September 21, 2026 • 12 mins
The asteroid belt between Mars and Jupiter is made up of sparse, tiny bodies of rock, ice, and metal that give us a window into our solar system's early days. Learn how Jupiter helped form the astroid belt and how asteroids might've given Earth its water (and life!) in today's episode of BrainStuff, based on this article: https://science.howstuffworks.com/asteroid-belt.htm
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Speaker 1 (00:01):
Welcome to BrainStuff, a production of iHeartRadio. Hey, BrainStuff. Lauren
Vogelbaum here. In the Star Wars film, The Empire Strikes Back,
Han Solo and the crew escape from the planet Hoth,
only to fly straight into an asteroid field. The field
is densely packed, and with huge, spinning rocks careening around
(00:25):
the Millennium Falcon, Han must deftly maneuver his spaceship to safety—
the odds of which, calculated by C-3PO, are only 3,720
to 1. Of course, Star Wars isn't known for its
scientific realism. If a spaceship launched from Earth toward our
(00:45):
solar system's asteroid belt and attempted to fly through it,
would it look the same, with dangerous debris flying everywhere,
putting the mission in jeopardy? As it turns out, navigating
through our asteroid belt wouldn't be nearly as dramatic. Only
a handful of asteroids are big enough to cause much
damage to a spacecraft, and there's way more space between
(01:06):
them than you might think. But that doesn't mean that
our solar system's main asteroid belt, located between the orbits
of Mars and Jupiter, is any less interesting than the
field in Star Wars. The more astronomers study the composition, activity,
and formation of asteroids in their orbit around the sun,
the more we understand about how the entire solar system
(01:28):
came to be. Some theories even suggest that life on
Earth started with asteroids colliding with our planet in its
early stages. And on the flip side, an asteroid's crash
was a major factor in the mass extinction of the
dinosaurs 65 million years ago. Today, let's talk about our
asteroid belt. Asteroids are remnants of our solar system's early days,
(01:53):
when the planets were still forming. Some are the oldest,
most unchanged objects we know of, untouched by processes like
volcanism that shaped larger bodies. There are several theories that
attempt to explain how the solar system began, but the
most widely accepted is known as the nebular theory. It
goes that our solar system started as a large, shapeless
(02:16):
cloud of gas, dust, and ice. But then something disrupted
the mass and set things in motion, perhaps the explosion
of a nearby star. If you've ever watched figure skating,
you've seen how skaters can spin much faster if they
pull their arms closer to their bodies. The more concentrated
their body mass is, the faster they're able to rotate.
(02:39):
The same thing happened with our solar system. the hypothetical
explosion nearby squeezed the unformed gas and dust together, which
began to spin faster and faster in a circle. As
the sun formed in the middle, the cloud started to
flatten out into a disk, sort of like a frisbee
or a pancake made up of tiny grains of dust. Eventually,
(03:02):
dust particles began to join together and form larger bodies
called planetismals. Even more matter collided with these planetesimals and
gravitationally stuck to them in a process called accretion. As
the bodies orbited and rotated and gravity brought in more
dust and gas, the planetesimals accreted into protoplanets, and then
(03:23):
into the planets we currently know and love. The planets
are wildly far apart. We measure this in astronomical units,
or AUs. An astronomical unit is the distance between the
Earth and the Sun, which is about 150 million kilometers.
For today's purposes, we're mostly interested in the area between
(03:44):
Mars and Jupiter. Mars lies about 1.5 AU from the Sun,
or 225 million kilometers away. Jupiter is about 5.2 AU
from the Sun, or 780 million kilometers. That means that
there's about 3.7 AU between Mars and Jupiter, or 555
(04:05):
million kilometers. It seems like there's enough room there for
another planet, right? So why isn't there? What happened in
that area during the formation of the solar system? Some
astronomers have suggested that a separate planet or protoplanet actually
formed between them, but that the impact of a high-speed
comet broke up and scattered this newly formed body to
(04:28):
create what we now know as the main asteroid belt. Today,
it's thought more likely that asteroids are leftover matter from
the solar system's formation that never successfully came together as
one planet. But if that's accurate, why did it never
come together? Well, Jupiter is big. By big, I mean
(04:50):
it's about two and a half times as massive as
the rest of the planets put together. It's much closer
to being like our sun than it is to being
like Earth or Mars. Jupiter's mass is plenty enough to
disturb any rocky matter that fell in between it and Mars.
