'asteroids'. By the end of the 19th century, over 450 asteroids had been identified, with the number growing to hundreds of thousands today, and possibly billions larger than 100 meters across. The script delves into the characteristics of asteroids, their composition, and the structure of the Main Belt, including the Kirkwood Gaps caused by Jupiter's gravitational influence.

05:05

🚀 Exploration and Nature of Asteroids

This paragraph discusses the exploration of asteroids by spacecraft like Dawn, which visited Ceres and Vesta, revealing their unique characteristics. Ceres, the largest asteroid, is described as having a rocky core and a water ice mantle, with a significant amount of water. The script also touches on the concept of asteroid families, formed from the disruption of larger parent bodies, and the surprising nature of asteroids as 'rubble piles' held together by gravity, as seen in the case of Itokawa visited by Hayabusa. The narrative corrects common misconceptions about the density of the asteroid belt and the physical properties of asteroids, emphasizing their diversity and the ongoing exploration and discovery process.

10:05

🌐 Asteroids' Orbits and Classification

The script explains the various classifications of asteroids based on their orbits relative to other celestial bodies. It introduces Mars-crossing, Apollo, and Aten asteroids, which have orbits that bring them close to or intersect with Earth's orbit, categorizing them as Near-Earth Asteroids. The concept of Lagrange points is introduced, explaining the Trojan asteroids found in these points ahead of and behind Jupiter. The script also mentions Earth's own Trojan asteroid, 2010 TK7, discovered through infrared observations. Additionally, it covers co-orbital asteroids like Cruithne, which maintain a relatively stable position near Earth due to their similar yet slightly tilted orbits. The paragraph concludes with a note on the naming convention of asteroids, transitioning from female goddesses to a system governed by the International Astronomical Union, including a personal anecdote about the asteroid named after the script's author, Phil Plait.

Keywords

💡Asteroids

Asteroids are small celestial bodies that orbit the Sun but are too small to be classified as planets. They are primarily composed of rock and metal and are found in various regions of the solar system, with the majority located in the asteroid belt between Mars and Jupiter. The script discusses the discovery and characteristics of asteroids, emphasizing their significance in understanding the solar system's composition and history.

💡Ceres

Ceres is the largest object in the asteroid belt and is unique for being nearly spherical due to its own gravity. It is mentioned in the script as the first celestial body found in the 'gap' between Mars and Jupiter, sparking the realization that this region was populated by a new class of solar system objects. Ceres is also highlighted for its potential to have a large amount of water, more than all the fresh water on Earth.

💡Main Belt

The Main Belt refers to the region of the solar system located between Mars and Jupiter, where the majority of asteroids are found. The script explains that the Main Belt has a structured distribution of asteroids, with gaps known as Kirkwood Gaps caused by gravitational interactions with Jupiter. The Main Belt is central to the video's theme of exploring the nature and distribution of asteroids.

💡Kirkwood Gaps

Kirkwood Gaps are areas within the asteroid belt that have very few asteroids due to gravitational resonances with Jupiter. The script describes how these gaps occur at distances from the Sun that correspond to simple multiples of Jupiter's orbital period, causing asteroids in these regions to be perturbed by Jupiter's gravity and thus preventing their stable existence there.

💡Asteroid Families

Asteroid families are groups of asteroids that share similar orbits and are thought to have originated from the same parent body that was disrupted by an impact. The script provides the example of the Eunomia family, which consists of over 400 silicaceous, rocky asteroids, illustrating how asteroid families can offer insights into the composition and history of the solar system.

💡Near-Earth Asteroids (NEAs)

Near-Earth Asteroids are asteroids whose orbits bring them close to Earth's orbit. The script mentions different types of NEAs, such as Apollo and Aten asteroids, and discusses the importance of monitoring these objects due to their potential impact risk to Earth. This concept is crucial to the video's broader message about the dynamic nature of the solar system and the threats it may pose to our planet.

💡Trojan Asteroids

Trojan asteroids are a group of asteroids that share an orbit with a planet but are located at the stable Lagrange points, 60° ahead or behind the planet in its orbit. The script explains that the first Trojan asteroid found was named Achilles and orbits 60° ahead of Jupiter, and it notes that Trojan asteroids have been found associated with other planets as well, including Earth.

💡Differentiation

Differentiation in the context of celestial bodies refers to the process by which heavier materials sink to the center while lighter materials form the outer layers, creating a distinct core, mantle, and crust. The script mentions that Ceres has likely undergone differentiation, indicating that it has a rocky core and an icy mantle, which is significant for understanding the internal structure and composition of large asteroids.

💡Rubble Pile

A rubble pile is a term used to describe asteroids that are not solid bodies but are instead collections of rocks and debris held together by gravity. The script uses the example of the asteroid Itokawa, visited by the Hayabusa spacecraft, to illustrate the concept of a rubble pile, emphasizing the dynamic and varied nature of asteroids.

💡Binary Asteroids

Binary asteroids are systems where two similarly-sized bodies orbit around a common center of mass. The script mentions that many asteroids have moons or are binary systems, with the example of Kleopatra, which has two moons, highlighting the diverse configurations that can exist among asteroids.

💡Co-orbital Asteroids

Co-orbital asteroids are those that have orbits that are very similar to Earth's but are slightly elliptical and tilted, allowing them to stay relatively near Earth in space without actually orbiting it. The script introduces Cruithne as an example of a co-orbital asteroid with Earth, showcasing the complex orbital relationships that can exist in the solar system.

Highlights

The discovery of Ceres in 1801 by Italian astronomer Giuseppi Piazzi, initially thought to be a planet.

The subsequent discovery of more celestial bodies like Ceres leading to the recognition of a new class of solar system objects called asteroids.

By the end of the 19th century, over 450 asteroids had been discovered, with the number growing to hundreds of thousands today.

Asteroids are generally rocky or metallic bodies that orbit the Sun, primarily between Mars and Jupiter.

The composition of asteroids includes carbonaceous, silicaceous, and metallic types.

The Kirkwood Gap, a region in the asteroid belt with very few asteroids due to Jupiter's gravitational influence.

Asteroid families, groups with similar orbits believed to have formed from a larger disrupted parent asteroid.

The vast emptiness of the asteroid belt, with asteroids being millions of kilometers apart on average.

Ceres, the largest asteroid, is nearly spherical and has a differentiated composition with a rocky core and an ice mantle.

The spacecraft Dawn's mission to visit Ceres and provide new insights into its composition and surface features.

The concept of rubble pile asteroids, which are loosely bound collections of rocks held together by gravity.

The discovery of Trojan asteroids, which share an orbit with a planet but at 60° ahead or behind.

The existence of Near-Earth Asteroids, including Apollo and Aten types, which have orbits that bring them close to Earth.

The naming convention of asteroids, which initially followed female goddesses and later expanded to include various themes.

The role of Jupiter in the formation of the asteroid belt, possibly consuming material that could have formed additional planets.

The potential existence of co-orbital asteroids with Earth, which have similar orbits but do not orbit our planet directly.

Transcripts

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.