You've heard about the main asteroid
belt and the faraway Kyper belt, but
there's a third group of unexplored
asteroids that could revolutionize
everything we know about the origins of
our solar system. For billions of years,
Jupiter has been shephering ancient
space rocks in its orbit around the sun.
These Trojan asteroids are pristine time
capsules that have witnessed our solar
systems entire history. From chaotic
beginnings to today's delicate
stability.
Despite numbering in the thousands, we
still haven't seen them up close. But
that's all set to change very soon.
I'm Alex Mccoan and you're watching
Astramm. Join me today as we take a
closer look at these peculiar relics,
what we know about them so far, and what
mysteries NASA hopes to untangle with
its ambitious Lucy mission set to bring
us face to face with the Trojans for the
first time in 2027.
And we'll even check out two very
special asteroids it has seen up close
on its way there.
Jupiter's first Trojan was discovered in
1906 by astronomer Max Wolf. He noticed
it remained fixed 60° ahead of Jupiter
as the planet orbited the sun. A century
earlier, Ysef Luigi Lrange had theorized
that small objects like asteroids could
remain stable at specific points in a
two-body system. Until Vulf's discovery,
Lrange's prediction had just been a
mathematical exercise. But now
scientists had evidence. He was right.
Now these gravitationally stable regions
bear his name and are known as Lrangee
points. Trojan asteroids cluster in two
swarms, one 60° in front of Jupiter at
the L4 point and 160° behind Jupiter at
the L5 point.
These asteroids are all named after
characters in Homer's Iliad, which tells
the story of the Trojan War. L4
asteroids are named after Greek heroes,
while L5 asteroids bear the names of
Trojan heroes. So, the two swarms are
known as the Greek camp and the Trojan
camp.
While Neptune, Mars, and Earth have
Trojans, too, Jupiters are the most
numerous with over 15,000 currently
confirmed, and some calculations
predicting there could be up to 260,000.
But how did they end up there? Why
aren't they in the main belt or the
Kyper belt along with the majority of
asteroids? To understand that we have to
go back 4 billion years to the origins
of our solar system.
The most common theory of how the outer
solar system came to be is the nice
model. It proposes that the giant
planets formed much closer together and
to the sun than their current positions.
Surrounding them was an outer disc of
icy bodies, asteroids, and
planetessimals.
Every now and then, a rocky body would
interact with the outermost planet's
orbit before being scattered inward by
the planet's gravity. This in turn moves
the planet outwards. This process
repeats as the rocky body approaches the
next planet to the sun and the next
until it finally reaches Jupiter.
Jupiter's gravity is so immense, these
rocks are sent into highly elliptical
orbits or even ejected from the solar
system. while Jupiter moves slightly
closer to the sun.
After millions of years of slight shifts
in orbital distance, the model predicts
something remarkable and sudden
happened, throwing the solar system into
chaos.
About 880 million years after our solar
system formed, Jupiter and Saturn
entered a dangerous rhythm where Jupiter
orbited the sun twice for every one
orbit by Saturn. This is known as a 21
orbital resonance and it caused a sharp
instability in the orbits of all the gas
giants. This instability pushed Neptune,
Uranus, and Saturn further out from the
sun, drove Neptune out past the orbit of
Uranus, and pulled Jupiter violently
closer to the sun, also known as the
jumping Jupiter theory.
Meanwhile, the neat disc of planet
decimals that had orbited the sun
together for eons were flung in all
directions. Some towards the sun, some
outward towards deep space. Some of
these ancient rocks became trapped in
Jupiter's stable lrangee points,
becoming the Trojans we see today. Newer
theories suggest the Trojans might have
been captured by giant planets falling
out of resonance rather than into it. If
this model is correct, the Trojans
aren't just random space rocks. They're
preserved samples of the very material
that built our giant planets. They could
be similar in composition to objects in
the distant Kyper belt, containing
pristine material that has remained
largely unchanged since the solar
systems formation 4.5 billion years ago.
Rocks bombarding into each other might
sound like a destructive way to build a
planet, but those pristine materials are
actually vital components of the
structures we see. It's thought that
asteroids bombarding Earth are a
prominent source of the planet's water,
hydrating its geology, which is ironic
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So far, our understanding of Trojans has
been limited to telescopic observations
on Earth and in orbit. We think most
range from a few kilome to over 200 km
in diameter, but we don't really know
for sure. To estimate their sizes,
scientists have to assume a specific
albido value, which we don't have yet.
So, it's possible our predictions are
way off.
We also know they have porous surfaces
containing several fine particle
silicates which support the theory that
these rocks are primordial and
originated in the Kyper belt. Still,
several questions about this diverse
group of primordial rocks remain. How
old are they really? What is their
precise composition? Do they harbor ice,
minerals, or organic compounds?
What are their masses and densities
like? What goes on under the surface? Do
they have any craters? Could Trojan
asteroids have rings or satellites of
their own?
That's why in 2021, NASA launched one of
the most ambitious missions in solar
system exploration history, a 12-year
Odyssey. The Lucy mission will fly by a
record-breaking 11 asteroids, including
three main belt asteroids, five Trojans,
and three of their satellites to answer
our burning questions about these
mysterious space rocks. 2.
On the 16th of October 2021, the Lucy
spacecraft lifted off from Cape
Canaveral aboard an Atlas 5 401 rocket,
beginning a journey so complex and
farreaching that it seems almost
impossible.