Its strong gravitational pull could have caused any potential protoplanets
to collide and break apart. leaving us with this large,
(05:14):
spread-out collection of stuff that orbits around the Sun in
an elliptical pattern, mostly in the same direction as the planets.
With its center around 2.7 AU from the Sun, the
main asteroid belt separates the rocky inner planets from the
massive gas giants farther out. The gravitational force of Jupiter
still affects the asteroid belt to this day. Its giant
(05:38):
mass disturbs the path of asteroids and creates large gaps
known as Kirkwood gaps. This happens due to orbital resonance,
which happens when orbiting bodies periodically line up and exert
gravitational force on each other. For example, an asteroid might
make two full orbits around the Sun in the time
it takes Jupiter to make one. So, every other orbit,
(06:01):
that asteroid would line up with Jupiter, and its orbit
would display a slight change. This has caused several different
groups of asteroids to cluster together over time, depending on
how frequently they circle the sun. It also leaves several
gaps where there aren't any asteroids. Due to orbital interactions
with Jupiter, and even more occasional ones with Mars, Earth,
(06:24):
and other objects, the asteroid belt is always changing. Asteroids
can get knocked or pulled out of the belt and
flung through space, even towards us. A network of observatories
and space telescopes, wrangled by organizations like NASA's Planetary Defense
Coordination Office, keeps track of near-Earth objects, that is, asteroids
(06:46):
and other things like comets that graze Earth's orbit and
pose a potential hazard. But large impacts are thought to
be extremely few and far between, on the scale of
hundreds of thousands, if not hundreds of millions, of years.
Most asteroids in the main belt are only the size
of a pebble. Despite the vast amount of space that
(07:07):
it takes up, astronomers estimate the total mass of the
entire belt to be less than 1,000th the mass of
the Earth, or less than half of the mass of
our moon, and even that might be a generous estimate.
The largest object in the belt is the dwarf planet Ceres,
which has a diameter of about 600 miles or 1,000 kilometers.
(07:28):
Only another 15 asteroids or so have a diameter over
a quarter of that. The largest object in the belt
currently defined as an asteroid is Vesta, which is a
little over half the size of Ceres. The majority of
the asteroids in the main belt fall under three main categories, carbonaceous, silicaceous,
or metallic. Carbonaceous, or C-type asteroids, make up about 75%
(07:53):
of all known asteroids. They're carbon-rich, made up of clay
and silicate rocks, like Earth's crust, very dark because they
absorb light, and are found on the outer edges of
the belt. Silicaceous or S-type asteroids make up about 17%
of all known asteroids. Their composition is mainly stony, and
(08:14):
they're found along the inner edge of the belt. The
remaining 8% of asteroids are metallic or M-type. These are
made mostly of denser metals like iron and nickel, and
are found in the middle region of the belt. the
different types may have formed up due to their distance
from the sun, or from actions like dense planetismals getting
(08:34):
their mantles stripped away by early collisions. Note that any
given asteroid can contain different types of material. Many have
smashed into each other over time, and a few, like Vesta,
even formed layers like Earth did. NASA's Dawn mission visited
Vesta from 2011 to 2012, and found evidence that objects
(08:56):
like Vesta were able to form a core, mantle, and
crust because of radioactive materials that they had created early on,
which basically melted the rest of the stuff they were
made up of, allowing the materials of different densities to
separate out, with metals sinking to the core. In case
you're planning a trip, don't worry. The internal radiation on
(09:17):
Vesta seems to have decayed since then. Many asteroids contain
water ice as well. We only discovered that asteroids currently
contain water ice about 20 years ago in the early 2000s,
when researchers observed tails coming off of asteroids in the
main belt, similar to comets' tails. Heat from the sun
(09:37):
causes the ice in them to evaporate, and a trail
of gas and dust is left behind as the object
moves through space. These are called main belt comets. We've
since learned that some asteroid tails are not made of water,
but of other materials shed due to small collisions or
a rapid rotation. These are part of a broader category
(09:59):
called activations. asteroids. The biggest revelation coming from the discovery
of so-called main belt comets is the possibility that icy
asteroids may have crashed into Earth and provided us with
our water, or even with life. Astronomers used to think
that ice from regular comets provided the Earth with its water.
