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.