Named after the famous homminin fossil
that revolutionized our understanding of
human evolution, Lucy aims to do the
same for our cosmic origins.
Over 12 years, Lucy will make three and
a half giant loops around the sun,
visiting both the L4 and L5 swarms of
Jupiter's Trojan asteroids, the first to
ever do so. To reach its targets, Lucy
relied on multiple gravity assists from
Earth. The spacecraft first swung past
Earth in October 2022, gaining momentum
and adjusting its trajectory. It flew by
the asteroid in late 2023 and flew by
Earth again in late 2024.
This slingshot set it on a course to fly
by Donald Johansson, a main belt
asteroid in April 2025.
From there, its new trajectory put it on
a course to reach Jupiter's L4 Trojan
Swarm by 2027.
Once there, Lucy will visit four Trojan
asteroids between 2027 and 2028.
Urbates, Polyle, Lucas, and Orus, which
will set it on a path back to Earth,
making Lucy the first spacecraft to
return to near Earth orbit from the
outer solar system.
After a third and final assist from
Earth in 2030, Lucy will be catapulted
towards Jupiter's L5 Trojan swarm where
it will encounter asteroids Patrick
Menuetius in 2033.
Lucy carries three key instruments to
help it study asteroids in
neverbeforeseen detail. LRALF combines a
multisspectral camera and an infrared
spectrometer to analyze surface
composition. The long range
reconnaissance imager captures high
resolution details while the thermal
emission spectrometer will measure
thermal properties of the surface. Lucy
is also kitted out with a high gain
antenna to communicate with Earth. By
measuring the Doppler shift in these
radio waves returning to Earth,
scientists will be able to determine how
much gravity and therefore mass each
asteroid has.
And so far, the mission hasn't been
without its challenges.
Shortly after launch, one of Lucy's
massive solar arrays failed to deploy
and latch. These arrays are critical for
powering the spacecraft in the dim outer
solar system. Encountering an anomaly
like this puts the whole mission at
risk. Luckily though, through months of
careful analysis and ingenuity, the
flight operations team managed to
address the issue, saving Lucy and
allowing her to continue on her journey,
which I've done a more in-depth video
about here.
While we're still a couple of years out
from the Trojan targets, Lucy has
already given us our first taste of
discovery. In November 2023, Lucy made
its first asteroid encounter, flying
past the smallest main belt asteroid
ever visited by a spacecraft,
Dinkesh. What scientists expected to be
a routine calibration flyby turned into
something extraordinary when Dinkesh
revealed two big surprises.
First, it was discovered to have a
previously unknown satellite, Salam. And
secondly, Salam turned out to be a
contact binary asteroid, meaning it's
made up of two separate bodies that
touch each other, probably formed when
two asteroids gently collided and fused
together. That's what gives it its
peanut-like shape. This is a big deal
because Selam is the first contact
binary satellite ever directly observed.
Lucy's initial observation showed that
Dinkanesh resembles the near Earth
asteroids Rayugu and Bennu, allowing
scientists to compare similar bodies in
different parts of the solar system.
In April 2025, Lucy completed its second
asteroid flyby. This time it soared
within 960 km of a 150 million-year-old
main belt asteroid. Donald Johansson
formed when two smaller bodies collided.
Donald Johansson proved to have a
strikingly complicated geology according
to NASA. It's also larger than
originally estimated, measuring 8 km
long and 3.5 km wide. At the time of
writing, NASA is still analyzing data
from Lucy's spectroscopers that will
provide more insight into this asteroid
binary.
But these main belt findings are more of
a bonus than the mission's bread and
butter. Like the Dinkesh flyby before
it, this was a systems test for the true
mission to the Trojan asteroids.
Lucy's mission is particularly exciting
because it might resolve a recent
discovery that left astronomers
perplexed. Astronomers assumed for years
that each swarm would have uniform
composition, but spectroscopic analysis
revealed the opposite. Not only do the
Greek and Trojan camps differ from each
other, but within each camp there's
remarkable diversity in color and
reflectivity.
Astronomers have identified distinct red
and less red spectral groups among the
Trojans similar to DT type and Ptype
asteroids. Their albidos range from
extremely dark to moderately reflective
and their color variations suggest
dramatically different surface
compositions.
Perhaps the most surprising is the
discovery of the Eurabates family in the
L4 swarm, which appears completely
anomalous compared to other Trojans.
This diversity hints that Trojans formed
in different regions before Jupiter
carried them. Lucy may confirm they're
essentially a cosmic sample platter.
Materials from across the solar system
gathered in one accessible location.
This would revolutionize our
understanding of how materials were
distributed during our solar systems
formation and potentially explain how
Earth acquired the necessary building
blocks for life.
In an age of instant gratification, Lucy
asks us to do something radical. Wait.
When Lucy arrives at the Trojans in
2027, it will begin a 5-year exploration
that won't be complete until 2033.
In this time, the gradual influx of new
information could trigger a Voyageresque
anticipation in the public for what
breakthroughs may emerge.
Still, the real work begins after the
mission ends. Students who were in
elementary school during launch could be
analyzing the mission's findings as
graduate students.
Space exploration is a
multi-generational commitment, but as
they say, good things require patience,
and this mission is no exception. For
now, all we can do is wait.
Thanks for watching. If you enjoyed this
video, you might want to check out my
other videos on asteroids like this one.
All the best and see you next time.
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