(10:19):
But recent observations have shown that comet water doesn't tend
to have much in common with our planet's water. If
asteroidal water is anything like ours, icy asteroids may provide
us with important insights into the Earth's formation and even
our own existence. And our main asteroid belt isn't unique.
(10:40):
Farther out, beyond Neptune, the Kuiper Belt exists some 30
to 50 AU from the Sun. It's another, even larger
belt of small objects left over from the solar system's formation.
It contains what's now recognized as the dwarf planet Pluto.
Just as Jupiter may have prevented a full planet from
forming where the main asteroid belt lies, Neptune may have
(11:03):
caused the creation of these small icy bodies that make
up the Kuiper Belt. Astronomers estimate the Kuiper Belt to
contain a total mass of about one-tenth the mass of Earth.
and it's slowly eroding out of orbit, with bodies flying
either out into space or deeper into the solar system
to become comets or asteroids. The idea of the Kuiper
(11:25):
Belt was first proposed by astronomer Gerard Kuiper in 1951,
but its existence wasn't confirmed until 1992, when astronomers observed
the first smaller Kuiper Belt objects. But what about outside
of our solar system? Having a sample size of one
solar system to look at around us means that we
(11:46):
can't assume anything here is universally normal, but we have
found asteroid belts around several other stars. Observing them may
help us better understand the process of planetary formation overall.
Even here, although over 1.56 million asteroids have already been discovered,
new-to-us asteroids are being identified all the time, presenting us
(12:09):
with new opportunities to learn, and Maybe in the future,
even visit, mine for materials, or use as ready-made platforms
for further exploration. Today's episode is based on the article
How Asteroid Belts Work on HowStuffWorks.com, written by John Fuller.
(12:30):
BrainStuff is a production of iHeart Podcasts in partnership with
HowStuffWorks.com and is produced by Tyler Klang. For more shows
from iHeart Podcasts, visit the iHeartRadio app. Apple Podcasts, or
wherever you listen to your favorite shows.
Welcome to BrainStuff, a production of iHeartRadio. Hey, BrainStuff. Lauren
Vogelbaum here. In the Star Wars film, The Empire Strikes Back,
Han Solo and the crew escape from the planet Hoth,
only to fly straight into an asteroid field. The field
is densely packed, and with huge, spinning rocks careening around
(00:25):
the Millennium Falcon, Han must deftly maneuver his spaceship to safety—
the odds of which, calculated by C-3PO, are only 3,720
to 1. Of course, Star Wars isn't known for its
scientific realism. If a spaceship launched from Earth toward our
(00:45):
solar system's asteroid belt and attempted to fly through it,
would it look the same, with dangerous debris flying everywhere,
putting the mission in jeopardy? As it turns out, navigating
through our asteroid belt wouldn't be nearly as dramatic. Only
a handful of asteroids are big enough to cause much
damage to a spacecraft, and there's way more space between
(01:06):
them than you might think. But that doesn't mean that
our solar system's main asteroid belt, located between the orbits
of Mars and Jupiter, is any less interesting than the
field in Star Wars. The more astronomers study the composition, activity,
and formation of asteroids in their orbit around the sun,
the more we understand about how the entire solar system
(01:28):
came to be. Some theories even suggest that life on
Earth started with asteroids colliding with our planet in its
early stages. And on the flip side, an asteroid's crash
was a major factor in the mass extinction of the
dinosaurs 65 million years ago. Today, let's talk about our
asteroid belt. Asteroids are remnants of our solar system's early days,
(01:53):
when the planets were still forming. Some are the oldest,
most unchanged objects we know of, untouched by processes like
volcanism that shaped larger bodies. There are several theories that
attempt to explain how the solar system began, but the
most widely accepted is known as the nebular theory. It
goes that our solar system started as a large, shapeless
(02:16):
cloud of gas, dust, and ice. But then something disrupted
the mass and set things in motion, perhaps the explosion
of a nearby star. If you've ever watched figure skating,
you've seen how skaters can spin much faster if they
pull their arms closer to their bodies. The more concentrated
their body mass is, the faster they're able to rotate.
