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.