When you look at a diagram of the solar system,
youâll 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 centuryâJanuary
1, 1801âthey 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 wasnât fuzzy. The object was given the name CeresâŚ
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 âasteroidsâ, 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 billionsâyes, billionsâof 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? Thereâs not really a hard-and-fast definition
of whatâs an asteroid and what isnât. But generally speaking, itâs a class of
smaller bodies that are rocky or metallic that orbit the Sun out to Jupiter. Objects
past Jupiter have special designations that weâll get to in the next episode. Over the centuries weâve 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 silicaceousâheavy on the silicon-based materials, yâknow, 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 Jupiterâs 12 year period. Any asteroid there would feel a repeated tug from
Jupiterâs 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 Jupiterâs period. In this way, the main belt is like Saturnâs 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 wouldnât even
be able to see another one with your naked eye. And despite their huge numbers, they donât
add up to much. If you took all the asteroids in the main belt and lumped âem together
theyâd be far smaller than our own Moon! Ceres is the biggest, at about 900 km across.
Itâs 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, itâs 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 Sunâs heat, or it might indicate cryovolcanoes. Dawn also visited Vesta, which is the third
largest but second most massive asteroid known. Vesta is roundâŚish, whatâs called an oblate
spheroid, flattened a bit like a ball someoneâs 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 thatâs
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 whatâs called a rubble pile: Individual rocks held together by their own gravity,
like a bag of gravel, or a car window thatâs 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 youâd expect for a loosely bound rock pile. Itâs 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. Itâs full of surprises, and we need to keep our minds
flexible. So hereâs 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 thatâs 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 planetâs
immense gravity altered the asteroidsâ 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 Earthâs orbit. We call those⌠Apollo asteroids.
Eh? Gotcha! Theyâre named after the asteroid Apollo, the first of its kind to be found. Some have orbits that are almost entirely
inside Earthâs 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 doesnât
mean theyâll hit us, because, for example, their orbits may be tilted, so their orbits
and the orbit of the Earth donât actually ever physically cross. But⌠some do have paths that literally intersect
Earthâs. That doesnât mean theyâll 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. Thatâs why we have surveys, observatories scanning
the skies, looking for them. This is a pretty important topic, and Iâll go into in more
depth in a future episode. Thereâs another category of asteroid that
exists due to a quirk of gravity. When a planet orbits a star, there are points along the
planetâs 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! Earthâs 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 itâs 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 Earthâs, but are slightly elliptical and tilted with respect
to ours. Because of this, they can stay relatively near the Earth in space, but donât really
orbit us; instead they sometimes get closer and sometimes recede. Itâs pretty weird,
but a natural outcome of orbital mechanics. Some people say these asteroids are moons
of Earth, but itâs better to say theyâre 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 missionâhers is 234750 Amymainzerâand Eleanor Helin, who discovered
quite a few asteroids and comets. Hers is 3267 Glo; for her nickname. And this one? Itâs 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 CafĂŠ.
And here I always thought Trojan asteroids were called that in a reference to the Trojan horse, in which the Greeks were hidden, much as Trojan asteroids might be thought of as "hiding" behind the planet.