(02:39):
The same thing happened with our solar system. the hypothetical
explosion nearby squeezed the unformed gas and dust together, which
began to spin faster and faster in a circle. As
the sun formed in the middle, the cloud started to
flatten out into a disk, sort of like a frisbee
or a pancake made up of tiny grains of dust. Eventually,
(03:02):
dust particles began to join together and form larger bodies
called planetismals. Even more matter collided with these planetesimals and
gravitationally stuck to them in a process called accretion. As
the bodies orbited and rotated and gravity brought in more
dust and gas, the planetesimals accreted into protoplanets, and then
(03:23):
into the planets we currently know and love. The planets
are wildly far apart. We measure this in astronomical units,
or AUs. An astronomical unit is the distance between the
Earth and the Sun, which is about 150 million kilometers.
For today's purposes, we're mostly interested in the area between
(03:44):
Mars and Jupiter. Mars lies about 1.5 AU from the Sun,
or 225 million kilometers away. Jupiter is about 5.2 AU
from the Sun, or 780 million kilometers. That means that
there's about 3.7 AU between Mars and Jupiter, or 555
(04:05):
million kilometers. It seems like there's enough room there for
another planet, right? So why isn't there? What happened in
that area during the formation of the solar system? Some
astronomers have suggested that a separate planet or protoplanet actually
formed between them, but that the impact of a high-speed
comet broke up and scattered this newly formed body to
(04:28):
create what we now know as the main asteroid belt. Today,
it's thought more likely that asteroids are leftover matter from
the solar system's formation that never successfully came together as
one planet. But if that's accurate, why did it never
come together? Well, Jupiter is big. By big, I mean
(04:50):
it's about two and a half times as massive as
the rest of the planets put together. It's much closer
to being like our sun than it is to being
like Earth or Mars. Jupiter's mass is plenty enough to
disturb any rocky matter that fell in between it and Mars.
Its strong gravitational pull could have caused any potential protoplanets
to collide and break apart. leaving us with this large,
(05:14):
spread-out collection of stuff that orbits around the Sun in
an elliptical pattern, mostly in the same direction as the planets.
With its center around 2.7 AU from the Sun, the
main asteroid belt separates the rocky inner planets from the
massive gas giants farther out. The gravitational force of Jupiter
still affects the asteroid belt to this day. Its giant
(05:38):
mass disturbs the path of asteroids and creates large gaps
known as Kirkwood gaps. This happens due to orbital resonance,
which happens when orbiting bodies periodically line up and exert
gravitational force on each other. For example, an asteroid might
make two full orbits around the Sun in the time
it takes Jupiter to make one. So, every other orbit,
(06:01):
that asteroid would line up with Jupiter, and its orbit
would display a slight change. This has caused several different
groups of asteroids to cluster together over time, depending on
how frequently they circle the sun. It also leaves several
gaps where there aren't any asteroids. Due to orbital interactions
with Jupiter, and even more occasional ones with Mars, Earth,
(06:24):
and other objects, the asteroid belt is always changing. Asteroids
can get knocked or pulled out of the belt and
flung through space, even towards us. A network of observatories
and space telescopes, wrangled by organizations like NASA's Planetary Defense
Coordination Office, keeps track of near-Earth objects, that is, asteroids
(06:46):
and other things like comets that graze Earth's orbit and
pose a potential hazard. But large impacts are thought to
be extremely few and far between, on the scale of
hundreds of thousands, if not hundreds of millions, of years.
Most asteroids in the main belt are only the size
of a pebble. Despite the vast amount of space that
(07:07):
it takes up, astronomers estimate the total mass of the
entire belt to be less than 1,000th the mass of
the Earth, or less than half of the mass of
our moon, and even that might be a generous estimate.
The largest object in the belt is the dwarf planet Ceres,
which has a diameter of about 600 miles or 1,000 kilometers.
(07:28):
Only another 15 asteroids or so have a diameter over
a quarter of that. The largest object in the belt
currently defined as an asteroid is Vesta, which is a
little over half the size of Ceres. The majority of
the asteroids in the main belt fall under three main categories, carbonaceous, silicaceous,
or metallic. Carbonaceous, or C-type asteroids, make up about 75%
(07:53):
of all known asteroids. They're carbon-rich, made up of clay
and silicate rocks, like Earth's crust, very dark because they
absorb light, and are found on the outer edges of
the belt. Silicaceous or S-type asteroids make up about 17%
of all known asteroids. Their composition is mainly stony, and
(08:14):
they're found along the inner edge of the belt. The
remaining 8% of asteroids are metallic or M-type. These are
made mostly of denser metals like iron and nickel, and
are found in the middle region of the belt. the
different types may have formed up due to their distance
from the sun, or from actions like dense planetismals getting
(08:34):
their mantles stripped away by early collisions. Note that any
given asteroid can contain different types of material. Many have
smashed into each other over time, and a few, like Vesta,
even formed layers like Earth did. NASA's Dawn mission visited
Vesta from 2011 to 2012, and found evidence that objects
(08:56):
like Vesta were able to form a core, mantle, and
crust because of radioactive materials that they had created early on,
which basically melted the rest of the stuff they were
made up of, allowing the materials of different densities to
separate out, with metals sinking to the core. In case
you're planning a trip, don't worry. The internal radiation on
(09:17):
Vesta seems to have decayed since then. Many asteroids contain
water ice as well. We only discovered that asteroids currently
contain water ice about 20 years ago in the early 2000s,
when researchers observed tails coming off of asteroids in the
main belt, similar to comets' tails. Heat from the sun
(09:37):
causes the ice in them to evaporate, and a trail
of gas and dust is left behind as the object
moves through space. These are called main belt comets. We've
since learned that some asteroid tails are not made of water,
but of other materials shed due to small collisions or
a rapid rotation. These are part of a broader category
(09:59):
called activations. asteroids. The biggest revelation coming from the discovery
of so-called main belt comets is the possibility that icy
asteroids may have crashed into Earth and provided us with
our water, or even with life. Astronomers used to think
that ice from regular comets provided the Earth with its water.
(10:19):
But recent observations have shown that comet water doesn't tend
to have much in common with our planet's water. If
asteroidal water is anything like ours, icy asteroids may provide
us with important insights into the Earth's formation and even
our own existence. And our main asteroid belt isn't unique.
(10:40):
Farther out, beyond Neptune, the Kuiper Belt exists some 30
to 50 AU from the Sun. It's another, even larger
belt of small objects left over from the solar system's formation.
It contains what's now recognized as the dwarf planet Pluto.
Just as Jupiter may have prevented a full planet from
forming where the main asteroid belt lies, Neptune may have
(11:03):
caused the creation of these small icy bodies that make
up the Kuiper Belt. Astronomers estimate the Kuiper Belt to
contain a total mass of about one-tenth the mass of Earth.
and it's slowly eroding out of orbit, with bodies flying
either out into space or deeper into the solar system
to become comets or asteroids. The idea of the Kuiper
(11:25):
Belt was first proposed by astronomer Gerard Kuiper in 1951,
but its existence wasn't confirmed until 1992, when astronomers observed
the first smaller Kuiper Belt objects. But what about outside
of our solar system? Having a sample size of one
solar system to look at around us means that we
(11:46):
can't assume anything here is universally normal, but we have
found asteroid belts around several other stars. Observing them may
help us better understand the process of planetary formation overall.
Even here, although over 1.56 million asteroids have already been discovered,
new-to-us asteroids are being identified all the time, presenting us
(12:09):
with new opportunities to learn, and Maybe in the future,
even visit, mine for materials, or use as ready-made platforms
for further exploration. Today's episode is based on the article
How Asteroid Belts Work on HowStuffWorks.com, written by John Fuller.
(12:30):
BrainStuff is a production of iHeart Podcasts in partnership with
HowStuffWorks.com and is produced by Tyler Klang. For more shows
from iHeart Podcasts, visit the iHeartRadio app. Apple Podcasts, or
wherever you listen to your favorite shows.
BrainStuff News
Host
Lauren Vogelbaum
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